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Geologic and Isotopic Constraints on the Age and Origin of Auriferous Quartz Veins in the Parcoy Mining District, Pataz, Perú

By
Andrew W. Macfarlane
Andrew W. Macfarlane
Department of Geology, Florida International University
,
Miami, Florida 33199
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Richard M. Tosdal
Richard M. Tosdal
U.S. Geological Survey
,
Menlo Park, California 94025
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César E. Vidal
César E. Vidal
Avenida Paseo La Castellana 827, Lima 33
,
Perú
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Jorge Paredes
Jorge Paredes
Minera Aurífera Retamas S.A., Lima
,
Perú
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Published:
January 01, 1999

Abstract

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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Contents

Special Publications of the Society of Economic Geologists

Geology and Ore Deposits of the Central Andes

Brian J. Skinner
Brian J. Skinner
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Society of Economic Geologists
Volume
7
ISBN electronic:
9781629490311
Publication date:
January 01, 1999

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