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ISOTOPE HYDROGEOCHEMISTRY IN EXPLORATION FOR

BURIED AND BLIND MINERALIZATION

 Anita S. Andrew^1,2, Graham R. Carr², Angela M. Giblin² and David J Whitford¹.

¹CSIRO Division of Petroleum Resources, ²CSIRO Division of Exploration and Mining

 Extracted from: Centre for Isotope Studies Research Report 1995-2000 (pp77-81)

INTRODUCTION

Buried and blind deposits, with no direct geological or geochemical manifestation at the surface, are becoming increasingly important targets in Australia. One of the key exploration challenges relates to assessing and ranking targets established from geophysical and other remotely sensed surveys. Sub-surface geology is reflected in the geochemistry of groundwaters (Giblin, 1996) and hydrogeochemical methods provide a particularly powerful technique in areas of poor surface exposure, deep weathering and where transported overburden obscures the underlying geology (Giblin, 1997). In such areas several hundred samples are used to define locally prospective areas although how these relate to a specific mineralization style may be difficult to determine.

 The question of proximity to an ore body is fundamental to mineral exploration and isotopic (S, Pb, Sr) methods are uniquely capable of contributing to an answer. The isotopic composition of ores and waters that interact with ores carries important information about the elemental source; S and Pb are direct ore indicators allowing straightforward interpretation of possible ore associations. The isotopic methods also provide unequivocal evidence for mixing. The isotopic compositions of S, Pb and Sr in rocks are unaffected by weathering and in natural waters are unaffected by precipitation, evaporation or dilution. Isotopic methods provide information that is complementary to that obtainable from major and trace element abundances.

The application of integrated isotopic studies to conventional hydrogeochemical interpretations was tested in several areas (Fig. 1); Menninnie Dam (Pb, Zn; Eyre Peninsula SA), Abra (Ag, Pb; Bangemall Basin WA), Benambra (Cu, Zn, Pb: Lachlan Fold Belt Vic), Goonumbla (Cu, Au; Lachlan Fold Belt NSW) and Kanmantoo (Cu, Pb, Zn, Au; Kanmantoo Fold Belt SA). These were chosen to include different deposit types, tectonic regimes, climatic and topographic environments and groundwater chemistry.

QUANTIFYING UNKNOWNS

 Hydrology: In exploration of new terrains detailed groundwater flow patterns and the spatial distribution of aquifers is generally unknown with groundwater movement commonly occurring in fractured bedrock aquifers. In such environments different aquifers can be characterized by field-measured chemical parameters (eg pH, Eh, salinity and reduced Fe) and laboratory-measured elemental abundances (Fig. 1).  The relative elevations of the water table can be used to make inferences about gross groundwater flow paths (Fig. 1).

Target Signatures: As a good starting point the target isotopic compositions of S and Pb can be established from existing metallogenic information. The isotopic composition of background sulphate derived from aerosol input, can be estimated for large areas of the Australian continent (Chivas et al., 1991) or by a regional survey of surficial sulphate minerals or sulphate in ground- and surface-waters. Background Pb isotopic ratio ranges can be either measured from local host rocks or estimated from the crustal growth curve.  Strontium isotope indicators of potassic alteration are defined by relative increases in ⁸⁷Sr/⁸⁶Sr against background values of ⁸⁷Sr/⁸⁶Sr, which can be estimated from regional geochronological data.

Fig. 1: Regional and plan view of Menninnie Dam showing interpreted hydrology determined by groundwater chemistry (DA = dilute aquifer; SAG = saline aquifer GRV; SAC = saline aquifer cavernous) and relative elevation of water table. Sulphur isotopes values measured from regional bores (left) and in prospect area (right).

Mineralization can be detected in the isotope chemistry of groundwaters.  The Pb isotopic composition of groundwaters from Abra, Menninnie Dam, Kamnantoo, Benambra and Currawang is a direct reflection of the local mineralization. 'Near-ore' Pb isotope signatures may reflect, indirectly, broad alteration zones but such an interpretation needs further testing.  The presence of mineralization can also be inferred from *³⁴Svalues with pure ore-signature found only in low-salinity relatively sulphate-poor waters (Abra, Benambra, Goonumbla). In higher salinity waters, the *³⁴Svalue is characteristic of mixing of ore-derived S with sulphate derived from modem aerosol fallout (Menninnie Dam, Fig. 1; Waring et al, 1997).

