Mineralogical and Isotopic Relations in the Port Radium Pitchblende Deposit, Great Bear Lake, Canada

Citation

Jory, Lisle Thomas (1964) Mineralogical and Isotopic Relations in the Port Radium Pitchblende Deposit, Great Bear Lake, Canada. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/W43G-ED67. https://resolver.caltech.edu/CaltechETD:etd-10112002-152503

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. This thesis integrates the data from field, petrographic, X-ray, and lead and uranium isotopic studies on the Port Radium pitchblende deposit, Great Bear Lake, Canada. The oldest rocks exposed are andesitic-dacitic bedded tuffs and flows of the Echo Bay group. Overlying detrital rocks of the Cameron Bay group were accumulated before intrusion of hypabyssal, dacitic porphyry bodies and granitic rocks. Sandstones of the Hornby Bay group lie unconformably on the above groups and on denuded granitic rocks. Folds are generally open in the Echo Bay and Cameron Bay groups; the Hornby Bay group is flat-lying. The Echo Bay group was metamorphosed to hornblende hornfels facies rocks, probably during intrusion of granitic rocks. Fault and fracture zones, most commonly striking northeasterly and dipping steeply north, were in places loci for the successive introduction of "giant quartz veins", diabase dykes, and complex vein mineralizations. In the development of the Port Radium deposit, six stages of metallic and non-metallic vein mineral deposition are recognized. Three periods of wall rock alteration are correlative with episodes of vein mineralization. Tuffs of the Lower Echo Bay subgroup are the predominant host rocks to the ore mineralization. Localization of the pitchblende is attributed to physical factors. Diabase sills, intruded after development of the Port Radium deposit, were fractured during late-stage movements on the veins. An age of 1820[plus or minus] 30 million years is assigned to a granitic rock at Port Radium on the basis of the lead-uranium isotopic analyses of seven zircon fractions. This sets a lower limit on the age of the Echo Bay and Cameron Bay groups and an upper limit on the age of Hornby Bay group and diabase dykes. In the zircon fractions, uranium and radiogenic lead concentrations increase with decreasing average grain size. All fractions show the discordant age pattern Pb [superscript 206]/U[superscript 238] < Pb [superscript 207]/U[superscript 235] < Pb[superscript 207]/Pb[superscript 206]. On a "Concordia" diagram, points representing the different zircon fractions define a chord intersecting the Concordia curve at 1815 and 50 million years. Microscopic and macroscopic samples of three pitchblende specimens from the Port Radium mine were analysed isotopically for lead and uranium. For a specimen of pitchblende in carbonate gangue, concordant lead-uranium ages were obtained on two microscopic samples. The age, 1445[plus or minus] 20 million years, is the only concordant age which has been measured for the Port Radium ores. It places a lower limit on the age of the "giant quartz veins", the diabase dykes, and possibly the Hornby Bay group, and an upper limit on the diabase sills. For a specimen of pitchblende in siliceous gangue, two analyses define a chord intersecting the Concordia curve at 1450 and 300 million years. In general, Port Radium pitchblende samples show evidence of past loss of lead; the lead was removed from the vein systems. Comparison of isotopic data on microscopic and macroscopic samples from the same specimen shows that the microscopic samples can be less discordant and have higher lead-lead ages. Analyses of a number of lead-uranium systems in the same specimen offers a possible means of determining the age of mineralization and the age of an episodic disturbance. Of eight samples of galena and one of chalcopyrite analysed isotopically for lead, three are ordinary lead, four J-type anomalous lead and two possibly mixtures of ordinary lead and radiogenic uranium lead derived from the pitchblende. The ordinary lead is finely disseminated through the other vein minerals and was probably introduced shortly after the pitchblende. The J-type lead occurs in late, lenticular veinlets and was not formed by mixing with radiogenic lead from the pitchblende ores. Because of the absence of mixing and because of considerations concerning the geochemical character of the source system for the J-type lead, this lead was probably introduced less than 300 million years ago. Disturbances to the lead-uranium systems of the zircon and pitchblende samples provide further isotopic evidence for processes probably active within the last 200-300 million years but not recognized on the basis of geological field data. Additional isotopic work to confirm and elaborate on these interpretations is suggested.

Thesis Files