New geochronological results and structural evolution of the Pataz gold mining district: Implications for the timing and origin of the batholith-hosted veins

The Paleozoic Pataz–Parcoy gold mining area is located in a right-stepping jog on the regional Cordillera Blanca fault, in northern Peru. Most of the 8 million ounces of gold production from this area has come from quartz–carbonate–sulfide veins hosted by the Pataz batholith. Despite a subduction zone setting since at least the Cambrian, the area records several periods of extension and its present structure is that of a rift and graben terrain. The Pataz district (the northern part of the Pataz–Parcoy area) is dominated structurally by northwest to north northwest-striking (NW–NNW) faults and northeast to east northeast-striking (NE–ENE) lineaments, both of which have been active periodically since at least the Mississippian (Early Carboniferous). NW–NNW faults control the margins of a central horst that exposes basement schist and the Pataz batholith, and step across NE–ENE lineaments. The Lavasen graben, to the east of the central horst, contains the Lavasen Volcanics, and the Chagual graben, to the west, contains an allochthonous sedimentary sequence derived from the Western Andean Cordillera.New SHRIMP zircon geochronological data indicate emplacement of the Pataz batholith during the Middle Mississippian, at around 338–336 Ma, approximately 10 Ma earlier than previous estimates based on 40Ar/39Ar geochronology. The calc-alkaline, I-type batholith comprises diorite and granodiorite, the latter being the major component of the batholith, and was emplaced as a sill complex within the moderately NE-dipping sequence of the Eastern Andean Cordillera. Moderate- to high-temperature ductile deformation took place on the batholith contacts during or shortly after emplacement. Following emplacement of the batholith, differential uplift occurred along NW–NNW faults forming the Lavasen graben, into which the Lavasen Volcanics were deposited. SHRIMP U–Pb in zircon ages for the Lavasen Volcanics and the Esperanza subvolcanic complex, which was intruded into the western margin of the graben, are within error of one another at ca 334 Ma. The ductile batholith contacts were cut by renewed movement on NW–NNW faults such that the margins of the batholith are now controlled by these steep brittle-ductile faults. The NW–NNW faults were oriented normal to the principal axis of regional shortening (ENE–WSW) during formation of the batholith-hosted, gold-bearing quartz–carbonate–sulfide veins. The misoriented faults were unable to accommodate significant displacement, leading to high fluid pressures, vertical extension in the competent batholith and formation of gold-bearing veins. Brittle failure of the batholith was most extensive in the northern Pataz district where the fault-controlled western contact of the batholith is offset by a swarm of NE–ENE lineaments.The timing of vein formation is not established, despite published 40Ar/39Ar ages of 312 to 314 Ma for metasomatic white mica, which are interpreted as minimum ages of formation. Gold-bearing veins formed during or shortly after uplift of the Pataz batholith and formation of the Lavasen graben; they were therefore broadly coeval with deposition of the Lavasen Volcanics and emplacement of the Esperanza subvolcanic complex. These K-rich, weakly alkalic, ferroan (A-type) magmas may provide a viable source for the ore fluid that deposited gold in the Pataz batholith.