Mineralization mechanisms in the Shangfanggou giant porphyry-skarn Mo–Fe deposit of the east Qinling, China: Constraints from H–O–C–S–Pb isotopes

Abstract

The giant Shangfanggou skarn-porphyry Mo–Fe deposit, one of the largest Mo deposits in the East Qinling Orogen, is hosted by the Neoproterozoic Luanchuan Group on the southern margin of the North China Craton. The mineralization occurs mainly as veinlets and in veinlet-disseminated form within altered schists, skarns, hornfelses, and/or marbles. The hydrothermal alteration and mineralization at Shangfanggou took place over four stages. Stage 1 involved potassic alteration and silicification in the porphyry, skarnization and magnetization in carbonate strata, and hornfelsization in schists. Quartz–molybdenite veins associated with phyllic alteration were produced in the porphyry during stage 2, along with sericitization, chloritization, serpentinization, and silicification in the skarns, hornfelses and/or marbles. Quartz–polymetallic sulfide veins related to phyllic alteration were produced during stage 3, and ore-barren quartz±calcite±pyrite veins associated with weak carbonatation were produced during stage 4. C, O, and D isotope compositions of hydrothermal calcite, dolomite, magnetite, phlogopite, and quartz from different stages of mineralization during the formation of the Shangfanggou deposit range from −12.9‰ to +8.8‰, −3.2‰ to +22.7‰, and −100‰ to −80‰, respectively. Sulfur isotope compositions of sulfides from the ores of the Shanfanggou deposit range from 3.6‰ to 4.4‰. 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 17.648 to 18.742 (apart from one magnetite with a value of 20.78), 15.526 to 15.690, and 38.197 to 39.823 in the Shanfanggou deposit. The evolution of the ore-forming fluids in the Shangfanggou deposit, as revealed by δ18O, δD, and δ13C isotope studies, was one of progressive modification of the magmatic fluids through fluid–wall rock interactions along conduits during fluid ascent, and this signifies the involvement of formation/strata water during the earlier stages of mineralization, and the predominance of meteoric waters over magmatic fluids in the later stages. The carbon, sulfur, and lead isotope compositions suggest mixing took place between the magmas and the host rocks within the region. Integrating the data obtained from studies of the regional geology, ore geology, fluid inclusions, and C–H–O–S–Pb isotope geochemistry, we conclude that the Shangfanggou Mo–Fe deposit is a porphyry-skarn type system that formed during a transition from compressional to extensional tectonics during the Jurassic–Early Cretaceous continental collision between the North China Craton and Yangtze Craton. The ore materials were sourced from magmatic ore-forming fluids that mixed with components derived from carbonaceous wall rocks during their ascent, and which were gradually mixed with meteoric waters, especially in the later stages.