©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 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 seawater. 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 MINERAL PROSPECT is centered on a pyritic quartz
+ phyllosilicate alteration zone located at 145°36'08" E,
41°39'32" S, midway between the Rosebery and Hellyer
mines in western Tasmania (Fig. 1). Following the original
detection of a few weak conductivity anomalies by a regional
Barringer airborne INPUT EM survey in 1975, the prospect
was subjected to an intensive gradient array-induced polarization and soil geochemical-based exploration program by
Electrolytic Zinc Co. and CSR Ltd. during the late 1970s and
mid-1980s (Sainty, 1984; Williams, 1985). Their work, which
was aimed at discovery of a volcanic-hosted massive sulfide
(VHMS) deposit, culminated in 12 short percussion drill
holes and 14 diamond drill holes totaling ~5,650 m, which delineated a 1,400 × 350 m pervasively hydrothermally altered
zone (Fig. 2) but did not intersect any base metal mineralized
zones. Subsequent systematic geophysical surveys, including
downhole and surface time-domain electromagnetic, reconnaissance magnetic induced polarization, and gravity surveys,
failed to produce further drilling targets.
† Corresponding
author: e-mail, wherrmann@iprimus.com.au
0361-0128/09/3837/775-18
The Boco system has been attributed to a low-temperature
Cambrian hydrothermal system that did not transport and deposit base metals (Green, 1986). Green’s suggestion, that isotope geochemistry could provide a fingerprinting technique
to distinguish low-temperature barren pyritic alteration zones
from higher temperature systems of greater base metal exploration potential, was tentatively based on relatively few
sulfur and oxygen isotope data. In 2001, we carried out a
more extensive study to fully characterize the sulfur and
whole-rock oxygen isotope distribution in and around the
Boco alteration system, aiming to elucidate its hydrothermal
genesis and establish whether sulfur and oxygen isotopes are
robust discriminators of such barren systems.
Methods
Visual logging of all existing diamond drill core (~5,300 m)
determined the distributions of primary volcanic facies and
extent of hydrothermally altered zones. Textural, mineralogic,
alteration intensity, and estimated pyrite-content data were
recorded using a graphic logging technique based on the format recommended by McPhie et al. (1993). Subsequently,
systematic short wavelength infrared (SWIR) analyses with a
PIMA-SP portable spectrometer of core samples spaced at
775
Submitted: June 24, 2009
Accepted: September 25, 2009
776
HERRMANN ET AL.
+ + + + + +
+ + + + +
+ + + + +
Mt Cripps + + + + +
HELLYER
+ + + + +
+ + + + + +
+ + + + + +
+ + +
+ + + +
+ + + + + +
+ + + + + +
10 km
5
Mt Block
+ + + + + + +0
+
+
+
+ + + +
BOCO
+ + + +
+ + +
Tyndall Group
+ + + +
+ + + + + + + Granitoids & porphyry
+ + + +
+ +
+ +
Andesites and basalts
+ +
CHESTER
+
TULLAH
Western volcano+
sedimentary sequences
Mt Black
+
+ +
Central Volcanic Complex
+ +
+ +
Eastern quartz-phyric
+ +
sequence
+ + +
+ +
Sticht Range Beds
+
ROSEBERY
Cambrian Mount Read Volcanics
QUE RIVER
146°E
HERCULES
Mt Read
HENTY
TASMANIA
42°S
Mount Read
Volcanics
Hobart
100 km
FIG. 1. Location of the Boco prospect in relation to the main lithostratigraphic units and mineral deposits in the central northern part of the Mount
Read Volcanics.
2- to 10-m intervals along drill holes BBP 250, 251, and 254
(Fig. 2) and micropetrographic examination of 27 thin-sectioned specimens enabled interpretation of phyllosilicate
mineral species and delineation of altered facies. The Spectral Geologist v. 2.0 software was used for SWIR spectral data
processing and interpretation.
Major and trace element analyses of 27 representative samples (W372–W399, which were also thin-sectioned and analyzed for oxygen ± sulfur isotopes) were carried out at the
Analabs laboratory at Welshpool, Western Australia, by X-ray
fluorescence methods on fused glass discs and pressed powder buttons. Major and trace element data for six earlier samples were acquired at the Tasmanian Department of Mines
laboratory for G.R. Green, ca. 1985.
Oxygen isotope compositions of 27 3- to 10-mg powdered
whole-rock samples (W372–W399) were determined at the
Department of Geosciences, University of Arizona, following
the method of Clayton and Mayeda (1963) on a Finnigan Mat
Delta S mass spectrometer. δ18O precision and accuracy is estimated to be about ±0.3 per mil. Sulfur isotope compositions
of pyrite in 22 150-µm-thick polished rock sections of the
same samples were determined to precision of ±0.5 per mil at
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 different pyrite grains in each sample. The initial five δ34Spyrite
values reported by Williams (1985), and 15 whole-rock oxygen
N
40
278
254
250
0
Boco
Siding
251
40
250
500 m
279
A’
n Hig
hway
A
5387000 mN
Murc
hiso
280
251
Ro
a
d
drill hole
Bo
co
road
y
railway
fault zone
Em
u
Ba
y
wa
il
Ra
outcrop boundary
5386000 mN
246
Quaternary fluvio-glacial cover
247
384000 mE
383000 mE
surface projection of altered zone
253
outcrop of altered zone
interbedded siltstone & greywacke
felsic volcaniclastics, lavas & sills
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.
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LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA
isotope analyses were obtained for G.R. Green by conventional means at CSL during the mid-1980s.
In order to obtain sample-specific oxygen isotope fractionation factors for fluid modeling, the mineral proportions in
each sample were estimated from whole-rock major element
composition data by the MINSQ method (Herrmann and
Berry, 2002), which is an iterative least-squares approach to
normative mineral calculations, following SWIR spectral and
petrographic analyses to determine the mineral species present. The sample-specific rock-water oxygen fractionation
factors were calculated by applying Zheng’s (1991, 1993) mineral-water fractionation factors to estimates of the mineral
proportions. Model curves of final rock δ18O relative to
water/rock ratio at various temperatures and assumed fluid
δ18O values were calculated for an open system, according to
equation 9 of Green and Taheri (1992). For modeling purposes, we have used a δ18O value of 0 per mil for Cambrian
seawater, based on the recent silicon isotopic evidence of
Robert and Chaussidon (2006), which resolved the longstanding debate on isotopic composition of the oceans
through time (e.g., Veizer et al., 1999; Muehlenbachs et al.,
2004).
Prospect Geology and Alteration Zonation
The Boco prospect is located 8 km north of Tullah, in the
northern Central Volcanic Complex of the Cambrian Mount
Read Volcanics (Corbett, 1992), at the southern end of the
Bulgobac plain. This area is largely covered by up to 100 m of
Quaternary fluvioglacial deposits. The Cambrian rocks in the
district retain well-preserved primary volcanic textures, despite Devonian deformation and lower greenschist facies regional metamorphism. The host rocks are dominantly massive
coherent rhyolites and less abundant dacites, interlayered
with thick units of massive rhyolitic pumice breccia and minor
polymictic felsic volcanic breccias and volcaniclastic sandstones. The coherent felsic rocks are sparsely feldspar phyric
to aphyric and generally massive. Locally flow-banded rhyolites, monomict hyaloclastite breccias, and some intrusive
hyaloclastite contact zones suggest they were emplaced as a
complex assemblage of overlapping flows, small domes, and
subvolcanic intrusive rocks. The pumiceous breccias exist in
20- to 100-m-thick massive units interpreted to represent
syneruptive deposits resedimented and deposited as subaqueous mass flows (McPhie and Allen, 1992). Numerous
thin sills and dikes of coherent mafic and intermediate rocks
ranging from medium-grained holocrystalline dolerite
through fine-grained amygdaloidal basalt to feldspar porphyritic-glomeroporphyritic andesite intersect the felsic
rocks. They commonly have finer grained quenched or more
amygdaloidal margins and sharp intrusive contacts, and sometimes include spalled fragments of the wall rock. Rare examples of fluidal peperitic margins suggest more or less synvolcanic emplacement of the mafic intrusives into semilithified
felsic volcaniclastic rocks. This volcanic facies association resembles, and it is probably contiguous southward with, the
Mount Black and Kershaw Pumice formations, which Gifkins
and Allen (2002) interpreted to have formed in a proximal
submarine intracaldera setting.
