Nd and Sr isotope study of hydrothermally altered granite at San Manuel, Arizona; implications for element migration paths during the formation of porphyry copper ore deposits

Abstract

A study of the Nd and Sr isotope composition of the Late Cretaceous monzogranite stock at San Manuel, Arizona, and its mineralized alteration halo was undertaken in order to assess the transport of Nd and Sr that accompanied the hydrothermal alteration. Sampling was along a 1-km transect perpendicular to the alteration zones extending from the potassically altered core to the propylitically altered fringe of the system. Large isotopic differences between the stock and the enclosing 1.4-b.y.-old granitic wall rock, coupled with the original homogeneity of the latter, provide optimal conditions for detecting evidence of chemical transport. The mineralizing porphyry stock has a lower 87 Sr/ 86 Sr ratios (0.708) and a higher epsilon Nd (-6) than the Precambrian granite wall rock (0.782, -11). The 87 Sr/ 86 Sr ratios of the altered wall rock decrease regularly from 0.782 to 0.715 from the propylitic alteration zone to the stock contact, indicating that material flow was primarily from the stock into the wall rock, consistent with an orthomagmatic model for porphyry copper ore genesis. The Sr in the ore zone is 75 percent stock derived, and evidence of stock-derived Sr is found up to 700 m into the wall rock. The Sr and Rb concentration variations are mainly controlled by the alteration mineralogy and do not correlate with the isotopic variations. The epsilon Nd values and the Sm and Nd concentrations of the altered wall rock show subtle changes that are mostly confined to the potassically altered zone. Only one wall-rock sample, located 0.5 m from the stock contact, is strongly disturbed. The results suggest that Sr isotopes may provide a useful tool for studying or locating fossil hydrothermal systems, whereas the relatively immobile Nd isotopes are reliable indicators of the magmatic provenance of altered intrusions.