Trace Element Geochemistry of Pyrite from Bitumen-Bearing Stratabound Cu–(Ag) Deposits, Northern Chile

Abstract

The Coastal Cordillera metallogenic province of northern Chile comprises several styles of mineralization, including stratabound Cu–(Ag), iron oxide–copper–gold (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, Tl, 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 Tl are the most abundant, reaching concentrations up to ∼2.8 (Cu), ∼0.25 (As), ∼2.4 (Mn), and ∼0.09 wt % (Tl). 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 (∼0.1 to 10) and Co/Cu ratios (∼0.0001 to 10) and higher Ag/Co values (∼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 ∼300 °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.

Auriferous pyrite formed by episodic fluid inputs in the Akeshi and Kasuga high-sulfidation deposits, Southern Kyushu, Japan

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Dynamic Interplay: Unveiling the Biosphere-Geosphere Nexus in Carbon Cycling

“…Geologists have long noticed the association of "black shales" with powerful geochemical anomalies represented by compounds of rare earths and metals (Mo, V, Re, Hg, Ni, etc.) (Yudovich, Ketris, 2015;Herazo et al, 2021). The question is, what connects these completely independent biosphere processes: "black shales" formation and geochemical association.…”

Pyrite geochemistry in a porphyry-skarn Cu (Au) system and implications for ore formation and prospecting: Perspective from Xinqiao deposit, Eastern China

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