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Book Chapter

Isotopic Studies of Northern Andean Crustal Evolution and Ore Metal Sources

By
Andrew W. Macfarlane
Andrew W. Macfarlane
Department of Geology, Florida International University, Miami, Florida 33199
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Published:
January 01, 1999

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 TDM equal to 1.43 to 2.06 Ga. Northern Perú basement rocks have much higher 206Pb/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.

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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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