The nature of Archaean gold‐bearing fluids as deduced from gold deposits of Western Australia

Abstract

Archaean gold deposits of Australia are most abundant in 2.8 ± 0.1 Ga granitoid‐greenstone terrains, where they form both vein‐type and disseminated occurrences. Two‐thirds of total gold production has come from metamorphic (early, peak or late) vein deposits within host rocks of metabasaltic composition and of greenschist to lower amphibolite facies grade. Wallrock alteration, isotopic readjustment and fluid inclusion data help to characterise the nature and importance of hydrothermal fluids in the generation of the vein‐type deposits. Although fluid access is controlled by major structures and more locally by permeable zones (e.g. hydraulically‐fractured country‐rocks), gold deposition is a result of fluid‐wallrock interaction within a suitable temperature range. Mineralising temperatures at least as high as 400°C (e.g. Mt Charlotte, Kalgoorlie) and slightly alkaline, reducing, H₂O‐CO₂‐rich fluids of low salinity, derived from lower in the sequence, are inferred. In this type of solution, gold transport is more likely as reduced sulphur complexes than as either gold bicarbonate or gold chloride complexes, based on both theoretical predictions and natural occurrences. Considering the associated nature of most species in the above temperature range, HAu(HS)₂ complexes are considered to be the most likely gold‐transporting agent. Such transport can explain the strong enrichment of gold relative to copper, silver and zinc, and can account for high antimony, arsenic, tungsten, mercury and boron with gold, the variety of host rock chemical environments seen at different deposits, and the close spatial association of pyrite alteration and gold. The important broad‐scale controls of gold accumulation in Archaean terrains are still speculative. Greenstone belts are an integral part of any model and may provide sources of gold and hydrothermal fluids (H₂O‐CO₂). They also provide suitable combinations of timing of structural and metamorphic events and suitable host rocks for ore deposition. Host rocks of tholeiitic composition are widespread and particularly favourable for gold deposition, whereas disseminated deposits in banded iron formation appear to be more common in older (≥ 3.0 Ga) Archaean terrains. The less auriferous nature of some c. 3.5 Ga terrains (e.g. Pilbara) may reflect different early Archaean magmatism with lower total gold, a sequence of tectonometamorphic events unfavourable for major gold deposit formation, or early alteration (e.g. silicification) non‐conducive to the later formation of suitable auriferous hydrothermal fluids.