Age and sources of gold mineralization in the Marmato mining district, NW Colombia: A Miocene-Pliocene epizonal gold deposit

摘要

Epigenetic gold mineralization occurs in the Marmato mining district, within the Calima Terrain of the Setentrional Andes, Colombia. Regional rocks associated with this mineralization include: graphite- and chlorite-schists of the Arquia Complex; metamorphosed during the Cretaceous, Miocene sandstones, shales and conglomerates of the Amaga Formation; as well as pyroclastic rocks (clasts of basalt, andesites and mafic lavas) and subvolcanic andesitic/dacitic bodies of the Combia Formation (9 to 6 Ma). The subvolcanic Marmato stock hosts mesothermal and epithermal low-sulfidation Au-Ag ores in the form of distensional veins, stockwork, and quartz veinlets within brecciate'd zones. Ore minerals are pyrite, sphalerite and galena with subordinate chalcopyrite, arsenopyrite, pyrrhotite, argentite and native gold/electrum.Sericitized plagioclase from a porphyry dacite yielded a K-Ar age of 5.6+-0.6 Ma, interpreted as the age of ore deposition. This is in close agreement with the age of reactivation of the Cauca-Romeral Fault System (5.6+-0.4 Ma), which bounds the Calima Terrain. A porphyry andesite-dacite (6.7+-0.1 Ma), hosting the Au-Ag veins, shows a measured ~(87)Sr/~(86)Sr between 0.70440 and 0.70460, epsilon_(Nd) between+2.2 and+3.2 and~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pbratios of 18.964 to 19.028; 15.561 to 15.570; and 38.640 to 38.745, respectively. The ~(87)Sr/~(86)Sr and epsilon_(Nd) values of rocks from the Arquia Group range from 0.70431 to 0.73511 and -12.91 to +10.0, respectively, whereas the corresponding Pb isotopic ratios (~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb) range from 18.948 to 19.652; 15.564 to 15.702; and 38.640 to 38.885, respectively. ~(87)Sr/~(86)Sr and epsilon_(Nd) values obtained on sulfides from the gold quartz veins, which occur at shallow and intermediate levels, range from 0.70500 to 0.71210 and from -1.11 to +2.40. In the deepest veins, epsilon_(Nd) values lie between + 1.25 and +3.28 and the ~(87)Sr/~(86)Sr of calcite and pyrite fall between 0.70444 and 0.70930. The ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)pb/~(204)pb rations of all mineralization are in the ranges 18.970 to 19.258; 15.605 to 15.726 and 38.813 to 39.208, respectively. Carbonates have an average ~(87)Sr/~(86)Sr ratio of 0.70445, which is within the range of values measured in the host dacite. The Sr isotopic data indicate that carbonic fluids have a restricted hydrothermal circulation within the host igneous body, while the Sr, Pb and Nd isotopic compositions of the sulfides suggest that the fluids not only circulated within the Marmato stock, but also throughout the Arquia Complex, inferring that these rocks offer a potential target for mineral exploration. Based on geological and geochronological evidence, the epizonal Marmato gold ores formed during the Miocene to Pliocene, as a result of cooling of the Marmato stock and reactivation along a crustal-scale fault zone related to thermal processes in an accretionary oceanic-continental plate orogen.