Mineralized breccia clasts: a window into hidden porphyry-type mineralization underlying the epithermal polymetallic deposit of Cerro de Pasco (Peru)

• Published in: Mineralium deposita 2018-10, Vol.53 (7), p.919-946
• Main Authors: Rottier, Bertrand, Kouzmanov, Kalin, Casanova, Vincent, Bouvier, Anne-Sophie, Baumgartner, Lukas P., Wälle, Markus, Fontboté, Lluís
• Format: Article
• Language: English
• Subjects: Apatite; Augite; Biotite; Breccia; Brines; Carbon dioxide; Chalcopyrite; Chemical evolution; Copper; Crystallization; Dikes; Earth and Environmental Science; Earth Sciences; Embankments; Erosion; Evolution; Feldspars; Fluid inclusions; Fluids; Galena; Geology; High temperature; High temperature fluids; Hosts; Hydrothermal systems; Isotopes; Lead; Low temperature; Magma; Mass spectrometry; Mass spectroscopy; Measuring instruments; Meteoric water; Mineral inclusions; Mineral Resources; Mineralization; Mineralogy; Molybdenite; Oxygen isotopes; Pyrite; Quartz; Rutile; Saline water; Silicates; Sphalerite; Sulfur; Temperature; Temperature effects; Titanite; Vapors; Veins (geology); Zinc
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-017-0786-9

Cerro de Pasco (Peru) is known for its large epithermal polymetallic (Zn-Pb-Ag-Cu-Bi) mineralization emplaced at shallow level, a few hundred meters below the paleo-surface, at the border of a large diatreme–dome complex. Porphyry-style veins crosscutting hornfels and magmatic rock clasts are found in the diatreme breccia and in quartz–monzonite porphyry dikes. Such mineralized veins in clasts allow investigation of high-temperature porphyry-style mineralization developed in the deep portions of magmatic–hydrothermal systems. Quartz in porphyry-style veins contains silicate melt inclusions as well as fluid and solid mineral inclusions. Two types of high-temperature (> 600 °C) quartz–molybdenite–(chalcopyrite)–(pyrite) veins are found in the clasts. Early, thin (1–2 mm), and sinuous HT1 veins are crosscut by slightly thicker (up to 2 cm) and more regular HT2 veins. The HT1 vein quartz hosts CO 2 - and sulfur-rich high-density vapor inclusions. Two subtypes of the HT1 veins have been defined, based on the nature of mineral inclusions hosted in quartz: (i) HT1 bt veins with inclusions of K-feldspar, biotite, rutile, and minor titanite and (ii) HT1 px veins with inclusions of actinolite, augite, titanite, apatite, and minor rutile. Using an emplacement depth of the veins of between 2 and 3 km (500 to 800 bar), derived from the diatreme breccia architecture and the supposed erosion preceding the diatreme formation, multiple mineral thermobarometers are applied. The data indicate that HT1 veins were formed at temperatures > 700 °C. HT2 veins host assemblages of polyphase brine inclusions, generally coexisting with low-density vapor-rich inclusions, trapped at temperatures around 600 °C. Rhyolitic silicate melt inclusions found in both HT1 and HT2 veins represent melt droplets transported by the ascending hydrothermal fluids. LA-ICP-MS analyses reveal a chemical evolution coherent with the crystallization of an evolved rhyolitic melt. Quartz from both HT1 and HT2 veins also contains secondary, low-temperature (~ 300 °C) brine and aqueous fluid inclusions that record the cooling of the system. Both vein types are locally crosscut and/or reopened by a pre-diatreme polymetallic event consisting of pyrite, sphalerite with “chalcopyrite disease,” galena, chalcopyrite, tetrahedrite–tennantite, and minor quartz. LA-ICP-MS analyses of mineral and high-temperature fluid inclusions hosted in HT1 and HT2 veins and in situ secondary-ion mass spectrometry oxygen isotope analy