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DESCRIBING ALTERED VOlCANIC ROCKS

A. Bubble-wa shardsDelicate bubble-wall and platy shards (S) have been preserved wichin a carbonate nodule in the proximal, carbonate zone beneath the Hrcules VHMS deposic. The carbonate nodule comprises quartz + calcite + chlorite-altered pumice breccia. Plae polarised light.Sample MR96-57, Cambrian Hrcules Pumice Formation, Central Volcante Complex, Mount Read Volcantes, Hrcules footwall, western Tasmania.B. Pumice shardsDelicate tube pumice clasts (/J) have beeen preserved in intensely quartz + chlorite ( muscovite)-altered pumice bteccia from the footwall to the Gossan Hill VHMS deposit. The tube vesicles have been coated in thin films of chlorite and filled with quartz, and vesicle walls have been altered to quartz. Plae polarised light. Sample 138752, Arcbaean Golden Grove Formation, Gossan Hill footwall, Western Australia.C. ShardsThis quartz nodule (Q) from the footwall, quartz + chlorite ( muscovite) zone contains delicate shard textures. Plae polarised light.Sample 138795, Arcbaean Golden Grove Formation, Gossan Hill footwall, Western Australia.-GSE 2.6 | Photographs of intensely altered pumice breccias with delicate primary textures.RBK 2.5 | Pairs of hand-specimen and thin-section photographs of increasing intensity of alteration in rhyolitic feldspar-phyric pumice breccia in the Hrcules -c-Them Central Volcanic Complex, western Tasmania. (A) Hand-specimen and (B) thin-section photographs of subtle, domainal, albite + sericite- and - cniorite-altered pumice breccia (sample MR96-63) showing excellent preservaron of volcanic textures. Plagioclase crystals are partly replaced by albite. ie-ricJ domains, tube vesicles and clast margins are lined with sericite and albite + quartz altered. In contrast, pumice clasts and shards in the chlorite-rich --s ="= pervasively sericite - chlorite alteres. The Al s 40 and CCPI 26. C) Hand-soeemen and D) thin-section photographs of weak. domainal, albite + fe- and sericite + chlorite-altered pumice breccia (sample MR96-54). Volcanic textures are well preserved in the albite-rich domains and poorly preserved in the s--* domains. Plagioclase crystals (P) are sericite albite opaques altered and have albite overgrowths or nodules (a/6), which locally preserve delicate textures. Elsewhere vesicles are coated in sericite and filled with albite. Pumice walls are albite + quartz altered and sericite chlorite + hematite fiamme > are abundant. The Al = 58 and CCPI = 37. (E) Hand-specimen and (F) thin-section photographs of modrate, pervasive, albite + sericite-altered pumice t ;a-ole MR96-48) with partly preserved pumice textures and plagioclase crystals. Sericite fiamme (F) and sericite + hematite stylolites are abundant. 3J6S or overgrowths of albite oceur around calcite and albite + hematite-altered plagioclase crystals (P). The Al = 70 and CCPI = 38. (G) Hand-specimen and (H) - - t . :-pr Dtographs of strong, pervasive, quartz + sericite + pynte-altered pumice breccia (sample MR9&-50), Priman/ volcanic textures are faint, with sparse srcte-afiered pumice clasts or fiamme (F). Plagioclase crystals (P) are polycrystalline-quartz pyrite altered. The Al = 98 and CCPI = 64. (I) Hand-specimen and- -scor photographs of intense, schistose, quartz + serice + pyrite-altered pumice breccia sampie UR96-4B). No rellct plagiociase or volcanic textures are ^ =:-- - seor.cn: in nar.d specimen irregular lenses o sericite resembie fiamme (F). This alteration facles is pervasive and strongly foliated. The Al = 99 and: - = 30.COMMON ALTERATION TEXTURES AND ZONATION PATTERNS I 39A. Spherulites and obsidian in rhyolitePink, isolated spherLilitcs and densely microspherulicic flow bands are enclosed in black obsidian in this flow-bandcd rhyolite. Spherulites are radiating aggregates or bundles of acicular and fibrous crystals. They vary in shape from spherical to bow-tie shaped sheafs and axiolitic bundles, and are commonly composed of feldspar or intergrowths of alkali feldspar, plagioclase, cristobalite or tridymke and clinopyroxene (Lofgren, 1971b). Spherulites are typically the product of high-temperature (above the glass-transition temperature) devitrification of silicic glass (Lofgren, 1971a). Sample NG4, recent Ngongotaha lava dome, Hendersons quarry, Rotorua, New Zealand.B. Lithophysae in rhyoliteThis red albite + quartz + hemarite-altered, flow-banded quartz + plagioclase-phyric rhyolite contains abundarte spherulites and lithophysae. The lithophysae are filled with layered quartz.Sample from the Lower Devenan Snowy River Volcantes, Flukes Knob rea, Victoria.C. Variles in basaltDark spots in this basalt outcrop are variles: radial orsheaf-like aggregates of plagioclase and pyroxene, olivineor iron oxides, and are similar to spherulites, but onlyoceur in mane facies (cf. Fowler et al., 1987; Williamsetal., 1982).Shirakawa quarry, Miocene Green Tuff Belt, Odate,Japan.D. Micropoikilitic texture in thin sectionThegroundmass of this rhyolite is densely micropoikilitic; comprising patches of optically continuous quartz, which endose variably orienced laths of sericitised albite. Poikilitic and micropoikilitic texture (snowflake texture) comprise an optically continuous crystal enclosing numerous randomly oriented inclusions of a different composition (Anderson, 1969). The boundaries between rhe micropoikilitic quartz domains in this sample are hghlighted by corteen rrations of sericite. Cross polarised light.Sample 133921, Cambrian Mount Black Formation, Central Volcante Complex, Mount Read Volcantes, Mount Black, western Tasmania.FIGURE 3.1 | Examples of high-temperature devitrification textures.

40 I CHAPTER3

A. Altered macroperliteRelict macroperlitic factures in this coherent dacite are enhanced by dark grey sericite + chlorite-altered zones along and adjacent to che perlitic fractures. The arcuate shape of the fractures is preserved in some reas. The perlite cores are pink albite + quartz + sericite altered. Cambrian Mount Black Formation, Central Vokanics Complex, Mount Read Volcantes, Fieman Road, western Tasmania.B. Relict perlite in thin sectionThe formerly glassy groundmass of this rhyolite preserves perlitic fractures. Perlitic fractures are a netvvork of fine typically concentric, arcuate fractures that endose glassy or originally glassy cores. Here, the perlitic fractures are filled with dark, mixed-layer smectite/chlorite and the groundmass adjacent to the fractures is clinoptilolite altered. The perlitic cores are partly glassy and partly smectite altered. Plae polarised light. Sample J6-737 m, Miocene Nishikurosawa Formation, Hokuroku Basin, Green Tuff Belt, Odate, Japan.C. Banded perliteThis finely flow-banded, plagioclase-phyric rhyolite contains an intersecting fracture netvvork of sub-parallel long fractures linked by short cross fractures (banded perlite) superimposed on the flow-banded texture. Sample KB257, Siluro-Devonian rhyolite, Ural Volcanics, Ural Ridges rea, New South Wales.D. Banded perlite in thin sectionIn thin section, concentrations of sericite hematite mark the relict perlitic fractures. The pal flow bands comprise a fine-grained mosaic of feldspar + quartz, whereas the darker bands consist of sericite + feldspar + quartz + chlorite. Disseminated fine-grained hematite occurs throughout the groundmass. Plae polarised light.Sample KB257, Siluro-Devonian rhyolite, Ural Volcanics, Ural Ridges rea, New South Wales.FIGURE 3.2 | Examplesof perlite.

Twotexturaltypesofselectivealterationoccur:disseminated alteracin (or selective-pervasive alteration), which refers to the replacement of selective pre-existing phases throughout the host rock; and domainal alteration, which refers to the alteration of parches, pods, or groups of clasts within the host rock (Fig. 3.3F, G and H). In addition, selective alteration may result in concentrically zoned alteration facies within clasts or alteration halos around clasts (Fig. 3.31, J, Kand L). Selective alteration can result in a parchy or mortled appearance (e.g. Alien, 1988).Common microscopic selective replacement textures are pseudomorphs, partial pseudomorphs (cleavage and rim texture, core and zonal texture, core and rim texture and skeletal texture), overgrowths on pre-existing components, and spheroids or nodules (Fig. 3.4B to L: Dimroth and Lichtblau, 1979; Craig and Vaughan, 1981; Ineson, 1989). Carbonate and zeolite nodules are common in submarine volcaniclastic facies and can have a wide variety of grainsizes from 0.2 to greater than 20 mm (Fig. 3.41 to L: Franklin et al, 1975; Lees, 1987; Khin Zaw and Large, 1992; Hill and Orth, 1994; Alien, 1997).Vein haloVein-halo alteration involves the replacement of either the whole rock (pervasive alteration: Fig. 3.3E) or specific pre-existing phases (selective alteration) in restricted reas, such as the halos around veins, intrusin contacts, or at stracigraphic contacts. Alteration progresses in fronts, moving out from fractures or contacts into the adjacent wall rock. Vein-halo alteration has also been termed infiltration metasomatism, vein-veinlec, reaction rim, vein-wall-rock, vein-envelope, veinlet-controlled and fracture-controlled alteration (e.g. Titley etal., 1978;Titley, 1982; Thompson andThompson, 1996; Doyle, 2001).Infill texturesInfill or open space-filling textures result from the precipitacin of new mineral phases from solution into open spaces or cavities such as pore spaces, vesicles, inter-clast space, vugs and fractures (Taylor, 1992). Infill textures are characterised by well-developed crysral faces, zoned crystals and mineral banding (Craig and Vaughan, 1981). Silicate, carbonates, oxides, sulfates and sulfides all oceur as void fill in altered volcanic rocks.Infill results from the precipitation of minerals from solution. The first mineral to be deposited forms a crust on the cavity walls and grows inwards, generally with the development of inward facing crystal faces. Common infill textures include incomplete infill, massive infill, and layered or banded infill (Fig. 3-6: Taylor, 1992). These cexcures can include fibrous, prismatic, spherulitic or equant crystal shapes and exist on a range of scales from micrometres to metres (Dimroth and Lichtblau, 1979; Taylor, 1992).COMMON ALTERATION TEXTURES AND Z0NAT10N PATTERNS | 41 Incomplete infillIncomplete or partial infilling of veins and cavities or dissolution of void fill can leave an open vug in the centre (e.g. Fig.3.5A). In many cases, the resulting infill texture contains well-formed crystals that project into chis vug.Massive infillMassive infill cexcures result from the continuous deposition of a mineral or aggregate of minerals until the cavity is filled (e.g. Fig. 3.5B). Massive infill commonly contains well-formed crystals, especially quartz, feldspar, fluorite, cassiterite, galena, sphalerite and chalcopyrite crystals. Massive, microcrystalline forms also exisr (Taylor, 1992).Layered infillLayered or banded infill textures result from the deposition of a succession of minerals inwards from the cavity or fracture wall (Bateman, 1951). Layered infill textures do not generally contain well-formed crystals, such as comb texture (e.g. Fig. 3.5H), but vary from thin layers of individual minerals to crustiform bands or colloform textures.Dissolution texturesDissolution textures are common in alrered volcanic rocks (Alien, 1990; Alien and Cas, 1990; Marsaglia and Tazaki, 1992; Gifkins and Alien, 2001). They form from the corrosin or leaching of pre-existing phases (either glass or mineral phases), wich or without minor replacement by new mineral phases (Fisher and Schmincke, 1984). For example, leaching of rhyolitic glass is commonly accompanied by crystallisation of muscovite or clay minerals that absorb leached ions from solution (Karkhanis et al., 1980).Dissolution textures include corrosin vugs or dissolution pits, stylolites and solution seams (e.g. Fig. 3.6: Pettijohn, 1957).Corrosin vugsDissolution or corrosin of volcanic glass or pre-existing minerals can crate open cavities or oversized pores (Fig. 3.6A to F) in which infill can oceur synchronous with dissolution or after dissolution (Hay, 1963; Sheppard et al., 1988). In some cases pseudomorphs of minerals or originally glassy particles, such as glass shards, form by dissolution and precipicacion (Riech, 1979; Sheppard et al., 1988). Riech (1979) recognised infill textured zeolites and calcite within clinopyroxenes, and proposed that clinopyroxenes were corroded during diagenesis, creating an open void that was subsequently filled with zeolites and calcite. Similarly, Hay (1963) recognised partial to complete dissolution of glass shards followed by the precipitation of authigenic minerals, especially zeolites, in the new cavities as well as in original pore space.42 I CHAPTER3

