The Marmato gold deposit consists of a structurally controlled epithermal vein system with a mineral assemblage dominated by pyrite, arsenopyrite, black iron (Fe) rich sphalerite, pyrrhotite, chalcopyrite and electrum in the Upper Zone (UZ), and a mesothermal veinlet system with a mineral assemblage dominated by pyrrhotite, chalcopyrite, bismuth minerals and visible gold in the MDZ.
The MDZ mineralization consists of a network of thin, less than 5 centimeters, sulfide veinlets, mainly pyrrhotite+chalcopyrite, hosted in weak argillic and deeper potassic alteration which is related to a previous event and rimmed by a thin sodium-calcitic alteration halo, which is related to the mineralization. Recent geological reports on MDZ (Sillitoe, 2019) concluded:
• Gold grade distribution in the Zona Baja (MDZ) mineralized orebody is unrelated to the presence of distinct porphyry phases and is entirely dependent on the intensity of structurally localized veinlets;
• Potassic alteration, represented chiefly by biotite, is progressively better preserved at depth in the Zona Baja, raising the possibility that early potassic alteration could also be gold bearing, but further work is required to confirm this theory;
• Gold distribution appears to be exclusively a product of veinlet intensity and orientation related to structural controls during orogenesis. The veinlets responsible for much of the Zona Baja gold are those containing quartz, pyrrhotite and tra ........
