Scheelite geochemistry in porphyry-skarn W-Mo systems: A case study from the Gaojiabang Deposit, East China

Scheelite (CaWO4), a hydrothermal mineral commonly displaying enrichment in Mo (up to 16%) and/or rare earth elements (REE), is the main economic mineral in the Gaojiabang porphyry-skarn type W-Mo deposit, East China. Based on microscopic observations and in-situ LA-ICP-MS and LA-MC-ICP-MS analysis, three groups of scheelite, each with different geochemical characteristics, can be recognized. This evidence provides a good basis for considering the behavior of some trace elements in scheelite and how they may constrain ore genesis in porphyry-skarn systems. P-group scheelite occurs inside the porphyry rocks in the form of vein-hosted or disseminated scheelite. These have the lowest (Sr-87/Sr-86)(i) values (0.7089-0.7108), lowest Mo concentration (mean 213 ppm; n = 51), LREE/HREE ratios of 16.33-84.91, and highest Sigma REE concentration (191-405 ppm) with downwards-sloping REE fractionation trends. S-group scheelite occurs in skarns, coexists with skarn minerals (garnet, diopside, epidote, etc.) and has the highest (Sr-87/Sr-86)(i) values (0.7103-0.7113), highest Mo concentration (mean 1,323 ppm; n = 29), highest LREE/HREE ratio (62.6-164), and lowest Sigma REE concentrations (68.5-112 ppm), also with strongly downwards-sloping REE fractionation trends. H-group scheelite occurs within veins hosted by hornfels and displays moderate (Sr-87/Sr-86)(i) (0.70104-0.70122), Mo (mean 806 ppm; n = 17), LREE/HREE ratio (16.32-42.55), and Sigma REE concentration (98.3-167.6 ppm). During hydrothermal precipitation of scheelite, changing redox state plays a major role in controlling Mo behavior. Both the precipitation of early skarn minerals and changing redox states of ore-forming fluids likely result in an increase in Mo, and corresponding decrease of HREE, in scheelite (particularly in S-group scheelite). The Sr-isotope study indicates that crustal materials provided the main source for W-Mo-bearing ore-forming fluids. Furthermore, both fluid mixing and fluid-rock interaction played an important role in the evolution of ore-forming fluids: Formation water or groundwater was likely involved in ore formation. A geochemical model is described combining data for the different types of scheelite, and the behavior of Mo, REEs and Sr-isotopes, to constrain the evolution of ore-forming fluids and constrain ore genesis in the porphyry-skam system at Gaojiabang. This study contributes to the fingerprinting of ore deposits using scheelite geochemistry.