Abstract

A study of the sulphur-isotope and minor-element composition of gold ores and country rock from the Hemlo deposit, Ontario, has been carried out on 10 drill cores from the mineralization zone and two occurrences of stratiform barite west of Hemlo. The work confirms that Mo, As, and Sb correlate strongly with Au, whereas Ba, and by inference barite, does not. A preferential association of heavy rare-earth elements with higher grades of Au was also established. The isotopic compositions of barite and pyrite reflect a deposit-wide thermal equilibration at 504 °C. Whole-rock sulphur isotopic compositions can be modelled as a mix of sulphate and sulphide end members, the former marked by a relatively constant isotopic composition of roughly 10‰ and the latter by variable and negative isotopic compositions in mineralized zones.These isotopic characteristics are interpreted as the result of (i) preliminary fluid–rock reactions in which some isotope exchange between sedimentary barite and hydrothermal sulphide has occurred and (ii) a later stage metamorphic event in which sulphur-isotope equilibrium is established between the precipitated, distinctly isotopically light pyrite and bedded barite. These two fractionation events, separated in space and time, are required to explain both the variable and distinctly negative δ34S values observed for the pyrite in the Hemlo ores (−15.9 to +1.0%o) and the uniform Δδ34S value of 12.7 ± 1‰ obtained for the coexisting barite and pyrite pairs, in widely different proportions on a deposit-wide scale. Both the production of light isotopic sulphide and the later precipitation of Au are thought to have been influenced by an increase in fluidgraphic.

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