Abstract Ore lead isotope provinces in the central Andes between 6 ° S and 32 °S correspond in part to broad differences in the ages and types of rocks exposed in each area. If these provinces reflect scavenging oflead from upper crustal rocks and reconcentration into ore deposits, ore lead isotope ratios reflect the average upper crustal composition in each region. If the ore metals have a deeper source, the provinces instead reflect differences in magma sources or generation processes among the provinces. Ores from province II (the high Andes of Perú) show steep lead isotope arrays indicative of source mixing. The igneous rocks in the Hualgayoc district in northern Perú overlap with the radiogenic end of the province I array and are representative of the nonradiogenic end of the province II mixing trends; exposed supracrustal rocks are candidates for the radiogenic end member. The origins of both isotopic signatures are investigated to examine the relationship between hydrothermal metal budgets and magma sources. The exposed crust in the northern Peruvian Andes consists of middle Cretaceous platform carbonates, sandstones, and shales that overlie a similar Jurassic sequence and probably a Precambrian to early Paleozoic metamorphic basement. The metamorphic basement and overlying sediments have broadly similar Pb-Sr-Nd isotope systematics. Whole-rock Pb isotope and U/P systematics of the sediments suggest U/Pb fractionation in the sediment source at approximately 1800 Ma, followed by evolution with elevated U/Pb ratios. ∊ Nd values of the metamorphic basement and Cretaceous sedimentary rocks range from −11.6 to −16.5, with T DM equal to 1.43 to 2.06 Ga. Northern Perú basement rocks have much higher 206 Pb/204Pb values than metamorphic basement terranes in eastern Colombia, southern Perú, and northern Chile, and their isotopes more closely resemble basement terranes to the east in Brazil. The sedimentary rocks were intruded in the middle to late Miocene by numerous felsic igneous bodies associated with hydrothermal Ag-Zn-Cu-Pb mineralization. The intrusive rocks are intermediate to high K andesitic intrusions and rhyodacitic volcanic domes. Fresh igneous rocks have rare earth element (REE) abundances less than 100 times chondrites, lack significant Ce and Eu anomalies, and are relatively depleted in Ti and Nb. The isotopic compositions and homogeneity of the igneous rocks with respect to Pb, Sr, and Nd suggest that they assimilated little shallow crust and were derived largely from deeper sources in the upper sub-Andean mantle or the lower sub-Andean crust. Because no exposed Andean basement rocks resemble the compositions of province I ores, and because subducted sediment has recently been shown to be an important source of lead in arc magmas, the role of subducted sediment in producing a province I-like signature is evaluated. A simple numerical model for the enrichment of a possible mantlewedge source region by subducted sediments is presented. The model suggests that subducted sediment can account for the lead isotope signature of province I ores, and that the quantity of subducted material along the Perú-Chile trench could produce a mantle source with this signature within a few million years of the onset of subduction.

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. 208 Pb/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.