Lithogeochemical and Stable Isotopic Insights Into Submarine Genesis of Pyrophyllite-Altered Facies at the Boco Prospect, Western Tasmania

Home , Mount Black (Tasmania), Mount Darwin (Tasmania), Mount Read (Tasmania), Mount Read Volcanics, Sticht Range

Lithogeochemical and Stable Isotopic Insights into Submarine Genesis of Pyrophyllite-Altered Facies at the Boco Prospect, Western Tasmania

WALTER HERRMANN,†,1 GEOFFREY R. GREEN,2 MARK D. BARTON,3 AND GARRY J. DAVIDSON1 1 ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart, Tasmania, Australia, 7001 2 Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania, Australia, 7018 3 Department of Geosciences, University of Arizona, Tucson, Arizona, USA, 85721

Abstract The Boco prospect is a large, fault dismembered, pipelike, hydrothermally altered zone in the Mount Read Volcanics of western Tasmania. It is a synvolcanic alteration zone hosted by felsic volcanic rocks formed in a subaqueous proximal intracaldera setting. Previous detailed geochemical and geophysical surveys and extensive drill testing have indicated it contains no economic metals. The strong to intense, pervasively quartz + phyllosilicate + pyrite-altered northern segment of the prospect is semiconcentrically zoned. Short wavelength infrared (SWIR) spectral analysis has revealed that phyllosilicate assemblages grade from phengitic white mica in the least altered peripheries, through normal potassic white mica, to central zones containing kaolinite, slightly sodic white mica, and pyrophyllite. Mass balance calculations indicate average net mass losses in the altered facies were about 10 to 30 g/100 g, mainly owing to loss of SiO2, which implies very high hydrothermal water-rock ratios. Whole-rock oxygen isotope compositions of the enclosing least altered felsic rocks (δ18O values 8.2–11.7‰) are indistinguishable from those of altered facies (9.6–11.8‰). We attribute the former to low-temperature diagenetic isotopic exchange with 0 per mil δ18O seawater in the peripheral least-altered zones, and the latter to exchange with 3 to 6 per mil δ18O hydrothermal fluids at high water/rock ratios and temperatures generally greater than 220°C, and locally greater than 270°C, in the intensely altered facies. Pyrite sulfur isotope compositions in the Boco altered facies (δ34S values 1.2–7.2‰) are distinctly lower than most Tasmanian massive sulfide deposits (6–15‰), compatible with a dominantly magmatic source of sulfur. The alteration mineral assemblages, estimated mass changes, and isotopic data show that the Boco alteration system was formed by a large volume of focused acidic hydrothermal fluid which had an oxygen isotope composition of 3 to 6 per mil δ18O at and temperature greater than 270°C. The slightly 18O-enriched fluid isotope composition suggests derivation from either mixed magmatic fluid and seawater or isotopically evolved sea - water. Its advanced argillic altered facies place Boco among a newly recognized class of southeast Australian Cambrian volcanic-hosted prospects and deposits. These include Chester, Basin Lake, Western Tharsis, and North Lyell in Tasmania, and Rhyolite Creek, Hill 800, and Mike’s Bluff in eastern Victoria. SWIR spectral analyses with field-portable spectrometers allow early discrimination of this type of hydrothermally altered system, and can potentially assist subsequent exploration in mapping facies zonation. Introduction The Boco system has been attributed to a low-temperature THE BOCO MINERAL PROSPECT is centered on a pyritic quartz Cambrian hydrothermal system that did not transport and de- + phyllosilicate alteration zone located at 145°36'08" E, posit base metals (Green, 1986). Green’s suggestion, that iso- 41°39'32" S, midway between the Rosebery and Hellyer tope geochemistry could provide a fingerprinting technique mines in western Tasmania (Fig. 1). Following the original to distinguish low-temperature barren pyritic alteration zones detection of a few weak conductivity anomalies by a regional from higher temperature systems of greater base metal ex- Barringer airborne INPUT EM survey in 1975, the prospect ploration potential, was tentatively based on relatively few was subjected to an intensive gradient array-induced polar- sulfur and oxygen isotope data. In 2001, we carried out a ization and soil geochemical-based exploration program by more extensive study to fully characterize the sulfur and Electrolytic Zinc Co. and CSR Ltd. during the late 1970s and whole-rock oxygen isotope distribution in and around the mid-1980s (Sainty, 1984; Williams, 1985). Their work, which Boco alteration system, aiming to elucidate its hydrothermal was aimed at discovery of a volcanic-hosted massive sulfide genesis and establish whether sulfur and oxygen isotopes are (VHMS) deposit, culminated in 12 short percussion drill robust discriminators of such barren systems. holes and 14 diamond drill holes totaling ~5,650 m, which de- Methods lineated a 1,400 × 350 m pervasively hydrothermally altered zone (Fig. 2) but did not intersect any base metal mineralized Visual logging of all existing diamond drill core (~5,300 m) zones. Subsequent systematic geophysical surveys, including determined the distributions of primary volcanic facies and downhole and surface time-domain electromagnetic, recon- extent of hydrothermally altered zones. Textural, mineralogic, naissance magnetic induced polarization, and gravity surveys, alteration intensity, and estimated pyrite-content data were failed to produce further drilling targets. recorded using a graphic logging technique based on the for- mat recommended by McPhie et al. (1993). Subsequently, systematic short wavelength infrared (SWIR) analyses with a † Corresponding author: e-mail, [email protected] PIMA-SP portable spectrometer of core samples spaced at

Submitted: June 24, 2009 0361-0128/09/3837/775-18 775 Accepted: September 25, 2009 776 HERRMANN ET AL.

+ + + + + + + + + + + 2- to 10-m intervals along drill holes BBP 250, 251, and 254 + + + + + HELLYER Mt Cripps + + + + + (Fig. 2) and micropetrographic examination of 27 thin-sec- + + + + + QUE RIVER + + + + + + tioned specimens enabled interpretation of phyllosilicate + + + + + + + + + mineral species and delineation of altered facies. The Spec- + + + + + + + + + + tral Geologist v. 2.0 software was used for SWIR spectral data + + + + + + Mt Block + + + + + + +0 5 10 km processing and interpretation. BOCO + + + + + + + + + + + Major and trace element analyses of 27 representative sam- + + + Tyndall Group + + + + ples (W372–W399, which were also thin-sectioned and ana- + + + + + + + Granitoids & porphyry + + + + + + lyzed for oxygen ± sulfur isotopes) were carried out at the + + CHESTER ++ Andesites and basalts Analabs laboratory at Welshpool, Western Australia, by X-ray TULLAH+ + Western volcano- fluorescence methods on fused glass discs and pressed pow- Mt Black + sedimentary sequences der buttons. Major and trace element data for six earlier sam- ROSEBERY + + + + Central Volcanic Complex ples were acquired at the Tasmanian Department of Mines + + Eastern quartz-phyric + + sequence + + + laboratory for G.R. Green, ca. 1985. + + Sticht Range Beds + Cambrian Mount Read Volcanics Oxygen isotope compositions of 27 3- to 10-mg powdered 146°E whole-rock samples (W372–W399) were determined at the HERCULES Mt Read Department of Geosciences, University of Arizona, following HENTY TASMANIA the method of Clayton and Mayeda (1963) on a Finnigan Mat Delta S mass spectrometer. δ18O precision and accuracy is es- 42°S Mount Read timated to be about ±0.3 per mil. Sulfur isotope compositions Volcanics of pyrite in 22 150-µm-thick polished rock sections of the

Hobart same samples were determined to precision of ±0.5 per mil at

100 km the University of Tasmania Central Science Laboratory (CSL), using a laser ablation technique (Huston et al., 1995). Most of these data are average results of ablations on two or three dif- FIG. 1. Location of the Boco prospect in relation to the main lithostrati- δ34 graphic units and mineral deposits in the central northern part of the Mount ferent pyrite grains in each sample. The initial five Spyrite Read Volcanics. values reported by Williams (1985), and 15 whole-rock oxygen

N 40 278

254 5387000 mN 250 A 0500250 m Boco Siding 279 40 251 A’

280 Highway

Murchison 251 drill hole road

Boco Road railway Railway

Bay fault zone

mu outcrop boundary E 5386000 mN

Quaternary fluvio-glacial cover 246 247 surface projection of altered zone

outcrop of altered zone

interbedded siltstone & greywacke 253 felsic volcaniclastics, lavas & sills 384000 mE 383000 mE

FIG. 2. Plan of the Boco prospect showing extent of altered zone and locations of drill holes. Line A-A' indicates location of the cross section depicted in Figure 3.

