©2009 Society of Economic Geologists, Inc. Economic Geology, v. 104, pp. 775–792 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 exten- sive 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 calcula- tions 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 hydrother- mal 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 com- position 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 sys- tem, 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