A- Pervasively altered rhyoliteIntense, pervasive, fine-grained K-feldspar + quartz alteration has completely replaced the groundmass and plagioclase phenocrysts in chis rhyolite. Sample 143286, Central Volcante Complex, Mount Read Volcantes, Mount Darwin, western Tasmania.B. Selectively altered phenocrystsSericite has selectively altered the coarse prismatic feldspar phenocrysts (F) in this latite. The pal green-grey, fine-grained groundmass is moderately and pervasively phengite + chlorite + ankerite altered, and the amygdales are quartz filled.Sample 144369, Ordovician Lake CowalVolcanics, Junee-NarKomine Volcante Belt, Endeavour 42 prospect, New South Wales.C. Selectively altered pumice clastsLarge pumice clasts (P) in this sample of crystal- and pumice-rich volcanicla.stic breccia have been selectively altered to orange albite + quartz, whereas the finer grained matrix has been altered to green sericite + chlorite + albite. The domainal alteration style enhances its clastic appearance.Sample 31993, Cambrian Mount Julia Member, Tyndall Group, Mount Read Volcantes, Comstock, western Tasmania.D. Selectively altered matrixIn this andesitic volcaniclastic breccia, the matrix is moderately and selectively epidotc altered. In contrast, tbe plagioclase-phyric clasts (C) are weakly chlorite + sericite altered.Sample 144805, Ordovician Mingelo Volcantes, Junee-Narromine Volcanic Belt, Peak Hill, New South Wales.FIGURE 3.3 | Examplesof replacementtexturesinhandspecimen.

COMMON ALTERATION TEXTURES AND ZONATION PATTERNS I 43. Vein halo:::r altere'd zones are restricted to 5 mm halos orpyrite vcinletsr^es adiacent to quartz + accinolitc __= reidspar porphyritic dacite. r-r TH386 271-i m, Cambro-Ordovician Trooper rormation, Seventy Mile Range Group, Mount '-iv- Subprovince, ThalangE 3nded, domainal alteraton faciesAse and disconcinuous pink and green bands in this _--.-e crystai-rich volcaniclastic sandsrone are defined i:~iains of albite + quartz chloricc, and chlorite -serate + magnetite alteraton facies, respectively. The yA are not obviously consistent with grainsize or : -,ent \'ariations; they altrnate on a 21 0 cm scale, in _:erally extensive (1020 m) and are commonly, but: iively, bedding parallel. --.:. 131982, Caminan Mount Julia Member, Tyndall - -.-. Mount Read Volcantes, Lyell Comstock, westernG. Patchy, domainal alteraton faciesTzs domainal, green epidote + quartz and grey albite - -iartz + hematite alteraton facies are distributed in- _: _...: parches with diffuse margins in this cohcrcni r .oclase-phyric dacite. .- ::'U MI42, Cambrian Mount Black Formation, '-al Volcante Complex, Mount Read Volcanics, Tidlah, v-1-' Tasmania.H. Domainal alteraton facies in pseudobrecciaDomainal red albire + quartz and dark green epidote + sricite + albite alteration facies in this sample of macro-rerte gives it a pseudo-polymictic and -clastic texture. However, the red apparent clasts have diffuse margins and identical phenocryst populations to the apparent atrix.Sample 47550, Cambrian Mount Black Formation, Central Volcanics Complex, Mount Read Volcanics, Pieman Road, western Tasmania.

:iGURE 3.3 | Examples of replacement textores in hand specimen, cont.44 I CHAPTER3

I. Zonation within clastsThe andesite and basalt clasts in this polymictic volcanic breccia are concentrically zoned, with sericite + quartr + calcite-akered rims, and chlorite-altered cores. Some of the larger clasts have an additional quarcz + sericite + chlorite-altered core zone. The matrix has been moderacely and pervasively quartz + sericite + calte chlorite altered.Cambrian Que-Hellyer Volcanics, Mount Charter Group, western volcano-sedimentary sequences, Mount Read Volcanics, Hellyer, western Tasmania.J. Zonation within clastsClasts in chis basaltic pebble conglomrate display heterogenous alteration facies, and some clasts are internaUy zoned. The bsale clast (B) has a fine-grained, pal green sericite-rich rim, and darker sericite + chlorite-altered core.Sample 134632, Cambrian Red Lead Formation correlate, Dundas, Kapi Creek, western Tasmania.K. Zonation within pillowsThis metamorphosed, amphibolice-grade lava-pillow has a typical triangular, draped shape and is concentrically zoned. The central red zone is coarse-grained, scapolite-poor and albite + hematite + sericite epidote altered. The average grainsize decreases, and scapolite grainsize and abundance increases, in consecutive zones towards the rim. Biotite + calcite + hornblende + microcline + scapolite + epidoce + quartz compuse the incer-pillow matrix.Proterozoic Corella Formation, Mary Kathleen Group, Malbon River, northuiest Queensland.L. Altered halos around clastsOrange albite + quartz alteration facies is distributed in a halo around a massive, albite-altered dacite clast (C) in chis crystal- and lichic-rich volcaniclastic sandscone. The more pervasive green-grey domain is sericite + chlorite + quartz + albite altered.Sample 132090, Cambrian Mount Julia Member, Tyndall Group, Mount Read Volcanics, Anthony Road, western Tasmania.FIGURE 3.3 | Examples of replacement texturas in hand specimen, cont.A. Microcrystalline texture in thin sectionThe groundmass of chis rhyolite is a microcrystalline mosaic of quartz + feldspar + sericite. Quartz phenocrysts (Q) have been recrystllised. Microcrystalline texture (aphanicic) is a fine-grained granular ccxture where the individual cryscals can be distinguished in thin section. In contrast, cryptocrystalline texture (phaneritic) is where che cryscals are too minute to be distinguished even with che aid of a microscope (Wiliiams ct al., 1982). Cross polarised light.Sample 133318, Cambro-Ordovician Mount Windsor Formation, Seventy Mile Range Group, Thalanga, Queensland.B. Pseudomorphs in thin sectionThe plagioclase phenocrysts in this sericite + quartz + tourmaline-altered andesite were pseudomorphed by tourmaline, and subsequently almost completely replaced by blue-green chlorite. Pseudomorphs are cryscals or aggregaces of crystals thac preserve the shape of a pre-existing mineral or particle (e.g. glass shard or pumice clast) (Spry, 1976). Plae polarised light. Sample 145199, Ordovician Forest Reefi Volcantes, Molong Volcante Belt, Black Rock, New South Wales.C. Pseudomorphs in thin sectionThis thin section of plagioclase + quartz + pyroxenc-phyric rhyolite shows an illite pseudomorph afcer pyroxene. Plagioclase phenocrysts have been altered to K-feldspar and the groundmass comprises a fine-grained mosaic of K-feldspar + quartz + chlorite + smectite. Cross polarised light.Sample KB495, Siluro-Devonian Coan rhyolite, Mount Hope Volcantes, Coan Gonn Peak, New South Wales.D. Cleavage and rim texture in thin sectionThe plagioclase crystals in this basalt have been selectively altered by sericite along cleavage planes. Cleavage and rim textures oceur by selective alteration of mineral grain boundaries and cleavages. It is common in plagioclase, in which montmorillonite, sericite or calcice form along the cleavage planes (Sales and Meyer, 1948). Plae polarised lighe.Sample SVD87a-104.9 m from the Camban Sterling Valley Volcanics, Mount Read Volcanics, Sterling Valley, western Tasmania.FIGURE 3.4 I Examples of replacement texturas in altered volcanic rocks.46 I CHAPTER3

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E. Core and zonal texture in thin sectionZones within plagioclase phenocrysts in chis subtly, .smectite t calcice-altered diorite have been selectively alcered to sericite. These incomplete pseudomorphs, tenned core and zonal texture, are particularly common in zoned feldspar, amphibole and mica crystals where the cores, or one or more zones in zoned minerals, are altered (Barker, 1990). In plagioclase crystals, like tliose pictured here, the calcic zones are typically altered to calcite or sericite (Sales and Meyer, 1948). Plae polarised light.Sample 152958, Pliocene-Pleistocene Luise Volcano, Lihir Island, New Ireland Province, Ladolam epithermal Au mine, Papua New Guinea.F. Core and zonal texture in thin sectionIn this example of core and zonal texture, the core zones of plagioclase phenocrysts have been altered to sericite. The groundmass of this plagioclase + clinopyroxene-phyric basalt was subtly and pervasively smectite + calcite-altered. Plae polarised light. Sample 152830, Pliocene-Pleistocene Luise Volcano, Libir Island, New Ireland Province, Ladolam epithermal Au mine. Papua Neto Guinea.G. Overgrowth texture in thin sectionA discontinulas K-feldspar overgrowth endoses a hematite-altered plagioclase phenocryst {P) in this strongly and pervasively albite + quartz + sericite-altered pumice breccia. K-feldspar nucleated on the plagioclase phenocryst, spread outwards filling vesicles, and replaced vesicle walls in the pumice clasts. Overgrowth textures are mineral rims that may be composed of one or more crystals of similar or different minerals. Cross polarised light.Sample 133814 from the Cambrian Hrcules Pumice Formation, Central Volcanic Complex, Mount Read Volcantes, Hrcules footwall, western Tasmania.H. Altered nodules in pumice brecciaBlue-green celadonite nodules have overprinted pumice clasts in this polymictic volcanic breccia. These nodules are composed of fine-grained aggregates of celadonite opal CT quartz, preserve uncompacted tube and round vesicle pumice textures, and are surrounded by pervasive smectite + mordenite + calcite alteracin facies. Sample FK2, Miocene Byobu-iwa Member, Tokiwa Formation, South Fossa Magna, Green TuffBelt, Fujikawa River, japan.FIGURE 3.4 | Examples of replacement textures in altered volcanic rocks, cont.I. Carbonate spheroidsLarge dolomice spheroids are encloscd in the strongly chlorite + quarcz + dolomite-altered matrix ot chis formerly plagioclase-phyric andesite. Nodules and spheroids are spherical domains of alteration, which may comprise radiating aggregates of fibrous crystals, fine internally concentric struccures, or mosaics of anhedral grains, with orwithout cores (Alien, 1997; HU and Orcli, 1995).Sample 35756, Cambrian Que-Hellyer Volcantes, Mount Charter Group, western volcano-sedimentary sequences, Mount Read Volcantes, Hellyerfootwall, western Tasmania.J. Carbonate spheroids in thin sectionin thin section, the dolomice spheroids display concentric zones and a coarse, radiating, fibrous cexture. This compositional zonacion in the spheroids probably indicaces mltiple stages of carbonate alteration (cf. Hill and Orth, 1995)- Plae polarised light. Sample 135756, Cambrian Que-Hellyer Volcanics, Mount Charter Group, western volcano-sedimentary sequences, Mount Read Volcanics, Hellyer footwall, western Tasmania.K. Carbonate spheroidsCarbonate spheroids are concentrated in individual beds in this strongly chlorite + carbonate + pyrite-alcered laminaced volcaniclastic sandstone. The larger spheroids, which are up to 2 mm in diameter, have coalesced. Sample 138601, Archaean Mb5 Golden Grove Formation, Luke Creek Group, Murchison Volcanics, Golden Grove, Western Australia.L. Carbonate spheroids n thin sectionThin section examinarion shows these carbonate spheroids are supported in a finc-grained quartz + sericicc + carbonate matrix. Plae polarised light. Sample 138601, Archaean Mb5 Golden Grove Formation, Luke Creek Group, Murchison Volcanics, Golden Grove, Western Australia.COMMON ALTERATION TEXTURES AND ZONATION PATTERNS |

