Formation of Large to Giant Porphyry Mo Deposits: Constraints From Whole-Rock and In Situ Mineral Geochemistry of Causative Magmatic Rocks in the Tongbai-Hong’an-Dabie Orogens, Central China

Abstract The Tongbai-Hong’an-Dabie orogenic belt (THD) contains over 3 million tonnes (Mt) of metal Mo. At least 10 porphyry Mo deposits developed in small to giant scales, forming a significant Mo mineralized belt in China. Ore genetic models of these porphyry Mo deposits have been widely discussed; however, key factors responsible for forming the large to giant porphyry Mo deposits still remain unclear. In this study, we investigated samples from three small deposits (Wangwan, Mushan, and Bao’anzhai) and two large to giant deposits (Tangjiaping and Shapinggou) in the THD. Comprehensive characterization of whole-rock compositions and in situ geochemical analysis of apatite, plagioclase, and biotite from these deposits were conducted to provide potential distinguishable indicators for evaluating the prospectivity of a given magmatic suite in continental collisional settings. The data show that the productive granites from the large to giant Tangjiaping and Shapinggou deposits are characterized by higher Nd-Hf-O isotopes than those in the small ones, as well as magma temperatures of 832° and 860°C. It is also noteworthy that a significant number of mafic igneous rocks were intruded in the THD, such as in the Tangjiaping deposit, coeval with the formation of the Mo deposits. Apatite within the contemporary mafic enclaves of the Lingshan granitic pluton is anticipated to demonstrate the key geochemical characteristics of these mafic rocks, with the apatite displaying high SO3 contents ranging from 0.45 to 0.91 wt %.These observations suggest that the formation of large to giant porphyry Mo deposits could be linked to the replenishment of the hotter mafic magma in the deep magma chamber in extensional tectonic settings, which might provide sufficient heat and sulfur in parent fertile magmas. The reverse zoning and complex core-to-rim variations in the plagioclase crystals from mafic enclaves provide further evidence for magma mixing processes in the deep magma chamber in the THD from the regional perspective. Magma oxygen fugacity in the Tangjiaping and Shapinggou deposits derived from biotite chemistry is comparable to that in the small ones, implying minimal controls on the large to giant Mo mineralization. The application of plagioclase-based hygrometer reveals that fertile magmas from Tangjiaping and Shapinggou are less hydrous with H2O contents of 3~4 wt %. However, estimates of absolute sulfur and fluorine contents in these mineralization-related melts from apatite SO3 and F contents using available partitioning models suggest that they are indeed significantly richer in S and F, with contents of 45 to 301 and 2,100 to 2,852 ppm, respectively. Moreover, these magmas are also characterized by lower magma viscosity with log ƞ values of 6.0 to 6.4 Pa s, as calculated from whole-rock and apatite compositions. In contrast to primary fertile chlorine-rich fluids responsible for porphyry Cu deposits, the primary ore-forming fluids in the Tangjiaping and Shapinggou in equilibrium with biotite are enriched in fluorine with intercept values (IV[F]) of 0.75 to 1.23. Also, log(fHCl/fHF) and log(fH2O/fHF) values derived from biotite and XF/XCl, XCl/XOH ratios and SO3 contents in apatite record that hydrous volatile exsolution from melts occurred earlier during the magmatic-hydrothermal evolution in the Tangjiaping and Shapinggou deposits. In detail, our modeling illustrates that primary fluids initially developed at high temperatures exceeding 765°C. Meanwhile, S and Cl were extracted from the melts into primary ore fluids, leading to a reduction in their concentrations within the melts, decreasing from 0.03 to 0.005 wt % for S and from 0.030 to 0.005 wt % for Cl. This process accounts for the generation of high-temperature, low-Cl, and S-rich ore-forming fluids.