0361-0128/98/000/000-00 $6.00 776 LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA 777 isotope analyses were obtained for G.R. Green by conven- lack of surface exposures and oriented drill cores. Broken tional means at CSL during the mid-1980s. core intervals indicating the drill holes intersected numerous In order to obtain sample-specific oxygen isotope fraction- faults, sparse and conflicting sedimentary facing indicators, ation factors for fluid modeling, the mineral proportions in and, not least, the extent of texturally destructive hydrother- each sample were estimated from whole-rock major element mal alteration, all present difficulties in correlating nondis- composition data by the MINSQ method (Herrmann and tinctive massive volcanic units. The most likely interpretation Berry, 2002), which is an iterative least-squares approach to is that the volcanic succession is upright and dipping at a low normative mineral calculations, following SWIR spectral and angle to the northwest (Herrmann, 1997). petrographic analyses to determine the mineral species pre- The Boco exploration prospect is centered on two subverti- sent. The sample-specific rock-water oxygen fractionation cal, pipelike, pervasively quartz + phyllosilicate + pyrite al- factors were calculated by applying Zheng’s (1991, 1993) min- tered zones (Figs. 2, 3). The altered zones lie on opposite eral-water fractionation factors to estimates of the mineral sides of a north-northeast–trending brittle fault, which is proportions. Model curves of final rock δ18O relative to topographically expressed to the south as an incised linear val- water/rock ratio at various temperatures and assumed fluid ley along upper Boco Creek and recognizable by its low mag- δ18O values were calculated for an open system, according to netic intensity in aeromagnetic data. This fault appears to equation 9 of Green and Taheri (1992). For modeling pur- have had dextral displacement of ~600 m, which separated poses, we have used a δ18O value of 0 per mil for Cambrian the two segments of the altered zone. Before dislocation, the seawater, based on the recent silicon isotopic evidence of alteration zones were probably contiguous in a roughly ellip- Robert and Chaussidon (2006), which resolved the long- tical 600 × 400 m zone, pitching steeply eastward to a verti- standing debate on isotopic composition of the oceans cal depth of greater than 400 m (the existing limit of drilling) through time (e.g., Veizer et al., 1999; Muehlenbachs et al., below the Quaternary erosion surface. The altered zone thus 2004). appears to cut across the volcanic units, perhaps localized on some preexisting fractured zone. There is evidence that per- Prospect Geology and Alteration Zonation vasive hydrothermal alteration was synvolcanic: nonaltered The Boco prospect is located 8 km north of Tullah, in the mafic dikes that intersect the altered zone contain spalled northern Central Volcanic Complex of the Cambrian Mount fragments of pervasively altered wall rock. These dikes have Read Volcanics (Corbett, 1992), at the southern end of the compositions similar to thin peperitic mafic sills that intruded Bulgobac plain. This area is largely covered by up to 100 m of nonlithified felsic volcaniclastic rocks. Quaternary fluvioglacial deposits. The Cambrian rocks in the Previous descriptions of the Boco altered facies, based on district retain well-preserved primary volcanic textures, de- visual drill core logging, microscopic petrography, and some spite Devonian deformation and lower greenschist facies re- major element analyses, reported “pyritic quartz-sericite” as- gional metamorphism. The host rocks are dominantly massive semblages (e.g., Sainty, 1984) which were regarded as identi- coherent rhyolites and less abundant dacites, interlayered cal to those associated with VHMS deposits at Rosebery and with thick units of massive rhyolitic pumice breccia and minor Hercules, except for the absence of chlorite (Williams, 1985). polymictic felsic volcanic breccias and volcaniclastic sand- However, systematic SWIR spectral analyses of 118 drill core stones. The coherent felsic rocks are sparsely feldspar phyric samples, at intervals of 1 to 10 m down three holes (BBPs to aphyric and generally massive. Locally flow-banded rhyo- 250, 251, and 254) on two cross sections through the northern lites, monomict hyaloclastite breccias, and some intrusive segment of the altered pipe, have subsequently revealed a hyaloclastite contact zones suggest they were emplaced as a semiconcentric zonation of phyllosilicate minerals that were complex assemblage of overlapping flows, small domes, and formerly nondifferentiated as “sericite.” These SWIR-inter- subvolcanic intrusive rocks. The pumiceous breccias exist in preted phyllosilicates include white micas of variable compo- 20- to 100-m-thick massive units interpreted to represent sition, pyrophyllite, and kaolinite. syneruptive deposits resedimented and deposited as sub- The background white micas, which are minor components aqueous mass flows (McPhie and Allen, 1992). Numerous of the felsic host volcanic rocks outside the altered zone, have thin sills and dikes of coherent mafic and intermediate rocks AlOH absorption features in SWIR spectra of >2,210 nm, in- ranging from medium-grained holocrystalline dolerite dicating slightly to moderately phengitic compositions. SWIR through fine-grained amygdaloidal basalt to feldspar por- AlOH features of white micas in the outer parts of the altered phyritic-glomeroporphyritic andesite intersect the felsic zone are mostly in the range 2,200 to 2,210 nm, comparable rocks. They commonly have finer grained quenched or more to normal potassic muscovite. Discrete zones near the center amygdaloidal margins and sharp intrusive contacts, and some- of the system contain white mica with AlOH absorption fea- times include spalled fragments of the wall rock. Rare exam- tures in the range 2,190 to 2,200 nm, suggesting slightly to ples of fluidal peperitic margins suggest more or less synvol- moderately sodic compositions (cf. Herrmann et al., 2001). In canic emplacement of the mafic intrusives into semilithified the upper central part of the northern segment, two drill felsic volcaniclastic rocks. This volcanic facies association re- holes (BBP 250, 251, Fig. 3) intersected a 10-m-wide zone of sembles, and it is probably contiguous southward with, the quartz + pyrophyllite + pyrite enclosed by zones containing Mount Black and Kershaw Pumice formations, which Gifkins low AlOH wavelength sodic white mica passing outward to and Allen (2002) interpreted to have formed in a proximal normal white mica ± kaolinite. submarine intracaldera setting. The kaolinite- and pyrophyllite-bearing argillic and ad- However, the geologic structure and detailed facies archi- vanced argillic assemblages have similarities to those de- tecture of the Boco area is poorly understood owing to the scribed in the Mount Read Volcanics at Western Tharsis

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A (WNW) BBP250 BBP251 A’ (ESE) Altered Facies:

pyrophyllite Base of weathering Na-mica,

AlOH λ <2200 nm L

L Kaolinite ± K-mica L

AlOH λ 2200-2210 nm L

K-mica, L pre-Quaternary surface

AlOH λ 2200-2210 nm L

L Volcanic Facies:

L L

300 m asl L rhyolitic coherent lava/sill

L L L L

L L L

L dacitic coherent lava/sill L

limit of strong feldspar L rhyolitic pumice breccia

destructive alteration L L

L felsic pumiceous-lithic breccia L

felsic volcaniclastic sandstone

mafic dyke

200 m asl

fault

050100 m 100 m asl 383500 mE

FIG. 3. Cross section through the northern segment of the Boco altered zone showing interpreted distribution of altered facies and graphic logs of volcanic facies intersected by drill holes BBP 250 and 251. Interpretation of the facies zonation in the deeper central parts of this section is partly speculative owing to the poor drill hole configuration, but systematic SWIR spectral analyses of cores from BBP254, 150 m to the north, indicate that the Na mica facies extends to at least 200 m below surface.

(Huston and Kamprad, 2001) and Basin Lake (Williams and are used in this paper for convenience; e.g., K mica facies. Davidson, 2004). Although argillic and advanced argillic al- There is a general increase in phyllosilicate content toward teration assemblages are key features of subaerial high-sulfi- the core of the system, reflecting increasing alteration inten- dation epithermal systems, similar styles of subaqueous syn- sity and greater SiO2 depletion. The original protoliths of all volcanic aluminous alteration are inferred from some ancient the altered facies are interpreted to be felsic volcanic rocks of Au-rich VHMS deposits—for example, LaRonde Penna, rhyolitic to dacitic compositions. Abitibi belt, Canada (Dubé et al., 2007). This style of alter- Microphotographs and SWIR spectra representative of ation is also increasingly recognized in some modern arcs and each altered facies are presented in Figures 4 and 5. back arcs (e.g., de Ronde et al., 2005; Paulick and Bach, 2006). Major Element Geochemistry and Figure 3 shows the interpreted distribution of altered facies Alteration Mass Changes on a cross section through the northern segment of the al- Lithogeochemical data from the Boco prospect include tered zone and Table 1 summarizes the characteristics of each major and trace element analyses of several hundred contigu- altered facies. All the altered facies (except the least altered ous drill core intervals and a smaller set of 33 specimens rep- facies) have assemblages containing 35 to 70 percent quartz, resenting various volcanic and altered facies (Table 2). The with varying proportions of phyllosilicates, and 1 to 5 percent early EZ Co. data showed that the Boco alteration system was disseminated pyrite, but insignificant chlorite and carbonate. associated with strong depletion of Na, Ca, and Sr and addi- Short facies names, indicating the main phyllosilicate species, tion of sulfur, and these geochemical trends assisted in tar-

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TABLE 1. Characteristics of Boco Altered Facies

Facies Mineral assemblage Texture Intensity Distribution

Least altered Albite, quartz, k-feldspar, ± minor Sparsely albite ± quartz porphyritic (rarely Weak Regional chlorite and white mica (high AlOH quartz amygdaloidal) with fine-grained wavelength, 2,210–2,220 nm, phengitic) micropoikilitic, spherulitic, or perlitic matrices.