FIGURE 3.4 I Examples of replacemenl textures in altered volcante rocks, cont.48 I CHAPTER3

A. Incomplete infill in fracturesIncomplete filling of fractures in this altered diorite Jefe sub-planar vugs. The pyrite fill has botryoidal surfaces, representing rounded shapes of either spherulitic radiating aggregates of fibrous crystals or fine-concentric intemal structures (cf. Jensen and Bateman, 1981). This massive plagioclase-phyrc diorite has been pervasively K-feldspar + pyrite (>quartz + illite) altered, and dissolution of primary mafic minerals produced a fine, spongy, porous texture.Sample 152959, Pliocene-Pleistocene Luise Vokano, Lihir Island, New Ireland Province, Ladolam epithermal Au mine. Papua New Guinea.B. Massive infillPal green epidote altered halos surround massive chlorice-fiUed amygdales {A) in this basalt sample. Sample 144753, Ordovician, Junee-Narromine Volcanic Belt, Boundary Prospect, Lake Cowal, New South Wales.C. Layered infill stringer veinThis banded vein consists of successive layers, from the vein wall to centre, of quartz, quartz and intergrown chalcopyrite and pyrite, and dolomite. A chin dolomite vein has overprinted the stringer vein at an oblique angle. These veins are hosted in strongly and pervasively sericite + chlorite + albite + pyrite-altered andesite. Cambrian Que-Hellyer Volcanics, westem vokano-sedimenta/y sequences, Mount Read Volcanics, Hellyer footwall, western Tasmania.D. Layered infill in amygdalesAmygdales {A) in this bsale clast, from a basalt-rnudstone peperite, contain concentric layers of quartz and calcite, which have grown inwards from the vesicle walls. The basalt groundmass has been pervasively sericite + chlorite + calcite altered. The clast grainsize decreases towards the clase rim, co the left of the field of view in this photograph.Sample 76836, Cambrian Que-Hellyer Volcanics, western volcano-sedimentary sequences, Mount Read Volcanics, Hellyer, western Tasmania.FIGURE 3.5 | Examples of infill textures in altered volcanic rocks.COMMON ALTERATION TEXTURES AND ZONATION PATTERNS I 49E. Layered i nfil 1 textiire in vesiclesVesicles (V) in this plagioclase-phyric pumice clast have been filled with roughly concencric layers of tan-coloured mordenite, dark smectite and clear clinoptilolite. The zeolites occur in clusters or aggregates of fine, radiating fibres. The originally glassy vesicle walls {W) have been replaced by mordenite + K-feldspar smectite. Plae polarised light.Sample OH8-387 m, Miocene Onnagawa Formation, Hokuroku Basin, Green TuffBelt, Odate, Japan.F. Layered infll textiire in amygdalesAmygdales in this subdy altered perlitic rhyolite havebeen filled with bands of fine-grained montmorilloniteand unknown radiating fibrous minerals. Pal polarisedlight.Simple J6-737 m, Miocene Nishikurosawa Formation,- :, Basin, Green TuffBelt, Odate, Japan.G. Layered infill in amygdalesThe amygdales in this palagonite-altered trachytic basalt i^r: from a crystal- and lithic-rich pumice breccia are EljsC with layers of montmorillonite and fibrous zeolites. Z'as.t polarised light.-.; OH8-387 '., Miocene Onnagawa Formation, Hkuroku Basin, Green TuffBelt, Odate, Japan.H. Comb textureTr.is example of comb texture shows layers of prominent sr=rry quartz + amethyst carbonate crystals projecting a-.vards from the vein or cavit)' wall. Simple T5, Cretaceous, andesite, Fresnillo epithermal trict, Mxico.

=iGURE3.5 | Examples of infill textures in altered volcanic rocks, cont.50 I CHAPTER3

A. Dissolution vugsThis hand specimen of polymictic breccia has a spongy or vuggy porous texcure due co che dissolution of primary mafic igneous minerals and glass. It has been incensely and pervasively adulara + Hite + pyrite altered with illite replacing plagioclase crystals, secondary K-feldspar in che altered matrix, and disseminated pyrite. Sampk 52726, Pliocene-Pleistocene Luise Volcano, Lihir hland, New Ireland Province, Ladolam epithermal Au mine. Papua New Guinea.B. Dissolution vugs in thin sectionIn thin section, irregularly shaped, empry, corrosin or dissolution vugs (V) are conspicuous in che matrix and clases. Some vugs cut across clase margis. Plae polarised light.Sample 152726, Pliocene-Pleistocene Luise Volcano, Lihir Island, New Ireland Province, Ladolam epithermal Au mine. Papua New Guinea.C. Filled dissolution vug in thin sectionCorrosin vugs, created by the dissolution of volcanic glass or pre-exiscing minerals, are commonly filled by subsequent mineral precipitacin from solution. Successive layers of montmorillonitc and zeolite have filled an irregular vug (V) in chis thin section. The vug oceurs in the matrix and in a basalt clast, crossing the clasc-matrix contacc. Plae polarised lighe. i Sample OH8-3S7 m, Miocene Onnagawa Formation, Hokuroku Basin, Green TuffBelt, Odate, Japan.D. Vuggy quartzThe prominent features n this quartz-rich sample are the corrosin vugs, which were generated by che dissolution of pumice clases and cryscals from this pumice and lichic tuff.Miocene rhyodatic pumice and lithic tuff, Pierina Au-Ag deposit, Per.FIGURE 3.6 | Examples of dissolution texturas in altered volcanic rocks.

COMMON ALTERATION TEXTURES AND ZONATION PATTERNS I 51E. Kaolinite + dickite-altered andesiteLarge (up to 4 mm), blocky feldspar phenociysts havebeen kaolinite altered in chis sample of massive, cohercntandesite.Miocene andesitic lava, Pierina Au-Ag deposit, Per.F. Vuggy quartzIn this sample of vuggy quartz, vvhich is equivalen: to rhe previous lcaolinite + dickite-altered andesite, the feldspar phenocrysts have been dissolved resulting n blocky vugs. The groundmass is composed dominandy o f quartz. Miocene andesitic lava, Pierina Au-Ag deposit, Per.G. Stylolite in thin sectionScylolites (5;) in this rhyoliric pumice breccia have conccntrated fine-grained opaques and sericite chlorite. The stylolites define che compaction foliation and are crenulated by che dominant regional cleavage (S) defined by alignment of sericite n che subdy albite + quartz + sericite-altered matrix.Sample 147422, Cambrian Kershaiv Pumice Formation, Central Volcanics Complex, Mount Read Volcanics, Rosebery, western Tasmania.E. Solucin seams in thin sectionThese analcime-filled solution seams occur in a smectite-lich fiamme, extending from the fiamme cerminations into the shard- and crystal-rich matrix of a cryscal-rich pumice breccia. They are incerpreted to have formed by dissolution and precipitation under the influence of lithostatic load during diagenesis. Plae polarised lighe. Sample FK7, Miocene Wadaira Tuff Member, Tokiwa Formation, South Fossa Magna, Creen TuffBelt, Wadaira, Japan.

