Gold accumulation in diagenetic framboidal pyrite from the Derwent Estuary, Tasmania

Recently Large et al. (2007; 2009) have suggested that diagenetic pyrite in black shale may be the source of gold in shale-hosted orogenic gold systems. Support for this theory includes laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) analyses conducted on different generations of pyrite from shale hosted orogenic gold deposits. The results show that diagenetic pyrite has a higher gold concentration than metamorphic pyrite and pyrrhotite after pyrite. It is suggested that gold and arsenic has been released from diagenetic pyrite during metamorphism to form orogenic gold deposits. One of the unresolved aspects of this model is the mechanism for the enrichment of gold and other trace metals in the diagenetic pyrite. This study focuses on resolving the mechanism by conducting LA-ICPMS analyses of diagenetic pyrites that form within a high metal environment: the Derwent Estuary, Hobart, Tasmania. Our results show a strong increase in the number of pyrite framboids significantly enriched in gold that are at and immediately below the base of the sediments anthropogenically enriched in base and precious metals. The chemical conditions found in this interval represent those that enhance uptake of gold by pyrite, either by releasing adsorbed or absorbed gold from adjacent particles or by enhancing the incorporation of gold into the lattice of diagenetic pyrite.