Modeling of the large Miocene epithermal and porphyry gold deposits of Colombia using Monte Carlo simulations

Abstract

The Cauca metallogenic belt is an inter-Andean area located along the Cauca-Romeral fault zone, which is made up of a group of twelve Miocene magmatic-hydrothermal Au-Ag-Cu mineral deposits positioned between the Western and Central Cordillera of Colombia. In addition to being a widely developed region in exploration and exploitation with known Au endowments of over 1,700 tons, this area represents an exceptional metallogenetic laboratory for modeling a typical Andean-type mineralization from a calc-alkaline source with high Sr/Y ratios. Efficiency processes such as ion and halogen transport, oxygen fugacity and sulfur content have been extensively studied with a geochemical approach; however, the quantification and modeling of these efficiency processes currently debated. Through multiple iterations using Monte Carlo simulations (N > 2 million), the modeled reservoirs corresponding to the upper crust, using a high flux of dacitic magma of 0.007 km3/year and efficiencies of 1 – 2 %, showed that gold endowments bear heavily with magmatic and hydrothermal Au deposits in the Cauca Metallogenic Belt. Outcomes including Au endowments up to 1,000 tons can be obtained for reservoirs below 400 km3 of hydrous melt in brief mineralization intervals between 40 to 120 ka and for volumes of 400 to 800 km3 in a 120 to 200 ka window. In contrast, the hypothetical reservoirs for the lower and middle crust, through a basaltic calc-alkaline magma flux between 0.0007 and 0.0011 km3/year, an efficiency of ~0.7%, and pressures below 5 kbar, showed sufficient available content of exsolvable H2O and gold to feed the reservoirs in the upper crust or to generate gold deposits from them, but in longer time intervals (>1 Ma).