The age and petrogenesis of reduced to weakly oxidized porphyry intrusions at the Marmato gold deposit, Colombia

Abstract

The Marmato gold deposit is a hybrid between a porphyry gold and a reduced intrusion-hosted gold deposit with epithermal features at shallow levels. It is adjacent to the contemporaneous Aguas Claras porphyry gold deposit. They are located in one of the premier gold-copper belts of Colombia, the Middle Cauca Belt (MCB), and associated with a Late Miocene magmatic arc. The Marmato Porphyry Group (MPG), defined as the Marmato and Aguas Claras Porphyry intrusions, was dated at 6.58 ± 0.07 Ma to 5.74 ± 0.14 Ma by U-Pb LA-ICP-MS in zircon. The MPG is composed of andesitic-dacitic porphyry intrusions which belong to the I-type granite series and the reduced or weakly oxidized ilmenite-series suites, with a calc-alkaline to high K calc-alkaline signature. 87Sr/86Sr ratios between 0.70435 and 0.70513 and 143Nd/144Nd values of 0.512688 to 0.512898 from the MPG suggest partial melting of the mantle wedge as source of the magma. An additional contribution by partial melting involving garnet in the deep crust is suggested by a (Dy/Yb)N ratio of 1 to 1.5 and flat REE diagrams. Crystal fractionation during magma ascent is indicated by high Sr/Y >55 ratios with suppression of plagioclase on the liquidus until the appearance of amphibole shown by EuN/Eu*<1. The presence of ilmenite indicates relatively reduced or weakly oxidized conditions for the magma, probably generated by contamination with carbon-bearing crust in graphitic schists of the Arquía Complex. In addition, high manganese values in the intrusions (~1010 ppm) and in ilmenite (6.2–14.7 wt% Mn) suggest contamination by mafic to ultramafic rocks during magma ascent. The relatively reduced or weakly oxidized conditions created by the degree of interaction of the ascending magma with the crust produced variations in the amount of Mn-ilmenite and magnetite in the MPG porphyries. These conditions influenced the fugacity of the resulting rocks and magmatic-hydrothermal fluids involved in the genesis of different types of deposit in the adjacent Marmato and Aguas Claras gold deposits. This supports a model of interaction in the upper crust rather than a fundamental difference in the initial magma. Compositional similarities of the MPG with intrusions related to other gold deposits in the MCB reflect similar process for the generation of productive intrusions in the Late Miocene. This study highlights the important of the MPG as an example of the features of magmas capable of producing Au deposits in the MCB during the Late Miocene, such as high H2O content, high Sr/Y ratio, I-type granite series and reduced or weakly oxidized ilmenite-series granitoid suite. These features result in variable magnetic susceptibilities, which are important criteria for the interpretation of magnetic surveys for gold exploration in the Middle Cauca Belt.