Sedimentary Signatures of Sulfides in the El Soldado Manto-Type Cu-(Ag) Deposit, Coastal Cordillera of Central Chile: A Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Study

Abstract The Coastal Cordillera of central-northern Chile hosts numerous stratabound (manto-type) Cu-(Ag) deposits, where sulfides are commonly associated with residual petroleum or pyrobitumen. El Soldado, the largest of these deposits in central Chile, exemplifies this relationship, raising questions about the provenance of metals and the role of hydrocarbons and hydrothermal fluids in ore formation. Although El Soldado is an important major source of Cu and Ag, the main sulfide phases—pyrite, chalcopyrite, bornite, and chalcocite—remain poorly constrained. Characterizing their trace element chemistry and compositional variability is key to better understanding the evolution of the deposit and the deportment of critical elements. To address this knowledge gap, we present a geochemical characterization of sulfides from El Soldado using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), in combination with electron probe microanalysis (EPMA) and micro-X-ray fluorescence (μXRF). Our results show that El Soldado sulfides—especially pyrite—contain a large variety of trace elements, including appreciable concentrations of Co, Ni, Pb, As, Ag, Mn, Mo, and Tl. Vanadium, Mn, Mo, and Tl can reach high concentrations in Cu-(Fe) sulfide phases (tens to hundreds of parts per million), suggesting a possible contribution from sedimentary sources in a basinal or marine environment. Microprobe analyses of pyrobitumen reveal high concentrations of S, Cl, Mn, Fe, Cu, and Ag, whereas μXRF maps show both homogeneous and irregular distribution of some of these elements at the grain scale (e.g., S and Cl). This suggests that the organic phase may have incorporated them through interaction with ore-forming hydrothermal fluids, acting as an important redox trap, and potentially providing certain components to the ore phases. Trace element ratios of Cu-(Fe) sulfides from El Soldado are compared with geochemical data from other ore deposits, including sediment-hosted copper deposits, iron oxide copper-gold (IOCG), iron oxide-apatite (IOA), and porphyry Cu systems. The Co/Ni (<1) and Mn/Fe (~0.00001–0.01) versus Mo (<100 ppm) relationships in pyrite, the In/Se ratio in chalcopyrite (~0.001–0.1), and the Ag/Bi ratio in bornite (~0.1–100) point to a potential sedimentary source for these metals in El Soldado, with possible contributions that include connate, basinal, marine, and meteoric fluids. Our data suggests that these geochemical signatures could help discriminate manto-type Cu mineralization, aiding in the exploration of new Cu resources in the Andean province and possibly elsewhere.