T290 Porphyry Copper and Related Mineral Deposits of Arizona, the Basin and Range Province, and Beyond

Wednesday, 25 September 2019
1:30 PM - 5:30 PM
Room 126ABC, North Building

PLUTONIC AND HYDROTHERMAL MINERAL STAGES IN METALUMINOUS CALC-ALKALIC HYDROUS OXIDIZED PORPHYRY CU-MO-AG, MINERAL SYSTEMS

Paper number: T290-16
Program: Technical Sessions
Time: 4:35 PM – 4:50 PM

Authors

Abstract

Hydrothermal mineral zoning in porphyry copper systems reveals that alteration and mineral zoning is not related to a single concentrically zoned alteration zoning event as portrayed in the classic Lowell-Guilbert model. Rather, when integrated to the associated metaluminous calc-alkalic hydrous oxidized epizonal intrusive sequences, each igneous rock system exhibits its own hydrothermal fluid release that is systematically and consistently cross-cut by other compositionally distinct younger rock and hydrothermal systems. Fluid releases from each rock system induce freezing and, as a result, little time-equivalent volcanism is present. The following generic magmato-hydrothermal, fractionation/unmixing sequence has been repeated in some 50 case histories in N and S America: Stage 1 hbld gabbro unmixes less mafic Stage 2 magma (hbld-mag-sphene qtz diorite) which exsolves Stage 2 [Fe(Cu)] hydrothermal fluid that forms proximal Fe-Cu skarns and widespread peripheral propylitic alteration. Mineral assemblages include: proximal mag (cpy-mo) with more distal, and Fe-rich chl-ep-cal, and local distal Ag-argentite-cal veins/pipes. Stage 3 bt-hbld granodiorite releases fluids which form the familiar porphyry Cu-Mo(Re)-Ag deposits. Mineral assemblages are cpy-mo-K feldspar-phlogopite potassic alteration in intrusions and gar-cpy in skarns. Py-qtz-ser typically form a fringing phyllic assemblage with chalcopyrite abundance maximized at the pH transition from potassic to phyllic alteration. Decompression of Stage 3 bt hbld granodiorite unmixes a Stage 4 bt granodiorite porphyry commonly associated with the Stage 3 Cu-Mo zones. Stage 3 bt granodiorite also exsolves a Stage 4 [CuMoAg(PbZnAuAsMn(U,F,V)] hydrothermal fluid. Mineral assemblages include: early qtz-mus-mo-py followed by cpy-bn-en-qtz-mus followed by tn-gn-sph-argentite-py-qtz-mus followed by alabandite-rhod-Mn oxides-bar-cal followed by minor Au-py-qtz-Ag-argentite-cal. When Stage 4 magma (bt granodiorite) decompresses, a Stage 5 qtz-feld porphyry magma is produced that is spatially associated with Stage 4 polymetallic metallization. Finally, Stage 5 qtz-feld porphyry exsolves a 'last gasp' barren py-alun-clay low pH vapor assemblage which is common in 'lithocaps' of the now spent porphyry sequence.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-340296

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