However, the geologic structure and detailed facies architecture of the Boco area is poorly understood owing to the
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777
lack of surface exposures and oriented drill cores. Broken
core intervals indicating the drill holes intersected numerous
faults, sparse and conflicting sedimentary facing indicators,
and, not least, the extent of texturally destructive hydrothermal alteration, all present difficulties in correlating nondistinctive massive volcanic units. The most likely interpretation
is that the volcanic succession is upright and dipping at a low
angle to the northwest (Herrmann, 1997).
The Boco exploration prospect is centered on two subvertical, pipelike, pervasively quartz + phyllosilicate + pyrite altered zones (Figs. 2, 3). The altered zones lie on opposite
sides of a north-northeast–trending brittle fault, which is
topographically expressed to the south as an incised linear valley along upper Boco Creek and recognizable by its low magnetic intensity in aeromagnetic data. This fault appears to
have had dextral displacement of ~600 m, which separated
the two segments of the altered zone. Before dislocation, the
alteration zones were probably contiguous in a roughly elliptical 600 × 400 m zone, pitching steeply eastward to a vertical depth of greater than 400 m (the existing limit of drilling)
below the Quaternary erosion surface. The altered zone thus
appears to cut across the volcanic units, perhaps localized on
some preexisting fractured zone. There is evidence that pervasive hydrothermal alteration was synvolcanic: nonaltered
mafic dikes that intersect the altered zone contain spalled
fragments of pervasively altered wall rock. These dikes have
compositions similar to thin peperitic mafic sills that intruded
nonlithified felsic volcaniclastic rocks.
Previous descriptions of the Boco altered facies, based on
visual drill core logging, microscopic petrography, and some
major element analyses, reported “pyritic quartz-sericite” assemblages (e.g., Sainty, 1984) which were regarded as identical to those associated with VHMS deposits at Rosebery and
Hercules, except for the absence of chlorite (Williams, 1985).
However, systematic SWIR spectral analyses of 118 drill core
samples, at intervals of 1 to 10 m down three holes (BBPs
250, 251, and 254) on two cross sections through the northern
segment of the altered pipe, have subsequently revealed a
semiconcentric zonation of phyllosilicate minerals that were
formerly nondifferentiated as “sericite.” These SWIR-interpreted phyllosilicates include white micas of variable composition, pyrophyllite, and kaolinite.
The background white micas, which are minor components
of the felsic host volcanic rocks outside the altered zone, have
AlOH absorption features in SWIR spectra of >2,210 nm, indicating slightly to moderately phengitic compositions. SWIR
AlOH features of white micas in the outer parts of the altered
zone are mostly in the range 2,200 to 2,210 nm, comparable
to normal potassic muscovite. Discrete zones near the center
of the system contain white mica with AlOH absorption features in the range 2,190 to 2,200 nm, suggesting slightly to
moderately sodic compositions (cf. Herrmann et al., 2001). In
the upper central part of the northern segment, two drill
holes (BBP 250, 251, Fig. 3) intersected a 10-m-wide zone of
quartz + pyrophyllite + pyrite enclosed by zones containing
low AlOH wavelength sodic white mica passing outward to
normal white mica ± kaolinite.
The kaolinite- and pyrophyllite-bearing argillic and advanced argillic assemblages have similarities to those described in the Mount Read Volcanics at Western Tharsis
777
778
HERRMANN ET AL.
A (WNW)
BBP250
A’ (ESE)
BBP251
Altered Facies:
pyrophyllite
Base of
weathering
Na-mica,
AlOH λ <2200 nm
L
Kaolinite ± K-mica
AlOH λ 2200-2210 nm
L
L
L
L
K-mica,
AlOH λ 2200-2210 nm
L
L
pre-Quaternary surface
L
L
Volcanic Facies:
L
L
rhyolitic coherent lava/sill
L
300 m asl
L
L
L
L
L
L
L
L
dacitic coherent lava/sill
L
L
limit of strong feldspar
destructive alteration
rhyolitic pumice breccia
L
L L
felsic pumiceous-lithic breccia
L
felsic volcaniclastic sandstone
mafic dyke
200 m asl
383500 mE
t
faul
100 m asl
0
50
100 m
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
Davidson, 2004). Although argillic and advanced argillic alteration assemblages are key features of subaerial high-sulfidation epithermal systems, similar styles of subaqueous synvolcanic aluminous alteration are inferred from some ancient
Au-rich VHMS deposits—for example, LaRonde Penna,
Abitibi belt, Canada (Dubé et al., 2007). This style of alteration is also increasingly recognized in some modern arcs and
back arcs (e.g., de Ronde et al., 2005; Paulick and Bach,
2006).
Figure 3 shows the interpreted distribution of altered facies
on a cross section through the northern segment of the altered zone and Table 1 summarizes the characteristics of each
altered facies. All the altered facies (except the least altered
facies) have assemblages containing 35 to 70 percent quartz,
with varying proportions of phyllosilicates, and 1 to 5 percent
disseminated pyrite, but insignificant chlorite and carbonate.
Short facies names, indicating the main phyllosilicate species,
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are used in this paper for convenience; e.g., K mica facies.
There is a general increase in phyllosilicate content toward
the core of the system, reflecting increasing alteration intensity and greater SiO2 depletion. The original protoliths of all
the altered facies are interpreted to be felsic volcanic rocks of
rhyolitic to dacitic compositions.
Microphotographs and SWIR spectra representative of
each altered facies are presented in Figures 4 and 5.
Major Element Geochemistry and
Alteration Mass Changes
Lithogeochemical data from the Boco prospect include
major and trace element analyses of several hundred contiguous drill core intervals and a smaller set of 33 specimens representing various volcanic and altered facies (Table 2). The
early EZ Co. data showed that the Boco alteration system was
associated with strong depletion of Na, Ca, and Sr and addition of sulfur, and these geochemical trends assisted in tar-
778
LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA
779
TABLE 1. Characteristics of Boco Altered Facies
Facies
Mineral assemblage
Texture
Intensity
Distribution
Least altered
Albite, quartz, k-feldspar, ± minor
chlorite and white mica (high AlOH
wavelength, 2,210–2,220 nm, phengitic)
Sparsely albite ± quartz porphyritic (rarely
quartz amygdaloidal) with fine-grained
micropoikilitic, spherulitic, or perlitic matrices.
Weak
Regional
K mica
Quartz, white mica (medium
AlOH-wavelength 2,200–2,210
nm, potassic), pyrite
Fine, granular mosaic of quartz with 10–30%
interstitial shreds or seams of mica defining
weak foliation; disseminated pyrite
Moderate to
intense
Patchy to pervasive
in outer parts of
Boco altered zone
Kaolinite ± K mica
Quartz, kaolinite, pyrite, ± white
mica (medium AlOH wavelength
2,200–2,210 nm, potassic)
Mosaic of quartz and mica with mm-scale
irregular patches of fine clays partly pseudomorphs after feldspar; disseminated pyrite
Strong to
intense
Pervasive in medial
and upper parts of
Boco altered zone
Na mica
Quartz, white mica (low AlOHwavelength 2,190–2,200 nm,
sodi-potassic), pyrite
Fine, granular mosaic of quartz with 10–30%
interstitial patches or seams of mica defining
weak foliation; disseminated to semimassive
pyrite
Strong to
intense
Pervasive in central
parts of Boco
altered zone
Pyrophyllite
Pyrophyllite, quartz, pyrite, ± low
AlOH-wavelength white mica
Foliated-sheared, fine-grained pyrophyllite
± white mica with sparse granular aggregates
of recrystallised quartz, disseminated pyrite,
and deformed quartz + pyrite veinlets
Intense
Pervasive in narrow
central core of
Boco altered zone
geting exploration drill holes (Sainty, 1984). A large proportion of those samples contain 1 to 5 wt percent sulfur, which
equates to approximately 1 to 5 vol percent pyrite. High sulfur proportions are generally associated with low Na2O, which
is a reasonable index of plagioclase destructive alteration (Fig.
6). Subsequent whole-rock lithogeochemical analyses, including complete major and some immobile trace elements
(Table 3), enable graphic representations of geochemical alteration indices, and quantitative estimation of the chemical
changes that occurred in each hydrothermal alteration facies.
Alteration indices and box plots
Figure 7A presents an alteration box plot of the alteration
index (AI) and chlorite-carbonate-pyrite index (CCPI)1 following the method of Large et al. (2001a). The least-altered
volcanic rocks plot in the least altered box, mainly in the lower
half because they are dominantly felsic. The K mica facies
data trend to higher AI values, with the most intensely altered
samples clustering with members of the kaolinite ± K mica facies and Na mica facies at AI > 90 and CCPI 30 to 60. The
single analysis of the pyrophyllite facies plots at slightly higher
CCPI of 63, essentially because of its low Na and K content.
