Summary of Pb isotopic compositions in epitermal precios metal deposits, Orcopampa área of Southern Peru, Berenguela area of Western Bolivia, and the Maricunga belt in north-central Chile

Abstract

The Mesozoic and Cenozoic Central Andes are divided into three Pb isotopic provinces, based upon the Pb isotopic compositions of ore minerals (MacFarlane et al., 1990). Macfarlane et al., 0990), furthermore, argue that the Pb isotopic compositions of the ore minerals reílect those of the igneous rocks associated with the deposits. Province I lies along the coast of Perú, Chile, and westernmost Bolivia. Mcsozoic and early Cenozoic volcapic and plutonic ares built upon a rifted and thinned continental margin dominate this province. Three subprovinces are distinguished based upon slight differences in Pb isotopic compositions. Province la includes northern and central Chile south of 19°S; province lb includes central Perú north of 13°S; whereas province le includes central and southern Perú between the two other subprovinces. Province II lies in the high Andes of central Perú and, perhaps, in northern Chile and Argentina, where miogeoclinal sedimentary rocks crop out and the crust underwent a lower magnitude of extension in the early Mesozoic. This region generally represents a back-arc position relative to the Mesozoic and early Cenozoic magmatic ares, and extensive magmatism related to the Andean cycle has only occurred since the Oligocene. Paleozoic ares are the dominant basement in this province. Province III lies in the Cordillera Oriental and Altiplano of Perú and Bolivia where Paleozoic, Mesozoic, and Cenozoic sedimentary rocks are multiply dcformed by thrust faults. Magma ti-e episodes of Triassic to Jurassic and Oligocene to Miocene age are documented. Proterozoic rocks of the Brazilian shicld are underthrust beneath the Cordillera Oriental, with the youngest shortcning episode beginning in the Oligocene. Province III is subdivided into two subprovinces: ma lies in southeastern Perú where both episodes of magmatism occurred, whereas IIIb lies in Bolivia where magmatism is primarily of Oligocene and Miocene age. Pb isotopic compositions for Province I are slightly less radiogenic than those from province II, whereas province III isotopic compositions are much more varied with consistently higher 207Pb/ 204pb and 208pb/204pb at a given 206pb/ 204pb_Province I Pb isotopic compositions (206pb/ 204pb =18.21-18.82; 207Pb/2<YíPb = 15.55-15.69; 2º8Pb/2º4Pb = 38.11-38.95) overlap with and extend below the average crustal growth curve of Staccy and Kramers (1975) on the uranogenic diagram (207pb/204pb versus 206pb/204Pb). Province lT Pb isotopic compositions (206pb/2<Yípb = 18.76-18.90; 207pb/204pb 15.62-15.73; 208pb/204pb 38.63-39.16) and Province III Pb isotopic compositions (206pb/204Pb 17.97-25.18; 207pb/204pb 15.51-16.00; 208pb/20/4pb 37.71-40.07) lie above the average crustal growth curve on the same diagram. The Pb isotopic compositions from these last two provinces require contribution from a high mu (238U/204pb) Proterozoic or Archean source. On the thorogcnic Pb isotopic variation diagram (208Pb/ 204Pb versus 206pb/204pb), isotopic compositions for province I, 11, and IIIa scatter along the average crustal growth curve of Stacey and Kramers (1975-) indicating that a time averaged Th/U ratio - 4 (the average cristal value) characterizes the Central Andes. Pb isotopic compositions for province IIIb are the most radiogenic and also the most heterogeneous. The variable radiogenic Pb isotopic compositions of province III suggcst heterogeneous upper cristal sources, whereas the isotopic compositions of province I probably reflect a mafic cristal lithospheric source, probably modified by subduction processcs. Province II isotopic compositions conceivably represent a mix betwecn the two model reservoirs.