Deciphering deep-seated, highly fractionated, and reduced granitic magma systems associated with world-class scheelite skarn ores: A case study of the Zhuxi deposit, South China

Abstract

The Zhuxi deposit, with 3.44 million tons (Mt) WO3 at 0.54%, is a world-class reduced scheelite skarn deposit and is spatially associated with highly fractionated and highly mineralized dikes, i.e., scheelite-bearing anorthite rock and albitite dikes. These scheelite-bearing dikes share similar rare earth element (REE) and Sr–Nd isotope compositions but display major element compositions distinct from those of the causative biotite monzogranite pluton associated with the Zhuxi deposit. The average compositions of apatite grains from anorthite rock, albitite, and the granitic pluton display roughly subparallel chondrite-normalized REE patterns. Additionally, subparallel chondrite-normalized REE patterns are displayed between the edges of scheelite grains from altered fine-grained granite and the cores of scheelite grains from albitite as well as between the edges of scheelite grains from albitite and scheelite grains from anorthite rock. Moreover, the in situ titanite (150.04 ± 0.39 Ma) and apatite (150.8 ± 1.7 Ma) U–Pb ages of the anorthite rock and the apatite U–Pb age (152.2 ± 2.6 Ma) of the albitite are consistent with the rock- and ore-forming ages (∼150 Ma) of the Zhuxi deposit. Furthermore, the water- and alkali-rich intergranular melt, which was extracted from the magma system, predated its solidification entirely and formed widespread intergranular albite rims surrounding plagioclase grains in the biotite monzogranite pluton where myrmekite is widespread.The scheelite-bearing anorthite rock and albitite dikes represent the products of highly fractionated melts that were saturated with volatiles and extracted from deep granitic magma reservoirs, which later crystallized to form the granitic pluton. Because W is highly incompatible and enriched in residual granitic magmas, large-tonnage W mineralization occurred above deep-seated causative granitic intrusions during the extraction of residual granitic melts if the parental magma exhibited metallogenic potential. Extensive W mineralization is directly associated with highly fractionated dikes rather than with granitic plutons, and these dikes could indicate pathways for W-bearing fluids.The exploration potential for reduced scheelite skarn deposits in deeply emplaced intrusions should not be confined to areas where granites are exposed at the surface but should include areas where granitic intrusions are possibly present at depth, particularly in regions with widespread, highly fractionated, highly mineralized discrete felsic dikes. If the causative granitic intrusions are exposed at the surface, most W orebodies and their spatially associated dikes could have been completely eroded. This is the reason for the disappearance of economic W orebodies adjacent to highly fractionated granitic plutons that share similar geochemical characteristics with granitic dikes surrounded by economic W orebodies. Thus, caution should be applied when assessing the W metallogenic potential of highly fractionated granitic rocks primarily according to their geochemical characteristics.