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Polliand,M., Fontboté, L., Schaltegger, U. (2002). The Perubar VHMS deposit, central Peru: formed in an upper Cretaceous intra-arc setting? XI Congreso Peruano de Geologia, Resúmenes, Sociedad Geológica del Peru, p. 103

The Perubar VHMS deposit, Central Peru:
formed in an Upper Cretaceous intra-arc setting?

M. Polliand, L. Fontboté, & U. Schaltegger

Section des Sciences de la Terre, University of Geneva, Switzerland

The Perubar VHMS deposit, situated 50 km east of Lima in the Cocachacra mining district, consists of four main massive sulfide bodies named Graciela, Juanita, Cecilia North and Cecilia South (Polliand and Fontboté, 2000; Vidal, 1987). The massive sulfide-barite orebodies are hosted by submarine volcano-sedimentary sequences traditionally attributed to the Casma Group (mid-Cretaceous?).

The Perubar VHMS deposit occurs in a sequence of submarine volcanic and associated sedimentary rocks deposited in rapidly subsiding sub-basins. The mineralization is spatially associated with submarine volcanic and volcaniclastic rocks, ranging from basaltic andesite to rhyolite. Zircon U/Pb data for an hydrothermally altered rhyolite flow located in the direct hangingwall of the deposit yielded a concordant age of 68.91±0.15Ma, thus indicating an Upper Cretaceous age for the volcanosedimentary host rocks.

Shortly after the VHMS mineralizing event, an abrupt subsidence regime took place, spliting the seafloor into deeply subsiding sub-basins delimited by volcano-tectonic faults. It resulted in the dislocation (downward translation, slumping and brecciation) of the Perubar VHMS deposit. Together with this fault-block subsidence, volcanic activity became more predominant. Subsequently, a major explosive volcanism event was responsible for the deposition of the Upper Unit volcanic/pyroclastic debris flow-like sequence (Polliand & Fontboté, 2000). This debris flow-like sequence most likely finds its origin along a major, N-S oriented, volcano-tectonic fault delimitating the eastern margin of the subsided sub-basins, where an ignimbrite unit subvertically cross-cut the footwall stratigraphy. Approximately one million year later, this same zone of weakness was intruded by a monzodiorite stock (zircon U/Pb concordant age of 67.88±0.16Ma, this study), which contact metamorphosed the ignimbrite unit. This monzodioritic stock probably represents an apophysis of the Canchacaylla monzogranite (hornblende K/Ar age of 66.7±2Ma, Vidal 1987), the latter being responsible for a more regional contact metamorphism.

The complex sedimentary, volcanic and tectonic evolution recorded in the Casma? rocks of the Cocachacra district suggests the existence of a local submarine caldera system at Perubar. Under extensional basin settings, it is more likely to develop piecemeal caldera collapse rather than piston-like subsidence, as mentioned by Kokelaar (1992). The chaotic fault-block subsidence recorded at Perubar could correspond to such a piecemeal caldera collapse.

The zircon U/Pb ages of this study support an arc setting for the Perubar VHMS deposit, since the volcano-sedimentary host rocks were deposited at the same time and location than part of the Peruvian Coastal Batholith. Thus, it is likely that the VHMS hosting volcano-sedimentary rocks at Perubar were deposited within a transtensional basin at the vicinity of an active volcanic arc, and not as generally assumed for the Casma Group during the development of the Peruvian Huarmey-Cañete extensional marginal basin (e.g., Atherton and Webb, 1989).

Atherton, M. P. and Webb, S. 1989. Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru; Journal of South American Earth Sciences, v. 2, no 3, p. 241-261.

Kokelaar, P. 1992. Ordovician volcanic and sedimentary record of rifting and volcanotectonism: Snowdon, Wales, United Kingdom, Geological Society of America Bulletin, v. 104, p 1433-1455.

Polliand, M., and Fontboté, L., 2000, The Perubar Ba-Pb-Zn VHMS deposit, Central Peru, in Sherlock, R. L., and Logan, M. A. V., eds., VMS Deposits of Latin America., Spec. Pub., Mineral Deposits Division, Geological Association of Canada, v. 2, p. 439-446.

Vidal, C. E., 1987, Kuroko-Type Deposits in the Middle Cretaceous Marginal Basin of Central Peru, Econ. Geol., v. 82, p. 1409-1430.

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