It is the pyrite contents, and to a lesser degree feldspar destruction reflected in depletion of Na and K, which are the
main influences on CCPI in these samples as chlorite is not a
significant component of hydrothermally altered facies in the
Boco system.
The data for least altered and altered samples are also
clearly demarcated on a box plot (Fig. 7B) combining AI and
the advanced argillic alteration index (AAAI)2 devised by
Williams and Davidson (2004). Most of the altered samples
cluster near the upper-right corner at the position of muscovite and K-feldspar at high values of AI and AAAI. This
AI
= 100(MgO + K2O) / (MgO + K2O + CaO + Na2O)
CCPI = 100(FeO + MgO) / (FeO + MgO + Na2O + K2O)
2 AAAI = 100SiO / (SiO + 10MgO + 10CaO + 10Na O)
2
2
2
1
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essentially reflects strong feldspar destruction and depletion
of Ca and Na, and minor depletion of Mg. This distribution of
altered facies data emphasizes the absence of chlorite in the
alteration assemblages, and dispels the chlorite-pyrite ambiguity inherent in the AI-CCPI boxplot.
Contrary to Williams and Davidson’s (2004) AI-AAAI box
plot of Basin Lake and other data, the Boco kaolinite- and pyrophyllite-bearing assemblages do not trend to lower AI values at high AAAI. AI is indeterminate for quartz, kaolinite,
and pyrophyllite and hence those minerals plot as lines along
the upper boundary at AAAI = 100. Any trace of mica or chlorite in these assemblages would produce high AI—hence, the
clustering at the upper right corner (unless the mica was significantly sodic).
Neither the AI-CCPI nor the AI-AAAI plot offers a reliable
means of discriminating between the Boco alteration assemblages and the quartz-sericite-pyrite assemblages typical of
proximal footwall alteration zones in Tasmanian volcanichosted massive sulfide (VHMS) systems. The main mineralogical difference is the absence of chlorite in the Boco system, at least to the depths so far tested by drilling. Hence,
chloritic VHMS-type footwall zones may be distinguishable
from Boco-type assemblages by higher CCPI and lower AAAI
values, at high AI. However, the existence of Cu-Au–bearing
chlorite-altered facies vertically below pyrophyllite facies in
some other Tasmanian deposits (e.g., Western Tharsis: Huston and Kamprad, 2001) suggests that a chlorite-based discriminant may not be universally applicable.
Mass changes related to hydrothermal alteration
We used the immobile element method of MacLean and
Barrett (1993) to estimate average mass changes of chemical
components in the altered facies. The first step in their procedure tests for, rather than assuming, immobility of monitor
elements and identifies magmatic affinity groups. Figure 8
shows scatter plots of the TiO2 and Zr data categorized by
volcanic lithofacies (A) and altered facies (B). The data for
779
780
HERRMANN ET AL.
Na-mica
ab + ks ± chl
qz
qz
pl
qz
A
py
D
0.5 mm
0.5 mm
prl
qz
qz
py
prl
py
K-mica
qz
py
qz
B
E
0.5 mm
0.5 mm
ka
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).
Mineral abbreviations: ab = albite, ks = K-feldspar, pl = plagioclase, qz =
quartz, prl = pyrophyllite, Na mica = low AlOH wavelength <2,190 nm sodipotassic white mica, K mica = medium AlOH wavelength 2,200–2,210 nm
potassic white mica.
qz
C
K-mica
0.5 mm
coherent rhyolites form a highly correlated linear trend (r =
0.88, n = 8 analyses) with a Ti/Zr ratio of about 6.5 that projects close to the origin of the plot and convincingly satisfies
MacLean and Barrett’s (1993) immobility test. Data for six
samples of altered coherent dacite also lie on a highly correlated (r = 0.98) trend, which projects above the origin of
the plot, intersecting the TiO2 axis at about 0.12 percent.
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However, this linear trend does not include the least altered
dacite sample, W398, which lies slightly above the regression
line. This does not necessarily indicate Ti or Zr mobility during alteration. It is probably due to minor primary compositional variability in the relatively few dacite samples analyzed.
The dacite unit represented by W398 was intersected by drill
hole BBP 279 east of the bisecting fault, and about 300 m
780
781
Na2O wt%
LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA
least altered facies (insignificant mica)
least altered facies (minor phengitic mica)
K-mica facies
kaolinite ± K-mica facies
6
5
4
Na-mica facies
3
pyrophyllite facies
2
1
0
0
1300
1600
1900
2200
1
2
3
4
5
6
Sulfur wt%
2500
Wavelength (nm)
FIG. 5. Stack of SWIR spectra typical of the Boco prospect altered facies.
FIG. 6. Scatter plot illustrating generally inverse relationship between sulfur and sodium in lithogeochemical data from the Boco prospect (data from
Sainty, 1984).
northeast of and 250 m lower than the main group of dacite
samples from units intersected by the upper sections of BBPs
250 and 251. It is therefore likely to be a different emplacement unit.
The least altered coherent rhyolites (W389, 392, 396; selected by petrographic observations and geochemical parameters) plot in a cluster centered on 0.24 percent TiO2 and
202 ppm Zr. However, the two least altered rhyolitic volcaniclastic rocks (pumice breccias) have very different immobile
element compositions (W393 and W397: 243 and 372 ppm
Zr, respectively), which nevertheless fall close to the trend of
coherent rhyolites. Nonaltered felsic rocks in the Mount
Read Volcanics rarely contain more than 300 ppm Zr (cf.
Crawford et al., 1992). The dissimilar Boco pumice breccia
samples seem to represent different compositional groups but
we have not investigated whether the disparity is due to magmatic, volcaniclastic, or diagenetic alteration processes. It
makes the selection of the least altered precursor problematic
and, accordingly, we have not pursued estimates of mass
change in these noncoherent volcanic facies.
Given the small data set, alteration mass changes were calculated on average compositions of altered facies in coherent
volcanic rocks, matched to judiciously selected precursors,
and based on Zr as the immobile component in all cases. The
estimates of absolute mass change are listed in Table 4 and
graphically presented in Figure 9. The compositional variation in dacites mentioned above presents a minor problem in
selection of a dacitic least altered precursor composition for
mass change calculations. To overcome it, we took alternative
approaches: (1) using the composition of sample W398, and
(2) deriving a precursor composition using the MacLean and
Barrett (1993) multiple precursor method, assuming linear
magmatic fractionation between the composition of dacite
W398 and the average composition of three least altered coherent rhyolites (W389, 392, 396). Both approaches are approximations but the outcomes are quantitatively similar (Fig.
9A, B).
The mass change estimates for average compositions (Table
4; Fig. 9) indicate that all of the altered facies underwent significant 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
mostly 3 m contiguous
split core samples
Partial suite of major elements, including SiO2, Fe,
CaO, Na2O, S and trace elements Cu, Pb, Zn, Ag,
Au, Sr, Ba, Mn, Co, Ni, Hg
EZ Co.