Strontium isotope ratios reflect the nature of the host rock and its alteration. High ⁸⁷Sr/⁸⁶Sr ratios in groundwaters from Abra, Menninnie Dam, Kanmantoo and Goonumbla reflect K-rich alteration related to mineralization.  At Menninnie Dam, ⁸⁷Sr/⁸⁶Sr ratios have been buffered by local carbonates; high ratios reflecting alteration are found in the carbonate-free suites.

The scale of hydromorphic dispersion is dependent on the local hydrology.  Lead isotopes provide localised target definition at Abra (Fig. 2), Menninnie Dam, Kanmantoo and Benambra with targets slightly larger than the mineralization itself.  Sulphur isotope values show greater variability but retain evidence for interaction with ores up to several kilometres down the hydromorphic gradient. For example, at Menninnie Dam (Fig. 1), ore-related S can be detected down the hydraulic gradient 1.1 km south of mineralization.   At Abra, ore-related S and Pb isotopic signatures have been detected up to 6 km from the main mineralized zone (Fig. 2) but the lack of a well-defined intervening dispersion plume makes the significance ambiguous. Strontium isotopes provide very broad target definition at Menninnie Dam and Kanmantoo in a pattern similar to that defined by sulphur isotopes. At Abra, very high ⁸⁷Sr/⁸⁶Sr ratios provide localized target definition (Fig. 2). At Benambra waters with isotopic signatures indicative of ores are restricted to the immediate ore environment.

Fig. 2. Contoured S, Pb and Sr isotope data for groundwaters around the Abra prospect. Waters collected from percussion holes except in the ore zone where they were also collected from DDHs.

APPLICATION TO EXPLORATION

Isotope geochemistry should be, an adjunct to conventional surveys.  Lead and S are only likely to have wide application in high - Pb, and high - S exploration situations. As a potential direct ore indicator, S isotope analyses might be carried out on a routine basis with major and trace element abundances. Groundwaters with S isotope and concentration anomalies might then be analysed for Sr to refine regional targets. Pb isotopes are most appropriate for prospect-scale exploration. Isotope hydrogeochemistry is best suited to areas of low relief and low rainfall.

Isotope hydrogeochemistry represents an exciting new exploration technology and has the potential to offer a cost-effective exploration technique applicable at both the local and regional scale. Despite only limited and largely empirical testing, isotope methods could be usefully added to conventional hydrogeochemical surveys. The isotopic composition of Pb and S provide robust targets that are significantly independent of the style of mineralization being sought. Sulphur and Sr isotope analyses will have application in regional target definition whereas Pb will have application in prospect scale evaluation.

 ACKNOWLEDGEMENTS

We acknowledge Aberfoyle Resources, Denehurst Limited, North Limited and Pasminco who supported this research through AMIRA. Terry DonneIly provided a helpful review.

 REFERENCES

ANDREW A.S., CARR G.R., GIBLIN A.M. & WHITFORD D.J. 1998. Isotope hydrogeochemistry in exploration for buried and blind mineralization. The State of the Regolith. Geological Society of Australia Special Publication 20, 222-225.

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. I. Sulphur. Palaeogeography, Palaeoclimatology, Palaeoecology 84, 309-332.

GIBLIN A.M. 1996. An application of groundwater geochemistry to the detection of prospective basement beneath Mesozoic cover in North Queensland. In: Mesozoic geology of the Eastern Australia plate conference, Brisbane, 23-26 September 1996. Geological Society of Australia, Extended Abstracts 43, 186-194.

GIBLIN A.M. 1997. Geochemistry of groundwaters in the vicinity of Stawell, Clunes, Ararat and Ballarat gold deposits. In: The Ausimm 1997 Annual Conference, Ballarat, 12-15 March, 1997 (Australasian Institute of Mining and Metallurgy, Carlton, 1997) Publ. Ser. 1/97,181-191.

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Last modified: April 17, 2003