K mica Quartz, white mica (medium Fine, granular mosaic of quartz with 10–30% Moderate to Patchy to pervasive AlOH-wavelength 2,200–2,210 interstitial shreds or seams of mica defining intense in outer parts of nm, potassic), pyrite weak foliation; disseminated pyrite Boco altered zone

Kaolinite ± K mica Quartz, kaolinite, pyrite, ± white Mosaic of quartz and mica with mm-scale Strong to Pervasive in medial mica (medium AlOH wavelength irregular patches of fine clays partly pseudo- intense and upper parts of 2,200–2,210 nm, potassic) morphs after feldspar; disseminated pyrite Boco altered zone

Na mica Quartz, white mica (low AlOH- Fine, granular mosaic of quartz with 10–30% Strong to Pervasive in central wavelength 2,190–2,200 nm, interstitial patches or seams of mica defining intense parts of Boco sodi-potassic), pyrite weak foliation; disseminated to semimassive altered zone pyrite

Pyrophyllite Pyrophyllite, quartz, pyrite, ± low Foliated-sheared, fine-grained pyrophyllite Intense Pervasive in narrow AlOH-wavelength white mica ± white mica with sparse granular aggregates central core of of recrystallised quartz, disseminated pyrite, Boco altered zone and deformed quartz + pyrite veinlets geting exploration drill holes (Sainty, 1984). A large propor- essentially reflects strong feldspar destruction and depletion tion of those samples contain 1 to 5 wt percent sulfur, which of Ca and Na, and minor depletion of Mg. This distribution of equates to approximately 1 to 5 vol percent pyrite. High sul- altered facies data emphasizes the absence of chlorite in the fur proportions are generally associated with low Na2O, which alteration assemblages, and dispels the chlorite-pyrite ambi- is a reasonable index of plagioclase destructive alteration (Fig. guity inherent in the AI-CCPI boxplot. 6). Subsequent whole-rock lithogeochemical analyses, includ- Contrary to Williams and Davidson’s (2004) AI-AAAI box ing complete major and some immobile trace elements plot of Basin Lake and other data, the Boco kaolinite- and py- (Table 3), enable graphic representations of geochemical al- rophyllite-bearing assemblages do not trend to lower AI val- teration indices, and quantitative estimation of the chemical ues at high AAAI. AI is indeterminate for quartz, kaolinite, changes that occurred in each hydrothermal alteration facies. and pyrophyllite and hence those minerals plot as lines along the upper boundary at AAAI = 100. Any trace of mica or chlo- Alteration indices and box plots rite in these assemblages would produce high AI—hence, the Figure 7A presents an alteration box plot of the alteration clustering at the upper right corner (unless the mica was sig- index (AI) and chlorite-carbonate-pyrite index (CCPI)1 fol- nificantly sodic). lowing the method of Large et al. (2001a). The least-altered Neither the AI-CCPI nor the AI-AAAI plot offers a reliable volcanic rocks plot in the least altered box, mainly in the lower means of discriminating between the Boco alteration assem- half because they are dominantly felsic. The K mica facies blages and the quartz-sericite-pyrite assemblages typical of data trend to higher AI values, with the most intensely altered proximal footwall alteration zones in Tasmanian volcanic- samples clustering with members of the kaolinite ± K mica fa- hosted massive sulfide (VHMS) systems. The main miner- cies and Na mica facies at AI > 90 and CCPI 30 to 60. The alogical difference is the absence of chlorite in the Boco sys- single analysis of the pyrophyllite facies plots at slightly higher tem, at least to the depths so far tested by drilling. Hence, CCPI of 63, essentially because of its low Na and K content. chloritic VHMS-type footwall zones may be distinguishable It is the pyrite contents, and to a lesser degree feldspar de- from Boco-type assemblages by higher CCPI and lower AAAI struction reflected in depletion of Na and K, which are the values, at high AI. However, the existence of Cu-Au–bearing main influences on CCPI in these samples as chlorite is not a chlorite-altered facies vertically below pyrophyllite facies in significant component of hydrothermally altered facies in the some other Tasmanian deposits (e.g., Western Tharsis: Hus- Boco system. ton and Kamprad, 2001) suggests that a chlorite-based dis- The data for least altered and altered samples are also criminant may not be universally applicable. clearly demarcated on a box plot (Fig. 7B) combining AI and the advanced argillic alteration index (AAAI)2 devised by Mass changes related to hydrothermal alteration Williams and Davidson (2004). Most of the altered samples We used the immobile element method of MacLean and cluster near the upper-right corner at the position of mus- Barrett (1993) to estimate average mass changes of chemical covite and K-feldspar at high values of AI and AAAI. This components in the altered facies. The first step in their pro- cedure tests for, rather than assuming, immobility of monitor

1 elements and identifies magmatic affinity groups. Figure 8 AI = 100(MgO + K2O) / (MgO + K2O + CaO + Na2O) shows scatter plots of the TiO2 and Zr data categorized by CCPI = 100(FeO + MgO) / (FeO + MgO + Na2O + K2O) 2 AAAI = 100SiO2 / (SiO2 + 10MgO + 10CaO + 10Na2O) volcanic lithofacies (A) and altered facies (B). The data for

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Na-mica

ab + ks ± chl qz qz pl qz

A 0.5 mm D py 0.5 mm

prl qz

py qz qz py prl

K-mica py

qz B 0.5 mm E 0.5 mm

py ka qz

FIG. 4. Micrographs of representative specimens of Boco altered facies. A. Least altered facies (W396). B. K mica facies (W388). C. Kaolinite K mica facies (W376). D. Na mica facies (W394). E. Pyrophyllite facies (W388). qz Mineral abbreviations: ab = albite, ks = K-feldspar, pl = plagioclase, qz = K-mica quartz, prl = pyrophyllite, Na mica = low AlOH wavelength <2,190 nm sodi- C 0.5 mm potassic white mica, K mica = medium AlOH wavelength 2,200–2,210 nm potassic white mica.

coherent rhyolites form a highly correlated linear trend (r = However, this linear trend does not include the least altered 0.88, n = 8 analyses) with a Ti/Zr ratio of about 6.5 that pro- dacite sample, W398, which lies slightly above the regression jects close to the origin of the plot and convincingly satisfies line. This does not necessarily indicate Ti or Zr mobility dur- MacLean and Barrett’s (1993) immobility test. Data for six ing alteration. It is probably due to minor primary composi- samples of altered coherent dacite also lie on a highly cor- tional variability in the relatively few dacite samples analyzed. related (r = 0.98) trend, which projects above the origin of The dacite unit represented by W398 was intersected by drill the plot, intersecting the TiO2 axis at about 0.12 percent. hole BBP 279 east of the bisecting fault, and about 300 m

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least altered facies (insignificant mica) 6

least altered facies (minor phengitic mica) wt% O 2 5

K-mica facies Na

kaolinite ± K-mica facies 4

Na-mica facies 3

pyrophyllite facies 2

0 012345 6 1300 1600 1900 2200 2500 Sulfur wt%

Wavelength (nm) FIG. 6. Scatter plot illustrating generally inverse relationship between sulfur and sodium in lithogeochemical data from the Boco prospect (data from FIG. 5. Stack of SWIR spectra typical of the Boco prospect altered facies. Sainty, 1984). northeast of and 250 m lower than the main group of dacite Given the small data set, alteration mass changes were cal- samples from units intersected by the upper sections of BBPs culated on average compositions of altered facies in coherent 250 and 251. It is therefore likely to be a different emplace- volcanic rocks, matched to judiciously selected precursors, ment unit. and based on Zr as the immobile component in all cases. The The least altered coherent rhyolites (W389, 392, 396; se- estimates of absolute mass change are listed in Table 4 and lected by petrographic observations and geochemical para- graphically presented in Figure 9. The compositional varia- meters) plot in a cluster centered on 0.24 percent TiO2 and tion in dacites mentioned above presents a minor problem in 202 ppm Zr. However, the two least altered rhyolitic volcani- selection of a dacitic least altered precursor composition for clastic rocks (pumice breccias) have very different immobile mass change calculations. To overcome it, we took alternative element compositions (W393 and W397: 243 and 372 ppm approaches: (1) using the composition of sample W398, and Zr, respectively), which nevertheless fall close to the trend of (2) deriving a precursor composition using the MacLean and coherent rhyolites. Nonaltered felsic rocks in the Mount Barrett (1993) multiple precursor method, assuming linear Read Volcanics rarely contain more than 300 ppm Zr (cf. magmatic fractionation between the composition of dacite Crawford et al., 1992). The dissimilar Boco pumice breccia W398 and the average composition of three least altered co- samples seem to represent different compositional groups but herent rhyolites (W389, 392, 396). Both approaches are ap- we have not investigated whether the disparity is due to mag- proximations but the outcomes are quantitatively similar (Fig. matic, volcaniclastic, or diagenetic alteration processes. It 9A, B). makes the selection of the least altered precursor problematic The mass change estimates for average compositions (Table and, accordingly, we have not pursued estimates of mass 4; Fig. 9) indicate that all of the altered facies underwent sig- change in these noncoherent volcanic facies. nificant losses of Si and Na, with additional small losses of Al,

