Geologic and Isotopic Constraints on the Age and Origin of Auriferous Quartz Veins in the Parcoy Mining District, Pataz, Perú
Andrew W. Macfarlane, Richard M. Tosdal, César E. Vidal, Jorge Paredes, 1999. "Geologic and Isotopic Constraints on the Age and Origin of Auriferous Quartz Veins in the Parcoy Mining District, Pataz, Perú", Geology and Ore Deposits of the Central Andes, Brian J. Skinner
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Gold-bearing quartz veins in the Parcoy mining district occupy brittle shear zones in the Pataz batholith, which we have dated at 329 ± 1 Ma by U-Pb in zircons. The batholith is emplaced in metamorphic rocks of the Proterozoic Marañón complex. The veins contain paragenetically early quartz, pyrite, and arsenopyrite, and later quartz, sphalerite, galena, and chalcopyrite. Both paragenetic stages contain important gold mineralization. Wall-rock alteration consists of quartz, sericite, and pyrite, with envelopes of propylitic alteration. In the Gigante vein, between 3,900 and 4,200 m, the early and late ore assemblages filled an en-echelon fault-fracture system of limited sinistral, oblique thrust slip. Higher grades of mineralization lie in dilational inflections in the fault system. The vein is offset sinistrally and normally to the north by east-west-striking faults and by minor normal faults parallel to the veins themselves.
Lead in galenas from the Parcoy district is isotopically homogeneous. Estimated corrections for in situ decay of U and Th in the batholith and the metamorphic basement suggest that the Pataz batholith provided most of the ore lead in the system. 208Pb/204Pb of Marañón complex samples are too high for the basement to have been a major lead source; however, lead isotope ratios of Pataz batholith samples are not greatly different from the basement rocks. Marañón complex metamorphic whole-rock samples have values of -8.9 to -12.3, with unusually high Nd contents (34-66 ppm). Depleted mantle model separation ages for the metamorphic rocks range from 2.06 to 1.43 Ga. Initial values of the Parcoy district granodiorites vary from -4.7 to -6.1, which indicate an addition of 35 to 70 percent ancient crustal material to a depleted mantle-derived parental melt, depending on the characteristics of the contaminant.
Coarse-grained hydrothermal muscovite gives a K-Ar age of 286 ± 6 Ma, suggesting that mineralization greatly postdated the emplacement of the host batholith, and was therefore unrelated to cooling of the batholith, as previously proposed. However, the batholith is clearly a composite feature; undated quartz monzonite porphyry intrusions that cut the Pataz batholith and felsic dikes that cut both the batholith and the mineralization indicate that magmatism occurred well after the batholith was emplaced. Further geochronology will be needed to explore any possible genetic link between these later intrusions and the Pataz gold mineralization.
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Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages:
1. Normal subduction extended into Eocene time.
2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera.
3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle.
4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab.
5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner