Lead isotopic compositions of ore deposition and their host rocks in Arizona: Implications for the crustal inheritance of metals.

Abstract

Pb isotopic compositions of twelve Late Cretaceous-Early Tertiary (LC-ET) igneous complexes in Arizona suggest that there are no isotopic distinctions between productive (those that host a porphyry copper deposit) and barren complexes. Pb isotopic compositions of the LC-ET igneous rocks are similar to those of the Proterozoic basement rocks in which they are located and indicate a regional coherence and inheritance of Pb isotopic compositions. Within the LC-ET igneous complexes, there is commonly a decreasing trend in 206Pb/204Pb from the oldest to youngest pluton. In Arizona, the sulfide mineralization in porphyry copper deposits is generally spatially and temporally associated with the youngest plutons. Pb isotopic compositions of the sulfide mineralization suggest that much of the mineralization was deposited from fluids with an isotopic composition similar to the plutons lowest in 206Pb/204Pb, (the younger plutons). The low 206Pb/204Pb values of the younger plutons and sulfide mineralization suggest derivation from a lower crustal source. Not all the sulfide mineralization has the same Pb isotopic composition. Some distal mineralization, later stage mineralization, and mineralization hosted in Precambrian rocks have Pb isotopic compositions higher in 206Pb/204Pb. The higher 206Pb/204Pb suggests that these mineralizing fluids interacted with upper crustal rocks. Ag/Au ratios of LC-ET igneous complexes and Middle to Late Tertiary metallic mineral districts in Arizona geographically correspond to Pb isotopic provinces. Pb isotopic compositions and model Th/U for sulfide mineralization from 50 of the metallic mineral districts mimic regional Pb isotopic trends defined by Early Proterozoic whole rocks. Crustal provinces with Th/U higher or lower than the crustal average (Th/U≅4) have a higher potential for Au-enriched ore deposits. Crustal provinces with Th/U≅4 have a higher potential for Ag-enriched ore deposits. The Pb isotopic data presented here are consistent with the crustal inheritance of metal ratios. Thus, the crustal inheritance hypothesis can be used as a viable exploration technique at the crustal-province scale. Mass spectrometers that also have the capability of rapid elemental analyses (ICPMS) and aeroradiometric data are useful tools for this exploration method.