TABLE 2. Summary of Boco Lithogeochemical Data

No. of analyses Elements analysed Source Reference

~645 Partial suite of major elements, including SiO2, Fe, EZ Co. Sainty, 1984 mostly 3 m contiguous CaO, Na2O, S and trace elements Cu, Pb, Zn, Ag, exploration report split core samples Au, Sr, Ba, Mn, Co, Ni, Hg

+ 6 Major elements SiO2 to P2O5; also CO2, S and H2O ; Tasmanian G.R. Green, writ. commun., 1997; trace elements Zr, Nb, Y, Cr, Ni, V, Sc, Sr, Rb, Ba, Department of Mines unpub. data, listed in Table 5 Cu, Pb, Zn, Ag, Au, As (excluding some trace element data)

27 Major elements SiO2 to LOI; trace elements Zr, CODES, MRV isotope Table 3, this study Nb, Y, Cu, Pb, Zn, As, Sb, Tl SPIRT project

0361-0128/98/000/000-00 $6.00 781 782 HERRMANN ET AL. CDT S 34 δ SMOW O 18 δ Arizona UTAS and ablation Mayeda, 1963 io) (‰) (‰) LOI Total Zr Nb Y Ti/Zr 5 O 2 OP 2 OK 2 MnO MgO CaO Na 3 O 2 Fe 3 O 2 Al 2 TiO 2 77.64 0.21 11.59 2.89 0.01 0.67 0.01 <0.05 3.98 0.02 2.94 99.96 180 12 34 7.0 8.2 3. Lithogeochemical and Isotopic Composition Data Acquired by CODES, 2001 ABLE T Alteration SiO K mica 75.19 0.39 12.56 3.72 0.00K mica 0.69 0.02 0.08 72.59 4.08 0.15 0.03 8.35 3.6510.01 100.41 0.00 211 0.11 10 0.01 0.06 34 2.30 11.1 0.01 6.42 10.3 100.01 123 1.9 6 17 7.3 8.6 –0.1 pumiceous/ K mica , qtz and Polymict volcaniclastic Rhyolitic volcaniclastic Fs phyric lithic breccia perlitic breccia, qtz and Fs phyric spherulitic banded perlite silicified rhyolite/ breccia breccia banded perlite banded perlite sandstone, and Py pumice breccia qtz amygd pumice breccia pumice breccia pumice breccia micropoikilitic pumice breccia S data are of pyrite 34 δ Note: All Sample HoleW372 Depth BBP250 facies Volcanic W373 59.0 BBP250 Rhyolitic W374 77.0 faciesW375 BBP250 Dacite; Fs phyric, 112.0mica BBP250 131.8 Dacite; Fs phyric Kao±K mica Polymict/pumice K (wt %)W376 70.91 (wt %) (wt %) (wt %) (wt %)W377 (wt %) mica K BBP250 0.56 75.53 (wt %) (wt %) (wt %) (wt %) 161.0W378 15.93 BBP250 (wt %) 0.41 (wt %) (ppm) 205.0 Dacite; Fs phyricW379 BBP250 (ppm) 13.42 2.33 (ppm) (rat 256.5 Dacite; Fs phyric BBP250 0.01 3.41 75.56 297.0 Rhyolite, Fs phyricW380 mica Kao±K 0.00 0.44 0.66 Rhyolite, Fs phyric, W382 mica K BBP250 13.03 70.62 Na mica 0.26 0.00 K mica 342.0 BBP251 0.54 3.42 0.00 0.10 Rhyolite, Fs phyricW383 15.37 60.6 0.00 4.10 0.11W384 BBP251 72.76 4.07 72.89 altered Least Rhyolite, aphyric, 0.50 3.77 0.02 BBP251 0.32 78.90 69.5 0.03 0.49 73.64 0.00 13.48 0.22 0.04 Na mica 92.5 14.26 4.72 0.28 1.06 Rhyolite, Fs phyric 11.61W385 0.13 13.80 5.15 3.51 3.74 99.34 Pyrite bands in 0.06 Pyrophyllite 100.47 3.93W386 BBP251 0.00 2.13 0.00 1.91 277 81.67 112.3 0.13 213 BBP251 0.00 75.04 0.04 0.03 0.13 0.44 0.18 12 3.54 142.6 Dacite; Fs phyric 0.33 Na mica 0.71 3.46 11 0.48 0.00 0.10 10.16 17.41W387 100.52 0.06 0.06 2.02 41 0.31 2.50W388 0.23 mica K BBP251 205 1.95 4.71 35 3.50 0.10 66.62 0.00 167.5 100.19 4.04 0.10 BBP251 0.00 12.1 10 3.58 0.25 11.5 182.5 Basalt dike 4.37 254 0.03W389 0.04 0.10 0.00 Rhyolite, aphyric, 0.02 BBP251 11.6 0.04 4.38 8.39 0.00 72.60 3.85 23 13 0.00 100.07 189.5 13.69W390 100.14 2.72 8.7 0.50 3.20 0.10 K mica 100.05 0.00 0.05 12.9 292 Rhyolite, aphyric, BBP251 226 13.51 26 99.87 2.80 0.98 263.0 174W391 0.03 3.74 13 14 Least altered 219 Least altered 0.02 2.9 10.6 12.7 0.03 BBP251 0.00 0.00 11 45.18 75.92 79.58 282.5 2.56 11W392 3.97 32 1.45 31 0.72 0.22 0.06 100.04 10.3 0.21 Rhyolitic, Fs phyric BBP251 24 20.94 99.77 4.3 12.25 2.26 0.07 163 11.99 29 K mica 359.5 13.0 14.00W393 6.6 282 2.31 0.02 0.25 1.91 Rhyolite, Fs phyric, 1.69 BBP251 11 7.6 7.2 0.02 19 7.7 Least altered 0.00 373.0 8.30 3.39W394 5.53 9.4 9.3 0.34 73.52 Rhyolitic, Fs phyric 14 BBP254 0.40 10.6 99.63 72.79 0.05 1.00 0.21 21 10.2 Least altered 250.0 0.37W395 0.33 174 0.03 3.66 13.28 3.33 74.76 2.4 Rhyolitic, Fs phyric 6.6 13.37 BBP254 <0.05 4.6 2.79 99.93 10 2.84 4.0 7.0 0.25 2.11 14.2 Na mica 318.5W396 3.81 1.89 3.69 229 11.35 0.10 0.67 Rhyolitic, Fs phyric BBP254 0.35 0.02 9.7 0.03 47 2.01 K mica 342.0 0.56 4.75W397 9.8 0.53 2.59 6 2.13 68.43 0.28 Rhyolite, Fs phyric, BBP254 100.54 1.04 99.94 100.07 8.6 2.19 0.49 Least altered 409.0 4.3 0.63W398 190 201 46 196 20.13 3.42 4.6 71.98 0.47 Rhyolitic, Fs phyric W399 BBP279 76.77 1.94 4.04 10 1.71 0.29 12 9.7 4.03 13.1 Least altered 471.0 BBP251 0.30 5Laboratory 1.98 0.00 0.0413.95 70.29 120.0 Dacite; (Fs) phyric 0.0512.58 28 3.83Method 31 0.41 0.18 1.703.08 Dacite; Fs phyric 29 3.472.66 9.1 2.1 altered Least 100.02 0.0314.95 0.07 0.05 0.01 64.75 99.47 6.6 43.2 207 6.7 2.732.06 0.63 mica Kao±K 0.44 0.49 0.28 248 0.06 6.6 99.79 0.61 69.65 13.87 5.51 9 0.12Detection limit 9.7 14 7.0 0.78 9.7 243 0.54 5.00 0.05 4.96 0.73 15.31 1.97 0.11 24 3.45 11 3.56 32 3.66 4.23 100.39 2.4 0.94 2.43 4.9 0.08 0.04 0.00 6.1 32 438 4.25 3.16 8.0 0.99 2.65 0.72 100.09 0.05 22 100.01 3.13 6.2 0.03 203 9.6 2.76 267 2.88 8.2 100.01 26 0.73 10 0.11 11 372 9.6 3.72 5.05 6.7 25 16 0.02 29 99.44 4.92 9.0 178 11.0 8.6 44 6.7 99.87 259 9 10.0 6.6 1.9 9.9 15 30 10.5 Analabs 5.1 41 14.8 0.7 12.5 8.5 10.4 X408 X408 11.4 0.05 X408 13.8 X408 0.01 X408 0.05 6.1 X408 0.01 X408 X408 0.01 X408 0.01 X408 X408 0.01 0.05 0.01 X401 0.01 X401 0.01 X401 Clayton Laser 5 3 3

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calciteankerite dolomite pyrite chlorite quartz, pyrophyllite, kaolinite muscovite 100 100 K-feldspar

90 A 90 B

mafic CCP Index CCP 80 Index AAA 80

least altered O) 70 2 70 rhyolites phengite O) 2 60 60 Na-mica dacitic 50 50

40 40 albite paragonite phengite +10MgO+10CaO+10Na 30 2 30 mafic rhyolitic chlorite / (SiO

20 2 20 anorthite

10 10 paragonite muscovite 100 SiO 100(FeO+MgO) / (FeO+MgO+NaO+K albite Na-mica K-feldspar calcite ankerite dolomite 0 0 0 102030405060708090100 0 102030405060708090100

100(MgO+K2O) / (MgO+K2O+CaO+Na2O) Alteration Index 100(MgO+K2O) / (MgO+K2O+CaO+Na2O) Alteration Index

Altered facies: least altered Na-mica K-Mica pyrophyllite Kaolinite +/- K-mica Green's data

FIG. 7. Box plots of alteration indices of Boco prospect lithogeochemical data: A. AI and CCPI indices (after Large et al., 2001); altered facies trend to high AI values, and variable CCPI values dependent largely on pyrite content. B. AI and AAAI indices (after Williams and Davidson, 2004); data for altered facies cluster at high AI and AAAI values reflecting absence of plagioclase feldspar, chlorite, and carbonate.

