Extremely fractionated magmas linked with W mineralization: Evidence from the Lyangar W-Mo deposit, South Tianshan Orogenic Belt

Tungsten (W) deposits are commonly related to the exsolution of magmatic-hydrothermal fluids from high-Si granites (SiO2 > 70%). However, whether the W-related high-Si granitic magma is produced via partial melting of metasedimentary source rocks or by high degree of fractional crystallization remains controversial. Here we present new geochronological and geochemical data on the intrusions associated with the Lyangar W-Mo skarn deposit in the Southern Tianshan Orogenic Belt, Uzbekistan. Our new U-Pb zircon age data show that the major intrusion exposed in the region are ca. 280 Ma biotite gabbroic diorite and biotite granite and about 260 Ma porphyritic granite and muscovite porphyritic granite. The molybdenite grains in the skarn rocks and orebodies show weighted Re-Os ages of 261.4 +/- 7.8 Ma and 261.1 +/- 3.8 Ma, respectively. In combination with the field contact, we confirm that the muscovite porphyritic granite is genetically related to the W mineralization. The gradual transition from the porphyritic granite to muscovite porphyritic granite, similar mineral assemblages and geochemical variations indicate that they are co-magmatic, and that the porphyritic granite represents less evolved member. Rhyolite-MELTS modeling further reinforces that the muscovite porphyritic granites can be produced by high degree of fractional crystallization (similar to 33%, including, similar to 1.2% biotite, similar to 27% plagioclase, similar to 2% alkali-feldspar, similar to 0.21% Fe-Ti oxides, and similar to 2.7% amphibole) of the porphyritic granite magma. On the basis of the positive epsilon(Hf)(t) values (+3.03 to +6.02), high-SiO2 contents and CIPW characters, the porphyritic granite is considered to have formed from dehydration melting at low pH(2)O of juvenile basaltic source rocks around 16 kbar and 850-1000 degrees C. Our study demonstrates that extreme fractional crystallization of granitic magma plays a significant role in W enrichment in the granitic melt.

Automated summary

Tungsten (W) deposits in the Southern Tianshan Orogenic Belt, Uzbekistan are related to high-Si granites which exsolve magmatic-hydrothermal fluids. The formation mechanism of these granites is still controversial. This study provides new geochronological and geochemical data, showing that the muscovite porphyritic granite is genetically related to W mineralization. The study suggests that extreme fractional crystallization plays a significant role in W enrichment in the granitic melt.