Geology and Genesis of the Ta'ergou Skarn - Quartz Vein Tungsten Deposit in the North Qilian Caledonian Orogenic Belt, Northwest China

摘要

The Ta'ergou tungsten deposit in Gansu province, northwestern China, is located in the western part of the North Qilian Caledonian orogen, and consists of scheelite skarn bodies and wolframite quartz veins. The tungsten-bearing skarn developed by the replacement of carbonate layers intercalated in the Precambrian schist and amphibolite whereas wolframite-quartz ore veins developed along a group of fractures that cut through horizontal skarn. The Ta'ergou tungsten deposit is genetically related to the Caledonian Yeniutan granodiorite intrusion and occurs ca. 500 m wide in the exo-contact zone 300 ~ 500 m apart from the intrusion. The granodiorite displays a lower grade of differentiation, low content of SiO_2 and high contents of mafic components. There are three types of fluid inclusions in the wolframite-quartz vein systems, i.e. aqueous, CO_2-H_2O and CO_2-rich. The homogenization temperature of aqueous inclusion ranges from 140 to 380 deg C and their salinities from 6.4 to 17.4 equivalent wt percent NaCl. Laser Raman spectroscopy shows that the inclusions contain a relatively high content of CO_2. The delta~(18)O values of skarn type sulfides range from +8.1 to + 12.7 per mil and those of quartz vein sulfides from +9.3 to + 14.9 per mil, similar to sulfides of the granodiorite with from +6.0 to +11.7 per mil. The delta~(18)O values of quartz are between +10.5 and +13.3 per mil and those of wolframite between +3.4 and +5.1 per mil. The delta~(18)O water values of ore forming fluids range from +0.6 to +6.4 per mil and suggest the mixture of magmatic fluids with meteoric water formed the ore-forming fluids. It has been proved that Precambrian strata in the west sector of North Qilian region are enriched in tungsten. We propose the strata were remelted to be tungsten-granitoid during subduction. The polymetallic tungsten was gradually accumulated into the roof pendants of the granite intrusion by fractional crystallization and then was deposited by hydrothermal fluids during meta somatism and infilling along fractures. On the other hand, the granite intrusion also acted as "heating machine" to make hydrothermal fluids leach out the metals from Precambrian strata and these metals joined the ore-forming hydrothermal system.