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Geology, alteration, and mineralization of the Cerro Corona porphyry copper-gold deposit, Cajamarca Province, Peru James, John


The Cerro Corona deposit is located in the Hualgayoc mining district of the Cajamarca province of northern Peru. The copper-gold mineralization is hosted by the Cerro Corona stock which is divided into two intrusive units: a pre or synmineralization phase (Quartz Diorite 1) and post-mineralization phase (Quartz Diorite 2). Intrusive phases are compositionally very similar and have fine grain crowded porphyritic textures. The quartz diorite is comprised of approximately 25 % plagioclase, 8 % biotite and 8 % hornblende phenocrysts and the remainder is a fine grained groundmass. ²°⁶Pb/²³⁸U analysis of zircon gives a Middle Miocene age (14.4 ±0.1 Ma) for the Cerro Corona quartz diorite. The Cerro Corona stock intruded the late Cretaceous Pariatambo formation which is composed of silty limestones. Skarning and mineralization of the Pariatambo formation is restricted to within twenty meters of the limestone diorite contact. Four hydrothermal alteration assemblages have been classified at the Cerro Corona deposit: 1) K-silicate, 2) Sericite-chlorite-clay (SCC), 3) Quartz-sericite-pyrite (QSP), 4) Orange clay (OC). K-silicate alteration is characterized by replacement of primary hornblende by hydrothermal biotite, replacement of plagioclase by potassium feldspar and the formation of potassium feldspar and leafy hydrothermal biotite in the groundmass and in veins. K-silicate alteration is interpreted to be the oldest alteration assemblage and it occurs commonly in the deeper levels of the Cerro Corona stock. SCC alteration is characterized by pale green colour and waxy texture due to the replacement of plagioclase and groundmass by sericite, chlorite, clay, and calcite. SCC alteration comprises a large area in Quartz Diorite 1 and is interpreted to have the occurred after K-silicate alteration. QSP alteration is characterized by the destruction of intrusive texture and the formation of massive quartz, sericite and pyrite. QSP alteration occurs in the upper levels of the stock in Quartz Diorite 1 and is interpreted to be the last stage in the hydrothermal sequence which is associated with mineralization. OC alteration is characterized by the replacement of plagioclase and groundmass by clay which is bright orange in colour where the alteration affects magnetite-hematite bearing rocks. OC alteration occurs throughout the Cerro Corona stock and may be related to supergene fluids. The main vein types classified at Cerro Corona are: 1) Biotite 2) K-feldspar 3) Magnetite 4) Quartz-oxide-sulphide (QOS) 5) Quartz-pyrite (QP) 6) Pyrite 7) Calcite. The majority of the copper mineralization occurs as chalcopyrite which forms in the Kfeldspar, Magnetite, QOS, QP and Pyrite veins. Two types of supergene mineralization occur at Cerro Corona: oxidized Au bearing rock and supergene Cu mineralized rock both of which occur in zones sub-parallel to the surface. Significant hypogene mineralization occurs throughout Quartz Diorite 1 above 3500m. Mineralization is interpreted to have occurred in two separate phases: 1) low Au/Cu ratio mineralization which occurs throughout Quartz Diorite 1 and is related to K-silicate alteration; and 2) high Au/Cu ratio mineralization which occurs in the center of the SCC altered zone and may be related to SCC alteration. The size, grade and mineralogy of the Cerro Corona deposit are characteristic of porphyry copper-gold deposits which occur throughout the world. The alteration and mineralization at Cerro Corona represents a magmatic-hydrothermal system which chemically changed and collapsed over time resulting in intense overprinting alteration assemblages and related mineralization.

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