RE-OS ISOTOPE SYSTEMATICS OF SULFIDES FROM FELSIC IGNEOUS ROCKS - APPLICATION TO BASE-METAL PORPHYRY MINERALIZATION IN CHILE

Dating sulfides and determining the source of ore-forming metals have been limiting factors in our understanding of processes that produce h ydrothermal ore deposits. Here we analyze sulfides from base metal por phyry deposits from Chile to demonstrate the potential of the Re-Os sy stem to determine both the age and the source of metals for hydrotherm al mineralization. Cogenetic chalcopyrite, bornite, pyrite, and sphale rite from El Teniente (ca. 5 Ma) have Re-187/Os-188 ratios from 0.3 to 21.8 and initial Os-187/Os-188 ratios from 0.17 to 0.22. A parageneti cally late pyrite that has an initial Os-187/Os-188 of 0.88 indicates that the ore deposit is not isotopically homogeneous in Os throughout the formation of the deposit. Pyrite samples from Andacollo (ca, 100 M a) have Re-187/Os-188 ratios from 15 to 3600, and different isochrons yield ages between 87 and 103 Ma, consistent with different sericite K -Ar ages. Initial Os-187/Os-188 ratios of 0.2 to 1.1 are similar to th ose of El Teniente. The relatively homogeneous Os-187/Os-188 ratios (s imilar to 0.19) for cogenetic sulfides from El Teniente suggest that m ost of the Os in the ore was from the causal intrusive, The late-stage pyrite probably incorporated Os leached from the country rock by mete oric fluids, which mixed with magmatic fluids at the periphery of the hydrothermal system. We demonstrate that the Re-Os isotope system can be a powerful geochronological tool in hydrothermal ore deposits. The large range in Re/Os ratios of sulfides permits the age of mineralizat ion to be well constrained, despite isotopic heterogeneities of the hy drothermal fluids.