FIGURE 3.6 | Examples of dissolution texturas in altered volcanic rocks, cont.52 | CHAPTER3Vuggy silica (quartz) alteracin facics is characterised by fine-grained, microcryscalline quartz and abundant open vugs or pores, which may be partly infilled (e.g. Fig. 3.6D, E and F). It is common in high-sulfidation epithennal systems and resulcs from the extensive leaching of all phases, excepr Si02 and TiO from volcanic roces by hot acid solutions (Whire and Hedenquist, 1990).StylolitesSrylolites are common in altered volcaniclastic rocks (Alien, 1990; Alien and Cas, 1990; Marsaglia and Tazaki, 1992; Gifkins and Alien, 2001). They are surfaces of dissolution associated with strain (pressure solution). They are roughly planar surfaces that exhibit mutual column and socket inter-digitation and may branch. Stylolites result from mechanical compaction and removal of elements by diffusion and precipitacin (Merino et al., 1983). They indcate volume loss and may form parallel or sub-parallel to bedding during burial, or at high angles to bedding during folding. Stylolites often contain a residue of insoluble material and minerals precipitated from solucin. Rccrystallisation, dissolution, grain growtb, grain orientation, pressure twinning, fracturing and residual accumularion of minerals along stylolites are common (Amstutz and Park, 1967).Irregular, anastomosing, bedding-parallel scylolices have been recognised in originally glassy volcanic facies, especially pumice breccias, in the Ylount Read Vblcanics (Alien, 1990; Alien and Cas, 1990; Gifkins and Alien, 2001). These are seams that concencrace fine-grained opaques and sericite at the margins of originally glassy clases, along tube vesicle walls in pumice clasts and in tbe matrix (Fig. 3.6D: Gifkins, 2001), These srylolites are interpreted as diagenetic compaction and dissolution fabrics that formed by the dissolucion of soluble componenes, parcicularly glass, and by che precipitation of clays and Fe-oxides as a result of pressure during burial (Alien, 1990; Alien and Cas, 1990; Gifkins, 2001).Solution seamsSolution seams are non-sutured, discontinuous mineral-filled seams that may form during diagenesis as a result of stress-related dissolution of soluble components and re-precipitation (Merino et al, 1983). Analcime-filled solution seams in pumice-rich rocks from the Green Tuff Belt (Japan) are anastomosing and roughly parallel co bedding. They oceur in the fine-grained matrix and within fiamme and pardy compacted pumice clasts (Fig. 3.6E: Gifkins et al., in press).Static recrystallisation texturesRecrystallisacion is the transformacin of a mineral or glass to a new grainsize, morphology or orientation of the same mineral species or minerals of the same composition (i.e. neomorphism, Folk, 1965)- Pre-existing minerals recrystallise to a new grainsize in an attempt to assume a more stable form by minimising the ratio of che surface rea to the volume during changed physical conditions (Yardley, 1989).Recrystallisation textures are produced by changes in the size, shape and arrangement of minerals in a rock. With increasing temperature, recrystallisation generally involves the change from fine to coarse grainsize (aggrading), except for static recrystallisation to hornfels and dynamic recrystallisation where large, strained grains are replaced by a mosaic of tiny, unstrained crysrals (Folk, 1965). Minerals may be directed or randomly orientated (non-directed: Spry, 1976). Directed textures oceur where recryscallisation is accompanied by stress (dynamic recrystallisation). Non-directed textures oceur where the pressure is equal in all directions (static recrystallisation).Common macroscopic and microscopic recrystallisation textures include mineral overgrowths, porphyroblasts, poikoblasts, and homfelsic, granoblastic, granophyric and decussate textures (Fig. 3.7).Dynamic recrystallisation texturesDirected fabrics or textures are common to regional metamorphic rocks where recrystallisacion is accompanied by stress (dynamic recrystallisation). These textures include subgrains .(granoblastic, porphyroblastic and poikiioblastic textures), foliations, layering and lineations.In common usage the term foliation is non-genetic and describes any planar, spaced or pervasive fabric in the rock. Foliations may form during diagenesis, metamorphism or tectonic deformation. Planar foliations are due to the preferred orientation of minerals, particularly micas, aligned perpendicular to the mximum compression direction (Yardley, 1989). Planar foliations are subdivided on the basis of grainsize and overall appearance of the altered rock. These include slaty cleavage, schistosity and gnessic layers.In fault zones or zones of intense duccile shear, two characteristic textures oceur: cataclastic and mylonitic textures. Cataclase refers to a fine-grained fault gouge breccia with an unfoliared matrix (Sibson, 1977). Ideally, cataclasis is mechanical ftagmentation without recrystallisacion, however this rarely oceurs in nature (cf. Sibson, 1977). Mylonite is a term used fot strongly foliated fine-grained rocks in which the grainsize has been reduced by recrystallisation (Bell and Etheridge, 1973).Deformation texturesDeformation textures are stress activated and develop in response to overburden pressures or regional tectonic stress. Because volcanic deposirs typically have high inidal porosities they are easily modified by mechanical compaction during burial, tectonic deformarion or, in the case of pyroclastic deposits, welding (Peterson, 1979; Alien, 1988; Branney and Sparks, 1990). Textural modification associated with compaction is essentially a result of increased pressure causing the re-arrangement and deformation of grains and reduction of intergranular pore space (Deelman, 1975).Deformation textures result from the rotation, brittle fracturing, flattening and distortion of exiscing grains or fabrics, especially clasrs that were previously altered to soft minerals (McBride, 1978; Galloway, 1979; Craig and Vaughan, 1981; Branney and Sparks, 1990).COMMON ALTERATION TEXTURES AND ZONATION PATTERNS I 53A. PorphyroblastsThe large, strongly altered, cordierite porphyroblasts give this rhyolite a distinctive coarsely sport}' texture. This rexture inspired the rerms dalmatianite, which was applied by early workers in the Noranda Camp, and the spotted facies, which was applied by Riverin and Hodgson (1980), for the cordierite-altered Amulet rhyolite and Millenbach andesite. The porphyroblasts are enclosed in a groundmass of chloritised biotite + sericite + quartz. Porphyroblasts are metamorphic crystals that are surrounded by a much finer grained matrix of other minerals (Spry, 1976). These large minerals have formed at the expense of the matrix and are the metamorphic equivalent of phenocrysts. Archaean Amulet Rhyolite Formation, Noranda, Abitibi greenstone belt, Amulet UpperA deposit, Canad.B. Porphyroblasts in gneissThis biotite + garnet gneiss is characterised by spotty 5 mm diameter garnet porphyroblasts in a mdium- to fine-grained quartz + feldspar + biotite groundmass. The garnet porphyroblasts commonly have a biotite rim. A gneiss is a rock with coarsely differentiated layering defined by the segregation of minerals of different composition (rypically dark and light minerals) in mdium- to coarse-grained, granular rocks. Layering forms parallel to the tectonic foliation and in this case deviates around the garnet porphyroblasts. The precursor is interpreted to have been a felsic volcaniclastic rock. Sarnple 154061, Proterozoic Potos gneiss, Harp prospect, Broken Hill Block, New South Wales.C. Porphyroblasts in thin sectionThis sample of garnet hornfels, from the contact zone berween rhyolite and a diorite intrusin, contains cuhedral garnet porphyroblasts in the biotite + muscovite * quartz groundmass. Porphyroblasts, like these, with well-developed crystal shapes are idioblastic or euhedral, whereas those with poorly developed crystal shape are xenoblastic or anhedral (Yardley, 1989). Plae polarised Light.Sample 140868, Cambro-Ordovician Mount Windsor Formation, east Thalanga, Mount Charter, Queensland.D. Poikiloblast in thin sectionThis thin section of amphibolite displays an amphibole roikiloblast with quartz and biotite inclusin trails. Toikiloblasts are porphyroblasts that contain numerous -.;lusions that may or may not show a preferred :;ntation (Barker, 1990). Poikiloblastic texture is malogous to poikilitic or micropoikilitic texture. Usually ths inclusions are minerals that also occur in the matrix Virdley, 1989). In this sample, the biotite inclusin riiis display snowball rotation indicating s}'ntectonic Sfowxh of the amphibole. Plae polarised light.pie 3215, Proterozoic Corella Formation, Mary - leen Group, Malbon River, northwest Qtieensland.

flU:..\S;...;

=CRE 3.7 | Examples of static recrystallisation textures in altered volcanic rocks.54 | CHAPTER3Common deformacin textures include intergranular cexcures and fabrics such as foliarions, lineations, and augen-srruccure, and intragranular textures such as strained, bent, kinked, flattened, twinned and broken grains (crystals or clasts), as well as irregular grain contacts (e.g. Fig. 3.8A, B and C: Dcelman, 1976; Spry, 1976). Deformation can modify pre-existing textures such as volcanic, hydration and devitrification textures (e.g. Fig. 3.8D to H). Fiamme and eucaxicic deformation textures are unique to volcanic facies (e.g. Fig. 3.9: Ross and Smith, 1960; Alien and Cas, 1990; McPhie et al., 1993; Gifkins et al., in press). Fiamme and eutaxitic textures are characteristic of, but are not rcstricted to, welded ignimbrites (e.g. Fig. 3.9A and B: Ross and Smith, 1960; Smith, 1960), welded pyroclascic fall deposits (e.g. Sparks and Wright, 1979), welded autobreccia (e.g. Sparks et al., 1993) and pyroclastic deposits that have undergone secondary welding as a result of contact with hot lava or ntrusions (e.g. Ross and Smith, 1960; Christiansen and Lipman, 1966; Schmincke, 1967; McPhie and Hunns, 1995). Similar fiamme and eutaxitic texture also occur in non-welded altered pumice-rich rocks (e.g. Fig. 3.9C and D: Fiske, 1969; Alien, 1988; Branney and Sparks, 1990; Gifkins et al., in press) and felsic lavas (e.g. Pichler, 1981; Alien, 1988).The terms fiamme and eutaxitic texture are used herein to describe the rock texture and not to imply any particular origin. Fiamme are flame-like, glassy or devitrified lenses, which define a pre-tectonic foliacin (cf. McPhie et al., 1993). Fiamme may have a wide variety of sizes (0.5 mm to 1 m), lengch to height ratios (up to 40:1), shapes (e.g. flame-like, bow ci, irregular branching and blocky) and incernal textures (aphyric, porphyricic, vesicular or stylolitic) (Gifkins et al., in press). Eutaxitic texture is the pre-tectonic foliation defined by the parallel alignment of fiamme (cf. Fritsch and Reiss, 1868; Ross and Smith, 1960; Smith, 1960). Eutaxitic texture typically impares a blotchy or streaky appearance to the rock duc to che colour contrast between the darker fiamme and paler matrix (e.g. Fig. 3.9A and C).3.2 | PSEUDOTEXTURESThe incomplece descruccion of primary cexcures and che combined effeces of a number of differenc overprinting alteration scyles (polyphase alceracion) can result in significant textural modification and che developmenc of false textures or pseudotextures (De Rosen-Spence et al., 1980; Alien, 1988; McPhie et al., 1993). Pseudotextures are alteration textures that modify or obscure primary volcanic textures and often lead to incorrect interpretation of the primary volcanic facies. Alien (1988) described examples of altered silicic lavas and autobreccias from Benambra, New South Wales, chat have the remarkably deceptive appearance of welded and non-welded pyroclastic facies and chinly bedded tuffaceous rocks.Pseudotextures can be subdivided into pseudoclastic textures (pseudobreccia, pseudogranular, false thin-bedded volcaniclastic) or false pyroclastic textures (false shards, false pyroclastic or eutaxitic: Fig. 3.10). However, strong pervasive alteration can also produce false massive textures that resemble either massive volcaniclascic or coherent facies (Alien, 1988; McPhie et al., 1993; Doyle and Huston, 1999;Doyle, 2001). Polyphase and patchy alteration of monomictic volcaniclastic facies can also result in false clast-supported and false polvmictic cextures.Pseudoclastic texturesThe mosr common pseudoclastic textures are pseudobreccia and false pyroclastic texture (also referred to as false eutaxitic texture). Other pseudoclastic textures include false thin-bedded volcaniclastic and pseudogranular textures.Pseudobreccias have the appearance of breccias, but form as a result of alteration of coherent facies (Carozzi, 1960; Alien, 1988). In outcrop they resemble coarse-grained, monomictic or polvmictic, clast- to matrix-supported breccias comprising angular to sub-rounded clasts in a fine-grained matrix (Fig. 3.10A, BandC).False pyroclastic textures occur in both coherent facies and in situ hyaloclastite. In ouccrop and hand specimen they may have a eutaxitic texture and contain abundant fiamme (e.g. Fig. 3.10D). In thin section they appear to contain splintery and arcuate fragments, which may closcly resemble pyroclastic glass shards (false shards: Fig. 3.10E).Both pseudobreccia and false pyroclastic texture develop as a result of two-phase alteration of fractured (perlicic or quench fractured) coherent or autoclastic facies and domain-controlled alteration of nodular devitrification cexcures in coherent facies (e.g. Fig. 3.11: Alien, 1988).Necworks of intersecting quench and/or perlitic fractures may control polyphase alteration in the fractured glassy parts of coherent lavas and ntrusions because they are permeable pathways for fluid flow. Initially glass immediately adjacent to the perlitic fractures is altered, then, as the fractures are filled, replacement fronts migrate away from che perlicic fractures towards the core. This may eicher obscure che concinuicy of che perlitic fractures or, if alteration is incomplete, enhance che perlitic fraecures. False shard cexcures develop eicher due co che preservacin of less altered, relatively siliceous slivers becween two or more fractures, or as alcered segments of the fractures themselves (e.g. Fig. 3.10E: Alien, 1988). The shape of false shards is a funecion of che shape of che fracture network. For example, cuspare false shards are produced from classical perlite, whereas those resembling flattened or welded shards result from banded perlite (e.g. Fig. 3.2D). False clasts develop where altered perlitic glass is pardy overprinced by a subsequent alteration phase, thereby preserving isolared relices of the earlier phase. Aiternatively, the earlier phase may be incomplete, leaving isolated kernels of glass that are subsequendy alcered co a different mineral assemblage.Pseudobreccia may also result from domainal or selective alteration of nodular devitrification textures: spherulites and lithophysae (e.g. Fig. 3.10F and G). Spherulites and lithophysae are typically recrystallised to quarrzo-feldspathic compositions, whereas the interstitial originally glassy domains are altered to phyllosilicate-rich assemblages (Alien, 1988). Consequently, the originally glassy and crystalline domains differ in alteration mineralogy and colour, and the spherulites appear as rounded siliceous clasts in a fine-grained phyllosilicate-rich matrix.Pseudogranular or sandy textures resemble well-sorced sandscones. These result from the recrystallisation of denselyCOMMON ALTERATION TEXTURES AND ZONATION PATTERNS | 55A. Augen schistThis sample of quarrz-augen schisr comprises large lenticular quarrz-rich domains (Q) encloscd in a strongly foliated, sericite + quartz chlorite-akered mairix. Augen texture is common in deformed, strongly porphyritic coherent and crystal-bearing volcaniclastic rocks. This augen schist ptobably resulted from che superposition of a strong regional cleavage on an altered pumice breccia. The cleavage anastomoses around comperent silicified pumice clasts.Sample 040617, Cambrian western volcano-sedimentary sequences, Mount Read Volcantes, Rosebery hanging watt, ivestern Tasmania.B. Broken crystals in andesiteThis deformed andesite contains broken plagioclase phenocrysts in a strongly foliated, sericite + chlorite + magnetite epidote-altered groundmass. Broken or fractured grains may rcsulc from mechanical pressure during tecconic deformation (McBride, 1978).Typically, feldspar crystals have been fractured along their cleavage planes, whereas quartz crystals have developed conchoidal fractures (Taylor, 1950; Sippel, 1968). Cross polarised lighe.Sample 144387, Ordovician Lake Cowal Volcanic Complex, Junee-Narromine Volcanic Belt, Lake Cowal, Gateway Prospect, New South Wales.C. Deformed grainsIn this amphiboite grade volcaniclastic siltstone, the quartz grains are deformed polycrystalline grains wirh undulse extinction, and elongated paraJlel to the regional cleavage. Cross polarised lighi. Sample GA9, Early Proterozoic Supra crustal succession, Bergslagen rnining district, Garpenberg, Sweden.D. Deformed clasts and pillowsPillow fragments and clasts in- this basaltic hyaloclastite were deformed and stretched parallel to the regional foliacin. The clase shapes are irregular and difficult to recognise as pillow or hyaloclastite fragments. Despite this, many clasts preserve an infernal zonation. Amphiboite, Proterozoic Corella Formation, Mary Kathleen Group, Malbon River, northwest Queensland.,:-:>"7p*s-",'r7"";:-::;-;;:ik i

