Scheelite Trace Elements and Sr-Nd-O Isotopes Influenced by Deposit-Scale Wall-Rock Reaction and Oxidation State

Abstract

Scheelite (CaWO4) is considered a useful mineral for classifying ore deposit types because its trace element concentrations, especially rare earth elements (REEs), As, Eu, Mo, and Sr, can inform on paragenesis. In this study, this concept is tested by integrating trace elements, in situ Sr and Nd isotopes, and δ18O data from two quartz-scheelite vein swarms cutting Paleozoic plutons in New Zealand. In both cases, the scheelite has been inferred by previous researchers to be of magmatic-hydrothermal origin. However, the scheelite in both deposits exhibits highly variable inter- and intrasample trace element concentrations, which we interpret to relate to different substitution mechanisms and oxidation states of the mineralizing fluids. Moreover, age-corrected scheelite in situ 87Sr/86Sr and 143Nd/144Nd values diverge from those of the host granitoids despite some veins occurring entirely within the plutons. Quartz-scheelite oxygen isotope thermometry indicates equilibration at relatively low temperatures (~170°–280℃), and the calculated δ18OH2O values suggest mixing between W-bearing fluids and meteoric waters. The scheelite trace element chemistries, therefore, reflect a complex interplay between local element reservoirs and oxidation states. Since these processes appear to have operated at the deposit scale, scheelite-based trace element paragenesis discrimination models should be applied with caution. For example, a metamorphic W-bearing fluid may acquire a magmatic-like trace element signature through interaction with granitoids shortly before scheelite crystallization; conversely, a magmatic-hydrothermal W-bearing fluid may develop a nonmagmatic signature by reacting with the metamorphic country rocks.