Zoned Base Metal Mineralization in a Porphyry System: Origin and Evolution of Mineralizing Fluids in the Morococha District, Peru

Abstract

Porphyry-related base metal vein and replacement mineralization (i.e., Cordilleran polymetallic mineralization) in the Morococha district, central Peru, is part of a large magmatic-hydrothermal system associated with the emplacement of several late Miocene porphyry intrusions and formation of important Cu-Mo mineralization. Zn-Pb-Ag-Cu veins overprint the giant Toromocho porphyry Cu-Mo deposit in the center of the district and display a typical concentric base metal zonation (Cu → Zn, Pb → Ag) covering an area of approximately 50km 2 .A detailed fluid inclusion study supports the hypothesis that base metal mineralization precipitated from cooled and evolved metal-rich, intermediate-density porphyry-type fluids. In early stages of Cordilleran base metal vein formation, fluid inclusions have low salinities of ~2 to 5 wt % NaCl equiv, CO2 contents of 3 to 10mol %, and homogenization temperatures (Th) of 380° to 340°C. They are similar to intermediate-density fluid inclusions trapped in a milky quartz vein predating Cordilleran polymetallic mineralization, with similar low salinities (3.0-3.8 wt % NaCl equiv) and low CO2 contents (6.5-8 mol %), but higher Th of ~420° to 410°C. During cooling of the intermediate-density fluids from 400° to 300°C, the lithostatic pressure regime changed to a hydrostatic one. The fluids underwent pressure drop as well as phase separation (i.e., unmixing) and lost most of their CO2. They acquired moderate salinities, in some cases intermediate (~up to 16 wt % NaCl) to brine compositions. However, the bulk of the magmatic fluid retained low salinity while it continued to cool under open-system conditions and precipitated tennantite-tetrahedrite, chalcopyrite, enargite, sphalerite, and galena. Upon cooling below 270°C, the fluids deposited abundant rhodochrosite and quartz, while following the boiling curve toward lower P-T conditions. These data record an evolution of mineral precipitation from deep (minimum depth of 2-1.5 km) to shallow environments (300-800 m). Oxygen, hydrogen, and carbon stable isotope data indicate that the hydrothermal fluids have a dominantly magmatic signature and were diluted by meteoric waters during the carbonate stage.Copper (5,000-18,000 µg/g), sulfur (up to 12,000 µg/g), and iron (2,100-6,000 µg/g) concentrations in the intermediate-density fluid inclusions in the milky quartz veins are approximately 5 to 10 times higher than in intermediate-density inclusions of the early Cordilleran base metal veins. The base metals Zn, Pb, and Mn have comparable concentrations between 100 and 1,000 µg/g for both types of fluid. These findings suggest that the fluids identified in Cordilleran polymetallic veins are compositionally similar to the porphyry-type fluids and could have derived from the latter after precipitation of Cu-and Fe-bearing sulfides in a deeper porphyry environment.The new data explain the commonly observed base and precious metal zonation patterns encountered in porphyry-centered districts (e.g., Bingham, Butte) and show that both porphyry and polymet...

Mineralogy, Fluid Inclusions, and Sulfur Isotopes of the Huanzala Deposits, Peru: Early Skarn and Late Polymetallic Replacement Style Mineralizations

“…Ohmoto, 1986). Similar results have been reported for sulfides from other porphyry-related polymetallic deposits such as the Main Stage veins at Butte (−3.7 to +4.8‰; Field et al, 2005), Pasto Bueno (−6.27 to +3.92‰; Landis & Rye, 1974), Cerro de Pasco (−3.7 to +4.2‰; Baumgartner et al, 2008), and Morococha (+1 to +4‰; Catchpole et al, 2015); the sulfur in each of these deposits has been attributed to a magmatic source. In these porphyry-related hydrothermal systems, most sulfide sulfur likely shares its origin with magmatic fluids that transport aqueous or gaseous species of sulfur to ore deposition sites.…”

Germanium enrichment in supergene settings: evidence from the Cristal nonsulfide Zn prospect, Bongará district, northern Peru

“…In addition, the porphyry-related epithermal polymetallic (BCordilleran^) deposits of Cerro de Pasco (Baumgartner and Fontboté 2008;Baumgartner et al 2009;Rottier et al 2016), San Gregorio in the Colquijirca district (Bendezú and Fontboté 2009) and the mines located in the Yauli Dome (e.g., San Cristobal, Carahuacra and Morococha; Beuchat et al 2004;Catchpole et al 2015) are also hosted by the Pucará Group. The Zn deposits of the Bongará district are considered to be stratabound MVT sulfide deposits, locally altered to nonsulfides after weathering processes (Fig.…”

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

“…Spatial and temporal link between porphyry-type mineralization and epithermal base metal deposits (i.e., Cordilleran polymetallic deposits) is recognized in several telescoped systems where base metal mineralization directly overprints high-temperature porphyry-style mineralization (e.g., Catchpole et al, 2011Catchpole et al, , 2015Masterman et al, 2005;Reed et al, 2013;Rusk et al, 2008a). At the world-class (Zn-Pb-Ag-Cu-Bi) Cerro de Pasco Cordilleran deposit a genetic link with a hidden porphyry system at depth has already been evoked by Baumgartner et al (2008), but no direct evidence reported.…”