Fluid inclusion and stable isotope studies of gold deposits in the Gamboke mineralized district, Pala area, Chad

Abstract

The Ganboke mineralized district in the Pala area in the southern part of Chad is largely located in the Precambrian greenstone belt. The gold mineralization occurs commonly as auriferous quartz veins (<0.6 m thick) together with networks or stockworks, which are spacially associated with shear zones trending N30°–45°E within greenstone. Paragenetic and micro-structural relationships of vein minerals reveal that gold-bearing veins might have emplaced during syn-deformation events within the shear zones. Gold occurs mainly as free grains (usually less than 0.5 mm in diameter) in white to grey quartz, together with finegrained disseminates of base metal sulfides. Au contents of vein quartz samples are variable within the concentration range of 0 to 2,946 ppm. Fluid inclusion study indicates that homogenization temperatures range from 200 to 400°C with salinities of less than 8 wt% eq. NaCl. The mean value of CO2 content is 17 mole percent. Phase separation of fluids by CO2 effervescence intensely occurred around 350°C and was linked to deposition of ore including gold. The estimated minimum pressure of mineralization is 0.8–1.6 kbars, based on the calculation of XCO2. Thermodynamic considerations show that ore minerals were dominantly deposited from the fluid evolved from an initial high temperature (near 380°C, log fS2=−5 atm) to a later low temperature (270°C, log fS2=−9 atm). The fluid's isotopic compositions (δD=−53.8 to −19.2‰ and δ18O=6.7 to 14.8‰) overlap the range of the metamorphic and primary magmatic waters and are similar to those of many plutonic-volcanic-hosted Archean gold deposits. However, more18O-depleted signature of the ore fluids for the deposits from the Ganboke South district indicate the involvement of meteoric water in the formation of ore deposits.