Assessing the role of bitumen in the formation of stratabound Cu-(Ag) deposits: Insights from the Lorena deposit, Las Luces district, northern Chile

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

“…In Lorena (Las Luces district), chalcocite is the dominant Cu sulfide and occurs as disseminated grains, aggregates and filling fractures, and amygdales (Figure D). Bitumen is often spatially associated with sulfides from both stages . In the Copiapó deposits, chalcopyrite and bornite partially replaced early pyrite and comprise the main Cu sulfide assemblage in Manto Cobrizo and La Culebra (Figure E, F).…”

“…At Lorena, the dominant hydrothermal alteration type is albitization with minor epidote, sericite, chlorite, and abundant late calcite. Zeolites, prehnite-pumpellyite, and minor chalcedony, as well as pyro­(bitumen), are also found filling vesicles and in the breccia matrix …”

“…In those deposits, sulfide mineralization has been attributed to two main consecutive stages, i.e., a pre-ore event followed by the main Cu ore stage. Pre-ore mineralization is characterized by a paragenesis of pyrite with magnetite, hematite and minor chalcopyrite, in close association with bitumen (Figure A–C). , The main Cu mineralization episode, on the other hand, consists mainly of chalcopyrite, bornite, chalcocite and minor sulfosalts (Figure D–F). In Lorena (Las Luces district), chalcocite is the dominant Cu sulfide and occurs as disseminated grains, aggregates and filling fractures, and amygdales (Figure D).…”

“…Stratabound or “Manto-type” Cu–(Ag) deposits, in particular, are currently the third largest source of copper in Chile after porphyry and IOCG deposits and are hosted within volcanic and volcano-sedimentary rocks of Jurassic to Cretaceous age located between Santiago and Antofagasta (Figure ). These deposits are divided into two groups, the first being hosted by volcanic rocks of Jurassic age in northern Chile (e.g., Mantos Blancos, Michilla, Las Luces), − and the second is hosted by volcanic and volcano-sedimentary rocks of Lower Cretaceous age in north-central Chile (e.g., El Soldado, Talcuna, Uchumí, La Culebra, Manto Cobrizo). − …”

“…Hypogene mineralization in stratabound Cu–(Ag) deposits comprises bornite, chalcocite, and chalcopyrite, with minor pyrite, covellite and digenite/djurleite. − In addition, variable amounts of “invisible” silver are contained in the Cu-bearing sulfides, and in some deposits Ag is recovered as a byproduct from Cu mining. , One of the most intriguing characteristics of Lower Cretaceous stratabound Cu–(Ag) deposits is the occurrence of variable amounts of solid hydrocarbons (bitumen) in close association with sulfide minerals. − ,− More recently, Herazo et al reported for the first time the presence of bitumen in the Lorena deposit, in Las Luces district, and provided insights on the role of solid hydrocarbons in the ore forming processes. In this study we focus on the geochemistry of pyrite from bitumen-bearing stratabound Cu–(Ag) deposits, northern Chile, in order to further constrain the conditions responsible for the early, pyrite-bearing pre-ore mineralization stage and evaluate the use of specific trace elements than could serve as an exploration tool for these deposits.…”