Source of the detrital components of uraniferous conglomerates, Quirke ore zone, Elliot Lake, Ontario, Canada
Large volumes of pyritic quartz-pebble conglomerate in the Elliot Lake district of Ontario contain at least 300,000 tonnes U3O8 recoverable at a grade of 0.1% (ref. 1), with much greater tonnages available at lower recovery grades. The Quirke zone alone contains in excess of 224,000 tonnes U3O8 at ∼0.1% (refs 2–4) and is, accordingly, one of the world's largest known uranium deposits. Situated 40 km north of the North Channel of Lake Huron, 400 km north-west of Toronto, ore conglomerates occur in the Matinenda Formation, locally the basal unit of the Huronian Supergroup, and, although their age is not well constrained, they are probably close to 2,350 Myr old.5 Uraninite, the most important primary ore mineral, occurs as poorly rounded to euhedral grains 0.05–0.2 mm across within the matrix between quartz pebbles. While its chemical and morphological characteristics have led most workers to consider that the uraninite is detrital5–7, there has hitherto been no attempt to characterize its source lithology, a matter of considerable significance as regards mineral exploration. This was the aim of the present study. We have examined the mineral and fluid inclusions observed in the quartz pebbles in the conglomerate and conclude that they were derived from a pegmatitic, potassic (alaskitic) granite. We have also demonstrated that in this source lithology, pebble-sized quartz was spatially associated with radioactive minerals, by inference, uraninite. The postulated characteristics of such a source are remarkably similar in several respects to the uraniferous alaskites of the Rössing deposit in Namibia.
Fluid inclusions in quartz from deep-seated granitic intrusions, south Norway
Abstract H2O, CO2, and H2OCO2 inclusions were observed in quatz from deep-seated granitic intrusions belonging to the Precambrian Farsund plutonic complex, south Norway. These inclusions represent solidus and/or sub-solidus fluids that were present in these rocks at some period between the initial melt and the present. Early CO2 and H2OCO2 inclusions with about 20 mole% CO2 contain up to 10 mole% CH4 in the CO2 phase and have densities from 0.96 to 0.85 g/cc. These inclusions are considered to most nearly approximate solidus vapour phases and suggest conditions of final solidification of the magma at 5 to 6 Kb and 700°C to 800°C. The H2O inclusions have salinities between 2 and 60 wt%; the majority contain 5 to 20 equivalent wt.% NaCl and have densities from 1.05 to 0.85 g/cc. Microthermometry indicates that other cations such as K+, Ca2+ and / or Mg2+ are present in these aqueous fluids. The H2O inclusions primarily represent fluids present at a post-magmatic stage of fracturing and healing of these rocks during uplift.
Phase equilibria in fluid inclusions from the Khtada Lake metamorphic complex
Abstract The Khtada Lake. British Columbia, metamorphic complex consists of high grade amphibolite and metasedimentary units with development of gneiss, migmatite and homogeneous autochthonous plutons. Maximum metamorphic conditions are estimated to have exceeded 5 kbar and 700°C. Fluid inclusions in matrix quartz are highly variable in density and composition, ranging from apparently pure CO 2 (gas or liquid or both at room temperature) through CO 2 + H 2 O ± CH 4 mixtures to inclusions which are entirely aqueous. They occur along cracks, in groups without planar features and as isolated inclusions. The latter and some which occur in groups, are interpreted to most nearly approximate, in density and composition, the fluids present during the peak of metamorphism. The density and fluid composition data are derived from direct observations of phase changes between − 180 and + 380°C and from the application of published experimental data in the system CH 4 -CO 2 -H 2 O-NaCl. The most dense, pure CO 2 inclusions indicate a pressure of entrapment at 5 kbar, if a temperature of 700°C is assumed. This is in close agreement with the minimum P - T estimates from the mineral assemblages. Methane was positively identified in inclusions in graphite-bearing specimens. Salt content is concluded to be about 5–6 wt% NaCl equivalent in the aqueous phase in both aqueous and CO 2 + H 2 O inclusions. There is evidence of immiscible separation of CO 2 -rich and H 2 O-rich fluids at temperatures at least as high as 375°C.