Abstract

The Coastal Cordillera metallogenic province of northern Chile comprises several styles of mineralization, including stratabound Cu-(Ag), iron oxide-copper- old (IOCG), iron oxide-apatite (IOA), and porphyry Cu deposits. Stratabound Cu-(Ag) or "Manto-type" deposits from the Coastal Cordillera of northern Chile formed during two main periods, i.e., the Late Jurassic and Early Cretaceous. These deposits are hosted in volcanic and volcano sedimentary rocks, and some of them are characterized by the presence of solid hydrocarbons (bitumen), which are closely associated with pyrite and Cu-(Fe) sulfides. Although pyrite is ubiquitous and in some cases abundant in this deposit type, its trace element content and mineralogical form of incorporation, i.e., nanoscale inclusions or solid solution, remains largely unknown. In this study, we focus on pyrite from bitumen-bearing stratabound Cu-(Ag) deposits from northern Chile to assess the formation conditions of these deposits by using a combination of electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Our results show that pyrite from stratabound Cu-(Ag) deposits hosts significant concentrations of Cu, As, Mn, Pb, T1, Co, Ni, V, and Mo that range from a few parts per million (ppm) to weight percent (wt %) levels. Among all elements analyzed, Cu, As, Mn, and TI are the most abundant, reaching concentrations up to similar to 2.8 (Cu), similar to 0.25 (As), similar to 2.4 (Mn), and similar to 0.09 wt % (TI). These elements occur dominantly in solid solution, although LA-ICP-MS depth profiles suggest the presence of micro- to nanosized mineral inclusions. Pyrite from stratabound Cu-(Ag) deposits is characterized by distinctively lower Co/Ni (similar to 0.1 to 10) and Co/Cu ratios (similar to 0.0001 to 10) and higher Ag/Co values (similar to 0.001 to 10) than pyrite from Cretaceous iron oxide-apatite (IOA) and IOCG deposits from the Coastal Cordillera metallogenic province. We interpret these variations as resulting from differences in physicochemical parameters of the pyrite-forming fluid, i.e., temperature, redox conditions, and source of metals/sulfur. Based on our data we conclude that pyrite from the studied stratabound Cu-(Ag) deposits formed at relatively low temperatures (100 to similar to 300 degrees C), reducing conditions, and with a strong sedimentary component as reflected in its trace element geochemistry. Therefore, our data supports the use of the trace element content of pyrite to fingerprint the formation conditions of ore mineralization in different deposit types with possible applications as a vectoring tool within the Coastal Cordillera metallogenic province and elsewhere.