exploration report
Sainty, 1984
6
Major elements SiO2 to P2O5; also CO2, S and H2O+;
trace elements Zr, Nb, Y, Cr, Ni, V, Sc, Sr, Rb, Ba,
Cu, Pb, Zn, Ag, Au, As
Tasmanian
Department of Mines
G.R. Green, writ. commun., 1997;
unpub. data, listed in Table 5
(excluding some trace element data)
27
Major elements SiO2 to LOI; trace elements Zr,
Nb, Y, Cu, Pb, Zn, As, Sb, Tl
CODES, MRV isotope
SPIRT project
Table 3, this study
0361-0128/98/000/000-00 $6.00
781
0361-0128/98/000/000-00 $6.00
782
BBP250 112.0
BBP250 131.8
BBP250
BBP250
BBP250
BBP250
BBP250 342.0
BBP251 60.6
BBP251
BBP251
BBP251 112.3
BBP251 142.6
BBP251 167.5
BBP251 182.5
BBP251 189.5
BBP251 263.0
BBP251 282.5
BBP251 359.5
BBP251 373.0
BBP254 250.0
BBP254 318.5
BBP254 342.0
BBP254 409.0
BBP279 471.0
BBP251 120.0
W374
W375
W376
W377
W378
W379
W380
W382
W383
W384
W385
W386
W387
W388
W389
W390
W391
W392
W393
W394
W395
W396
W397
W398
W399
Note:
69.5
92.5
161.0
205.0
256.5
297.0
77.0
Rhyolitic pumiceous/
lithic breccia
Dacite; Fs phyric,
perlitic
Dacite; Fs phyric
Polymict/pumice
breccia, qtz and
Fs phyric
Dacite; Fs phyric
Dacite; Fs phyric
Rhyolite, Fs phyric
Rhyolite, Fs phyric,
spherulitic
Rhyolite, Fs phyric
Rhyolite, aphyric,
banded perlite
Rhyolite, Fs phyric
Pyrite bands in
silicified rhyolite/
breccia
Dacite; Fs phyric
Polymict volcaniclastic
breccia, qtz and
Fs phyric
Basalt dike
Rhyolite, aphyric,
banded perlite
Rhyolite, aphyric,
banded perlite
Rhyolitic volcaniclastic
sandstone, and Py
Rhyolitic, Fs phyric
pumice breccia
Rhyolite, Fs phyric,
qtz amygd
Rhyolitic, Fs phyric
pumice breccia
Rhyolitic, Fs phyric
pumice breccia
Rhyolitic, Fs phyric
pumice breccia
Rhyolite, Fs phyric,
micropoikilitic
Rhyolitic, Fs phyric
pumice breccia
Dacite; (Fs) phyric
Dacite; Fs phyric
All δ34S data are of pyrite
Detection limit
Laboratory
Method
BBP250
W373
59.0
BBP250
W372
Depth Volcanic facies
Hole
Sample
0.56
0.21
15.93
11.59
70.62 0.54 15.37
72.89 0.49 14.26
72.76 0.32 13.48
78.90 0.22 11.61
75.56 0.44 13.03
75.53 0.41 13.42
70.91
77.64
3.74 0.00
3.72 0.00
17.41 1.95 0.00
8.39 13.69 0.00
72.60 0.50 13.51
75.19 0.39 12.56
75.04 0.33
66.62 0.25
1.91 0.03
2.50 0.00
4.07 0.03
3.74 0.00
5.15 0.00
2.13 0.00
72.79
72.59
76.77
68.43
0.41
0.29
0.30
0.49
0.25
0.21
0.33
0.15
0.22
2.31
14.95
13.95
12.58
20.13
11.35
13.28
13.37
0.06
0.07
0.01
0.00
0.28
0.10
0.35
0.00
0.02
0.94
0.72
0.78
0.63
0.49
0.18
0.63
0.56
0.53
0.11
0.34
5.53
0.40
0.72
0.69
0.00
0.03
0.48
0.04
1.06
0.44
0.13
0.71
0.50
0.26
0.66
0.67
4.10
1.69 3.39
0.08 4.08
0.05 0.98
0.06 2.26
0.10 4.37
0.10 2.80
0.13 3.54
0.23 4.04
0.31 3.50
0.10 3.58
0.13 3.93
0.11 3.77
0.10
3.16
0.03
1.97
0.61
0.12
0.05
1.94
1.04
2.19
0.01
0.37
4.25
3.45
3.66
5.51
3.83
4.04
4.03
2.30
3.69
3.13 2.88
0.73 3.72
2.43
4.96
0.73
0.28
1.98
3.42
0.47
0.06
2.79
1.00
3.33 2.84
0.03 <0.05 3.81
0.07
0.02
0.00
0.00
2.02
0.00
0.06
0.10
0.00
0.06
0.00
0.00
0.00
0.01 <0.05 3.98
0.11
0.02
0.05
0.08
0.04
0.05
0.03
0.04
0.05
0.01
0.03
0.67
0.02
0.05
0.03
0.03
0.02
0.04
0.02
0.06
0.10
0.03
0.02
0.04
0.04
0.02
0.02
0.05
0.01
0.05
0.01
0.01
0.01
0.01
0.05
0.01
0.01
99.34
99.96
100.19
100.14
100.07
100.05
99.77
99.63
99.47
99.79
5.05
4.92
0.01
99.44
99.87
2.76 100.01
0.99 100.09
2.65 100.01
3.56 100.39
2.73
1.70 100.02
3.47
6.42 100.01
2.13 100.07
4.75 99.94
2.59 100.54
3.66 99.93
3.65 100.41
3.97
8.30
3.20 99.87
2.56 100.04
4.71
3.85
4.38
2.72
3.46 100.52
3.51 100.47
4.72
2.94
Analabs
X408 X408 X408 X408 X408 X408 X408 X408 X408 X408 X408
5.00 0.11
4.23 0.00
2.06
3.08
2.66
1.71
2.01
2.11
1.89
8.35 10.01
12.25
Least altered 64.75 0.44 13.87
Kao±K mica 69.65 0.54 15.31
Least altered 70.29
Least altered 71.98
K mica
Na mica
Least altered 74.76
Least altered 73.52
K mica
K mica
Least altered 75.92
Least altered 45.18 1.45 20.94 14.00 0.25
K mica
79.58 0.21 11.99 1.91 0.00
K mica
K mica
Pyrophyllite
Na mica
0.01
0.01
3.42 0.00
3.41 0.00
2.33
2.89
9
15
16
10
11
22
11
9
14
6
12
5
10
6
10
19
10
11
11
13
14
13
11
10
11
12
12
30
41
44
25
29
26
32
24
32
17
31
29
28
46
34
21
47
29
14
26
31
32
24
23
35
41
34
5
3
3
X401 X401 X401
178
259
372
203
267
438
243
207
248
123
196
201
190
229
211
282
174
219
163
254
226
292
174
205
213
277
180
14.8
12.5
6.6
8.6
6.7
6.7
6.2
6.1
8.0
7.3
6.7
43.2
6.6
13.1
11.1
7.0
8.6
7.7
6.6
12.7
13.0
6.6
7.6
12.9
11.5
12.1
7.0
Arizona
Clayton
and
Mayeda,
1963
10.4
11.4
10.5
10.0
9.9
11.0
9.6
9.6
8.2
8.6
9.7
7.0
9.7
9.1
10.3
9.8
9.7
10.2
9.7
10.3
9.4
9.3
10.6
10.6
8.7
11.6
8.2
UTAS
Laser
ablation
13.8
6.1
8.5
5.1
0.7
1.9
9.0
–0.1
4.9
2.4
6.6
1.9
4.6
2.1
14.2
4.3
7.2
2.4
4.6
4.0
4.3
2.9
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Zr
Nb
Y
Ti/Zr δ18OSMOW δ34SCDT
(wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (ppm) (ppm) (ppm) (ratio)
(‰)
(‰)
Least altered 73.64 0.28 13.80
Na mica
81.67 0.18 10.16
Kao±K mica
K mica
Na mica
K mica
K mica
K mica
Kao±K mica
K mica
Alteration
facies
TABLE 3. Lithogeochemical and Isotopic Composition Data Acquired by CODES, 2001
782
HERRMANN ET AL.
783
LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA
calcite
ankerite
dolomite
pyrite
chlorite
AAA Index
A
90
mafic
80
100(FeO+MgO) / (FeO+MgO+NaO+K 2O)
70
60
dacitic
50
40
phengite
30
rhyolitic
20
10
0
paragonite
albite
0
muscovite
K-feldspar
Na-mica
B
90
80
70
least altered
rhyolites
60
ica
phengite
-m
Na
50
40
albite
paragonite
30
mafic
chlorite
20
anorthite
10
0
10
20
30
40
50
60
70
80
90 100
100(MgO+K2O) / (MgO+K 2O+CaO+Na 2O)
Alteration Index
muscovite
K-feldspar
quartz, pyrophyllite, kaolinite
100
100 SiO 2 / (SiO 2+10MgO+10CaO+10Na2O)
CCP Index
100
calcite
0
ankerite
dolomite
10
20
30
40
50
60
70
80
90 100
100(MgO+K2O) / (MgO+K 2O+CaO+Na 2O)
Alteration Index
Altered facies:
least altered
Na-mica
K-Mica
pyrophyllite
Kaolinite +/- K-mica
Green's data
0.6
TiO2 %
TiO2 %
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.
