Garnet texture, geochemistry, and geochronology revealing molybdenum mineralization in the Northern Qinling Belt, Central China

Abstract

The Northern Qinling Belt (NQB) develops some medium-sized molybdenum deposits, but their mineralization ages are unclear, which hinders to establish the metallogenesis and later mineral prospecting. The newly discovered Sigou Mo–W deposit in the NQB has garnet alteration, but the ages of skarn alteration and mineralization are unknown. Disseminated and veined garnet (Grt I and II) were identified at Sigou, with the former having three sub-types (I-A, I-B, and I-C) based on their texture, and they all show dark and light zones or oscillatory zoning under backscattered electron (BSE) images. These four types of garnet are grossular–andradite–spessartine solid solution series, with dark zones being Si, Ca, and Al enrichment and Fe, Mn, and Ti depletion than light zones. Furthermore, their dark and light zones show similar geochemical features, e.g., LREE depletion, HREE enrichment, positive Eu anomalies (1.12–5.12), and low Th (<0.52 ppm) and U contents (0.01–7.37 ppm). Uranium and REE incorporated into garnet dark and light zones are mainly controlled by crystal chemistry, such as isomorphic substitution of divalent cation (e.g., Mn2+ or Ca2+) in the dodecahedral position, but surface sorption during Grt I-A formation also plays a controlling role. These different types of garnet formed in different pH and oxygen fugacity (fO2) conditions, and fO2 decreased from Grt I to II during their formation. Garnet I-A and II yielded a lower intercept 206Pb/238U age of 148.4 ± 4.2 Ma, consistent with that of Grt II (150.6 ± 4.6 Ma). These ages are close to the emplacement ages of granite porphyry in the Sigou Mo–W deposit and regional small granitic plutons and formation of associated molybdenum deposits, indicating close relationship between the Sigou deposit and the granite porphyry. High-salinity magmatic fluid derived from granite porphyry interacted with host rocks (e.g., marble) to form different types of garnet at different pH and fO2 conditions. The continuous fluid–rock interaction probably led to ore-forming fluids evolving to form subsequent scheelite and molybdenite mineralization for the Sigou deposit. In combination of published regional data and this study, small granitic plutons and molybdenum mineralization in the NQB mainly formed at ca. 154–146 Ma and 152–144 Ma, respectively. Regional geology and geochronological evidence suggest that fractionated small granitic intrusions with contemporaneous or older ages than the Manling dioritic rocks may have potential for molybdenum mineralization in the west and southwest of the Mangling intrusive complex.