Abstract

The Dabaoshan polymetallic deposits in the Nanling Range, South China, consist of porphyry and skarn-type Mo mineralization genetically related to Jurassic porphyritic intrusions and adjacent strata-bound Cu-Pb-Zn mineralization hosted in mid-Devonian limestone. Porphyry Mo mineralization is characterized by the superposition of multiple generations of crosscutting quartz-bearing veins including: barren quartz veins (V1), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralization (V2), quartz-pyrite veins with muscovite alteration (V3), and late base metal mineralization with argillic alteration (V4), as well as limestone-hosted strata-bound Cu-Pb-Zn mineralization (VS). Fluid inclusion petrography and microthermometry combined with cathodoluminescent textures and trace elements in quartz reveal changes in pressure and temperature of the hydrothermal system that formed the deposit.

V1 and V2 veins are dominated by low-salinity (1–6 wt % NaCl equiv), CO2-bearing (4–10 mol %) two-phase inclusions with about 35 vol % bubble trapped in the one-phase field above the solvus of the fluid. V1 veins are dominated by CL-bright granular quartz mosaics with higher CL intensity than any other vein type. This quartz also contains more Ti than any other vein generation (24–89 ppm), while Al, Ge, and Li concentrations overlap with other vein generations. Molybdenum ore-hosting V2 veins are also dominated by CL-bright granular quartz mosaics, but V2 veins display slightly less CL intensity and correspondingly lower Ti concentrations (10–65 ppm). V3 veins have a broader spatial distribution than V1 and V2 veins, extending from inside the porphyries out into the adjacent limestone. These veins are also dominated by low-salinity (2–6 wt % NaCl equiv), CO2-bearing (4–7 mol %) two-phase inclusions, these containing about 45 vol % bubble trapped above their solvus. V3 veins are dominated by CL-dark quartz with euhedral growth zones of oscillating CL intensity and systematically lower Ti concentrations than previous vein generations (1.5–12 ppm). Minor V4 veins cut all the above vein generations and also occur as late infill in V3 veins. Low-salinity (4–7 wt % NaCl equiv), CO2-bearing (4–5 mol %) two-phase inclusions with about 20 vol % bubble prevail in V4 veins. V4 veins have the lowest CL-intensity quartz of all vein types, with euhedral growth zones of oscillating CL intensity and the lowest Ti concentration of all vein types (0.54–5.3 ppm).

Intersections of fluid inclusion isochores with Ti-in-quartz isopleths indicate that the hydrothermal system evolved from near-magmatic pressures and temperatures of 2.7 ± 0.2 kbars and 650° ± 40°C for V1 veins to 1.9 ± 0.2 kbars and 530° ± 40°C for V2 veins to 0.65 ± 0.2 kbars and 400° ± 40°C for V3 veins. V4 veins, which lack rutile, formed as the system cooled to 250° to 300°C at maximum pressures of 0.40 to 0.65 kbar.

Unlike nearly all other reported porphyry-type ore deposits, quartz-bearing veins from the Dabaoshan porphyry Mo deposit contain few halite-bearing or vapor-dominated fluid inclusions in any vein type. The dearth of such fluid inclusions, coupled with the abundance of two- and three-phase CO2-bearing low-salinity fluid inclusions in all vein types is evidence that the formation conditions of V1 to V4 veins remained above the V-L surface in the H2O-NaCl-CO2 system, such that fluid unmixing rarely occurred in the Dabaoshan hydrothermal system. This is further supported by only rare evidence for quartz dissolution textures in all vein types, implying that pressures were dominantly higher than zone of retrograde quartz solubility. Taken together, the fluid inclusions, CL textures, and quartz trace element data indicate that the Dabaoshan porphyry Mo deposit is one of the deepest formed porphyry-type ore deposits, having formed at depths of 6 to 7 km below surface. The extreme depth and lack of fluid unmixing inhibited Cu precipitation in the porphyry system, and instead allowed Cu to remain in solution to precipitate with Pb and Zn upon interaction with the surrounding Devonian limestone.

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