A
0.5
0.6
B
0.5
least altered dacite
0.4
0.4
0.3
0.3
least altered dacite
W398
ac
ite
s
n
0.0
least altered rhyolites
least altered rhyolites
s
lite
o
hy
0.1
0.1
tr
ren
he .88
o
c =0
r
0
high Py
content
0.2
c
r = ohe
0. ren
98 t
d
0.2
s
os
sl
as
m
et
50
100
150
200
250
300
350
400
0.0
450
Zr ppm
Volcanic facies:
0
50
100
150
200
250
300
350
400
450
Zr ppm
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
evident in the pyrophyllite-rich sample W383, but potassium
seems to have remained essentially unchanged in all the other
altered facies. Mass gains are generally insignificant for the
0361-0128/98/000/000-00 $6.00
averaged compositions, although some individual samples
show significant additions of Si ± Fe; e.g., W390, which contains 5 to 10 percent pyrite, and W382, which is an unusually
siliceous sample from the Na mica facies.
783
0361-0128/98/000/000-00 $6.00
Avg W389, 392, 396
Avg W378, 382
Avg lst altd rhyolite
Avg Na mica alt rhyolite
784
Avg W374, 377, 385
Avg W374, 377, 385
Avg W373, 376, 399
Avg W373, 376, 399
Avg W373, 399
W378
W382
Avg W378, 382
W383
Avg K mica alt dacite
Avg K mica alt dacite
Avg kao±K mica alt dacite
Avg kao±K mica alt dacite
Avg kao±K mica alt dacite
W378 Na mica alt rhyolite
W382 Na mica alt rhyolite
Avg Na mica alt rhyolite
W383 Pyrophyllite
Avg lst altd rhyolite
Avg lst altd rhyolite
Avg lst altd rhyolite
Avg lst altd rhyolite
Synthetic lst alt dacite
W398 lst alt dacite
Avg K-mica alt dacite W374, 385
Synthetic lst alt dacite
W398 lst alt dacite
0.24
0.25
0.39
0.48
0.47
0.55
0.55
13.16
11.82
13.69
13.60
13.27
15.54
15.62
2.50
3.83
4.38
3.63
3.58
3.54
3.28
0.06
0.00
0.10
0.00
0.00
0.01
0.01
0.51
0.09
0.83
0.55
0.61
0.81
0.69
0.67
0.00
2.54
0.06
0.04
0.03
0.02
3.72
0.21
3.28
0.68
0.91
0.32
0.42
3.73
3.15
3.10
3.79
3.66
3.79
3.91
0.05
0.03
0.10
0.06
0.05
0.03
0.02
1.61
3.47
4.20
3.66
3.56
4.78
4.82
100.06
100.05
99.60
100.20
100.23
99.80
99.61
–20
–23
27
–5
–18
–17
–17
–5
–5
0
0
0
0
0
0
0
0
0
–1
–4
–1
–3
–3
–3
–1
–2
–3
–1
1
1
1
–2
–3
–1
–1
–2
0
0
0
0
0
0
0
0
0
–1
0
0
0
0
0
0
0
0
–1
–1
–1
–1
–3
–3
0
–2
–3
–4
–4
–4
–4
–3
–3
–1
–3
–3
–3
–1
0
–1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
2
1
–1
–2
0
–1
–2
–29
–31
24
–11
–30
–32
–20
–16
–18
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI
Net
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
73.81
77.22
67.00
73.68
74.08
70.39
70.28
202
228
184
220
217
263
268
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total
Zr
(wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (ppm)
Mass changes were calculated by MacLean and Barrett’s (1993) “reconstituted composition” method, using Zr as the immobile component
Abbreviations: Avg = average, alt = altered, lst alt = least altered, Kao = kaolinite
Derived from
Facies
Precursor
W398 and Avg lst alt Rhy by M&B multiple precursor method
Avg W374, 377, 385
Avg W374, 385
Avg W373, 376, 399
Avg W373, 399
Synthetic lst alt dacite
Avg K mica alt dacite
Avg K mica alt dacite
Avg kao±K mica alt dacite
Avg kao±K mica alt dacite
Mass change estimates
Derived from
Facies
Compositions used in mass change calculations
TABLE 4. Average Compositions and Mass Change Estimates
784
HERRMANN ET AL.
LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA
compositions of K mica-altered facies (W374, 385) with K
mica + kaolinite-altered facies (W373, 399); these are all altered coherent dacite samples, most likely from a single volcanic unit.
The 400 × 600 m elliptical × 400 m deep pipelike, prefaulted shape of the Boco altered zone occupied a volume of
about 75 × 106 m3, equivalent to about 200 × 106 t (assuming S.G. of ~2.7). Accordingly, a conservative average mass
loss of 10 g/100 g SiO2 from the known Boco alteration system implies removal of about 20 × 106 t of silica during hydrothermal alteration. That would have required a very large
amount of water. The solubility of silica in aqueous fluid at
340°C is about 1,700 ppm at vapor pressure, and 3,200 ppm
at 1 kbar (Fournier, 1985). If the hydrothermal fluid flowing
into the altered zone contained no silica but became saturated to carry ~3,200 ppm silica at its outflow, a water/rock
mass ratio of about 30 would be required to remove 10 g/100
g of SiO2 from the rock. Hence, even at maximum silica solubility, the mass change estimates indicate a hydrothermal
water/rock mass ratio of at least 30; i.e., about 600 × 106 t of
water.
10
A
Average K-mica alt Dacite
0
-10
-20
Precursor: synthetic lst alt Dacite
-30
Precursor: W0398
-40
Si
Ti
Al
Fe Mn Mg Ca Na
K
P LOI Net
10
B
Average K-mica + Kao alt Dacite
0
-10
-20
Precursor: synthetic lst alt Dacite
-30
-40
Precursor: W0398
Si
Ti
Al
Fe Mn Mg Ca Na
K
P LOI Net
10
Average Na- mica altered Rhyolite
C
0
-10
-20
-30
Precursor avg lst alt Rhyolite
Absolute Mass Changes (g/100g)
-40
Si
Ti
Al
Fe Mn Mg Ca Na
K
P LOI Net
10
D
Pyrophyllite: W0383
0
-10
-20
-30
Precursor: avg lst altd Rhyolite
-40
Si
Ti
Al
Fe Mn Mg Ca Na
K
P LOI Net
Components
FIG. 9. Bar graphs representing estimated absolute mass changes due to
alteration in the four hydrothermally altered facies. All facies appear to have
undergone net mass losses of 10 to 30 g/100 g, principally due to losses of silica and sodium.
The overall pattern of mass change is of net mass losses between 10 and 30 g/100 g, dominated by SiO2 losses of between about 5 and 20 g/100g. The limited data suggest mass
losses were lowest in the outer K mica facies, and generally
greater in the medial to proximal altered facies. This inwardprogressive mass loss is particularly evident in comparison of
0361-0128/98/000/000-00 $6.00
785
Whole-Rock Oxygen Isotope Compositions
Twenty-seven new δ18Owhole rock analyses of drill core samples of altered and least altered felsic volcanic rocks from
holes BBP 250, 251, 254, and 279 (Fig. 2) have yielded δ18O
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.
The range of data from the altered zone is essentially similar
to that of Green and Taheri (1992), who obtained results between 9.7 to 11.8 per mil in 15 samples, mainly of peripheral
least altered zones, and found no apparent difference between the background and altered rocks (Table 5).
Comparison of oxygen isotope data with alteration indices
calculated from major element geochemical data (including 6
of G.R. Green’s δ18O analyses) indicates that there is no significant correlation between the δ18Owhole rock values and intensity of alteration in this system (Fig. 10C). The range and
distribution of the δ18O values in least altered background
samples is similar to those of samples from the intensely altered zone.
The least altered background δ18O values (9–12‰) for all
but one of the least altered Boco samples are typical of nonaltered felsic volcanics in the Mount Read Volcanics. For example, least altered andesites in the peripheral footwall of the
Hellyer deposit have background values of 11.3 ± 0.9 per mil.
The least altered Boco data are also similar to δ18O values in
the hanging-wall sequence above the northern lens of the
Rosebery deposit, and in outcrops between Hercules and
Mount Read (W. Herrmann, 1998, unpub. data). The single
exception (W387, 7.0‰) is from a narrow basaltic dike—one
of several mafic units intersected by BBP251 and other Boco
drill holes. Although surrounded by intensely quartz-sericitepyrite altered felsic volcanics and enclosing some spalled fragments of altered rock, the mafic dikes are nonaltered, indicating that they were emplaced after pervasive hydrothermal
alteration of the country rocks. The relatively low δ18O value
is typical for mafic rocks (e.g., Faure, 1986) and probably represents the primary magmatic oxygen isotope composition. It
implies negligible isotopic exchange with either diagenetic or
785
786
Frequency
HERRMANN ET AL.
