Origin of the Muguayuan veinlet-disseminated tungsten deposit, South China: Constraints from in-situ trace element analyses of scheelite

Abstract

The Late Triassic Muguayuan W deposit is located in the middle of the Jiangnan Orogen, South China. This deposit is characterized by veinlet-disseminated W mineralization that developed in the Sanxianba granitic porphyry stock. The ore minerals are mainly scheelite with minor molybdenite and wolframite. Scheelite mineralization was closely related to greisenization and phyllic alteration, and took place in two stages. Stage I involved scheelite ± wolframite ± molybdenite + quartz veinlet and disseminated mineralization, whereas Stage II resulted in scheelite + quartz + sericite veinlet mineralization. Sulfide and quartz + calcite ± pyrite veinlets formed during the post-ore stage. Scheelites from the two mineralization stages have different textures and compositions. Cathodoluminescence (CL) images of Stage I scheelites reveal two generations of growth (I-a and I-b). Stage I-a scheelite is dark under CL with oscillatory zoning, and has light rare earth element (LREE)-enriched chondrite-normalized patterns, negative Eu anomalies, and high total REE contents. Stage I-b scheelite forms rim overgrowths on Stage I-a scheelite, is bright under CL, and shows positive Eu anomalies and relatively low REE contents. Although Stage II scheelites are nearly uniform under CL, they can be subdivided into two generations according to their REE systematics. Stage II-a scheelite yields middle REE (MREE)-enriched chondrite-normalized patterns, with negative Eu anomalies, whereas Stage II-b scheelite has MREE-depleted patterns with positive Eu anomalies. Minor amounts of apatite formed in both stages of mineralization. Stage I apatite contains 1370–1930 ppm Mn and 97.7–127 ppm Sr, whereas Stage II apatite has lower Mn (111–158 ppm) and higher Sr (2170–4690 ppm) concentrations. The distinct trace elements compositions of the scheelite and apatite from the two stages identify two ore-forming fluids that had different origins and compositions. The ore-forming fluids in Stage I-a were relatively reduced magma-derived fluids with high Mo, Mn, Nb, and Ta, and low Sr. Fluid modeling shows that the initial fluids of Stage I-a were LREE-enriched with negative Eu anomalies, similar to the Sanxianba granitic porphyry. Precipitation of early apatite and scheelite, as well as plagioclase decomposition, altered the fluid composition and led to relative depletions in REE, Nb, and Ta, and increases of Eu and Sr in the Stage I-b fluids. Cooling of these fluids and the addition of recycled meteoric water led the fluids to become relatively oxidized and Sr-rich; Stage II scheelite precipitated from these fluids. Precipitation of Stage II-a scheelite resulted in the Stage II-b fluids becoming progressively MREE-depleted. Extensive alteration, especially greisenization and phyllic alteration, led to plagioclase decomposition, which provided the Ca necessary for scheelite mineralization. This process was important in generating the W mineralization in the Muguayuan deposit, and perhaps for other granite-hosted, veinlet-disseminated scheelite deposits in the Jiangnan Orogen.