w?Q

'

FIGURE 3.8 | Exampies of deformation textures, deformed clasts and pre-existing textures in altered volcanic rocks.

56 I CHAPTER3

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/E. Deformed clastsLens-shaped siliceous clasts (Q in this volcaniclastic breccia have been rotated and stretched into che strong tectonic cleavage. The fine-grained matrix has been foated and chlorite + sericke + quartz altered. Sample 133520, Cambro-Ordovician Trooper Creek Forrnation, Seventy Mile Range Group, Mount Windsor Subprovince, central Thalanga, Queensland.F. Folded pumice clastThis sample of rhyolite-, pumice- and crystal-rich breccia contains a folded tube pumice clasr with an axial planar cleavage (S) defined by aligned sericite. The pumice clast has been albite + quartz + sericite altered. Plae polarised light.Sample KB304B, Siluro-Devonian Ural Volcanics, Ural Ridges rea, New South Wales.G. Deformed relict perliteRelict perlitic fractures in this jigsavv-fic andesitic breccia are elongate and flattened, especially adjacent to competent phenocrysts. The groundmass has been sericite + chlorite + calcite + albite altered and the perlitic fractures chlorite filled. Plae polarised light. Sample 76902, Cambrian Que-Hellyer Volcanics, western volcano-sedimentary sequences, Mount Read Volcanics, Hellyer, western Tasmania.H. Deformed grainsIn this sample of volcaniclastic sandstone, strongly deformed fcldspar grains and clasts have been rotated parallel to the strong cleavage. Plae polarised light. Sample 133520, Cambro-Ordovician Trooper Creek Forrnation, Seventy Mile Range Group, Mount Windsor Subprovince, central Thalanga, Queensland.

FIGURE 3.8 | Examples of deformation (extures, deformed clasts and pre-existing textures in altered volcanic rocks, cont.COMMON ALTERATION TEXTURES AND ZONATION PATTERNS I 57- ?-ne and eutaxitic texture in welded._- -_.;-- -.- pbsidian lenses or fiamme (F) arealigned _--.:.. :: subaerial welded rhyolitic ignimbrite. Bcaiirae are commonly incerpreted as flatcened pumice __-: 7 : iime in chis sample are interpreted to result : i-tic deformacin, flatcening and sintering :.;: r hot glassy pumice clases dnring welding Sehh. 1960). The bedding-parallel alignment of tcaai, iiongare fiamme and glass shards defines the _._:.. [excure. Sample OW7, Pleistocene Owaboroa ignimbrite, Wbittanga Group, Coromandel Volcante Zone, Owharoa Falls, New Zealand.B. Fiamme in thin sectionIn chin seccin, che brmer pumice clases, fiamme (F), lack uncompacted vesicles, have feachery terminations and are enclosed in domains of cuspate and platy shards (5), and quarrz, feldspar and biocite crystal fragmenes. Alchough some shards have preserved bubble-wall shapes, ochers were plastically deformed and compacted, especially adjacent to crysrals. Plae polarised light. Sample OW1I, Pleistocene Owahoroa ignimbrite, Whitianga Group, Coromandel Volcante Zone, Owharoa Falls, New Zealand..,.. m#

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C. Fiamme and eutaxitic texture in non-welded pumice brecciaDark, plagioclase-phyric, wispy, chlorite-rich fiamme are enclosed in pal domains of quartz + chlorite + pyrite-alcered pumice clasts, shards and crystal fragmenes in chis non-welded rhyolitic pumice breccia. The bedding-parallel alignment offiamme defines the eutaxitic texture. Alteration and compaction of pumice clasts during diagenesis formed these apparenc welding cextures. Sample 133809, Cambrian Hrcules Pumice Formation, Central Volcanic Complex, Mount Read Volcanics, Hrcules fbotwall, westem Tasmania.D. Fiamme in thin sectionIn thin section, the chlorite fiamme (F) have feathery terminations and lack internal cextures ocher chan sparse plagioclase phenocryscs and hematite-rich stylolites. The pal quartz + chlorite + pyrite-altered domains contain uncompacted tube pumice clasts (P). Plagioclase crystals are dusted with hematite and sericite. Plae polarised light.Sample 133811, Cambrian Hrcules Pumice Formation, Central Volcanic Complex, Mount Read Volcanics, Hrcules footwall, westem Tasmania.

FIGURE 3.9 | Examples offiamme and eutaxitic texture.

A. Pseudobreccia in perlitic rhyoliteSericitc + chlorite-akcred perlitic fractures and pink albite-altered perlitic cores result in the pseudoclastic texture in this plagioclase-phyric coherent rhyolite. Sample BBP248-504.7 m from the Cambrian Central Volcante Complex, Mount Read Volcantes, Boco, uiestern Tasmania.B. Pseudobreccia in macroperlitic dacitePolyphase alteration of macroperlite in this plagioclase-phyric dacite has resulted in darle chlorite + epidote-rich domains along and adjacent to the perlitic fractures. This overprinted and cnclosed earlier, pal grey albite + sericite-altered perlitic cores. The chlorite + epidote-rich domains resemble the matrix in a matrix-supported breccia. Locally. the arcuate perlitic fractures are well defined.Cambrian Sterling Valley Volcantes, Mount Read Volcanics, Sterling Saddle, ivestern Tasmania.C. False clastsIn this coherent andesite, plagiockse phenocrysts have been extensively replaced by epidote, pyroxenes by chlorite and the groundmass domains by green epidote + chlorite and orange albite. The domainal distribution of the alteration facies gives the andesite a patchy pseudoclastic texture. The false clases have both sharp and diffuse margins, which are transgressed locally by altered plagiockse phenocrysts.Sample 145147, Ordovician ForestReefs Volcanics, Molong Volcanic Beh, Cooramilla, New Soutb Wales.D. False pyroclastic textureThe most conspictious feature of this sample is the vvispy dark green chlorite-rich lenses that resemble fiamme. However, these lenses are aligned in the tectonic cleavage and oceur in an evenly porphyritic rhyolite. The lenses are interpreced to result from domainal chlorite and scricite + quartz + biotite alteration of the groundmass in a coherent quartz + plagioclase-phyric rhyolite. Sample 140727, Cambro-Ordovician Mount Windsor Formation, MountWindsor Subprovince, centralThalanga, Queensland.E. False shards in thin sectionPolyphase chlorite + biotite and K-feldspar alteration of perlitic fractures, and their subsequent deformation have resulted in irregular arcuate and platy shapes, which resemble shards (arrows) in this quartz + plagiockse + pyroxene-phyric coherent rhyolite. Plae polarised light.Sample KB499, Siluro-Devonian Coan rhyolite, Mount Hope Volcanics, Mount Hope rea, New Soutb Wales.

COMMON ALTERATION TEXTURES AND ZONATION PATTERNS I 59F. Pseudoclastic texture in devitrified rhyoliteThis rhyolite conrainssilicified nodules thatare composed of coalesccd spherulites in a fine-grained sericire-altered groundmass. These nodular devicrification ccxturcs give a clastic appearance to the hand specimen and outcrop. Late Devonian Bunga Beds, Boyd Volcante Complex, Bengunnu Point, New South Wales.G. False clasts in thin sectionThe clastic texture in this rhyolite comes from the uneven distribution of strongly chlorite + hematite-altered spherulites in a strongly foliated, chlorite + sericite + feldspar + hematite-altered groundmass. The foliation has wrapped around the altered spherulites, which have preserved fibrous textures and quartz cores. Plae polarised light.Sarnple KB536D, Siluro-Deponan MountHope Volcanics, Boolahbone tank, Mount Hope rea, New South Wales.H. Pseudogranular textureThis altcrcd dacite has a fine granular texture in hand >pecimen and rescmbles massive sandstone. However, in thin section it has a densely microspherulitic groundmass in which the recrystallised quartz + albitc spherulites are separared by cuspate sericite-rich domains. The fine-grained, densely packed spherulites give the hand specimen its sandy texture.Sample MR25, Cambrian Kershaw Pumice Formation, Central Volcanics Complex, Mount Read Volcanics, Mount Read summit, western Tasmania.

