Stratabound Ore Deposits of Hualgayoc, Cajamarca, Peru

Hualgayoc is one of the typical "complex mining districts" of the Central Andes, characterized by the influence of many superimposed geologic events and ore-forming processes It has been worked since Spanish colonial times. Initially, Hualgayoc was famous for the silver ores which were extracted principally from the upper parts of the ore deposits. The zones of oxidation and supergene enrichment of many deposits have already been removed by early mining. With time, the production of the district became more polymetallic (zinc, lead, and copper as well as silver). Many mines are being operated by several small companies and some mining ventures of the "mediana mineria". There are up to five flotation plants, with a total capacity of about 800 t/day. Although this important mining district has been the object of a large number of geologic studies by many geologists, we are far from understanding the origin of many of its ore deposits, especially those of the stratabound type. The purpose of this chapter is to describe the geologic, mineralogic and geo- metric characteristics of the stratabound ore deposits of this important district to improve our understanding of the complicated geologic setting of the "mantos". Hualgayoc is situated in northern Peru, at a height of between 3500 m and 2400 m, near, and on the east zone of the continental divide of the Cordillera Occidental

Formation of intra-arc volcanosedimentary basins in the western flank of the central Peruvian Andes during Late Cretaceous oblique subduction: field evidence and constraints from U–Pb ages and Hf isotopes

During late Early to Late Cretaceous, the Peruvian coastal margin underwent fast and oblique subduction and was characterized by important arc plutonism (the Peruvian Coastal Batholith) and formation of volcanosedimentary basins known as the Western Peruvian Trough (WPT). We present high-precision U–Pb ages and initial Hf isotopic compositions of zircon from conformable volcanic and crosscutting intrusive rocks within submarine volcanosedimentary strata of the WPT hosting the Perubar massive sulfide deposit. Zircons extracted from both the volcanic and intrusive rocks yield concordant U–Pb ages ranging from 67.89±0.18 Ma to 69.71±0.18 Ma, indicating that basin subsidence, submarine volcanism and plutonic activity occurred in close spatial and temporal relationship within the Andean magmatic arc during the Late Cretaceous. Field observations, satellite image interpretation, and plate reconstructions, suggest that dextral wrenching movements along crustal lineaments were related to oblique subduction. Wrench tectonics is therefore considered to be the trigger for the formation of the WPT as a series of pull-apart basins and for the emplacement of the Coastal Batholith. The zircon initial εHf values of the dated magmatic rocks fall between 5.5 and 7.4, and indicate only very subordinate influence of a sedimentary or continental component. The absence of inherited cores in the zircons suggest a complete lack of old basement below the WPT, in agreement with previous U–Pb and Sr isotopic data for batholithic rocks emplaced in the WPT area. This is supported by the presence of a most likely continuous block of dense (~3.0 g/cm3) material observed beneath the WPT area on gravimetric crustal cross sections. We suggest that this gravimetric anomaly may correspond to a piece of lithospheric mantle and/or oceanic crust inherited from a possible Late Permian–Triassic rifting. Such young and mafic crust was the most probable source for arc magmatism in the WPT area