Magmatic evolution of the southern Centinela district, northern Chile: insights from PGE geochemistry and zircon fertility tools

Abstract

Copper is one of the most widely used metals in our society and it is essential for a broad range of applications. Globally, nearly three quarters of this metal is sourced from porphyry copper deposits. For this reason, is important to understand the processes that control their formation. Recent studies have shown that the platinum group elements (PGE) are sensitive indicators of sulphide saturation in evolving magma systems, which may be a key in controlling the fertility of Cu and Au in porphyritic deposits. The Centinela District, located in northern Chile, is part of the middle Eocene to early Oligocene metallogenetic belt, which contains a number of renowned deposits including Chuquicamata and La Escondida. The district is host to at least ten identified mineralized porphyries, including both Cu-only, Cu-Au and sub-economic bodies. Here I present the results of whole-rock geochemistry (including the PGE) and zircon geochronology of the Polo Sur and Penacho Blanco porphyry copper deposits of the southern Centinela District, northern Chile. This project aims to test the hypothesis that the timing of sulfide saturation, relative to volatile exsolution, is a key factor controlling the type (Cu or Cu-Au) and fertility of porphyry systems from the southern Centinela District. If a parent magma reaches sulfide saturation before volatile exsolution, an immiscible sulfide melt or solid will form, which will extract chalcophile metals, including Cu and Au, and may trap them at depth so they are unable to enter the volatile ore-forming phase, resulting in a barren system. In contrast, if the fraction of immiscible sulfide to form is small, the time difference between these events is short, or the magma does not become sulphide saturated, most of the metals will remain in the melt, and be available to enter the fluid phase to form an economic Cu or Cu-Au deposit. The results from zircon and whole-rock geochemistry suggest that both deposits share a similar source and that the magmatic activity continued for ca. 5-6 Myr., shifting into more oxidizing conditions as it evolved. Platinum group elements geochemistry show contrasting results between the two deposits: Penacho Blanco deposit shows a steady decrease in Pd and Pt, whereas Polo Sur shows a scattered pattern when Pt or Pd are plotted against MgO. Although the samples span from dioritic to rhyodacitic compositions, it was not possible to establish the onset of sulfide saturation for these deposits. However, the data for Penacho Blanco suggest that sulphide saturation occurred before the MgO of the melt fell to 3.7 wt.%. Zircon geochemistry indicates an increase in the fertility conditions as the deposits became younger, reaching a peak in the most fractionated samples from the TYC prospect. PGEs geochemistry allows the Penacho Blanco and Polo Sur deposits to be classified as Cu dominated porphyries. Although it is not possible to accurately establish the timing of the sulphide saturation for these deposits, the rate of depletion of Pd with decreasing MgO at Penacho Blanco suggests that the percentage sulphide precipitation was greater than Cadia (Au-Cu) but less than El Abra (Cu only), consistent with it being Cu-dominant deposits.