8
A
Least altered facies
n = 20
6
felsic volcanics
4
2
mafic intrusive
0
6
7
8
9
10
11
12
13
18
Frequency
δ O ‰
8
B
Altered facies
n = 21
6
4
2
0
6
7
8
9
10
40
50
60
70
11
18
δ O‰
13
12
13
18
δ O ‰
C
12
11
10
9
8
7
6
30
80
90
100
Alteration Index (AI)
Altered facies:
least altered
Na-mica
K-Mica
pyrophyllite
Kaolinite +/- K-mica
Green's data
FIG. 10. Graphic representations of Boco whole-rock oxygen isotope compositions: 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
per mil and 8.2 to 11.7 per mil, respectively. The single sample of a mafic intrusive rock has a δ18O value of 7 per mil. C. This plot of whole-rock δ18O values against AI values shows there is no significant correlation between oxygen isotope composition and intensity of alteration.
hydrothermal fluids, which is consistent with the interpreted
timing and mode of emplacement.
There is, therefore, no indication of an oxygen isotope
anomaly, gradient, or halo associated with the Boco alteration
zone, at least within 500 m of the periphery of the system.
Nevertheless, the δ18O data offer some constraints on the
compositions, temperatures, and potential volumes of hydrothermal fluids in the system. Figure 11 presents samplespecific oxygen isotope models for the least altered, K mica,
0361-0128/98/000/000-00 $6.00
and pyrophyllite facies. Each case illustrates relationships between final rock δ18O and atomic water/rock ratios at temperatures from 0° to 400°C for alternative fluid δ18O values of 7
and 0 per mil. These, respectively, represent approximate isotopic compositions of magmatic water and seawater sources
(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
rock-water fractionation factors do not differ greatly, and the
δ18Orock-water/rock ratio (w/r) curves vary only slightly. In
other words, the calculated facies-specific whole-rock fractionation factors account for differences of only about 1 per
mil. Hence, the contrasting mineral assemblages would have
had relatively little effect on final δ18Orock values, which were
primarily dependent on fluid isotopic compositions, temperatures, and water/rock ratios.
The isotopic models for least altered rhyolite samples (e.g.,
W392; Fig. 11A, B) indicate that the observed δ18O ratios
(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,
isotopic exchange with large amounts of water of either
source, at temperatures between about 200° and 400°C,
could produce the observed rock δ18O. The feldspar-rich
composition of the least altered rhyolites and marine depositional setting favors the first interpretation; i.e., essentially
low temperature seawater-diagenetic alteration involving
small amounts of 0 per mil fluid. As noted above, these are
typical background δ18O values for nonhydrothermally altered felsic rocks in the Mount Read Volcanics.
Similarly, the model curves for an intensely pyrophyllite facies altered rhyolite from the core of the Boco alteration zone
(e.g., W383; Fig. 11C, D) indicate the observed 9.8 per mil
δ18O value could be attributed to low temperature and low
water-rock conditions. However, the complete absence of
feldspar and estimated chemical mass losses of Na2O and
K2O and loss of ~20 g/100 g SiO2 suggest that very large
water/rock ratios were required to produce this altered facies.
A water/rock mass ratio of 30 (alluded to in the previous discussion of silica solubility) equates to an oxygen molecular
water/rock ratio of about 17. Therefore, high water/rock ratios must be considered in modeling of δ18O data for the
inner intensely altered facies. At water/rock ratios greater
than 4, final rock δ18O compositions are determined by isotopic fractionation, temperature, and the isotopic composition of the fluid, but not by small variations in water/rock ratios.
Accordingly, at high water/rock ratios, the δ18O value of 9.8
per mil observed in the pyrophyllite facies could be produced
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 hydrothermal settings with high activities of water at about 270°
to 360°C (Hemley et al., 1980). At the sample-specific fractionation factors estimated for sample W383, this pyrophyllite-stability temperature range equates to fluid δ18O values
between 3.2 and 5.7 per mil—i.e., possibly a mixture of seawater and magmatic water.
It could be argued that the pyrophyllite at Boco is not a hydrothermal phase, but represents metamorphosed kaolinite.
Kaolinite will dehydrate to pyrophyllite at ~270°C in the
786
0361-0128/98/000/000-00 $6.00
787
228.2
264.5
441.5
452.0
489.5
245.7
276.6
316.5
395.0
204.5
329.5
391.0
304.5
140.2
93.5
265.2
300.4
B100604 BBP278
B100605 BBP278
B100606 BBP278
B100607 BBP278
B100608 BBP278
B100609 BBP280
B100610 BBP280
B100611 BBP280
B100612 BBP280
B100613 BBP253
B100614 BBP253
B100615 BBP253
GRG1
GRG2
GRG3
GRG4
GRG5
Laboratory
Method
183.9
B100603 BBP278
BBP246
BBP247
BBP251
BBP251
BBP254
173.9
B100602 BBP278
Pyrite
Pyrite
Pyrite
Pyrite
Pyrite
Rhyolite,
Fs phyric
Rhyolite,
Fs phyric
Rhyolite,
Fs phyric
Rhyolite,
patchy chl
Monomict
Rhy breccia
Vesicular
mafic dyke
Rhyolite, aphyric
Rhyolite,
Fs phyric
Rhyolite,
Fs phyric
Rhyolitic v/
clastic siltstone
Sericitic flow
banded Rhyolite
Rhyolitic, Fs
phyric pumice
breccia
Rhyolite,
flow banded
Rhyolite, aphyric
Rhyolitic
volcaniclastic
Depth Description
151.0
Hole
B100601 BBP278
Sample
77.57
0.20
0.24
0.20
0.26
0.26
13.03
13.98
13.17
12.63
12.14
12.74
Tasmanian Dept. of Mines
XRF XRF XRF
K mica
K mica
Na mica
K mica
K mica
Least altered 74.49
Least altered
Least altered
Least altered 73.26
K mica
K mica
K mica
Least altered
Least altered 74.01
Least altered
Least altered 72.69
Least altered
Least altered
0.26
XRF
1.68
2.20
1.63
2.68
2.36
2.59
XRF
0.04
0.03
0.01
0.04
0.07
0.05
XRF
0.23
0.42
0.56
0.17
0.19
0.11
XRF
0.70
0.43
0.05
0.98
1.82
0.95
XRF
2.31
3.06
0.10
3.03
2.07
2.42
XRF
4.40
3.55
4.13
5.12
5.74
5.85
XRF
0.02
0.03
0.02
0.04
0.04
0.05
99.92 260
XRF
1.73
2.63
XRF
XRF
98.83 195
99.83 240
2.68 100.12 180
1.54 100.50 260
2.74 100.12 240
2.58
XRF
18
14
13
10
11
11
XRF
18
30
26
28
29
24
6.1
6.0
6.7
6.0
6.5
6.0
UTAS
Clayton
and
Mayeda
(1963)
11.6
10.2
11.7
11.7
11.5
11.7
9.7
11.1
10.7
11.2
11.8
11.1
10.1
10.5
10.4
UTAS
Conventional
0.2
4.7
–1.2
–0.7
–0.1
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Zr
Nb
Y
Ti/Zr δ18OSMOW δ34SCDT
(wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (ppm) (ppm) (ppm) (ratio)
(‰)
(‰)
Least altered 72.32
Least altered
Alteration
facies
TABLE 5. Lithogeochemical and Isotopic Composition Data Acquired by G.R.Green (mid-1980s)
LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA
787
788
HERRMANN ET AL.
