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The Himalayan Mianning–Dechang REE belt, western Sichuan, SW China, is approximately 270 km long and 15 km wide, and contains total reserves of > 3 Mt of LREE, including one giant (Maoniuping), one large (Dalucao) and a number of small-medium REE deposits (Muluozhai, Lizhuang) and occurrences. The belt is tectonically located within a continental collision zone, i.e., the eastern Indo-Asian collision zone. REE mineralization is associated with Himalayan carbonatite–alkaline complexes, which consist of carbonatitic sills or dykes and associated alkaline syenite stocks. Available dating data define a Himalayan metallogenic epoch (40–10 Ma). The magmatic–hydrothermal REE systems were controlled by Cenozoic large-scale strike-slip faulting and the resultant tensional fissure zones, developed in a transform phase from a transpressional to a transtensional regime. Alteration is characterized by fenitization, which formed fenite halos enveloping the REE orebodies. Associated REE mineralization occurs as complex vein systems consisting of various veinlets, stringer and stockwork zones, and breccia-pipe systems. The REE orebodies show various shapes from plate-like, lenticular to pipe-like in different districts. Ore types are dominated by pegmatitic, carbonatitic, breccia, and stringer (stockwork), and disseminated ores, mainly composed of barite + fluorite + aegirine-augite + calcite + bastnaesite assemblages. Bastnaesite precipitation (275–325 °C) appears in three assemblages: early clastics-bearing barite + arfvedsonite + calcite + bastnaesite; intermediate quartz + fluorite + barite + bastnaesite; and late fluorite + barite + bastnaesite assemblages. These were followed by low-temperature pyrite + galena + sphalerite assemblages. Fluid inclusion studies indicate that gangue quartz and fluorite hosts numerous melt/fluid inclusions with a large proportion of sulfate (i.e., BaSO 4 , K 2 SO 4 , and CaSO 4 ), in addition to CaCO 3 and CaF 2 , yielding abnormally high (up to 750 °C) but wide-ranging homogenization temperatures (750–125 °C). The similarity of Sr–Nd isotopic compositions for gangue minerals with those of the host carbonatite–syenite suggests that ore-forming fluids, especially the F − , HCO 3 2− , SO 4 2− components, were derived from carbonatite–syenite magmas. The O–D–C isotopic data from gangue calcite, fluorite and quartz indicate that the fluids were of orthomagmatic origin, but also involved mixing with an external fluid. On the basis of our synthesis, we propose a possible genetic model for REE mineralization, in which the hydrothermal system temporally underwent a complicated evolution from separation of high-T sulfate-bearing NaCl–KCl brine resulting in effective precipitation of REE-fluorocarbonate and sulfate, to subsequent mixing with low-temperature meteoric water precipitating minor sulfide assemblages. Spatially, this sequence of events generated a three-tier REE mineralized architecture, in which pipe-like breccia orebodies grade downwards from shallow to deep structural levels into large veinlet ore systems, and veinlet-disseminated orebodies. |
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