Abstract

Porphyry-style Cu (Au, Mo) mineralization is widely associated with igneous suites with Sr/Y > 40 and low heavy rare earth element (HREE) content. This geochemical signature is commonly attributed to garnet or amphibole fractionation in the lower crust where mantle derived melts evolve into intermediate composition arc suites. However, the presence of a residuum related to mineralized porphyry systems where amphibole and garnet are stable is generally inferred from geochemistry and has nowhere been documented directly. We here report on two occurrences of late Miocene garnet-bearing granodiorite porphyry at the Tesorito Au-Cu prospect in the Quinchía district and El Poma Au-Cu prospect in the Middle Cauca Au-Cu porphyry belt in Colombia. These rocks provide proof that garnet was stable in the lower crust at the time of porphyry mineralization. At both studied sites, garnet-bearing I-type granodiorite porphyries were intruded by intramineralization garnet-free plagioclase phyric granodiorite porphyries that were cut by quartz and K-feldspar veins and affected by biotite alteration. Garnet-bearing porphyries predate intramineralization porphyries by 0.4 to 2 m.y.

Garnet occurs as unzoned grains, commonly included in large plagioclase phenocrysts, that in some cases also contain inclusions of igneous epidote; the latter indicating pressures of >1 GPa for early crystallized phases. Large plagioclase phenocrysts exhibit two growth stages separated by a resorption boundary. High-Al amphibole phenocrysts as well as titanite and magnetite formed after the first stage but prior to the second stage of plagioclase growth at pressures of ca. 0.9 GPa.

Whole-rock geochemical compositions of garnet-bearing porphyries are consistent with only limited garnet fractionation whereas some of the intramineralization porphyries have Y <11 ppm and Sr/Y >40, which is, together with other trace element indicators, taken as evidence for significant garnet fraction at depth during porphyry mineralization. HREE content of zircon in garnet-bearing porphyries are higher than that of intramineralization porphyries, which supports the interpretation that garnet fractionation in the lower crust was significant during the formation of the intramineralization porphyries.

The phase relationships and geochemical data reflect a geodynamic change from an environment that permitted rapid ascent of oxidized garnet-bearing porphyries prior to Au-Cu mineralization to a regime that favored melt evolution in the lower crust where garnet was a residual phase and at the same time allowed the formation of large mid- to upper-crustal magma chambers. The latter are widely inferred to be essential for porphyry mineralization to occur.

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