Archaean carbon reservoirs and their relevance to the fluid source for gold deposits

Abstract

It is commonly assumed^1,2 that seawater-derived carbon in altered volcanic rocks is the only major pre-metamorphic carbon-in-carbonate reservoir in Archaean greenstone belts. Thus carbonates from Archaean gold deposits with stable isotope ratios too negative (median δ¹³C ≈ −3‰) to have been derived from such carbon (δ¹³C usually assumed to be near 0‰) have been interpreted as being derived from a local felsic magmatic source¹. Barley and Groves³, however, recognize two carbonate-alteration styles that predate regional metamorphism and gold mineralization in the Norseman–Wiluna greenstone belt of Western Australia: evidence is summarized below. These are seafloor alteration and fault-controlled regional alteration, which, as shown below, have completely different carbon isotope compositions. The latter (median δ¹³C = −4.8‰) implies a major juvenile carbon (CO₂) flux from the mantle during greenstone-belt evolution, and is thus of fundamental importance in itself. However, here we show why the mere existence of two carbon-in-carbonate reservoirs and variation in carbon isotope ratios of carbonates between individual gold deposits in one area, negate some of the fundamental assumptions used to support magmatic-fluid models¹.