Microthermometry and Raman analysis of fluid inclusions in different generations of quartz veins of the Athabasca Basin

The Athabasca Basin (Canada) contains the highest grade unconformity-type uranium deposits in the world. Underlying the Athabasca Group sedimentary rocks of the Dufferin Lake zone are variably graphitic pelitic schists (VGPS), altered to chlorite and hematite (Red/Green Zone: RGZ), and locally bleached near the unconformity during paleoweathering and/or later fluid interaction, leading to a loss of graphite near the unconformity. Fluid inclusions were examined in different generations of quartz veins, using microthermometry and Raman analysis, to characterize and compare the different fluids that interacted with the RGZ and the VGPS. In the VGPS, CH4-, N2- and CO2-rich fluids circulated. CH4- and N2-rich fluids could be the result of the breakdown of graphite to CH4/CO2, whereas N2-rich fluid is interpreted to be the result of breakdown of feldspars/micas to NH4+/N2. In the RGZ, highly saline fluids interpreted to be basinally derived have been recorded. The circulation of the two types of fluids (carbonic and brines) occurred at two different distinct events: 1) during the retrograde metamorphism of the basement rocks before the deposition of the Athabasca Basin for the carbonic fluids, and 2) after the deposition of the Athabasca Basin for the brines. Thus, in addition to possibly be related to graphite depletion in the RGZ, the brines can be linked to uranium mineralization.

Organic matter, isotopic composition of calcite veins in basement basalt and pore water at DSDP Leg 78A Holes

Sediments of the Barbados Ridge complex, cored on DSDP Leg 78A, contain low concentrations of acid-insoluble carbon (0.05-0.25%) and nitrogen (C/N 1.5-5) and dispersed C1-C6 hydrocarbons (100-800 ppb). The concentrations of organic carbon and 13C in organic carbon decrease with depth, whereas the concentration of dispersed hydrocarbons increases slightly with depth. These trends may reflect the slow oxidation of organic matter, with selective removal of 13C and slow conversion of the residual organic matter to hydrocarbons. Very minor indications of nitrogen gas were observed at about 250 meters sub-bottom at two of the drilling sites. Basement basalts have calcite veins with d13C values in the range of 2.0 to 3.2 per mil and d18O-SMOW values ranging from 28.5 to +30.6 per mil. Interstitial waters have d18O-SMOW of 0.2 to -3.5 per mil and dD-SMOW of -2 to -15 per mil. The oxygen isotopic composition of the calcite veins in the basement basalts gives estimated equilibrium fractionation temperatures in the range of 11 to 24°C, assuming precipitation from water with d18O-SMOW in the range of +0.1 to -1.0 per mil. This suggests that basalt alteration and precipitation of vein calcite occurred in contact either with warmer Campanian seawater or, later, with pore water, after burial to depths of 200- 300 meters. Pore waters from all three sites are depleted in deuterium and 18O, and dissolved sulfate is enriched in 34S at Sites 541 and 542, but not at Site 543.

Isotopic analyses of DSDP Hole 45-395A samples from veins containing gypsum (Table 1)

Gypsum and halite crystals, together with saponite and phillipsite, were found in a vein in a basalt sill 625 m below the sea floor at DSDP Site 395A, located 190 km west of the crest of the Mid-Atlantic Ridge. The delta34S value of the gypsum (+19.4?) indicates a seawater source for the sulfate. The delta18O values of the saponite (+19.9?) and phillipsite (+18.1?) indicate either formation from normal seawater at about 55°C or formation from delta18O-depleted seawater at a lower temperature. The gypsum (which could be secondary after anhydrite) was formed by reaction between Ca[2+] released from basalt and SO4[2-] in circulating seawater. The halite could have formed when water was consumed by hydration of basalt under conditions of extremely restricted circulation. A more probable mechanism is that the gypsum was originally precipitated as anhydrite at temperatures above 60°C. As the temperature dropped the anhydrite converted to gypsum. The conversion would consume water, which could cause halite precipitation, and would cause an increase in the volume of solids, which would plug the vein and prevent subsequent dissolution of the halite.