0.6 0.6 % % 2 2

TiO A

TiO B 0.5 0.5 least altered dacite least altered dacite W398

0.4 0.4

0.3 0.3

net mass loss high Py 0.2 0.2 content

coherent dacites least altered rhyolites least altered rhyolites r = 0.98 0.1 0.1

coherent rhyolites r = 0.88 0.0 0.0 0 50 100 150 200 250 300 350 400 450 0 50 100 150 200 250 300 350 400 450 Zr ppm Zr ppm Volcanic facies: Altered facies: coherent rhyolite coherent dacite least altered Na-mica rhyolitic volcaniclastics polymictic volcaniclastics K-Mica pyrophyllite Green's data K-mica + kaolinite Green's data

FIG. 8. Scatter plots of TiO2 and Zr data (excluding mafic intrusive sample) with different symbols categorized by volcanic facies (A), and altered facies (B).

Fe, and Ca in some facies. A small loss of ~3 g/100 g K2O is averaged compositions, although some individual samples evident in the pyrophyllite-rich sample W383, but potassium show significant additions of Si ± Fe; e.g., W390, which con- seems to have remained essentially unchanged in all the other tains 5 to 10 percent pyrite, and W382, which is an unusually altered facies. Mass gains are generally insignificant for the siliceous sample from the Na mica facies.

0361-0128/98/000/000-00 $6.00 783 784 HERRMANN ET AL. LOI Total Zr LOI Net 5 5 O O 2 2 OP OP 2 2 OK OK 2 2 nent MnO MgO CaO Na MnO MgO CaO Na 3 3 O O 2 2 Fe Fe 3 3 O O 2 2 Al Al 2 2 TiO TiO 2 2 SiO SiO 67.00 0.39 13.69 4.38 0.10 0.83 2.54 3.28 3.10 0.10 4.20 99.60 184 g/100 g g/100 g g/100 g g/100 g g/100 g g/100 g g/100 g g/100 g g/100 g g/100 g g/100 g g/100 g 4. Average Compositions and Mass Change Estimates 4. Average ABLE T e method precursor multiple M&B by Rhy alt lst and Avg W398 Mass changes were calculated by MacLean and Barrett’s (1993) “reconstituted composition” method, using Zr as the immobile compo Mass changes were calculated by MacLean and Barrett’s = average, alt altered, lst least Kao kaolinite Abbreviations: Avg FaciesSynthetic lst alt dacit K mica alt daciteAvg K mica alt daciteAvg kao±K mica alt daciteAvg kao±K mica alt daciteAvg W374, 377, 385 Avg W373, 376, 399 Avg W374, 385 Avg lst altd rhyolite W373, 399Avg Avg Na mica alt rhyoliteAvg Derived from W378, 382 Avg Mass change estimates W389, 392, 396 Avg Facies K mica alt daciteAvg K mica alt daciteAvg kao±K mica alt daciteAvg W374, 377, 385 Avg kao±K mica alt daciteAvg W374, 377, 385 Avg kao±K mica alt dacite W373, 376, 399Avg Avg W373, 376, 399 Avg Synthetic lst alt dacite 73.68 70.39 W373, 399W378 Na mica alt rhyolite Avg Derived from W398 lst alt dacite Synthetic lst alt dacite 0.48W382 Na mica alt rhyolite 0.55 W398 lst alt dacite Na mica alt rhyolite W378Avg 13.60 15.54 74.08 70.28 W382 3.63 3.54 0.47 0.55W383 Pyrophyllite K-mica alt dacite W374, 385 (wt %) Avg W378, 382 Avg 0.00 (wt %) 13.27 0.01 73.81 15.62 Precursor –5 (wt %) –17 (wt %) –18 3.58 0.55 0.24 3.28 77.22 0.81 (wt %) –5 (wt %) 0 (wt %) 13.16 0.00 –17 0.25 0.06 0.01 0.03 0 (wt %) 0 (wt %) W383 2.50 11.82 0 0.61 lst altd rhyolite Avg 0.68 (wt %) 0.69 0.32 (wt %) lst altd rhyolite 0 Avg –2 3.83 (wt %) 0.06 –1 0.04 –3 3.79 0.02 lst altd rhyolite Avg 3.79 (ppm) –3 0.00 0.51 –1 0.91 0.06 –3 0.42 0.03 –1 –2 0.09 0.67 3.66 –2 3.66 3.91 4.78 –3 0 0.00 100.20 3.72 0 0.05 0 0.02 99.80 220 0 –5 0.21 –23 3.73 263 3.56 4.82 0 lst altd rhyolite Avg 0 100.23 3.15 27 0.05 0 0 0 99.61 0 217 0 0.03 1.61 268 –2 0 0 –3 100.06 0 3.47 –3 –4 –3 202 100.05 –3 –3 –1 –3 –1 228 –3 1 –20 1 –3 0 1 0 0 0 0 0 0 0 0 0 0 0 –1 0 0 0 0 –1 –1 –1 0 0 –2 –1 –1 –16 –2 –30 –1 0 –20 –18 –4 –4 –32 –4 –1 –1 –1 –1 0 0 0 –4 0 1 1 –3 –11 –31 2 0 24 1 –29 Compositions used in mass change calculations

0361-0128/98/000/000-00 $6.00 784 LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA 785

10 compositions of K mica-altered facies (W374, 385) with K Average K-mica alt Dacite A mica + kaolinite-altered facies (W373, 399); these are all al- 0 tered coherent dacite samples, most likely from a single volcanic unit. -10 The 400 × 600 m elliptical × 400 m deep pipelike, pre- faulted shape of the Boco altered zone occupied a volume of -20 about 75 × 106 m3, equivalent to about 200 × 106 t (assuming S.G. of ~2.7). Accordingly, a conservative average mass Precursor: synthetic lst alt Dacite -30 loss of 10 g/100 g SiO2 from the known Boco alteration sys- Precursor: W0398 tem implies removal of about 20 × 106 t of silica during hy- -40 drothermal alteration. That would have required a very large Si Ti Al Fe Mn Mg Ca Na K P LOI Net amount of water. The solubility of silica in aqueous fluid at 10 340°C is about 1,700 ppm at vapor pressure, and 3,200 ppm Average K-mica + Kao alt Dacite B at 1 kbar (Fournier, 1985). If the hydrothermal fluid flowing 0 into the altered zone contained no silica but became satu- rated to carry ~3,200 ppm silica at its outflow, a water/rock -10 mass ratio of about 30 would be required to remove 10 g/100 g of SiO2 from the rock. Hence, even at maximum silica solu- -20 bility, the mass change estimates indicate a hydrothermal × 6 Precursor: synthetic lst alt Dacite water/rock mass ratio of at least 30; i.e., about 600 10 t of -30 Precursor: W0398 water.

-40 Whole-Rock Oxygen Isotope Compositions Si Ti Al Fe Mn Mg Ca Na K P LOI Net δ18 10 Twenty-seven new Owhole rock analyses of drill core sam- Average Na- mica altered Rhyolite ples of altered and least altered felsic volcanic rocks from C holes BBP 250, 251, 254, and 279 (Fig. 2) have yielded δ18O 0 values in the range 8.2 to 11.6 per mil (Table 3; Fig. 10A, B). A single sample of a basaltic dike has a δ18O value of 7 per mil. -10 The range of data from the altered zone is essentially similar to that of Green and Taheri (1992), who obtained results be- -20 tween 9.7 to 11.8 per mil in 15 samples, mainly of peripheral least altered zones, and found no apparent difference be- -30 tween the background and altered rocks (Table 5). Precursor avg lst alt Rhyolite Comparison of oxygen isotope data with alteration indices -40 Si Ti Al Fe Mn Mg Ca Na K P LOI Net calculated from major element geochemical data (including 6 10 of G.R. Green’s δ18O analyses) indicates that there is no sig- δ18 Pyrophyllite: W0383 D nificant correlation between the Owhole rock values and in- (g/100g) 0 tensity of alteration in this system (Fig. 10C). The range and distribution of the δ18O values in least altered background -10 samples is similar to those of samples from the intensely altered zone. δ18 -20 The least altered background O values (9–12‰) for all but one of the least altered Boco samples are typical of non-

-30 altered felsic volcanics in the Mount Read Volcanics. For ex- Precursor: avg lst altd Rhyolite ample, least altered andesites in the peripheral footwall of the

