A Reevaluation of the Timing and Temperature of Copper and Molybdenum Precipitation in Porphyry Deposits

Abstract

Abstract The timing and temperature at which copper-iron and molybdenum sulfide deposition occurs in porphyry deposits remain controversial. Petrologic estimates indicate that veins and wall-rock alteration zones containing copper-iron sulfides form in a wide temperature range from ~350° to 650°C. Most sulfides are hosted in potassium(K)-silicate–altered rock and quartz A veins or in early-halo alteration selvages formed above ~450°C. In contrast, cathodoluminescence (CL) imaging of A veins indicates that copper-iron sulfides are contained within a primary lucent (bright and gray)-CL quartz and are crosscut by microfractures filled with younger dull (dark and medium-gray)-CL quartz in direct contact with copper-iron sulfides. These observations have been interpreted as supporting late copper-iron sulfide introduction together with dull-CL quartz at moderate temperatures of ~300° to 450°C, based on fluid inclusion estimates. We provide new CL, QEMSCAN, and petrographic data and images of vein quartz as well as petrologic data of altered wall rock from Haquira East (Peru), Encuentro (Chile), and Batu Hijau (Indonesia) porphyry deposits, which were formed at conditions ranging from deep to shallow (~2–10 km). At all three deposits, dull-CL quartz in microfractures is ubiquitously observed crosscutting all generations of high-temperature lucent-CL quartz veins. Each lucent-CL vein type hosts distinct sulfide populations, crosscuts the others, and coexists in space within the copper and molybdenum ore zones. Within this ore zone, the dull-CL quartz only contains copper-iron sulfides where it transects old A veins and early halos, molybdenite where it transects young molybdenite-bearing quartz veins, and both copper-iron sulfides and molybdenite in younger B veins. Furthermore, where the dull-CL quartz crosscuts igneous or barren (deep) quartz veins, it typically lacks copper and molybdenum. Therefore, dull-CL quartz has no particular spatial or genetic affinity with copper-iron sulfides or molybdenite. We propose that copper was introduced and precipitated at high temperatures in stability with K-silicate alteration. In shallow porphyry deposits, most copper was introduced with lucent-CL quartz in A veins, likely formed via adiabatic decompression from magmatic lithostatic to hydrostatic conditions at ~450° to 600°C. In deep deposits, most copper is introduced with quartz-poor early halos, likely formed at a temperature range similar to that of A veins but during an early stage of retrograde silica solubility. The inferred timing and temperature of copper precipitation are consistent with available solubility experiments for copper-bearing solutions that suggest copper precipitation may start at a high temperature of ~600°C, and ~90% precipitates before it cools down to ~400°C. Much of the molybdenum is introduced and precipitated with discrete pulses of molybdenite-bearing quartz veins that crosscut and postdate copper-bearing A veins and early halos and, to a lesser degree, with B veins that may carry both copper and molybdenum. Whereas molybdenite-bearing and barren (deep) quartz veins form at relatively high temperatures of ~550° to 650°C, copper-molybdenum–bearing B veins likely form at lower temperatures near ~500°C. Copper precipitation and local copper remobilization from older veins and halos continued during the formation of copper-iron sulfide veinlets, named C veins, and during the precipitation of dull-CL quartz following K-silicate alteration. C veins and even younger pyrite-rich D veins may have chlorite or sericite selvages and are composed of dull-CL quartz that formed at ~450° and 300° to 450°C, respectively. Microfractures form through all lucent-CL quartz veins because of the thermal contraction of high-temperature quartz at the onset of sustained cooling after K-silicate alteration has ceased. The fluid that migrated through these microfractures was initially in retrograde silica solubility, which causes dissolution and corrosion of the older lucent-CL quartz. The formation of C veins may overlap in time with the initial stage. At a later stage and temperatures below <450°C, the fluid precipitates dull-CL quartz in microfractures and dissolution zones within older lucent-CL quartz. In copper-iron sulfide-bearing A and B veins and molybdenite-bearing quartz veins, corroded lucent-CL quartz and the younger dull-CL quartz infill can often be observed in contact with older sulfides because quartz sulfide grain boundaries are preexisting discontinuities, and they are preferentially opened during volume contraction. Collectively, these observations and estimates are consistent with silicate phase petrology and numerous observations that most copper-iron sulfides precipitate in K-silicate alteration zones or in early halos with K-feldspar-muscovite-biotite assemblages.