Variations in pyrite textures with proximity to gold deposits at St Ives: how this effects the pyrite trace metal content

A series of mineralized and non-mineralised samples of pyritic black shale from the St. Ives gold district were analysed to test the model proposed by Large et al. (2011) on an Archean aged deposit. The focus of the study is the regionally extensive Kapai slate horizon. The samples were analysed using traditional petrographic techniques and laser ablation ICP-MS. Hand specimen and polished section analyses show that the distal specimens have distinctly different pyrite textures than those proximal to mineralisation. Distal specimens generally have fine grained euhedral pyrite that periodically form nodules. These pyrite types conform well to the bedding and are interpreted to be of diagenetic origin. Proximal to the deposits the pyrite was coarse grained and euhedral and does not conform to any earlier bedding structures. Laser ablation ICPMS analyses show linear trends in the trace metal distributions for diagenetic pyrites with the most trace metal enriched pyrite distal to the mineralisation, which is what is expected in the model proposed by Large et al. However, pyrites within the mineralised zones vary significantly from the trend observed in the diagenetic pyrite suggesting a second fluid source. Thus we conclude that though the processes proposed by Large et al. may have been (2011) occurring in the Archean much of the metal endowment was brought from a second fluid source. These data can be used to better understand the fluid history of the district and help build a model of the mineralisation.