The Nuevo Chaquiro Cu-Au-(Mo) Porphyry Deposit, Middle Cauca Belt, Colombia: Geology, Alteration, Mineralization

Abstract

Nuevo Chaquiro is a newly discovered blind porphyry Cu-Au-(Mo) deposit located in the Middle Cauca belt, Colombia, approximately 60 km south-southeast of Medellin. The most recent resource estimate was stated as 566 M tonnes (t) averaging 0.64% Cu, 0.31 g/t Au, 4.34 g/t Ag, and 127 ppm Mo. The geology of Nuevo Chaquiro consists of a Miocene volcanic sequence of andesitic tuffs, agglomerates, and flows (Combia Formation) intruded by small stocks and dikes of diorite and quartz diorite also of Miocene age. Sparsely occurring outcrops at Nuevo Chaquiro consist of a 1.2 × 0.8 km zone of pervasive sericitic alteration with locally well developed quartz-Fe oxide stockworks. The sericitic alteration grades downward into sericite-chlorite alteration and then predominantly potassic (biotite-magnetite) alteration at depths of 250 to 400 m. Calcic-potassic alteration, containing actinolite and epidote, occurs at greater depths. A high-grade sheeted vein complex occurs immediately adjacent to the upper contact of an Early quartz diorite intrusion. Quartz veinlet density in this area reaches up to 80% quartz by volume, accompanied by up to 5 vol % chalcopyrite. Later Intermineral porphyries created a second orebody slightly to the west but contiguous with the Early orebody. Here Cu-Au-Mo mineralization in an inverted bowl shape is associated with potassic alteration that manifests itself as secondary flooding of the tuff wall rock matrix by hydrothermal biotite, accompanied by disseminations and hair veinlets of chalcopyrite, magnetite, pyrite, and molybdenite. The porphyry system is also accompanied by intermediate-sulfidation carbonate–quartz-base metal veins, typically with sericitic alteration selvages, which occur on the upper western flank of the deposit. At Nuevo Chaquiro, multiple intrusions of Early and Intermineral porphyry contain calcic-potassic and potassic alteration, with a later overprint by sericite-chlorite alteration centered on intrusion apexes. The amount of sericite-chlorite overprinting varies with each intrusion. Collectively, these intrusions are all overprinted by a single, later sercitic (phyllic) alteration event that is superimposed as a broad cap upon the entire system. There is a consistent paragenesis for the alteration/mineralization associated with each intrusion, suggesting a relatively stable underlying shallow magma chamber that fluxed geochemically similar pulses of magmatic/hydrothermal fluid with each porphyry. In many aspects, Nuevo Chaquiro is a “typical” Au-rich porphyry copper system. However, the contrast between the mineralization styles between the Early and Intermineral orebodies as well the timing differences in copper deposition suggest that differences in porphyry mineralization styles also occur at Nuevo Chaquiro.