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 50km2. 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 μ g/g), sulfur (up to 12,000 μ g/g), and iron (2,100–6,000 μ g/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 μ g/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 polymetallic mineralization can precipitate from similar magmatic-hydrothermal fluid pulses.