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

Abstract

Stratabound Cu-(Ag) deposits located in north-central Chile have been grouped into two NS parallel belts according to the age of their volcanic and volcanoclastic host rocks. Deposits from the Cretaceous belt are characterized by an unusual assemblage of Cu sulfides with migrated hydrocarbons, whereas the presence of organic matter has not yet been recognized in deposits from the Jurassic belt. Here we report the first evidence of pyrobitumen in a Jurassic stratabound Cu-(Ag) deposit and evaluate its role in ore formation. We present a micro-analytical study of pyrobitumen associated with Cu-(Fe) sulfides in the Lorena deposit, located in northern Chile. Our approach involved a combination of surface and drill core sample characterization, (FE)-SEM observations, EMPA and mu-XRF mapping of pyrobitumen. In addition, we determined the speciation of sulfur in pyrobitumen by using synchrotron mu-XANES. Two main events of hypogene mineralization were identified at Lorena deposit, i.e., a pre-ore and a main Cu ore stage. The pre-ore stage is characterized by globularly-shaped pyrobitumen and minor pyrite, and a widespread albite alteration of the andesite host rock. Pre-ore pyrite was replaced by chalcopyrite, bornite and chalcocite, providing evidence for a late Cu inception. In the main Cu ore stage, chalcocite is often associated with sharp-edged, angular pyrobitumen, most likely indicating re mobilization or a later pulse of hydrocarbons. EMPA data show that pyrobitumen at Lorena is enriched in Fe (0.02-1.74 wt%), Cu (0.02-0.87 wt%), S (0.01-0.43 wt%) and Cl (0.01-0.4 wt%). Other elements detected include V, Co, Ni, Cr, Sb, As, Ag and Au. Both Ag and Au were only detected in a few spots, with Au reaching up to 600 ppm. Desktop and synchrotron mu-XRF mapping shows that S is homogeneously distributed within pyrobitumen, with lower S concentrations observed at grain rims. On the other hand, EMPA-WDS X-ray maps show that Cu precipitated on pyrobitumen grain surfaces, filling fractures and forming veinlets. In addition, textures showing the presence of micro-to nano-sized pyrobitumen inclusions within both pre-ore pyrite and main ore chalcocite grains point, unequivocally, to the interaction of hydrothermal fluids with hydrocarbons during the formation of this deposit. The high S concentrations measured in pyrobitumen and the presence of organosulfur compounds detected by mu-XANES can be attributed to an organic source. The pre-ore assemblage is interpreted as formed from a petroleum-aqueous fluid (connate waters?) mixture that migrated from the sedimentary source rock into the Jurassic host rocks where pyrite and pyrobitumen precipitated in the andesite. Late Cu-bearing hydrothermal fluids interacted with this pyrobitumen-pyrite assemblage to form Cu sulfides, either by replacement of pyrite or precipitation. We conclude that some of the S contained in pyrobitumen could have been released during interaction with the Cu-bearing hydrothermal fluid contributing, at least partially, to the S budget required for sulfide mineralization in the main ore stage. Our results provide evidence supporting pyrobitumen-fluid interaction as a key process in the formation of the Lorena deposit and possibly other stratabound Cu-(Ag) deposits in Chile.