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Heterogeneous melt and hypersaline liquid inclusions in shallow porphyry type mineralization as markers of the magmatic-hydrothermal transition (Cerro de Pasco district, Peru)

Published inChemical geology, vol. 4467, p. 93-116
Publication date2016
Abstract

Recently identified occurrences of porphyry-style mineralization evidence the link of the world's second largest known epithermal base metal Cerro de Pasco deposit (Peru) to a porphyry system emplaced at depth. They consist of (i) quartz-monzonite dykes and (ii) the south-western part of the large diatreme-dome complex adjacent to the main ore bodies of Cerro de Pasco, and (iii) stockwork of banded quartz-magnetite-chalcopyrite-(pyrite) porphyry-type veinlets crosscutting trachyte porphyritic intrusion cropping out at surface in the central part of the diatreme-dome complex. The latter porphyry-type mineralization observed at the same erosion level as the main epithermal base metal carbonate-replacement ore bodies is the subject of this work. Geological constraints indicate a shallow emplacement level (depth < 1 km, P < 270 bar), implying rather unusual low-pressure and high-temperature environment for the formation of porphyry-style mineralization. The banded porphyry-type veinlets record a multiphase history of formation with two successive high-temperature (N600 °C) stages, followed by a lower-temperature (b350 °C) stage.More than 90% of the quartz in veinlets precipitates during the first two high-temperature stages. Stage 1 is characterized by the entrapment in hydrothermal quartz of inclusions containing variable proportions of both silicate melt and metal-rich hypersaline (N90 wt% NaCl eq.) liquid, hereafter referred to as heterogeneous silicate melt inclusions (HSMIs). The latter are rarely described in porphyry-type mineralization. We suggest that during stage 1, the inclusions result from heterogeneous entrapment of an evolved hydrous rhyolitic melt mixed with a hypersaline fluid phase at low pressure (270 bar) and high temperature (N600 °C). The stage 2 is marked by the entrapment of metaland sulfur-rich hypersaline liquid inclusions, with salinity around 70 wt% NaCl eq., originated from the adiabatic ascent of magmatic hypersaline fluids transferred fromdeeper parts of the system. The lower-temperature stage 3 is characterized by an important temperature drop from N600 °C to b350 °C as revealed bymicrothermometry of aqueous two-phase liquid-vapor (L-V) inclusions. Quartz textures revealed by SEM-CL imaging allowascribing the sulfide precipitation to the low-temperaturemineralization stage 3. In-situ SIMS 18O/16O isotope analyses of quartz across the veinlets are indicative of a magmatic signature of the fluids during the first two stages; while quartz from stage 3 has oxygen isotopic compositions suggestive of minor contribution of meteoric waters to a predominantly magmatic aqueous fluid (~10 vol.% of meteoric input), which probably triggered Cu-Fe sulfide precipitation in the stockwork. High metal and sulfur contents of HSMIs and hypersaline liquid inclusions determined by LA-ICP-MS are interpreted to represent the fluid composition prior to the main sulfide precipitation event. The similar Pb-Zn ratio of the bulk ore extracted from the epithermal ore bodies at Cerro de Pasco and the HSMIs and hypersaline liquid inclusions suggest a common source of the fluids associated with the different mineralization styles at Cerro de Pasco.

Citation (ISO format)
ROTTIER, Bertrand et al. Heterogeneous melt and hypersaline liquid inclusions in shallow porphyry type mineralization as markers of the magmatic-hydrothermal transition (Cerro de Pasco district, Peru). In: Chemical geology, 2016, vol. 4467, p. 93–116. doi: 10.1016/j.chemgeo.2016.10.032
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