A irisI. Pseudogranular texture in thin sectionRecrystallised micropoikiliric textures in the groundmass of this aphyric rhyolite rcsemble sand-si?.ed, rounded grains in a well-sorted quartzo-feldspathic sandstone. The margins of the micropoikilitic domains are marked by concentrations of sericite, which enhance the granular texture. Plae polarised light.Sample 147448, Cambrian Kershaw Pumice Formation, Central Volcanics Complex, Mount Read Volcanics, Rosebery, western Tasmania.

1 mm

rIGURE 3.10 | Examples of pseudotexlures in altered volcante rocks, cont.60 | CHAPTER3^-...-.,,,-..,,.. -y.-;--yy^1 cm

J. False thin-bedded volcaniclastic textureFiow banding in chis microspherulicic plagioclase-phyric dacice is defined by alcernacing pal albice + quarcz and darker albice + sericite + quarcz layers. The planar, repecicive, chin fiow banding resembles chin bedding in clascic facies such as cuffaceous silcscones. Sample 76772, Cambrian Que-Hellyer Volcantes, western volcano-sedimentary sequences, Mount Read Volcantes, Hellyer, western Tasmania.K. False thin-bedded volcaniclastic texture in thin sectionIn thin section, chis finely flow-banded rhyolite resembles a chin-bedded volcaniclascic facies wich fraecured plagioclase cryscals. However, axiolicic and bow-cie shaped spherulicic texcures are locally preserved in che groundmass. Cross polarised lighe. Sample KB132A, Siluro-Devonian Ural Volcanics, Ural Ridges rea, New South Wales.L. False volcaniclastic texture in thin sectionFraecured and broken plagioclase cryscals, and re-cryscallised spherulices in che groundmass of chis flow-banded rhyolice concribuce co ics pseudoclastic cexcure. Cross polarised lighe.Sample 133837 Cambrian Mount Black Formation, Central Volcante Complex, Mount Read Volcanics, Mount Read summit, western Tasmania.M. False polymictic, matrix-supported textureThe dark chlorite-rimmed clases in chis plagioclase-phyric basalcic breccia appear co be supporced in a composicionally differenc, pal calcite + chlorice-altered macrix. However, che rnatrix comprises jigsaw-fic, blocky and splintery clasts of perlicic bsale thac are idencical co che darker clases. The chloricc-rimmed clases appear subrounded, because chlorice alceracion of the clast margins and adjacenc macrix has obscured the blocky and splineery shapes.Sample 76833, Cambrian Que-Hellyer Volcanics, western volcano-sedimentary sequences, Mount Read Volcanics, Hellyer, western Tasmania.FIGURE 3.10 | Examples of pseudotextures in altered volcanic rocks, cont.

COMMON ALTERATION TEXTURES AND ZONATION PATTERNS I 61N. False polymictic textureOverprinting domainal albite + hematice and epidote alteration facies in this plagioclase + hornblende-phyric dacite gives the sample a heterogenous appearance. The abundanceof pink, albitised plagioclase phenocrysts in the red and green domains is equivalent, although they appear more abundant in the epidote-altered domains due to the colour contrast between the phenocrysts and epidote-altered groundmass. Colour difFerenccs between the two alteration facies and more prominent phenocr)'sts in the epidote-altered domains obscure the massive, coherent texture and uniform composicin of this sample. Sample 147557, Cambrian Mount Black Formation, Central Volcante Complex, Mount Read Volcantes, Fieman Road, western Tasmania.O. False matrix-supported textureClasts in this monomiede dacite breccia display igsaw-fic cexcure. However, feldspar + quarez + sericite alteration facies has replaced the groundmass adjacent ro the quench fractures becween clasts, enhancing the matrix, and imparting an apparent matrix-supported texture. The feldspar + quartz + sericite-alcered matrix has been more resistant to weathering than che sericite + chlorite-alcered clases and forms ridges on the outerop. The larger clases are perlicic, plagioclase + homblende porphyritic with planar and curviplanar margins typical of clasts produced by quench fragmentacin. Cambrian, Mount Black Formation, Central Volcanic Complex, Mount ReadVolcanics, Tullah lakeside, western Tasmania.P. False matrix-supported textureIn this in situ andesitic hyaloclastite, blocky chlorite-altered plagioclase + pyroxene-phyric clasts appear to be supported in a pyrite + quartz + sericite-rich macrix. However, thin section inspection reveis relict clasts wich jigsaw-fit textures preserved in che false matrix domains.Sample 144710, Ordovician Lake Cowal Volcanics, Junee-Narromine Volcanic Belt, Boundary Prospect, New South Wales.Q. False coherent textureThis albite + quartz + sericite- and chlorite + epidote-lcered pumice and rhyolite breccia resembles a coherent, :e!dspar porphyritic facies. The albite-altered plagioclase ;rystals, pseudophenocryscs, are evenly distributed in i fine-grained, sericite-rich false groundmass. On thert side of che photograph, pervasive, massive albite + ruara alteracin facies obscures che plagioclase crystalsn 3. pseudo-aphyric texture. Sample 147402, Cambrian Kersbaw Pumice Formation,-nitral Volcanic Complex, MountRead Volcanics, Rosebery, atern

- 3 JRE 3.10 | Examples of pseudotextures n aliered volcanic rocks, cont.

S'EAFLOOR-AND BURIAL-RELATED ALTERATION I 99typically sharp (Peacock, 1926; Lofgren, 1971a; Fisher and Schmincke, 1984).PaiagonitePaiagonite is a dull, resinous, yellow-orange co brown wax-like subsrance formed from hydrous altered sideromelane (basaltic) glass (Fig. 5.1). It is a mineraloid mixture of relict hydraced glass, nonrronice, montmorillonite and other sheet silicates (Hay and lijima, 1968b; Honnorez, 1969; Jakobsson and Moore, 1986). Eggleton and Kcller (1982) described paiagonite as a transitional alteration phase between volcanicglass and smectite; however, the end product may not always be smectite.There are two main varieties; gel-palagonite and fibro-palagonite (Peacock, 1926). Gel-palagonite is isotropic, dark brown and commonly banded, forming directly adjacent to unaltercd glass (Peacock, 1926; Zhou and Fyfe, 1989). Fibro-palagonite is orange-yellow, transparent and birefringent (Zhou and Fyfe, 1989).Paiagonite is widespread in submarine basaltic facies and common around the edges of glassy grains in basaltic cufFs, in pillow rinds, along fractures in glass, and in originally glassy vesicle walls (Moore, 1966; Baragar et al., 1977; Friedman and Long, 1984). Partly altered basaltic pillows typicallyA. Gel-palagonite in pillow basaltThe sideromelane groundmass of chis plagioclase + augite-phyric basalt is altered to yellow-brown paiagonite adjacent to the vesicle (V). The gel-palagonite exhibits banding parallel to the vesicle wall and perpendicular contraction cracks. Plae polarised light. Sample 153254, Miocene Waitakere Group, Muriwai, Nortbland regin, New ZeaLtnd.B. Palagonite-altered basalt clast rindThe basalt clast in this polymictic conglomrate has a thin palagonicised rind. The plagioclase-phyric clast is concentrically zoned with an unaltered sideromelane core (C), yellow-brown gel-palagonite altered zone (P) and a brown fibro-palagonite rim (i?). The conglomrate matrix includes palagonitised basaltic shards and crystal fragments. Plae polarised light.Sample 131562, Tertiary Macqiiarie Plains volcanics, Btislyy Park, Tasmania.C. Banded paiagoniteThe palagonite-altered rind on this basalt clase displays fine conccncric banding. Plae polarised light. Sample 131562, Tertiary Macquarie Plains volcanics, Bushy Park, Tasmania.FIGURE 5.1 | Photomicrographs of paiagonite.

100 ym -._____\ iiH

100 I CHAPTER5have glassy cores successively surrounded by concentric zones of gel-palagonite and fibro-palagonite ( smectite), which are enhanced by bands of fine Fe- and Ti-oxides (Fig. 5-2: Dimroth and Lichtblau, 1979; Zhou and Fyfe, 1989),Palagonites have variable compositions with 10-20 wt% H,0 (Brey and Schmincke, 1980; Eggleton and Keller, 1982; Pichler et al., 1999). Compared with sideromelane, Fe2* is oxidised, K20, FeO, Ti02 and Cl may be locally gained, and Na20, A1203, SiO, and CaO lost (Baragar et al., 1977, 1979; Jakobsson and Moore, 1986; Zhou and Fyfe, 1989). However, whole-rock compositions are not significantly changed, except for H20. Palagonitisation is typically accompanied by the growth of authigenic minerals in open pore spaces (Fig. 5.2) and these commonly account for the elements lost from the glass (e.g. Baragar et al., 1979; Jakobsson and Moore, 1986).Gnesis of palagoniteZhou and Fyfe (1989) and others have proposed a rwo-stage solution-precipitation mechanism for palagonitisation of sideromelane based on physical characteristics, chemical changes and the presence of etch or dissolution pits at alteration fronts. The first stage is Ti constant: glass is dissolved and gel-palagonite formed. There is a dramatic reduction in the glass volume due to the loss of greater than 60% of the SiO,, Al2Qj, MgO, CaO and Na20. The second stage is volume constant: gel-palagonite is replaced by fibro-palagonite, and zeolites begin to fill adjacent fractures and vesicles. CaO and Na20 are lost, and K20 and SiO,, A1,03 and MgO are gained from solution. Titanium and Fe3' are localised into nearby fracture-filling clay and oxide minerals.The rate of palagonitisation is tempera ture dependent and doubles with every 12C increase in temperature (Jakobsson and Moore, 1986). Palagonitisation proceeds rapidiy at temperatures above 50C and up to 150C (Jakobsson, 1972, 1978). Jakobsson and Moore (1986) noted that palagonitisation of glass varied from less than 40% at 60C, through 90% at 100C and was complete at temperatures above 120C. They also found that both gel- and fibro-palagonite occurred below 87C, but only fibro-palagonite occurred above this tempetature.The thickness of palagonite rinds is time and temperature dependent. Palagonite rinds in pillow basalts systematicallyincrease in thickness with time and doubles for every 8C temperature increase (Moore, 1966; Jakobsson and Moore, 1986).PerlitePerlite is a textural term referring to networks of fine fractures or cracks that range from concentric arcuate fractures enclosing cores of glass (classical perlite; e.g. Fig. 5-3A and B) to long sub-parallel fractures linked by short cross fractures (banded or ladder perlite) (Fig. 3.2C and D: Ross'and Smith, 1955; Friedman et al., 1966; Alien, 1988). Perlitic fractures are a common feature of glassy rock fragments, felsic lavas and synvolcanic sills, and also occur in the glassy rinds of mafic to intermedate lavas.Felsic perlites typically contain 26.5 wt% H20 compared with non-hydrated obsidian, which contains a few tenths of one percent (Ross and Smith, 1955; Noh and Boles, 1989). In addition to gains in H20, perlites typically gain K,0, and lose NajO and to a lesser degree CaO and S02 (Lipman et al., 1969; Fisher and Schmincke, 1984; Noh and Boles, 1989). Iron is oxidised, volatile components Cl, and F2 may be lost, and SO18 isotope vales modified by interaction with external fluids (Lipman, 1965; Jezek and Noble, 1978; Cerling et al., 1985). These compositional changes are most intense along the perlitic fractures (Jezek and Noble, 1978; Fisher and Schmincke, 1984).Gnesis of perliteA debate contines over the origin of perlite and the importance of hydration (Ross and Smith, 1955; Ftiedman and Smith, 1958; Friedman et al., 1966) versus cooling contraction (Marshall, 1961; Yamagishi and Goto, 1992). The formation of perlite is favoured by hydration of rapidiy cooled glass (i.e. glass with a high degree of under cooling) either during cooling or later at low temperatures (Friedman etal., 1966; Noh and Boles, 1989; Drysdale, 1991). However, it is also possible that perlitic fractures form in response to strain inherited from rapid cooling contraction, during the conversin of melts to glass, and associated volume changes (Ross and Smith, 1955; Friedman et al., 1966; Davis and McPhie, 1996).

