A reconnaissance study of stable isotope ratios in archaean rocks from the yilgarn block, Western Australia

Abstract

Isotopic compositions of sulphur, carbon, and oxygen have been determined for constituents from a total of 103 samples of sedimentary rocks, mafic and ultramafic igneous rocks, nickel ores, and gold ores from the Archaean Yilgarn Block. Sulphides in the bulk of the sedimentary rocks have δ³⁴S values close to 0‰ and appear to have precipitated from solutions which incorporated magmatic sulphur (either juvenile or derived from older rocks). There is no evidence for widespread sulphate reduction. δ³⁴S values of sulphides in the nickel deposits and associated mafic/ultramafic igneous rocks are within the magmatic range. The small, high‐grade deposits of the Kambalda‐Nepean‐Scotia type have small positive δ³⁴S values, and the large, low‐to‐medium grade dunite‐associated deposits of the Mount Keith‐Perseverance type have small negative δ³⁴S values. Sulphides in the Kalgoorlie gold ores are enriched in ³²S relative to those in their host dolerite, supporting an epigenetic origin for the gold, under moderately high fO₂ conditions. The δ¹³C values do not provide unequivocal evidence for the source(s) of the reduced carbon (kerogen) in the sedimentary rocks. Whilst they are compatible with biogenic derivation, it is not possible to rule out contributions from pre‐biotic organic ‘soup’ or from hydrothermal solutions of deep‐seated origin. Carbonate in the sedimentary rocks are predominantly in epigenetic, sulphide‐bearing veinlets. In many cases, their δ¹³C values suggest precipitation from hydro‐thermal solutions containing magmatically derived CO₂. In only two samples are the petrographic features and δ¹³C values compatible with marine carbonates. Talc‐carbonate altered ultramafic igneous rocks have δ¹³C values consistent with their incorporation of magmatically derived CO₂. The ?δ¹³C (carbonate‐kerogen) values for most of the sedimentary rock studied fall in a narrow range around +10‰, suggesting isotopic exchange between oxidized and reduced carbon species at moderately high temperatures (>250°C). δ¹⁸O values of carbonate from both sedimentary rocks and igneous rocks are mainly within the range +7.2‰ to +18.0‰. If the values are primary they are consistent with the formation of carbonate from hydrothermal solutions of magmatic and/or metamorphic origin. However, it is also possible the δ¹⁸O values are the result of post‐depositional equilibration with meteoric waters.