30
δ18Orock ‰
25
20
30
A
δ18O
0ºC
fluid
50ºC
25
= 0‰
20
B
δ18O
0ºC
fluid
50ºC
100ºC
= 7‰
100ºC
15
least altered
facies
W392, 9.6‰
200ºC
15
300ºC
10
200ºC
10
5
300ºC
5
400ºC
0
0.01
30
δ18Orock ‰
25
20
0.1
1
10
50ºC
0ºC
C
100
δ18Ofluid = 0 ‰
100ºC
400ºC
0
0.01
30
0.1
25
D
20
δ18Ofluid = 7 ‰
1
0ºC
10
50ºC
100
100ºC
pyrophyllite
facies
W383, 9.8‰
200ºC
15
15
200ºC
10
300ºC
5
300ºC
10
400ºC
5
400ºC
0
0.01
0.1
1
10
100
30
δ18Orock ‰
25
20
0
0.01
0.1
1
10
100
30
E
δ18O
fluid
25
F
20
δ18Ofluid = 7 ‰
50ºC
0ºC
= 0‰
100ºC
15
0ºC
50ºC
100ºC
K-mica
facies
W388, 9.7‰
200ºC
15
300ºC
200ºC
10
300ºC
5
10
400ºC
5
400ºC
0
0.01
0.1
1
10
100
0
0.01
0.1
water / rock ratio (atomic)
1
10
100
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
al., 1980). Regional metamorphic temperatures in the QueHellyer Volcanics, only 10 km northeast of Boco, have been
estimated at about 300° to 320°C (Offler and Whitford, 1992)
but may locally have been as low as 200°C (R.F. Berry, pers.
commun., 2001) and therefore metamorphic breakdown of
kaolinite may not have fully proceeded. However, the facts
that kaolinite still exists in the medial kaolinite ± K mica facies at Boco and that it is apparently symmetrically distributed around the core of the system suggest that kaolinite and
pyrophyllite are both primary hydrothermal phases.
The K mica facies, which occupies the greater part of the
Boco alteration system, does not offer firm temperature constraints. White mica stability is not strongly temperature dependent under hydrothermal conditions; it is mainly controlled by pH and activity of potassium (e.g., Schardt et al.,
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2001). The model curves in Figure 11E and F indicate that
the observed range of isotopic compositions (δ18O values
8.2–11.7‰) are equally consistent with isotopic exchange
with 0 per mil fluid at ~200°C, or alternatively, a 7 per mil
fluid at 320° to 500°C, at the high water/rock ratios implied
by the intensity of alteration. However, it is reasonably assumed that the K mica and Na mica facies were produced by
a fluid of similar oxygen isotope composition to that implied
by the pyrophyllite facies, i.e., ~3 to 6 per mil δ18O. On that
basis, the observed ranges of whole-rock δ18O values and fractionation factors for the K mica facies equate to equilibration
temperatures between 220° and 450°C.
Pyrite Sulfur Isotope Compositions
Sulfur isotope analyses of five pyritic samples from the Boco
alteration zone in the mid-1980s (Table 5) showed that δ34Spyrite
788
Discussion
18
The whole-rock δ O data and mineral assemblages reported in this study together suggest that the core of the Boco
alteration system attained temperatures of at least 270°C and
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,
comparable to that interpreted for the Basin Lake system
(Williams and Davidson, 2004). An alternative source could
be evolved seawater, isotopically modified by rock reactions,
along the lines of the convecting seawater system of the
Hokuroku basin, as modeled by Cathles (1983). He calculated that inflowing 0 per mil seawater would initially become
isotopically lighter by equilibration with the rocks at low temperatures near the sea floor, and subsequently become heavier as temperatures increase and water-rock fractionation factors decrease. Cathles’ models indicate that between ~2000
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.
The existence of kaolinite and pyrophyllite at Boco indicate
acidic conditions (pH <4), at least in the central zones. In
subaerial epithermal environments, low pH fluids are typically attributed to condensation of volatiles into shallow
groundwaters: i.e., magmatic-derived gases such as HCl, HF,
H2S, and SO2 in high-sulfidation epithermal systems (e.g.,
White and Hedenquist, 1990; Arribas, 1995; Giggenbach,
1996), or steam, CO2 and H2S from deeply boiled neutralchloride waters in low-sulfidation epithermal systems (Cooke
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789
4
A
Py in least altered facies
n=7
3
2
felsic volcanics
1
mafic
intrusive
0
-5
Frequency
values ranged from –1.2 to +4.7 per mil (Williams, 1985).
Green (1986), and Green and Taheri (1992) noted that these
values were distinctly lower than sulfides at the Hercules PbZn deposit, and they proposed that such low δ34Spyrite values
could be used to distinguish low-temperature barren pyritic
alteration zones from higher temperature base metal-associated systems.
We have analyzed sulfur isotopes in pyrite from an additional 22 samples (which were also analyzed for whole-rock
δ18O and major and trace elements) from in and adjacent to
the Boco alteration system. The results are presented in Table
3 and Figure 12.
The new δ34S data overlap with the previous data (Williams,
1985) and extend to a maximum of 14.2 per mil. There are
poorly correlated inverse relationships between pyrite δ34S
and geochemical measures of alteration intensity (Fig. 12C).
The pyrite δ34S values from the least altered facies, with AI
value of less than 70, have a broad range from 2.4 to 14.2 per
mil. The lowest δ34S value of these (sample W387; 2.4‰) is
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 ablation analysis, the two most distal samples (W380, 398) have
δ34S values around 14 per mil, suggesting that this may be the
background level in nonhydrothermal, diagenetically altered,
felsic rocks. Pyrites from altered facies samples, with AI of
greater than 70, mostly have δ34S values of less than 5 per mil.
The entire range of pyrite δ34S values in the northern segment of the Boco altered zone is –1.2 to +7.1 per mil, with a
mean of 3.3 per mil (n = 19).
Frequency
LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA
0
5
10
15
34
δ S ‰
4
B
Py in altered facies
n = 20
3
Mt Read Volcanics
VHMS deposit s: 6-15‰
2
1
0
-5
15
0
5
10
15
34
δ S ‰
C
34
δ S‰
10
r = 0.6
2
5
0
-5
30
40
50
60
70
80
90
100
Alteration Index (AI)
Altered facies:
least altered
Na-mica
K-Mica
pyrophyllite
Kaolinite +/- K-mica
Green's data
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 altered facies have a δ34S range of –1.2 to +7.2 per mil. C. Pyrite δ34S values
plotted against AI values show a weak negative correlation (r = 0.62) between
sulfur isotope composition and intensity of alteration.
and Simmons, 2000). In convecting hydrothermal sea-floor
systems, the acidity of fluids may increase due to the precipitation of magnesium hydroxysulfate as the descending seawater is heated above 250°C, and by subsurface metal precipitation (Bischoff and Seyfried, 1978). Reactions with volcanic
rocks generally buffer the fluid to maintain pH in the range 5
to 7 at 200° to 400°C (Lydon, 1996). However, in fluid-dominated focused flow zones, where the flux of water outstrips
the buffering capacity of the wall rocks, fluid pH may decrease to ~3, as in sea-floor black smoker fluids. The latter
789
790
HERRMANN ET AL.
case may apply to Boco, where the argillic and advanced
argillic kaolinite- and pyrophyllite-bearing mineral assemblages show that the acidic fluids were not entirely neutralized by wall-rock reactions, and must therefore have been focused along some structures of high permeability.
These new implications of hydrothermal fluid temperature,
pH, and oxygen isotope composition are partly inconsistent
with a previous interpretation that the Boco alteration zone
was produced by a fluid of seawater origin at temperatures of
less than 200°C (Green and Taheri, 1992). That interpretation was originated by Solomon et al. (1988) to explain the absence of base metals in some other barren systems in the
Mount Read Volcanics, including Chester, Howard’s Anomaly
and Basin Lake, which also exhibit low δ34S values in pyrite
(mostly <5‰). These pyrite sulfur isotope compositions contrast with the typical range of 6 to 15 per mil in the stratiform
massive sulfide deposits such as Rosebery, Hercules, and Que
River (Solomon et al., 1988). Those deposits evidently derived their sulfur largely from Cambrian seawater, at a time
when oceanic sulfate δ34S values peaked at 30 to 35 per mil
(Claypool et al., 1980). The Solomon et al. (1988) concept of
the low δ34S systems was that late-stage, relatively cool
(<200°C), oxidized seawater of neutral pH convecting
through felsic rocks, from which most of the reducing FeO
had previously been exhausted, would not continue to inorganically reduce residual seawater sulfate, nor transport sufficient base metals to form a massive sulfide deposit, but would
be capable of leaching primary magmatic sulfur from the volcanic country rocks. That interpretation did not account for
the low pH fluids implied by the advanced argillic facies in
those systems, because the kaolinite- and pyrophyllite-bearing assemblages were only recently recognized, with the advent of portable SWIR spectrometers. Our new mineralogic
data and interpretation of whole-rock δ18O compositions infers that the Boco hydrothermal fluid was either a highly
evolved seawater or included a magmatic component, to account for its low pH and 3 to 6 per mil δ18O composition,
which opens the possibility that the low δ34S compositions of
pyrite may also reflect a partially magmatic source of sulfur. A
non-seawater origin of Boco sulfur has been previously postulated by mineral exploration geologists: CSR’s decision to
withdraw from exploration of the Boco area was partly based
on a limited number of low δ34S data and anomalously high
fluorine data that pointed to a magmatic source of sulfur and
hydrothermal fluid, (Williams, 1985). Likewise, Randell (1991)
noted Boco’s similarity to low δ34S values at the Chester pyrite
deposit (–3.9 to 0.4‰) and suggested a magmatic source.