Absolute Mass Changes Hellyer deposit have background values of 11.3 ± 0.9 per mil. -40 Si Ti Al Fe Mn Mg Ca Na K P LOI Net The least altered Boco data are also similar to δ18O values in Components the hanging-wall sequence above the northern lens of the FIG. 9. Bar graphs representing estimated absolute mass changes due to Rosebery deposit, and in outcrops between Hercules and alteration in the four hydrothermally altered facies. All facies appear to have Mount Read (W. Herrmann, 1998, unpub. data). The single undergone net mass losses of 10 to 30 g/100 g, principally due to losses of sil- exception (W387, 7.0‰) is from a narrow basaltic dike—one ica and sodium. of several mafic units intersected by BBP251 and other Boco drill holes. Although surrounded by intensely quartz-sericite- pyrite altered felsic volcanics and enclosing some spalled frag- The overall pattern of mass change is of net mass losses be- ments of altered rock, the mafic dikes are nonaltered, indi- tween 10 and 30 g/100 g, dominated by SiO2 losses of be- cating that they were emplaced after pervasive hydrothermal tween about 5 and 20 g/100g. The limited data suggest mass alteration of the country rocks. The relatively low δ18O value losses were lowest in the outer K mica facies, and generally is typical for mafic rocks (e.g., Faure, 1986) and probably rep- greater in the medial to proximal altered facies. This inward- resents the primary magmatic oxygen isotope composition. It progressive mass loss is particularly evident in comparison of implies negligible isotopic exchange with either diagenetic or

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8 and pyrophyllite facies. Each case illustrates relationships be- A Least altered facies tween final rock δ18O and atomic water/rock ratios at temper- n = 20 δ18 6 atures from 0° to 400°C for alternative fluid O values of 7 Frequency and 0 per mil. These, respectively, represent approximate iso- felsic volcanics topic compositions of magmatic water and seawater sources 4 (as indicated by Muehlenbachs et al., 2004; Robert and Chaussidon, 2006). Although there are great contrasts in the mineral assemblages of the altered facies, their specific whole 2 mafic intrusive rock-water fractionation factors do not differ greatly, and the 18 δ Orock-water/rock ratio (w/r) curves vary only slightly. In 0 other words, the calculated facies-specific whole-rock frac- 6 7 8 9 10 11 12 13 18 tionation factors account for differences of only about 1 per δ O ‰ mil. Hence, the contrasting mineral assemblages would have 18 8 had relatively little effect on final δ Orock values, which were B Altered facies primarily dependent on fluid isotopic compositions, tempera- n = 21 tures, and water/rock ratios. 6 Frequency The isotopic models for least altered rhyolite samples (e.g., W392; Fig. 11A, B) indicate that the observed δ18O ratios 4 (9.6–11.8‰, avg, 10.6‰) could be produced by isotopic exchange with small amounts of either seawater or magmatic water (w/r <0.2) at low temperatures (<100°C). Alternatively, 2 isotopic exchange with large amounts of water of either source, at temperatures between about 200° and 400°C, δ18 0 could produce the observed rock O. The feldspar-rich 6 7 8 9 10111213 composition of the least altered rhyolites and marine deposi- 18 δ O ‰ tional setting favors the first interpretation; i.e., essentially δ18 O ‰ low temperature seawater-diagenetic alteration involving 13 C small amounts of 0 per mil fluid. As noted above, these are 12 typical background δ18O values for nonhydrothermally al-

11 tered felsic rocks in the Mount Read Volcanics. Similarly, the model curves for an intensely pyrophyllite fa- 10 cies altered rhyolite from the core of the Boco alteration zone

9 (e.g., W383; Fig. 11C, D) indicate the observed 9.8 per mil δ18O value could be attributed to low temperature and low 8 water-rock conditions. However, the complete absence of 7 feldspar and estimated chemical mass losses of Na2O and K2O and loss of ~20 g/100 g SiO2 suggest that very large 6 30 40 50 60 70 80 90 100 water/rock ratios were required to produce this altered facies. Alteration Index (AI) A water/rock mass ratio of 30 (alluded to in the previous dis- Altered facies: cussion of silica solubility) equates to an oxygen molecular least altered Na-mica water/rock ratio of about 17. Therefore, high water/rock ra- δ18 K-Mica pyrophyllite tios must be considered in modeling of O data for the Kaolinite +/- K-mica Green's data inner intensely altered facies. At water/rock ratios greater than 4, final rock δ18O compositions are determined by iso- FIG. 10. Graphic representations of Boco whole-rock oxygen isotope com- topic fractionation, temperature, and the isotopic composi- positions: A, B. There is a substantial overlap in the ranges of least altered felsic volcanic rocks and hydrothermally altered facies; δ18O values of 9.6 to 11.8 tion of the fluid, but not by small variations in water/rock ra- per mil and 8.2 to 11.7 per mil, respectively. The single sample of a mafic in- tios. trusive rock has a δ18O value of 7 per mil. C. This plot of whole-rock δ18O val- Accordingly, at high water/rock ratios, the δ18O value of 9.8 ues against AI values shows there is no significant correlation between oxy- per mil observed in the pyrophyllite facies could be produced gen isotope composition and intensity of alteration. by isotopic exchange with 0 per mil fluid at ~200°C or a 7 per mil fluid at a little over 400°C. Pyrophyllite is stable in hy- hydrothermal fluids, which is consistent with the interpreted drothermal settings with high activities of water at about 270° timing and mode of emplacement. to 360°C (Hemley et al., 1980). At the sample-specific frac- There is, therefore, no indication of an oxygen isotope tionation factors estimated for sample W383, this pyrophyl- anomaly, gradient, or halo associated with the Boco alteration lite-stability temperature range equates to fluid δ18O values zone, at least within 500 m of the periphery of the system. between 3.2 and 5.7 per mil—i.e., possibly a mixture of sea- Nevertheless, the δ18O data offer some constraints on the water and magmatic water. compositions, temperatures, and potential volumes of hy- It could be argued that the pyrophyllite at Boco is not a hydrothermal fluids in the system. Figure 11 presents sample- drothermal phase, but represents metamorphosed kaolinite. specific oxygen isotope models for the least altered, K mica, Kaolinite will dehydrate to pyrophyllite at ~270°C in the

0361-0128/98/000/000-00 $6.00 786 LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA 787 CDT S 0.2 4.7 –1.2 –0.7 –0.1 34 δ SMOW 9.7 ‰) (‰) and ventional O 10.4 10.1 11.1 11.2 11.1 11.5 11.7 10.2 (1963) UTAS UTAS Clayton Con- Mayeda 18 δ LOI Total Zr Nb Y Ti/Zr 5 O 2 OP 2 OK 2 MnO MgO CaO Na 3 O 2 Fe 3 O 2 Al 2 TiO 2 5. Lithogeochemical and Isotopic Composition Data Acquired by G.R.Green (mid-1980s) ABLE T Alteration SiO Fs phyric Fs phyric Fs phyric patchy chl Rhy breccia mafic dyke Fs phyric Fs phyric clastic siltstone banded Rhyolite phyric pumice breccia flow banded volcaniclastic Samplealtered B100601 Hole BBP278 Depthaltered72.32B100602 151.0 Description BBP278 Rhyolite, Least 0.26altered B100603 173.9 12.74 BBP278 Rhyolite, Least facies 2.59altered B100604 183.9 0.05 BBP278 Rhyolite, Least 0.11B100605 228.2altered72.69 (wt %) 0.95 (wt %) (wt %) BBP278 Rhyolite, Least (wt %) (wt %) (wt %)altered 2.42 (wt %) 0.26 (wt %)B100606 264.5 (wt %) (wt %) (wt %) (wt %) 5.85 12.14 BBP278 (ppm) Monomict Least (ppm) 2.36 (ppm) 0.05B100607 441.5 (ratio)altered74.01 0.07 2.58 BBP278 Least B100608 Vesicular ( 0.19 BBP278 99.92 452.0 0.26mica 260 1.82B100609 489.5 Rhyolite, aphyric 12.63 Least altered BBP280 Rhyolite, Least 2.07 2.68 11B100610 245.7 5.74 0.04 BBP280 Rhyolite, K 24 0.04 0.17B100611 276.6 2.74 0.98 6.0 BBP280 Rhyolitic v/ 100.12 3.03 240B100612 316.5 5.12 10.5 BBP280 Sericitic flow 11 0.04 K mica 395.0altered 1.54 K mica B100613 Rhyolitic, Fs 29 100.50 BBP253 260 6.5 77.57B100614 204.5 Least alteredaltered 0.20 10 BBP253 73.26B100615 Rhyolite, Least 13.17 11.8 0.24 BBP253 329.5 1.63 28 13.98 Rhyolite, aphyric 391.0 2.20 0.01 altered Least GRG1 Rhyolitic Least 6.0 74.49 0.03 0.56GRG2 0.20 BBP246 0.42 0.05GRG3 10.7 13.03 BBP247 304.5 0.43GRG4 0.10 1.68 BBP251 140.2 PyriteGRG5 3.06 4.13 0.04 BBP251 Pyrite 3.55 0.02 93.5 0.23 BBP254 265.2Laboratory 0.03 Pyrite 2.68 0.70 300.4 PyriteMethod 100.12 2.63 2.31 180 Pyrite 99.83 K mica 4.40 240 13 K mica 0.02 14 1.73 26 Na mica K mica 98.83 30 K mica 6.7 195 6.0 18 11.7 18 11.7 6.1 11.6 Dept. of Mines Tasmanian XRF XRF XRF XRF XRF XRF XRF XRF XRF XRF XRF XRF XRF XRF XRF

