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.
Trace-element characteristics of different pyrite types in Mesoarchaean to Palaeoproterozoic placer deposits
A comparative in situ LA-ICP MS trace-element study on pyrite from three different, variably auriferous, Archaean to Palaeoproterozoic palaeoplacer deposits in the Ouro Fino Syncline (Quadrilátero Ferrífero; Brazil), the Elliot Lake area north of Lake Huron (SE Canada) and several deposits within the Witwatersrand Basin (South Africa) revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits. Especially the Ni and Au concentrations as well as Co/Ni and Mo/Ni ratios were found to be systematically different in detrital compact, detrital porous and post-sedimentary/hydrothermal pyrite grains from different source areas. High Co/Ni ratios and low Au concentrations are typical of post-sedimentary pyrite, which is hydrothermal in origin. In contrast, relatively low Co/Ni ratios and high Au contents characterise detrital porous banded and concentric pyrite grains (Au > 1 ppm), which are syn-sedimentary in origin. In the Elliot Lake area and the Witwatersrand Basin, detrital compact rounded pyrite is characterised by high Co/Ni ratios, which is in agreement with derivation from a hydrothermal source. Low Au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks. In contrast, derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the Ouro Fino Syncline because of low to intermediate Co/Ni ratios. High Au contents may indicate a genetic relationship between pyrite and gold there. Systematic differences exist between the three areas with respect to Au, Ni, Co, Mo and Cu distributions in detrital pyrite, which reflects differences in the provenance. A predominantly mafic/ultramafic source is indicated for the Ouro Fino, a felsic source for the Elliot Lake, and a mixed felsic–mafic provenance for the Witwatersrand pyrite populations. Independently of pyrite type, the higher Au endowment of the studied Witwatersrand and Ouro Fino conglomerates are also reflected by an overall higher Au concentration in the respective pyrite grains compared to the relatively Au-poor samples from Elliot Lake. In general, a strong positive correlation between Au and Pb levels in the various pyrite grains is noted. Analogous to Pb, which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete PbS phases, Au is considered to be present mainly in the form of discrete Au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice.
VLA stereoscopy of solar active regions. 2: Altitude, relative motion, and center-to-limb darkening of 20 centimeter emission
A newly developed method of stereoscopic correlation is applied to solar radio maps at a wavelength of 20 cm. On the basis of stereoscopic correlations between radio maps obtained on 6 different days we establish the presence of 66 radio source components associated with 22 active regions. We find the following statistical results for active region source structures at 20 cm: 1. The observed lifetime is consistent with an exponential distribution having an e-folding time scale of greater than 18 days. 2. The average altitude of 20 cm sources is 25 +/- 15 Mm; 90% of the sources are found in heights less than 40 Mm. 3. The average diameter of discrete source structures is 48 +/- 15 Mm, implying a vertical/horizontal aspect ratio of q(sub A) approx. = 0.5. 4. No significant source motion has been found with respect to the standard differential rotation rate of Omega = 13.45 deg - ((3 deg (sin(exp 2)B)). 5. We find a statistical limb darkening, which can be described by the relation (T(sub B)(alpha))/T(sub B)(0)) = 0.4 + ((0.6(cos(exp 2) alpha)) for sources with T(sub B) greater than 0.5 MK. 6. The degree of source polarization is 15% +/- 10% and is independent of source location. 7. Bright sources (greater than or approximately equal to 0.5 MK), or equivalently, long-lived sources (greater than or approximately equal to 5 days) show a systematic variation of their altitude as function of the center-limb distance.