Abstract:

The newly discovered Lubuzhen gold-silver deposit in Ngamring County, Tibet is situated in the southern Lhasa microterrane and belongs to the west segment of the Gangdisi polymetallic metallogenic belt. The ore bodies, controlled by near NWW fractures, occur as veins and lens in the Eocene epoch volcanics of the Pana Formation and contain mainly breccia, quartz veins and altered rocks of coexisting ores. The major ore minerals in the deposit are natural gold, silver tellurite, etc. Moreover, based on the mineral assemblages and cutting relationship and alteration features, we identified three main types of hydrothermal veins: quartz-pyrite vein of the quartz-pyrite stage (S1), quartz-polymetallic sulfides vein of the quartz-polymetallic sulfides stage (S2), and quartz-carbonatation vein of the quartz-carbonation stage (S3). On the basis of detailed geological fieldwork, we selected quartz veins of different mineralization stages for the fluid inclusion study, including petrographic characteristic observation, micro temperature measurement, compositional analysis, and hydrogen and oxygen isotope analysis. Fluid inclusions in hydrothermal veins in all three stages consist of liquid-gas (liquid- and gas-rich) biphasic and daughter mineral-bearing (carbonate minerals) multiphasic fluid inclusions, with homogenization temperature, salinity (wt%, NaCl eqv.) and density ranging in 310-330 ℃, 5.0%-10.1% and 0.60-0.80 g/cm³, respectively, in S1, in 240-280 ℃, 3.0-7.0 and 0.70-0.90 g/cm³, respectively, in S2, and in 121-215 ℃, 1.0%-5.0% and 0.85-1.00 g/cm³, respectively, in S3. According to laser Raman spectroscopic analysis, the gas phase contained small amounts of CO₂, N₂, and CH₄, and the daughter minerals included some carbonate minerals. Hydrogen and oxygen isotopic analysis revealed that the δ${{\text{D}}_{{{\text{H}}_{2}}\text{O},\text{V-SMOW}}}$ values of fluid inclusions in quartz veins were from -106.1‰ to -97.5‰, and δ¹⁸${{\text{O}}_{{{\text{H}}_{2}}\text{O},\text{V-SMOW}}}$ values ranged from -7.33‰ to -7.13‰, which suggests that the origin of ore-forming fluids is mainly derived from circulating warm groundwater in the Pana Formation and with minor portion from magmatic activities. Our studies suggest that in the Lubuzhen Ag-Au deposit, ore-forming fluids move from the deep closed system to the shallow open system and quickly break through the critical state of decompression boiling, causing phase separation as minerals and metal sulfides precipitating from ore-bearing hydrothermal fluids. As heavy precipitation occurs, temperature and salinity of ore-bearing fluids decrease rapidly which ends the metallogenesis process for metal minerals. In brief summary, the ore-forming fluids in the Luobuzhen Ag-Au deposit contain small amounts of CO₂, N₂, and CH₄ gases and can be characterized by low to moderate homogenization temperature, mid-range salinity and low to mid-range density, all are the characteristics of typical epithermal deposit.

Key words: fluid inclusions, epithermal deposit, gangdese polymetallic metallogenic belt, Luobuzhen, oligocene epoch