Abstract

The Diyanqinamu porphyry Mo deposit in the southern Greater Khingan Range of the Central Asian orogenic belt contains 800 million metric tons (Mt) of ore with an average grade of 0.097% molybdenum. The deposit is hosted in Late Jurassic volcanic rocks of tuff, andesite, and volcanic breccia. Multiple-stage hydrothermal activities have resulted in propylitic, phyllic, and argillic alteration in this deposit. Five stages (I–V) of hydrothermal activity are identified. Stage I is represented by a mineral assemblage of epidote, chlorite, and magnetite, with some discontinuous barren veinlets of quartz + K-feldspar ± fluorite ± magnetite ± epidote ± chlorite. Stage II is marked by occurrence of quartz + fluorite + molybdenite + magnetite ± pyrite ± sericite ± siderite veinlets/veins with phyllic halos. Stage III consists of fluorite + siderite + quartz + molybdenite + pyrite ± ankerite ± calcite ± chalcopyrite veins that are commonly related to phyllic alteration and dissemination of fluorite in the altered rocks. Stage IV has an assemblage of fluorite + quartz + pyrite ± ankerite ± calcite ± molybdenite ± chalcopyrite ± sphalerite ± galena in coarse veins (10–20 mm wide). Stage V consists of narrow (≤5-mm wide) veinlets of calcite + fluorite + pyrite ± quartz. Molybdenite mainly occurs in Stages II and III.

Re-Os dating results for molybdenite samples from these two stages yielded an isochron age of 156.2 ± 4.2 Ma (2σ, MSWD = 0.96, n = 10). Most molybdenite samples have high δ34S values (≥8.4‰) relative to other sulfide minerals (i.e., galena, sphalerite, pyrite, and chalcopyrite) of Stages II to V (δ34S = 2.5–8.3‰, n = 22). Molybdenite also has low 207Pb/204Pb and 208Pb/204Pb ratios relative to other sulfide minerals although there are minor overlaps. In a diagram of 206Pb/204Pb versus 207Pb/204Pb, these Pb isotope data display a positive trend transecting the growth curves of crustal lead, which could be invoked by mixing of crustal and mantle sources with distinct Pb isotopes. In combination with the S isotope data and mineral paragenesis, we suggest that magmas were the main source of molybdenum, whereas other metals (i.e., Pb, Zn, and Cu) were possibly sourced from the country rocks.

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