GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 290-11
Presentation Time: 4:35 PM


RASMUSSEN, Jan C. and KEITH, Stanley B., MagmaChem Exploration, PO Box 950, Sonoita, AZ 85637

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.