Using PGE geochemistry to assess magma fertility in the Polo Sur and Penacho Blanco porphyry copper deposits, Centinela District, Northern Chile

Abstract

The Centinela District is host to several mineralized porphyries, including both Cu-only and Cu-Au deposits. This study focusses on the known Polo Sur and Penacho Blanco porphyry Cu +/- Mo deposits as cases of study to assess their Cu-Au fertility.

PGE are sensitive indicators of sulphide saturation in silicate melts due to their strong partitioning into sulphide melts. They also have relatively low mobility in hydrothermal fluids, minimizing any effects of addition during hydrothermal alteration. If a parent magma reaches sulphide saturation early in its history, an immiscible sulfide melt will extract chalcophile metals trapping them at depth, potentially preventing them from entering the volatile ore-forming phase. In contrast, if sulphide saturation occurs close to the time of volatile saturation, or does not occur, then metals will be available to enter the fluid phase to form a deposit.

Thirty igneous rock samples from both deposits were analysed for major and trace elements. A subset of these samples were analysed to determine their PGE concentrations. Preliminary results suggest that sulphide saturation may occurr before the MgO content of magmas at Penacho Blanco fell below 3.5 wt.% but the timing of sulfide saturation, if it occurred at all, at Polo Sur is unclear.