The role of phyllosilicate partial melting in segregating tungsten and tin deposits in W-Sn metallogenic provinces

Most tungsten (W) and tin (Sn) deposits are associated with highly evolved granites derived from the anatexis of metasedimentary rocks. They are commonly separated in both space and time, and in the rare cases where the W and Sn mineralization are part of a single deposit, the two metals are temporally separate. The factors controlling this behavior, however, are not well understood. Our compilation of whole-rock geochemical data for W- and Sn-related granites in major W-Sn metallogenic belts shows that the Sn-related granites are generally the products of higher-temperature partial melting (~800 °C) than the W-related granites (~750 °C). Thermodynamic modeling of partial melting and metal partitioning shows that W is incorporated into the magma formed during low-temperature muscovite-dehydration melting, whereas most of the Sn is released into the magma at a higher temperature during biotite-dehydration melting; the Sn of the magma may be increased significantly if melt is extracted prior to biotite melting. At the same degree of partial melting, the concentrations of the two metals in the partial melt are controlled by their concentration in the protolith. Thus, the nature of the protolith and the melting temperature and subsequent evolution of the magma all influence the metallogenic potential of a magma and, in combination, helped control the spatial and temporal segregation of W and Sn deposits in all major W-Sn metallogenic belts.

Early Oligocene partial melting via biotite dehydration melting and prolonged low-pressure–low-temperature metamorphism of the upper High Himalaya Crystalline Sequence in the far east of Nepal

Early Oligocene partial melting and prolonged low-pressure–low-temperature (low-P/T) metamorphism were investigated in migmatites and orthogneisses from the upper High Himalaya Crystalline Sequence (HHCS) in the far east of Nepal. The migmatites were formed by biotite dehydration melting at c. 800°C from 33 to 25 Ma. Cordierite was only produced at shallow crustal levels at pressures <6 kbar. After Early Oligocene partial melting, the low-P/T metamorphism continued until 17 Ma during exhumation of the cordierite-bearing migmatites. Early Oligocene biotite dehydration melting in the upper HHCS occurred at different times and locations from the Early Miocene muscovite dehydration melting in the underlying HHCS and the metamorphic discontinuity was accompanied by thrusting of the High Himalayan Discontinuity at c. 27–19 Ma. Pervasive partial melting and prolonged low-P/T metamorphism in the upper HHCS is more compatible with a lateral southwards channel flow of the upper HHCS along the High Himalayan Discontinuity, whereas current channel flow models explaining the exhumation of the HHCS as driven only by the coupled activity of the Main Central Thrust and South Tibetan Detachment have faced difficulties in explaining the timing of the low-P/T metamorphism observed in the upper HHCS.Supplementary material: The representative cordierite compositions from the cordierite migmatites, the far east of Nepal are available at https://doi.org/10.6084/m9.figshare.c.4068815

Petrogenesis of Mesozoic, Peraluminous Granites in the Lamoille Canyon Area, Ruby Mountains, Nevada, USA

Two groups of closely associated, peraluminous, two-mica granitic gneiss were identified in the area. The older, sparsely distributed unit is equigranular (EG) with initial εNd ∼ − 8·8 and initial 87Sr/86Sr ∼0·7098. Its age is uncertain. The younger unit is Late Cretaceous (∼80 Ma), pegmatitic, and sillimanite-bearing (KPG), with εNd from −15·8 to −17·3 and initial 87Sr/86Sr from 0·7157 to 0·7198. The concentrations of Fe, Mg, Na, Ca, Sr, V, Zr, Zn and Hf are higher, and K, Rb and Th are lower in the EG. Major- and trace-element models indicate that the KPG was derived by muscovite dehydration melting (&lt;35 km depth) of Neoproterozoic metapelitic rocks that are widespread in the eastern Great Basin. The models are broadly consistent with anatexis of crust tectonically thickened during the Sevier orogeny; no mantle mass or heat contribution was necessary. As such, this unit represents one crustal end-member of regional Late Cretaceous peraluminous granites. The EG was produced by biotite dehydration melting at greater depths, with garnet stable in the residue. The source of the EG was probably Paleoproterozoic metagraywacke. Because EG magmatism probably pre-dated Late Cretaceous crustal thickening, it required heat input from the mantle or from mantle-derived magma.