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30 30 0ºC 50ºC 0ºC 50ºC 25 A 25 B 100ºC δ18O = 0 ‰ δ18O = 7 ‰ ‰ 20 fluid 20 fluid 100ºC least altered rock 200ºC O 15 15 facies 18 W392, 9.6‰ δ 300ºC 10 200ºC 10 400ºC 5 300ºC 5

400ºC 0 0 0.01 0.1 1 10 100 0.01 0.1 1 10 100 30 30 0ºC 50ºC 0ºC 50ºC 100ºC 25 C 25 D δ18 Ofluid = 0 ‰ δ18

‰ O = 7 ‰ 20 100ºC 20 fluid 200ºC pyrophyllite rock

O 15 15 facies 18 300ºC W383, 9.8‰ δ 200ºC 10 10 400ºC 300ºC 5 5 400ºC 0 0 0.01 0.1 1 10 100 0.01 0.1 1 10 100 30 30 0ºC 50ºC 25 E 25 F 0ºC 50ºC 100ºC δ18 18 Ofluid = 0 ‰ δ O = 7 ‰

‰ 20 20 fluid 100ºC 200ºC K-mica rock facies

O 15 15

18 300ºC W388, 9.7‰ δ 10 200ºC 10 400ºC 300ºC 5 5 400ºC 0 0 0.01 0.1 1 10 100 0.01 0.1 1 10 100 water / rock ratio (atomic) water / rock ratio (atomic)

FIG. 11. Paired diagrams illustrating theoretical relationships between final rock δ18O values and water/rock ratios at various temperatures from 0º to 400°C for alternative fluid δ18O values of 0 (left) and 7 per mil (right), which approximate seawater and magmatic water sources, respectively. Curves for the least altered facies (A, B), pyrophyllite facies (C, D), and K mica facies (E, F) were calculated from fractionation factors specific to the mineral proportions existing in representative samples W392, W383, and W388, respectively. The gray horizontal bands indicate the ranges of δ18O values observed in each altered facies. presence of quartz, and at ~300°C without quartz (Hemley et 2001). The model curves in Figure 11E and F indicate that al., 1980). Regional metamorphic temperatures in the Que- the observed range of isotopic compositions (δ18O values Hellyer Volcanics, only 10 km northeast of Boco, have been 8.2–11.7‰) are equally consistent with isotopic exchange estimated at about 300° to 320°C (Offler and Whitford, 1992) with 0 per mil fluid at ~200°C, or alternatively, a 7 per mil but may locally have been as low as 200°C (R.F. Berry, pers. fluid at 320° to 500°C, at the high water/rock ratios implied commun., 2001) and therefore metamorphic breakdown of by the intensity of alteration. However, it is reasonably as- kaolinite may not have fully proceeded. However, the facts sumed that the K mica and Na mica facies were produced by that kaolinite still exists in the medial kaolinite ± K mica fa- a fluid of similar oxygen isotope composition to that implied cies at Boco and that it is apparently symmetrically distrib- by the pyrophyllite facies, i.e., ~3 to 6 per mil δ18O. On that uted around the core of the system suggest that kaolinite and basis, the observed ranges of whole-rock δ18O values and frac- pyrophyllite are both primary hydrothermal phases. tionation factors for the K mica facies equate to equilibration The K mica facies, which occupies the greater part of the temperatures between 220° and 450°C. Boco alteration system, does not offer firm temperature constraints. White mica stability is not strongly temperature de- Pyrite Sulfur Isotope Compositions pendent under hydrothermal conditions; it is mainly con- Sulfur isotope analyses of five pyritic samples from the Boco 34 trolled by pH and activity of potassium (e.g., Schardt et al., alteration zone in the mid-1980s (Table 5) showed that δ Spyrite

0361-0128/98/000/000-00 $6.00 788 LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA 789 values ranged from –1.2 to +4.7 per mil (Williams, 1985). 4 Green (1986), and Green and Taheri (1992) noted that these A Py in least altered facies values were distinctly lower than sulfides at the Hercules Pb- n = 7

Frequency 3 34 Zn deposit, and they proposed that such low δ Spyrite values could be used to distinguish low-temperature barren pyritic alteration zones from higher temperature base metal-associ- 2 ated systems. felsic volcanics We have analyzed sulfur isotopes in pyrite from an addi- 1 tional 22 samples (which were also analyzed for whole-rock mafic δ18O and major and trace elements) from in and adjacent to intrusive the Boco alteration system. The results are presented in Table 0 -5 0 5 10 15 3 and Figure 12. 34 The new δ34S data overlap with the previous data (Williams, δ S ‰ 1985) and extend to a maximum of 14.2 per mil. There are 4 poorly correlated inverse relationships between pyrite δ34S B Py in altered facies and geochemical measures of alteration intensity (Fig. 12C). n = 20 3 The pyrite δ34S values from the least altered facies, with AI Frequency Mt Read Volcanics value of less than 70, have a broad range from 2.4 to 14.2 per VHMS deposit s: 6-15‰ δ34 mil. The lowest S value of these (sample W387; 2.4‰) is 2 from a nonaltered mafic dike that cuts across and postdates the main Boco alteration zone. In contrast, although some of the least altered facies have insufficient pyrite for laser abla- 1 tion analysis, the two most distal samples (W380, 398) have δ34S values around 14 per mil, suggesting that this may be the 0 background level in nonhydrothermal, diagenetically altered, -5 0 5 10 15 34 felsic rocks. Pyrites from altered facies samples, with AI of δ S ‰ greater than 70, mostly have δ34S values of less than 5 per mil. The entire range of pyrite δ34S values in the northern seg- 15 34 ment of the Boco altered zone is –1.2 to +7.1 per mil, with a δ S ‰ C mean of 3.3 per mil (n = 19). 10 r = 0. 62 Discussion The whole-rock δ18O data and mineral assemblages re- 5 ported in this study together suggest that the core of the Boco alteration system attained temperatures of at least 270°C and 0 involved a fluid that was isotopically heavier than seawater, with a δ18O value possibly up to ~6 per mil. A possible source of such fluid could be mixed seawater and magmatic water, -5 30 40 50 60 70 80 90 100 comparable to that interpreted for the Basin Lake system Alteration Index (AI) (Williams and Davidson, 2004). An alternative source could Altered facies: be evolved seawater, isotopically modified by rock reactions, least altered Na-mica along the lines of the convecting seawater system of the K-Mica pyrophyllite Hokuroku basin, as modeled by Cathles (1983). He calcu- Kaolinite +/- K-mica Green's data lated that inflowing 0 per mil seawater would initially become isotopically lighter by equilibration with the rocks at low tem- FIG. 12. Sulfur isotope compositions of Boco pyrite. A. Pyrites in least altered volcanic rocks have a δ34S range of 2.4 to 14.2 per mil. B. Pyrites in al- peratures near the sea floor, and subsequently become heav- tered facies have a δ34S range of –1.2 to +7.2 per mil. C. Pyrite δ34S values ier as temperatures increase and water-rock fractionation fac- plotted against AI values show a weak negative correlation (r = 0.62) between tors decrease. Cathles’ models indicate that between ~2000 sulfur isotope composition and intensity of alteration. and 5000 years, the upflowing fluid would have attained oxygen isotope composition between 0 and 5 per mil δ18O, and temperature of 200° to 300°C. and Simmons, 2000). In convecting hydrothermal sea-floor The existence of kaolinite and pyrophyllite at Boco indicate systems, the acidity of fluids may increase due to the precipi- acidic conditions (pH <4), at least in the central zones. In tation of magnesium hydroxysulfate as the descending seawa- subaerial epithermal environments, low pH fluids are typi- ter is heated above 250°C, and by subsurface metal precipita- cally attributed to condensation of volatiles into shallow tion (Bischoff and Seyfried, 1978). Reactions with volcanic groundwaters: i.e., magmatic-derived gases such as HCl, HF, rocks generally buffer the fluid to maintain pH in the range 5 H2S, and SO2 in high-sulfidation epithermal systems (e.g., to 7 at 200° to 400°C (Lydon, 1996). However, in fluid-dom- White and Hedenquist, 1990; Arribas, 1995; Giggenbach, inated focused flow zones, where the flux of water outstrips 1996), or steam, CO2 and H2S from deeply boiled neutral- the buffering capacity of the wall rocks, fluid pH may de- chloride waters in low-sulfidation epithermal systems (Cooke crease to ~3, as in sea-floor black smoker fluids. The latter

