A DISCUSSION ON THE TIMING AND TEMPERATURE OF COPPER AND MOLYBDENUM PRECIPITATION IN PORPHYRY DEPOSITS

The timing and temperature at which copper and molybdenum sulfide deposition occurs in porphyry deposits remain a subject of debate. Observations of cathodoluminescence (CL) images of quartz veins have revealed low-temperature dull-CL quartz commonly in contact with Cu-sulfides. We provide new cathodoluminescence (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 (~10 to 2 km). We show several examples of copper and molybdenum sulfides that are not entirely contained in dull-CL quartz and contact lucent-CL quartz or are observed within hydrothermal K-feldspar-muscovite-biotite early halo wall-rock replacements with no quartz. Dull-CL quartz in microfractures is observed throughout, cross-cutting all types of older lucent-CL quartz, including different generations of cross-cutting copper and/or molybdenite-bearing veins as well as barren veins and quartz phenocrysts in porphyry dikes. Therefore, the dull-CL quartz has no particular spatial or genetic affinity with copper sulfides and molybdenite. We propose that the bulk of the copper was introduced and precipitated at high temperature in stability with K-silicate alteration and the formation of lucent-CL quartz in A-veins and younger B-veins or hosted in early halo alteration with minor quartz formed at similar timing than A-veins. Much of the molybdenum was introduced and precipitated with discrete pulses of molybdenite-quartz veins that post-date Cu-bearing A-veins and early halos and to a lesser degree with B-veins that may carry both Cu and Mo. Copper precipitation and local copper remobilization might have continued during C-veins formation and the precipitation of dull-CL quartz following or at the waning stages of K-silicate alteration. Much of the dull-CL quartz introduced at this stage precipitated in microfractures created by both thermal contraction of older lucent-CL quartz during sustained rock cooling below the 573°C alpha-beta phase transition of quartz and by quartz dissolution due to retrograde solubility at the end of K-silicate alteration.