Mineralogy and geochemistry of stratabound copper - silver deposits hosted by the La Quinta Formation, northern Colombia

The “La Quinta” Formation is a volcano-sedimentary unit formed by continental, oxidized redbeds interlayered with subaerial felsic-to-mafic lava flows. This lithostratigraphic unit, which outcrops in the Perij. Range (northern Colombia), hosts two different stratabound Cu (±Ag) mineralization styles: basalt-hosted and sediment-hosted. The former style of mineralization comprises native copper and silver, along with minor amounts of Cu-sulfides hosted flow-top mafic lavas. Conversely, the latter mineralization style primarily consists of chalcopyrite, bornite, and digenite hosted by sedimentary strata. Electron Probe Micro-Analyzer (EPMA) analyses indicate that Cu-sulfides in both mineralization styles host silver. On the other hand, the assessment of mineral assemblages suggests a low-temperature and epigenetic origin for the Cu (±Ag) mineralization in both basaltic flows and sedimentary strata. Also, the identified hydrothermal alterations comprise four distinct stages occurring in the next chronological order: (1) Na-rich, (2) K-rich, (3) Ca-Si-rich, and (4) Ca-CO2-rich alterations. The hypogene ore minerals were observed paragenetically associated with the Ca-Si-rich alteration. In sediment-hosted deposits, mineralization is closely associated with charcoal clasts and possibly pyrobitumen. These forms of organic matter may serve as possible sources of reduced sulfur, which may trigger the precipitation of Cu-sulfides upon contact with mineralizing fluids. Additionally, a strong alteration involving Ca, Mg, Na, K, and Fe is evidenced by lithogeochemistry analyses. These analyses also suggest a depletion of K, Rb, and Cs contents in the samples with hydrothermal alteration, and also evidence no significant variation of Si and Rare Earth Elements (REE), implying these elements remained immobile during the different stages of hydrothermal alteration. Despite the above, most major and trace elements evidence a wide range of variation, implying that the alteration reactions were primarily controlled by lithological factors. Moreover, the supergene oxidation of hypogene ore minerals resulted in the formation of spionkopite, anilite, covellite, and other secondary Cu-bearing minerals. Overall, the characteristics of the Cu (±Ag) deposits hosted in the “La Quinta” Formation are similar to those described for Chilean Manto-type Cu deposits and Volcanic Redbed Cu deposits.