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

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 hyper...

Geothermal energy and ore-forming potential of 600 °C mid-ocean-ridge hydrothermal fluids

“…S2) show that two and occasionally three immiscible fluids were trapped simultaneously within individual fractures upon healing: vapor-rich and brine end members, sometimes with melt inclusions. Such multiphase entrapment is common in hydrothermal systems (e.g., Audétat et al, 2008;Kodera et al, 2014;Rottier et al, 2016;Mernagh and Mavrogenes, 2019), as verified by experimental studies (Bodnar et al, 1985) and by observations of fluid inclusions in boiling geothermal systems (e.g., Belkin et al, 1985). The variation in vapor:solid proportions in the secondary FIAs is therefore ascribed to accidental physical mixing between the vaporrich and brine end member fluids prior to inclusion sealing.…”

Textural, fluid inclusion, and in-situ oxygen isotope studies of quartz: Constraints on vein formation, disequilibrium fractionation, and gold precipitation at the Bilihe gold deposit, Inner Mongolia, China

“…The occurrence of crystallized SMI is not common in porphyry Au deposits. Their formation requires high temperatures but relatively low pressure to promote the exsolution of volatiles (Harris et al, 2003;Rottier et al, 2016). The UST and dendritic quartz at Bilihe have experienced heterogeneous entrapment of melt and vapor phases, thus making the melt inclusions in them difficult to homogenize (Harris et al, 2003;Rottier et al, 2016) even at high temperatures (> 950°C) as measured by Jiang (2011).…”

“…DOI: https://doi.org/10.2138/am-2021-7823. http://www.minsocam.org/ (Taylor, 1974;Fekete et al 2016;Rottier et al, 2016Rottier et al, , 2018Li et al, 2018;Cernushi et al, 2018). Besides, the volumes of QA3 and QB3 are too small and only occur as a minor part of A and banded veins respectively, with no separated veins formed by them.…”

Silicate-replacive high sulfidation massive sulfide orebodies in a porphyry Cu-Au system: Bor, Serbia

“…The most important example is Butte (Montana) where a swarm of major open fractures was filled by high-sulfidation sulfide minerals forming the Main Stage Veins (Weed 1912;Meyer et al 1968). At Cerro de Pasco, polymetallic Zn-Pb-Cu ores with massive pyrite are formed at the contact between a diatreme and the adjacent limestone by carbonate replacement, which caused the neutralization of low-pH fluids (Baumgartner et al 2008;Rottier et al 2016). More enigmatic deposits are dominated by massive sulfide orebodies of irregular, tabular, or pipe-like shape lacking obvious evidence of open space creation or reaction with readily dissolvable carbonate host rocks.…”