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SULPHUR
ISOTOPE VALUES OF SURFICIAL SULPHATES:
A
NEW EXPLORATION TOOL?
Anita S. Andrew1, Allan R. Chivas2
and Andrew J. Bryce1 1CSIRO
Division of Exploration Geoscience 2Research
School of Earth Sciences, Australian National University Extracted from: Centre for Isotope Studies Research
Report 1991-1992 (pp1-4). INTRODUCTION
Gypsum and alunite are widespread in the Australian regolith. They occur in ancient, deeply weathered profiles, more modem pedoderms and are also found in modern lacustrine sediments and adjacent dunes. The isotopic composition of sulphur in such minerals has the potential to yield information about the chemical and isotopic composition of Australian brines as well as the potential as an indicator for buried mineralization in weathered terrains. Sulphur
isotope studies (Bird et al., 1989; Chivas et al., 1991) show that
generally no correlation exists between *34S values
for regolith and underlying bedrock types, suggesting that sulphate derived from
bedrock weathering is only a minor component of surficial deposits. Bedrock
lithologies containing significant sulphur such as sulphide-rich rocks and ores,
or sulphate-bearing evaporates,
however, often provide an identifiable *34S
signature. Collaborative research between ANU, CSIRO and others on this topic was part of the SLEADS (Salt Lakes, Evaporites and Aeolian DepositS) project (Chivas et al., 1987). The better understanding of the interplay between atmospheric processes, the southern ocean and weathering, resulting from this project, has led to a novel and potentially important exploration tool. RESULTS
Sample
locations are shown in Figure 1 and all *34S data
are reported as per mil () variations relative to the Caρon Diablo troilite
(CDT) standard. At the Elura Pb-Zn-Ag deposit (NSW), the *34S
signature of ores is identical with that of sulphates from the gossan profile
(Taylor et al., 1984), but all samples of weathered bedrock from 1-2 m
depth display the regional (airborne) sulphate signature. Samples of surficial
sulphate from >4 m depth and up to 100 m from mineralization have mixed
bedrock-regional signature; at 200 m and beyond, the ore signature cannot be
detected irrespective of sample depth. On a regional scale in both the Yilgarn Block and central South Australia, the *34S values of surficial sulphate vary regularly with distance from the sea in the 500 to 1000 km interval. Airborne sea-salt sulphate ("cyclic sulphate") with a *34S value of +21 is the dominant source of sulphate near coastlines and in places extending hundreds of kilometres inland. The other important source of sulphate is also airborne, but is thought to derive from mainly marine, volatile biogenic sulphur compounds (probably with a mean *34S value of ~0). The entire variation observed can generally be accounted for by a decrease in "cyclic sulphate" component of airborne-derived sulphate from ~100% near coastlines to ~55% in the interior of the continent.
Fig. 1: Map of Australia showing locations of surficial
sulphate samples. The
regular pattern of *34S values
of surficial sulphate may be used as a basis for exploration for sulphide ore
deposits. In the Yilgarn block (Fig. 2), where bedrock sulphides generally have *34S values
<5, the *34S value
for lacustrine sulphates ranges from +23.1 to +13.4 with the highest values
in the southwest and the lowest in the northeast, but show no relation to the
distribution of underlying granites and greenstones. The *34S values
of gypsum and alunite from soils overlying sulphide-bearing gold deposits at
Davyhurst and Callion, however, are 1 to 2 lower and clearly distinguished
from the general trend predicted for these locations. In
central South Australia, *34S values
of lacustrine gypsum decrease regularly from +20.1 near the coast to +16.5
across Proterozoic igneous rocks of the Gawler Craton and the Proterozoic
quartzose strata of the Stuart Shelf but show no correlation with bedrock
lithology. The trend is continued by gypsum from Tertiary units overlying the
Cretaceous Bulldog Shale (*34S ranges
from +16.7 to +14.6) and by surficial gypsum (*34S ranges
from +15.2 to +14.2) from within the Eromanga Basin. The exception to this
trend occurs when bedrock sources contain abundant sulphur with a distinctive *34S
signature. Secondary sulphates from weathered outcrops of sulphide-rich Bulldog Shale (with primary sulphide *34S values of ~-40) have *34S values of -10.2 to +2.1. Creeks and lakes that drain, or are located within, the Bulldog Shale have gypsum with *34S values between +9.4 and +11.9. In another example from the Eyre Peninsula, alunite samples from acid lakes have *34S values as low as +4.5 resulting from mixture of sulphate from underlying Tertiary palaeochannels and basement sulphides (*34S ranges from -17 to -6).
Fig.
2: Map
of Yilgarn Block showing contours of *34S
for surficial gypsum.
Most contoured values are for gypsum but where analysis has been on
alunite or dissolved sulphate the values have been recalculated for gypsum that
would be in isotopic equilibrium with the measured sulphate species. The
present investigation has shown that even where relatively sulphide-rich
basement rocks are weathered, the adjacent large playas are dominated by
"cyclic sulphate" sources, and that basement sources barely influence
the regional *34S
signature. In addition, the *34S
measurements establish the sources of sulphur from both sea-salt and marine
biogenic sulphur and clearly support delivery of salts to the Australian
landscape as aerosols following established wind patterns.
In smaller lakes (e.g. Lake Yaninee on the Eyre Peninsula), however, and
particularly in regolith profiles, the presence of basement sulphides may be
detected. The *34S technique can be applied to the search for economic minerals that occur as, or are associated with, sulphides and sulphates with *34S values that are distinct from the *34S values of regional sulphates (typically +15 to +20). Because the regional surficial *34S pattern can be established by both sulphate minerals and the dissolved sulphate of groundwaters, the technique may be applied to prospect evaluation by sampling either regolith or groundwater. REFERENCES BIRD
M.l., ANDREW A.S., CHIVAS A.R. & LOCK D.E. 1989. An isotopic study of
surficial alunite in Australia: 1. Hydrogen and sulphur isotopes.
Geochimica et Cosmochimica Acta 53, 3223-3237. CHIVAS
A.K., McCULLOCH M.T., LYONS W.B., DONNELLY T.H. & COWLEY .J.A. 1987.
Isotopic tracers of the source of salts. SLEADS (Salt Lakes, Evaporites and
Aeolian DepositS) Workshop '87, p. 10. Australian National University, Canberra. CHIVAS
A.K, ANDREW A.S., LYONS W.B., BIRD M.I. & DONNELLY T.H. 1991.
Isotopic constraints on the origin of salts in Australian playas. 1.
Sulphur. Palaeogeography, Palaeoclimatology, Palaeoecology 84, 309-332. TAYLOR G.F., WILMSHURST J.R., TOGASHI Y. & ANDREW A.S. 1984. Geochemical and mineralogical haloes about the Elura Zn-Pb-Ag ore body, western New South Wales., Journal of Geochemical Exploration 22, 265-290.
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