Lead isotope provinces of the Central Andes inferred from ores and crustal rocks

Abstract

The central Andean Cordillera of Peru, Bolivia, northwestern Argentina, and northern Chile may be divided into three fundamental geologic provinces, based on the age and dominant geology of the exposed rocks. We define these provinces as follows:Province I: The coastal volcanic arc, comprising the Jurassic-early Tertiary coastal batholith complex and related volcanic rocks of the Western Cordillera and coastal belt. Based on differences in the isotopic composition of ore lead, province I is further subdivided into subprovince Ia, comprising the coastal volcanic belt of northern and central Chile; subprovince Ib, in central Peru north of 13 degrees S; and subprovince Ic, in southern Peru and northernmost Chile between 19 degrees S and 13 degrees S.Province II: The Jurassic and Cretaceous miogeosynclinal sedimentary belt that dominates the geology of the high Andes of Peru.Province III: The Eastern Cordillera of southeastern Peru, central Bolivia and northwestern Argentina, made up mainly of early Paleozoic clastic sediments. We have also divided this into two subprovinces due to different ore lead isotope characteristics: subprovince IIIa in Bolivia and northern Argentina, and subprovince IIIb in southeastern Peru.These provinces are bordered by noncordilleran units of the Brazil and Guyana Shields, the Arequipa massif, and the metamorphic terranes of southern Chile and central-western Argentina.We present 45 new analyses of ore minerals from 31 mining districts and two of host rocks from province III and summarize our results together with other data available in the literature. Isotopic compositions of ore lead from the above provinces conform to neighboring but distinct fields on Pb isotope covariation diagrams.Lead from province I ores has 206 Pb/ 204 Pb = 18.21 to 18.82, 207 Pb/ 204 Pb = 15.55 to 15.69, and 208 Pb/ 204 Pb = 38.11 to 38.95. Lead from province II ores is less variable, with consistently higher 206 Pb/ 204 Pb and 208 Pb/ 204 Pb than the province I ores and slightly higher average 207 Pb/ 204 Pb. We consider the province I and II ore leads to reflect different degrees of mixing between magmatic, upper mantle-derived lead and crustal sources. Most assimilation of crustal lead is probably magmatic and results from incorporation of lead from subducted sediments at the magma source and/or from the crust during ascent of the parent magma from the upper mantle. Province III ore leads are much more variable than those of provinces I and II, having 206 Pb/ 204 Pb = 17.97 to 25.18, 207 Pb/ 204 Pb = 15.51 to 16.00, and 208 Pb/ 204 Pb = 37.71 to 40.07; they have consistently higher 207 Pb/ 204 Pb and 208 Pb/ 204 Pb for given values of 206 Pb/ 204 Pb than ore leads from the other provinces. Province III leads indicate more radiogenic, heterogenous source regions and suggest that fusion of continental crust has played an important role in the genesis of magmas and associated ore deposits. This is substantiated by the metaluminous to strongly peraluminous character of much of the magmatism in province III.Detailed comparisons of lead isotope data available in the literature for Andean volcanic rocks with compositions of ores are problematic because analyses are rarely reported from related ores and igneous bodies. Where such data exist, ore Pb isotope compositions correlate closely with those in the associated intrusions.