Fresh glassChabazile["SI Hydrated and oxidised glass [WrF] ph||DSta L^3 with oerlitic fractures -^ I ""'"PslleI Clay-altered glass | Gel-palagoniteFibro-palagonite Pore spaceFIGURE. 5.2 | Sequence of palagonite alteration and zeolite cementation stages in phonolitic glass fragments (afler Brey and Schmincke, 1980, in Fisher and Schmincke. 1984). (A) Glassy shards, perhaps with montmorillonite rim cements. (B) Hydration and development of perlitic fractures accompanied by partial dissolution and alteration of glass shards to gel-palagonite. (C) Complete dissolution and alteration of hydrated glass shards to gel-palagonite, accompanied by the precipitation of zeolites on to glass surfaces. (D) Alteration of gel-palagonite to fibro-palagonite and precipitation of zeolites into open spaces.

A, Perlite in thin sectionroundmass of chis quartz latite exhibits dassical perlicic fractures comprising intersecting ao overlapping arcuarc cracks. The perlitic fractures sedase cores of unaltered and locally oxidised glass. Arenare glassy false shard textures occur where perlitic - ;r_re intersea (arrow), Amygdales have bccn filied whh zeolires. Plae polarised light. SampU ET7-4, Werelcisend Formation, Pilchard Gorge, Eiendeka, Namibia.B. Perlite in partly altered rhyoliteWell-developed perlitic fractures are abundan: in chis parth- glassy rhyolite. The perlitic fractures llave been iined wirh fine-grained, dark green to brown smectites, enhancing the fracture pattern. Perlite cores have been partly altered to smectites and zeotes. Amygdales have been filied with crisrobalite. Plae polarised light. Sample 147582, Miocene Nishkurosawa Formation, Hokuroku Basin, Green TuffBelt, Ocate, Japan.C. Relict perlite and amygdales in altered rhyoliteIn this diagenetically altered rhyolite, relict perlitic fractures are conspicuous where glass adjacent to the fractures has been altered to dark green mixed layer smectite-chlorite. Elsewhere in the pervasively zeolite altered domains the perlitic fractures have been obscured. The amygdales have been filied with layers of cristobalitc and fibrous chlorite. Plae polarised light. Sample J6-735 m, Miocene Nishikurosaiva Formation, Hokuroku Basin, Green TuffBelt, Ocate, Japan.D. Relict perlite in altered basaltIn this hydrothermally altered jigsaw-fit basalcic breccia, perlitic fractures are only weakly discernable due to mltiple overprinting alteration facies. The pervasive sericite + quartz + pyrite and nodular carbonate alteration facies obscure the perlitic fracture pattern. Plae polarised light.Sample 76833, Cambrian Que-Hellyer Volcanics, western volcanoseclimentary sequences, Mount Read Volcanics, western Tasmania.FIGURE 5.3 | Photomicrographsoffresh and altered perlite.SEAFLOOR- AND BURIAL-RELATED ALTERATION |

102 | CHAPTER5Alteration of perlitePerlite commonly undergoes subsequenc alteration to diagenetic mineral assemblages that include smectite, Fe-oxides, zeolites, K-rich gel-like glass, low-cristobalite, K-feldspar, chlorite, sericite and carbonate (Noh and Boles, 1989). Alteration begins by dissolution of hydrated glass and crystallisation ol smectite, carbonate or Fe-oxides alone perlitic fractures (e.g. Noh and Boles, 1989). This commonly accentuates the fracture pattern (e.g. Fig. 538). As alteration progresses, glass dissolution with continued precipitation advances inwards and the perlitic fractures become diffuse and indistinct (e.g. Fig. 5.3C, D and Alien, 1988). Dissolution of remaining giassy cores is succeeded by formacin of zeolites, such as clinoptilolice or mordenitc, or gel-like glass, which are ultimately replaced by K-feldspar (e.g. Noh and Boles, 1989).5.3 | DIAGENESIS (GLASS TO ZEOLITE FACIES)Diagenesis cncompasses the low-remperature and low-pressure alteration processes that occur during progressive burial of sediments and rocks. It can be defined as the processes (excluding weachering) that change their character and composicin, between the moment of deposicin, and che onset of mecamorphism (Larsen and Chilingar, 1979).Submarine diagenesis involves low-cemperature processes, ranging from bottom water temperacures up to crystallisation of unequivocally metamorphic minerals such as laumontite, wairakice, chlorite and pumpellyite (Winkler, 1979; Bohlke et al., 1980). It is impossible to define a unique pressure and remperature range that would characterise the transicin between diagenesis and metamorphism, because of che greatly contrasting degrees of mineral stability that characterise different rock types and the wide range of conditions under which the common diagenetic minerals crystallise. Generally, diagenesis in submarine settings oceurs at pressures of 0.1 to 10 MPa (1 bar to 1 kbar) and temperatures ranging from 0 co 250C (Ale and Honnorez, 1984; Morrow and Mcllreath, 1990; Alt, 1995b). Temperatures and pore water salinities increase, and seawater-rock ratios decrease with burial depth (Hanor, 1979; Alt-Epping and Smith, 1997).Submarine diagenesis encompasses compaction, dissolution and leaching of components, precipitation of new minerals, and recrystallisation in response to changes in pressure, temperature and chemical conditions in the subseafloor. New minerals directly replace glass, form mineral overgrowths, fill primary and secondary pore spaces, and form cements, all of which dramatically reduce the porosity and permeability and promote lithification.With increasing diagenesis, porosity and permeability typically decrease. However, reversis in chis trend can occur during fraecuring or if a major componenr of che rock becomes under saturaced and secondary porosity is formed by dissolution. This can occur where deeply buried sediments are infiltrated by fresh or brackish ground water, or can be due to the relase of water of crystallisation from clay minerals (Morrow and Mcllreath, 1990).

The process of dissolution involves corrosin or leaching of pre-existing phases (either glass or mineral phases), with or wichouc minor replacement by new minerals (Morrow and Mcllreath, 1990). It is a complex process involving many distinct reaction steps and pathways. It can modify glass and most primary igneous minerals. Dissolution may ultimatelv lead co che formation of secondary porosicy (e.g. dissolution vugs), replacement of glass and minerals, and developmenc of solucin seams or stylolites (Amsrutz and Park, 1967; Marsaglia and Tazaki, 1992).Despite changes in mineral assemblage, many pre-exiscing cexcures (primary volcanic, high-cemperacure devitrification and hydracion textures) are preserved and sometimes enhanced during diagenesis. Figure 5.4 shows some examples of textures in unaltered volcanic rocks, and their diagenetically altered and in some cases metamorphosed equivalents.Submarine diagenesis may involve mltiple stages or episodes of diagenesis (Bohlke ec al., 1980; Morrow and Mcllreach, 1990). Diagenesis of mosc ancient sedimencary successions involved repeaced exposure co diagenetic realms as they underwenc cycles of subsidence and uplift. Generally, however, the imprint of the first stages of diagenesis is preserved because of che large inicial porosity reduccin and lichificacion (Morrow and Mcllreath, 1990).Diagenetic mineralsThere are chree main cypes of minerals typical of seafloor weachering and diagenesis in volcanic successions: layered silicates, zeolices and carbonates. Figure 5.5 provides escimaces of their formation temperatures.Layered silicatesThe layered silicates include clay minerals, mixed-layered minerals, micas, chlorite and prehnite. The common clay minerals in volcanic facies can be divided in to two groups: (1) smectites (e.g. montmorillonite, noncronireandsaponice), and (2) illitc group clay minerals (e.g. celadonite, glauconice and illice).Smeccices are swelling clay minerals chat readily exchange Ca and Na cations. They typically resulc from the alceracion of volcanic grains under alkaline condirions where Mg and Ca ions are available (Deer ec al., 1966). Smectites form rims on glass surfaces, replace boch felsic and mafic glass, and pseudomorph glass shards and olivine crystals (Sheppard and Gude, 1968; Schmincke and von Rad, 1976; Viereck et al., 1982). Smectites initially forms blebs and web-like arrays on glass surfaces, and become better crystallised as diagenesis proceeds (Hein and Scholl, 1978). The cerm bentonite refers ro felsic cuff that is composed of almost pur smectite (Gary et al., 1974).In concrasc, che illice group are K- and Al-rich minerals chac typically form in neucral co alkaline conditions from the breakdown of feldspars and micas (Deer et al., 1966). They typically occur as vesicle fill and pseudomorphs of felsic glass shards and pumice (Schmincke and von Rad, 1976; Iijima, 1978). Celadonite and glauconite are less common than ilute.SEAFLOOR- AND BURIAL-RELATED ALTERATION I 103A. Flow bandingThis devitrified flow-banded plagioclase-phyric rhyolite contains alternating darle and light flow bands. The dark bands are dominantly obsidian, whereas the pal bands contain fine spherulites and lithophysae. Sample NG1, 122 | CHAPTER5Subtle, patchy mordenite + smectite-chlorite alteration faciesSample No.J6-294

Alteration faciessubtle, patchy mordenite + smectite-

chlorite

Alteration zoneclinoptilolite + mordenite zone

LocationYoneshiro River

FormationOnnagawa Formation

SuccessionGreen Tuff Belt

Volcanic faciespumice breccia

Relict mineralsplagioclase + quartz

Relict texturastube pumice clasts, bubble-wall shards,

crystal fragments, non-vesicular volcanic

clasts

Primary compositionrhyolite

Lithofaciesgraded bed

Interpretationsyneruptive, mass-flow emplaced pumice

breccia

Alteration mineralspartly glassy, mordenite + saponite +

montmorillonite + smectite-chlorite +

K-feldspar + pyrite

Alteration texturassaponite films in vesicles, mordenite

saponite filled vesicles and pora space,

smectite-chlorite fiamme, disseminated

pyrite

Distributionpatchy

Preservationexcelleni

Alteration intensitysubtle

Timingearly

;]ZcibII - ' * j Zcne II 5] Sbtob chfile ?or>e shard-rh rnudslcnc W~\ PolyniitcCuinceGrecoa -2and lamnate) sitisione ^t^I InternedaM crysa'-ritri purnioc ilili^ ssndsOTe and shaiiMcti SillSIOiM ^"jPufnce bisccia. sondsfdM 1'and 5h(d-rLh sesioneB Pumice and litle &'ecc>3. and siiar d-rtCC sillsiow HCcherenl and aulo-biecciaied f hyoe QAnfle&e 3ti feasait intrusin; Ccheien! basaJt aM bss&lc bomb breeda ElmerMddsd ralymtfic conglomeris and sandKoneHK1

