Abstract

Sulfate minerals, hematite, and isotopically fractionated sulfides are relatively rare in rocks of Archean age. Several Archean Au deposits, however, contain one or more of these features indicative of oxidized hydrothermal fluids. They include Hemlo, McIntyre-Hollinger, Macassa, Lake Shore, and Ross which are all in Ontario, Canada, and Kalgoorlie in Australia. Examples of lesser deposits are in Hislop Township and in the Matachewan area, Ontario, Canada. With the exception of Hemlo, these are quartz-carbonate vein deposits spatially and temporally associated with felsic plutonism. Because metamorphic fluids and Archean surface waters were mostly reducing, the most feasible general source for the fluids was felsic magmas.Parent magmas were relatively oxidized, hydrous, and CO 2 rich. The oxidized character may have favored the incorporation of Au into the partial melt and its partitioning into magmatic aqueous fluids. High contents of CO 2 are conducive to the separation of an aqueous fluid from magma at greater depth than otherwise. Fluid inclusion data support the formation of major Archean Au vein deposits at deeper crustal levels than more recent magmatic-hydrothermal mineralization, such as porphyry Cu deposits. CO 2 -rich fluids can unmix into two phases at relatively high temperatures and pressures: a saline aqueous liquid and a low-salinity, H 2 O-CO 2 liquid. This can cause partitioning of Cl-complexed base metals from other metals including Au, Sb, and As. The extensive carbonatization around most deposits has consistent delta 13 C values in the range of juvenile-magmatic CO 2 .

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