0361-0128/98/000/000-00 $6.00 789 790 HERRMANN ET AL. case may apply to Boco, where the argillic and advanced acidic, moderately oxidized, magmatic fluids gradually mixed argillic kaolinite- and pyrophyllite-bearing mineral assem- with seawater away from the core of the system. blages show that the acidic fluids were not entirely neutral- The submarine high-sulfidation style model is also applica- ized by wall-rock reactions, and must therefore have been fo- ble to Boco in respect of its alteration assemblages, sulfur iso- cused along some structures of high permeability. topes, and apparent semivertical structural control on fluid These new implications of hydrothermal fluid temperature, flow. Disproportionation of SO2 in a magmatic vapor plume, pH, and oxygen isotope composition are partly inconsistent as described by Rye et al. (1992), and its condensation into or with a previous interpretation that the Boco alteration zone entrainment of convecting seawater, could partly account for was produced by a fluid of seawater origin at temperatures of the heavier than seawater δ18O fluid composition, the lighter less than 200°C (Green and Taheri, 1992). That interpreta- than typical Cambrian VHMS-type seawater-sulfate-reduced tion was originated by Solomon et al. (1988) to explain the ab- δ34S compositions of pyrite, and the low pH-related altered sence of base metals in some other barren systems in the facies. Even so, we are reluctant to press this interpretation Mount Read Volcanics, including Chester, Howard’s Anomaly too far, given the absence of diagnostic high-sulfidation state and Basin Lake, which also exhibit low δ34S values in pyrite minerals and sulfates (e.g., alunite and barite) at Boco. De- (mostly <5‰). These pyrite sulfur isotope compositions con- termination of hydrothermal temperatures and fluid compo- trast with the typical range of 6 to 15 per mil in the stratiform sitions could help to resolve the fluid-source and genetic un- massive sulfide deposits such as Rosebery, Hercules, and Que certainties but direct estimates of hydrothermal temperatures River (Solomon et al., 1988). Those deposits evidently de- have been thwarted by lack of workable fluid inclusions, and rived their sulfur largely from Cambrian seawater, at a time a more oblique approach by oxygen isotope geothermometry when oceanic sulfate δ34S values peaked at 30 to 35 per mil on quartz-white mica mineral pairs has not been pursued. (Claypool et al., 1980). The Solomon et al. (1988) concept of Irrespective of the source of fluids, the Boco system un- the low δ34S systems was that late-stage, relatively cool doubtedly had considerable energy, sufficient to remove in (<200°C), oxidized seawater of neutral pH convecting the order of 20 Mt of silica, and deposit 3 Mt of pyrite. through felsic rocks, from which most of the reducing FeO Nevertheless, the extensive exploration drilling at Boco, to had previously been exhausted, would not continue to inor- at least 400 m below surface, failed to intersect any mineral- ganically reduce residual seawater sulfate, nor transport suffi- ized zones. It is possible that mineralization did occur at cient base metals to form a massive sulfide deposit, but would higher levels in the hydrothermal system, now eroded, but we be capable of leaching primary magmatic sulfur from the vol- have no indication of its original vertical extent or its depth canic country rocks. That interpretation did not account for below the sea floor. The new isotopic data reported here con- the low pH fluids implied by the advanced argillic facies in firm Green’s (1986) preliminary conclusion that a combina- those systems, because the kaolinite- and pyrophyllite-bear- tion of sulfur and whole-rock oxygen isotope geochemistry ing assemblages were only recently recognized, with the ad- can be used to identify this type of nonmineralized alteration vent of portable SWIR spectrometers. Our new mineralogic system from potentially more economically prospective al- data and interpretation of whole-rock δ18O compositions in- tered zones associated with stratabound polymetallic VHMS fers that the Boco hydrothermal fluid was either a highly deposits such as Rosebery and Hellyer. Furthermore, that evolved seawater or included a magmatic component, to ac- discrimination can be based on relatively few analyses early in count for its low pH and 3 to 6 per mil δ18O composition, an exploration program. Even more practically in the explo- which opens the possibility that the low δ34S compositions of ration context, portable SWIR spectrometry can reliably pyrite may also reflect a partially magmatic source of sulfur. A identify the distinctive high-sulfidation style argillic and ad- non-seawater origin of Boco sulfur has been previously pos- vanced argillic phyllosilicate assemblages, which were for- tulated by mineral exploration geologists: CSR’s decision to merly overlooked as “sericite.” withdraw from exploration of the Boco area was partly based Boco thus joins a relatively recently recognized category of on a limited number of low δ34S data and anomalously high Cambrian volcanic-hosted alteration systems and deposits in fluorine data that pointed to a magmatic source of sulfur and southeastern Australia that are characterized by advanced hydrothermal fluid, (Williams, 1985). Likewise, Randell (1991) argillic-altered facies and indications of magmatic fluid in- noted Boco’s similarity to low δ34S values at the Chester pyrite volvements, possibly representing sea-floor massive sulfide deposit (–3.9 to 0.4‰) and suggested a magmatic source. high-sulfidation hybrids (Large et al., 2001b). In the Tasman- The magmatic source concept for barren or weakly miner- ian Mount Read Volcanics, the class includes Chester (Boda, alized systems in western Tasmania has recently been revived 1991), Basin Lake (Williams and Davidson, 2004), Western by Williams and Davidson (2004), in discussion of advanced Tharsis (Huston and Kamprad, 2001), and North Lyell argillic alteration facies and low δ34S values (–1.4 to +6.9‰) (Bryant, 1975). And in the Victorian Mount Wellington fault associated with the small, low-grade Cu-Au-Ag deposit at zone, it includes Rhyolite Creek (Raetz et al., 1988), Hill 800 Basin Lake. The types of sulfide mineralization and advanced (Morey et al., 2002) and Mike’s Bluff (J. Bartlett, writ. com- argillic alteration at Basin Lake are similar to those of many mun., October 1999). Most of the prospects remain uneco- high-sulfidation epithermal systems formed in subaerial envi- nomic, despite moderate to intensive exploration. North Lyell ronments, but its sulfur isotope values are intermediate be- is an important exception: its four main orebodies produced tween those of high-sulfidation epithermal systems and Tas- about 5.4 Mt of ore averaging 5.3 percent Cu, 34 g/t Ag and manian Cambrian massive sulfide deposits. Williams and 0.43 g/t Au (Corbett, 2001). These Tasmanian and Victorian Davidson (2004) consider that it formed in a submarine, prospects and deposits may be members of an emerging Cambrian synvolcanic, magmatic hydrothermal system, where category of Au-rich VMS deposits that are characterized by

0361-0128/98/000/000-00 $6.00 790 LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA 791 aluminous alteration, including deposits such as the world- barren or subeconomic alteration system from more prospec- class Archean Au-rich LaRonde Penna and Bousquet 2-Du- tive systems associated with stratiform, base metal-rich magami deposits, Canada (Dubé et al., 2007). That style of VHMS deposits such as Rosebery and Hellyer, even from a hydrothermal system has clear, mineralized analogues in few analyses. The Boco prospect has low δ34S values in com- modern arc and back-arc settings of the west Pacific, such as parison with economic Tasmanian stratiform VHMS deposits, the Brothers Volcano (de Ronde et al., 2005) and the Pac- and its δ18O values are at background levels, unlike the 18O manus hydrothermal system (Paulick and Bach, 2006). In depleted proximal zones typical of many VHMS deposits. recognition of that, we share Williams and Davidson’s (2004) opinion that this class of Tasmanian and Victorian Cambrian Acknowledgments pyrophyllite-kaolinite aluminous alteration systems, with low This research was part of the CODES 2000-2002 project δ34S values and indications of magmatic fluid input, may yet entitled “Isotopic response of fluid flow in submarine vol- have significant exploration potential. canic-hosted hydrothermal systems and its implications for prospectivity: Mount Read Volcanics case study.” An Aus- Conclusions tralian Research Council Strategic Partnership with Industry- The results of the lithogeochemical, mineralogical, and iso- Research and Training (SPIRT) grant, supported by Mineral topic characterization presented in this study allow some con- Resources Tasmania, Pasminco Australia Ltd., and Goldfields straints on the origin of the altered zone at the Boco prospect, Exploration Pty. Ltd, funded the project. We sincerely thank and comparison with some other mineralized systems in the Keith Harris at the University of Tasmania Central Science Mount Read Volcanics. Laboratory for his assistance with laser ablation sulfur isotope analyses. We acknowledge David Huston, Patrick Mercier- 1. The Boco aluminous alteration zone was produced by a Langevin, and Robert Seal for their meticulous and construc- Cambrian synvolcanic hydrothermal system in a proximal tive reviews of the draft manuscript. subaqueous (marine?) felsic volcanic setting. 2. The alteration system is concentrically zoned with outer REFERENCES quartz-white mica-pyrite assemblages grading inward through Note: Unpublished Tasmanian mineral exploration company reports (e.g., quartz-white mica-kaolinite-pyrite to quartz-pyrophyllite- references to Herrmann, 1997; Randell, 1991; Sainty, 1984; and Williams, 1985) are accessible via the Mineral Resources Tasmania (MRT) website at pyrite facies. www.mrt.tas.gov.au/portal/page?_pageid=35,832417&_dad=portal&_schem 3. The major chemical mass changes associated with hy- a=PORTAL. The MRT Document Search page caters for about 15 search drothermal alteration were losses of Si, Na, Ca, Mg, K (in criteria but the simplest way to find these references is by using the “Report order of decreasing magnitude) and small gains in sulfur. no.” criterion at the bottom of the search page. 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