Weak, pervasive mordenite + smectite alteration faciesSEAFLOOR-ANDBURIAL-RELATED ALTERATION | 123HK2Sample No.OH8-369

Alteration faciesweak, pervasive mordenite + smectite

Alteration zoneclinoptilolite + mordenite zone

LocationOdate city

FormationOnnagawa Formation

SuccessionGreen Tuff Belt

Volcanic faciespumice + lithic breccia

Relict mineralsplagioclase + quartz

Relict texturestube pumice clasts, bubble-wall shards,

crystal fragments, non-vesicular volcanic

clasts

Primary compositionrhyolite

Lithofaciesgraded bed

Interpretationsyneruptive, mass-flow emplaced pumice

breccia

Alteration mineralsmordenite + smectite-chlorite + K-feldspar

+ calcite + pyrite + glauconite

Alteration texturessmectite films in vesicles, mordenite

smectite filled vesicles and pore space,

mordenite-altered glass shards and

vesicle walls, smectite-chlorite fiamme,

disseminated pyrite, microcrystalline lithic

clasts

Distributionpervasive

Preservationexcellent

Alteration intensityweak

Timingearly

Alteration stylediagenetic

'.v.y.j I":&e52d rcarins andRvl IntefbMdeS aysial-nch puovce t-l-iJsan3slowand;r-rd-r :.h jiltsri:Pumise an lithi: rreria.anfl snatfMCi sftstoie ~~1 CtWerere and atfo-j bfeccstsa ydile Andesle and basan Coieren tiasalt and basaltic ccmb trece 3 Qlnietbedde pciymlic conglomrale and sa^iof e

124 I CHAPTER5Subtle, pervasive smectite-chlorite + mordenite + analcime alteration faciesHK3Sample No.OH8-511

Alteration faciessubtle, pervasive smectite-chlorite + mordenite + analcime

Alteration zoneanalcime zone

LocationOdate city

FormationOnnagawa Formation

SuccessionGreen Tuff Belt

Volcanic faciespumice breccia

Relict mineraisplagioclase + quartz

Relict texturastube pumice clasts, crystal fragments

Primary compositionrhyolite

Lithofaciesmassive

Interpretationsyneruptive, mass-flow emplaced pumice breccia

Alteration minerais Alteration texturesDistributionsmectite-chlorite + mordenite + analcime +sericite + pyriteanalcime solution seams, smectite-chloritefiamme, mordenite filled vesicles, analcimereplacing mordenite + smectite-alteredpumice clastspervasive

Preservationgood

Alteration intensitysubtle

Timingearly

Alteration stylediagenetic

..".OH-8 -

7me injzorell '**-*I Zore III ^h| Ssrile * chirote zcne___Mudstcmp^j Inlefbsddea marine andmFoljniclic cornee txcCGS lid famnaiefl siltslons prn Inierbsddea erystal-nci parios fc--J sarcstore and sratd-rch sillstone ^1 Pumice breccia, sairfstwe ''and shard-rich sitlstoneSPunvce and ;hc bieccB, and shard-rich Sltsicne Ccherem arel autc-bteccated ryolite Andrse anO basa! Inltu&cns Cq Cohetea; basad and bndSC j bomb breccia QlnioibeddM pSynWc corifllomeaie atxj $aris:or.e

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SEAFLOOR-AND BURIAL-RELATED ALTERATION | 125Weak, pervasive analcime + mordenite alteration faciesHK4Sampie No.OH8-537

Alteration faciesweak, pervasive analcime + mordenite

Alteration zoneanalcime zone

LocationOdate city

FormationOnnagawa Formation

SuccessionGreen Tuff Belt

Volcanic faciespumice breccia

Relict mineralsplagioclase

Relict texturas Primary compositiontube pumice clasts. bubble-wall shards,crystal fragmentsrhyolite

Lithofaciesgraded bed

Interpretationsyneruptive, mass-flow emplaced pumice breccia

Alteration mineralsanalcime + mordenite + clinoptilolite + smectite-chlorite + pyrite + sericite

Alteration textures Distributionmordenite and clinoptilolite filled vesicles, mordenite and analcime altered vesicle walls, analcime overgrowths on plagioclase crystal fragments pervasive

Preservationmodrate

Alteration intensityweak

Timingearly

Alteration stylediagenetic

v ": iniefteddfidui shard-nch mutis (eneHPolymiCipuRicO Cieeoa and lamnales sltslons [vTl Interteddeci crvsiaWiti pontee L--:JS3f"1^i-~ ?.n -y-ii- ci' ulti-torrsPumice bfeccia, santislore and snatt-riti slsoneH Conten sndauo-biecciaied rttyoe aAndesB ard N&3H nlrusons Cciief ent bsse; ana bisaiiu Cono breecia ntnlerbsddK cclvncllc conglomrale ar-3 sarWslore

126 | CHAPTER5Strong, pervasive quartz + sericite alteration faciesHK5Sample No.OH8-794

Alteration faciesstrong, pervasive quartz + sericite

Alteration zonesericite + chlorite zone

LocationOdate city

FormationNishikurosawa Formation

SuccessionGreenTuffBelt

Volcanic faciespumice + lithic breccia

Relict mineralsplagioclase

Relict texturasvolcanic clasts

Primary compositionrhyolite

Lithofaciesgraded bed

Interpretationsyneruptive, mass-flow emplaced pumice breccia

Alteration minerals Alteration texturesDistributionquartz + K-feldspar + sericite + chlorite+pyritepseudomorphs after plagioclase crystalsand clasts in pervasive crystalline matrix,quartz-filled dissolution vugspervasive

Preservaronpoor

Alteration intensitystrong

Tmingearly

Alteration stylehydrothermal

SlnlerbsJdeO roanw wd shard-f icn mudsone SPotwrtCflc nne brecha ai latrJnated stltslcne rr^l IntebeSded cryslal-ch pumee el^j sandslone ar; Br &cda, an-j sfiaic-f i tf sistcne [Tjl Conemni and suio---' biscciaiM fftycjtteQlnteriedd-d wlyrnciic co^gtomBra'.e &id sa"ds:one

SEAFLOOR- AND BURIAL-RELATED ALTERATION I 133r - = = s a bite - quartz + chlorite alteration faciesCVC1suijle. pervasive albite + quartz + chlorite: ---.r:-Mount Black Mount Black Formation Central Volcanic Complex massive, plagioclase-phyric rhyolite plagioclase - - - .-i- porphyritic, micropoikilitic- - :-::;-:- "volite- 3: - \ massivecoherent faciesalbite + quartz + sericite + chlorite +hematitealbite + quartz sericite chloritepseudomorphs of plagioclase,micropoikilitic albite + quartz, interstitialchlorite, disseminated hematitepervasiveexcellentsubtlepre-S2diagenetic

- ->:itensitystyleMr.t-,A13392.V. /'

* =>v.

::.frailea-.! n&fsd popfcyv

mOwen Conglwncrate

- =Eastem quartz-piyrc smu&ics

wiiifeSprFormaSof.--r.Tii.-iir-iGnw

Kersna Pur.co FcoubIci

/:.:. ;y,.Fcfmaltoi

FTf7Stertag VS?ey Volcanes

';Quartz f fel&aar-

AHste zcneEpdote zona HydrKfceimsl afleraiionzones

GeochemistrySi02 74.58K?04.34Cu2Al56

Ti020.27PA0.03Pb3CCPI22

Al20313.85s** '(VAJte Spw Formaran and Gurdas GmupCemralVok^nitCcmpfex l.'"i.*.*| Kets'naw Puricei i ^ ' j Pormaon ~"ISlerhnsValley , * Volcantes ^*\ Quartz * fetsper. I Wtyc rhysfflaAStleontGeochemistrySi02 74.01K?01.82Cu2Al26

Ti02 0.23 Al203 12.21p2o5S0.03 0.01Pb Zn419CCPI Ti/Zr29 5.31

Fe203 2.17 MnO 0.09Total100.61ThZr12 258

MgO 0.49Rb76Nb16

CaO 2.41 Na20 4.07Sr Ba113 513Y36

Hand specimen photographPhotomicrograph (xn)TiOj-Zr immobile elemsnl plot

calcita 100I 70-----pyrito, m&gnelile ar3 sKlsrite---------------gnvertte dtfc-nile uemofce chlorite1-----------------1 ____1 _------------------1 1 I 1 _____I-_a_J---------------------------1i i i

1-i L i.jj Andesito-basalt 1 l 1 i 7J . _>_ _ U -i i ii i ii i il i ii i ii i l _i |------|_ -l l lseridiei . i _ _ i_(chngfle)"L ! L l l 1

1 - -' i i .....i i. .14756?

1474! *' iUto M\j 1475=2-741| ^ ,j3g , Rhyotite,

ii i iii l 1 1 1 1 1 i K-lfrCapar

Zr (ppm)O 10 30 30 40 50 60 70 80 .90 100 Al (Ishikawa Alleralion ndex)SEAFLOOR- AND BURIAL-RELATED ALTERATION I 135domainal albite + quartz + sericite with sericite + hematite chloritefaciesCVC3arse-c147410

"K.-e-=modrate, domainal albite + quartz +

sericite

~~-~ialbite zone

'120R-524.5 m

- _Kershaw Pumice Formation

~ ~r- : I "Central Volcanic Complex

-13" : =:.t=graded, plagioclase-phyric pumice breccia

:; r - -"; =plagioclase

-? r r :_-estube pumice clasts, fiamme, plagioclase

crystal fragments, blocky rhyolite clasts

: -=- ::_::s;tionrhyolite

_- rfirtlnormaily graded

- r ;"t ; :_syn-eruptive, mass-flow-emplaced pumice

breccia

- f- :--------5r3!Salbite + quartz + sericite + chlorite +

hematite + calcite

- 7": :_ :':.-essericite fiamme, hematite stylolites, albite

veins, recrystallised albite + quartz +

sericite pumice clasts and matrix, albite +

sericite + calcite altered plagioclase

" : _; _" :'domainal

=;35srv3tonpoor

- t :- ntensitymodrate

_- -;pre-S2

MsaSon stylediageneiic

;::oanHandraBdedpciyry

;;;;;;Owen CaigOme'ate

L =Easlern quarz-phyrc S&quence

':':':':':'.'rute Scur Formaban

CaOlVdCfcC*.

Kersr^v Ptrnce Fconatico

^f^r^~Mounl BlickFiynidfo"

Sterlng Vatey Vccancs

;Quaitt * falflspaf-

Albezoie=p"do!e zonaHydrolhsinal alienen zorms

i

h

GeochemistrySi02 76.08K200.88Cu1Al16

Ti020.19PA0.03Pb4CCPI26

Al20310.66S0.01Zn27T/Zr5.43

Fe203 MnO1.67 0.09Total99.93Th Zr10 210

MgO0.47Rb34Nb13

CaO Na202.394.71Sr Ba144280Y37

Hand specimen photographPhotomicrograph (xn)TiO-Zr mmobile elemenl plol

^----------------- pyiw.magiwiiteantfiisrle ---------

calcie.n*iu ootontoe

moie chlorl

100 i-* 901lili _ _l__ J. w_l__l __:._II I

c 80J- ' L < J

0I 7011