The magmatic source concept for barren or weakly mineralized systems in western Tasmania has recently been revived
by Williams and Davidson (2004), in discussion of advanced
argillic alteration facies and low δ34S values (–1.4 to +6.9‰)
associated with the small, low-grade Cu-Au-Ag deposit at
Basin Lake. The types of sulfide mineralization and advanced
argillic alteration at Basin Lake are similar to those of many
high-sulfidation epithermal systems formed in subaerial environments, but its sulfur isotope values are intermediate between those of high-sulfidation epithermal systems and Tasmanian Cambrian massive sulfide deposits. Williams and
Davidson (2004) consider that it formed in a submarine,
Cambrian synvolcanic, magmatic hydrothermal system, where
0361-0128/98/000/000-00 $6.00
acidic, moderately oxidized, magmatic fluids gradually mixed
with seawater away from the core of the system.
The submarine high-sulfidation style model is also applicable to Boco in respect of its alteration assemblages, sulfur isotopes, and apparent semivertical structural control on fluid
flow. Disproportionation of SO2 in a magmatic vapor plume,
as described by Rye et al. (1992), and its condensation into or
entrainment of convecting seawater, could partly account for
the heavier than seawater δ18O fluid composition, the lighter
than typical Cambrian VHMS-type seawater-sulfate-reduced
δ34S compositions of pyrite, and the low pH-related altered
facies. Even so, we are reluctant to press this interpretation
too far, given the absence of diagnostic high-sulfidation state
minerals and sulfates (e.g., alunite and barite) at Boco. Determination of hydrothermal temperatures and fluid compositions could help to resolve the fluid-source and genetic uncertainties but direct estimates of hydrothermal temperatures
have been thwarted by lack of workable fluid inclusions, and
a more oblique approach by oxygen isotope geothermometry
on quartz-white mica mineral pairs has not been pursued.
Irrespective of the source of fluids, the Boco system undoubtedly had considerable energy, sufficient to remove in
the order of 20 Mt of silica, and deposit 3 Mt of pyrite.
Nevertheless, the extensive exploration drilling at Boco, to
at least 400 m below surface, failed to intersect any mineralized zones. It is possible that mineralization did occur at
higher levels in the hydrothermal system, now eroded, but we
have no indication of its original vertical extent or its depth
below the sea floor. The new isotopic data reported here confirm Green’s (1986) preliminary conclusion that a combination of sulfur and whole-rock oxygen isotope geochemistry
can be used to identify this type of nonmineralized alteration
system from potentially more economically prospective altered zones associated with stratabound polymetallic VHMS
deposits such as Rosebery and Hellyer. Furthermore, that
discrimination can be based on relatively few analyses early in
an exploration program. Even more practically in the exploration context, portable SWIR spectrometry can reliably
identify the distinctive high-sulfidation style argillic and advanced argillic phyllosilicate assemblages, which were formerly overlooked as “sericite.”
Boco thus joins a relatively recently recognized category of
Cambrian volcanic-hosted alteration systems and deposits in
southeastern Australia that are characterized by advanced
argillic-altered facies and indications of magmatic fluid involvements, possibly representing sea-floor massive sulfide
high-sulfidation hybrids (Large et al., 2001b). In the Tasmanian Mount Read Volcanics, the class includes Chester (Boda,
1991), Basin Lake (Williams and Davidson, 2004), Western
Tharsis (Huston and Kamprad, 2001), and North Lyell
(Bryant, 1975). And in the Victorian Mount Wellington fault
zone, it includes Rhyolite Creek (Raetz et al., 1988), Hill 800
(Morey et al., 2002) and Mike’s Bluff (J. Bartlett, writ. commun., October 1999). Most of the prospects remain uneconomic, despite moderate to intensive exploration. North Lyell
is an important exception: its four main orebodies produced
about 5.4 Mt of ore averaging 5.3 percent Cu, 34 g/t Ag and
0.43 g/t Au (Corbett, 2001). These Tasmanian and Victorian
prospects and deposits may be members of an emerging
category of Au-rich VMS deposits that are characterized by
790
LITHOGEOCHEMICAL AND STABLE ISOTOPIC INSIGHTS INTO GENESIS OF THE BOCO PROSPECT, WESTERN TASMANIA
aluminous alteration, including deposits such as the worldclass Archean Au-rich LaRonde Penna and Bousquet 2-Dumagami deposits, Canada (Dubé et al., 2007). That style of
hydrothermal system has clear, mineralized analogues in
modern arc and back-arc settings of the west Pacific, such as
the Brothers Volcano (de Ronde et al., 2005) and the Pacmanus hydrothermal system (Paulick and Bach, 2006). In
recognition of that, we share Williams and Davidson’s (2004)
opinion that this class of Tasmanian and Victorian Cambrian
pyrophyllite-kaolinite aluminous alteration systems, with low
δ34S values and indications of magmatic fluid input, may yet
have significant exploration potential.
Conclusions
The results of the lithogeochemical, mineralogical, and isotopic characterization presented in this study allow some constraints on the origin of the altered zone at the Boco prospect,
and comparison with some other mineralized systems in the
Mount Read Volcanics.
1. The Boco aluminous alteration zone was produced by a
Cambrian synvolcanic hydrothermal system in a proximal
subaqueous (marine?) felsic volcanic setting.
2. The alteration system is concentrically zoned with outer
quartz-white mica-pyrite assemblages grading inward through
quartz-white mica-kaolinite-pyrite to quartz-pyrophyllitepyrite facies.
3. The major chemical mass changes associated with hydrothermal alteration were losses of Si, Na, Ca, Mg, K (in
order of decreasing magnitude) and small gains in sulfur.
These changes are mineralogically apparent in the destruction of feldspars and introduction of 2 to 3 percent disseminated pyrite. They imply very large water/rock ratios in the
hydrothermal system.
4. Alteration box plots based on the AI, CCPI, and AAAI indices reflect those chemical and mineralogical changes. However, they do not differentiate between the sericitic, argillic,
and advanced argillic assemblages. Nor do they reliably discriminate between this type of barren system and quartzwhite mica-pyrite assemblages in proximal footwall zones associated with VHMS deposits elsewhere in the Mount Read
Volcanics.
5. There is no distinction between background whole-rock
δ18O values and those of the altered zone.
6. Whole-rock δ18O data and mineral assemblages together
suggest that the core of the Boco alteration system attained
temperatures of at least 270°C and involved an acidic fluid
that was isotopically heavier than seawater, with a δ18O ratio
possibly up to about 6 per mil. Based on those results, we propose two possible origins for the hydrothermal fluids that
formed aluminous altered facies at Boco: mixed magmatic
and seawater fluids, or evolved convecting seawater.
7. δ34S values of pyrite in the alteration zone range from
–1.2 to +7.1 per mil with a mean of 3.3 per mil. The δ34S values increase abruptly outside the alteration zone to local
background values of about 14 per mil.
This study demonstrates that pyrite δ34S and whole-rock
isotopic techniques in combination with SWIR spectrometry can be used to discriminate this type of potentially
δ18O
0361-0128/98/000/000-00 $6.00
791
barren or subeconomic alteration system from more prospective systems associated with stratiform, base metal-rich
VHMS deposits such as Rosebery and Hellyer, even from a
few analyses. The Boco prospect has low δ34S values in comparison with economic Tasmanian stratiform VHMS deposits,
and its δ18O values are at background levels, unlike the 18O
depleted proximal zones typical of many VHMS deposits.
Acknowledgments
This research was part of the CODES 2000-2002 project
entitled “Isotopic response of fluid flow in submarine volcanic-hosted hydrothermal systems and its implications for
prospectivity: Mount Read Volcanics case study.” An Australian Research Council Strategic Partnership with IndustryResearch and Training (SPIRT) grant, supported by Mineral
Resources Tasmania, Pasminco Australia Ltd., and Goldfields
Exploration Pty. Ltd, funded the project. We sincerely thank
Keith Harris at the University of Tasmania Central Science
Laboratory for his assistance with laser ablation sulfur isotope
analyses. We acknowledge David Huston, Patrick MercierLangevin, and Robert Seal for their meticulous and constructive reviews of the draft manuscript.
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