Host rock alteration and its application as an ore guide at the Midwest Lake uranium deposit, northern Saskatchewan

CIM Bulletin, Vol. 77, No. 868, 1984

J. HOEVE, Principal Research Scientist, Saskatchewan Research Council, Saskatoon, Saskatchewan

In the unconformity-type uranium deposit at Midwest Lake, mineralization is associated with extensive host rock alteration, which affects the metamorphic basement as well as the overlying Athabasca Group sandstones. Host rock alteration is manifested by (1) transformation of clay minerals [kaolinite into illite andchlorite (sudoite)}; (2) bleaching due to removal of hematite from the latentic paleoweathering profile and the overlying red bed, Athabasca Group sandstones; (3) loss of coherence of the sandstone resulting from corrosion of quartz cement; and (4) in the immediate vicinity of high-grade ore, development of a clay envelope due to residual enrichment of clay minerals caused by massive dissolution of quartz. Clay mineral transformation and bleaching are the most prominent and the most widespread alteration effects, forming haloes that extend for several kilometres along the strike of the mineralized structure and also constitute distinct alteration pipes that rise for several hundred metres above the unconformity to be truncated at the present top of the bedrock. Although the two haloes are largely congruent features, cross-cutting relationships indicate that clay mineral alteration represents an earlier phase in the history of the deposit than the large-scale bleaching that gave rise to the superimposed bleaching halo. Because clay mineral alteration and bleaching are intimately tied to the mineralization and extend well beyond the limits of the actual orebodies, host rock alteration may be used as an ore guide in exploration and drill-hole evaluation. In this application a third hydrothermal discharge centre was identified, as yet untested for ore, which is located between the hydro-thermal centres associated with the two known orebodies of the deposit. It is estimated that host rock alteration may enlarge a potential drill-target by a factor 10 - 20X. Information on the configuration of bleaching haloes can be obtained from systematic core logging, whereas diagnostic clay mineral alteration is readily identified by means of automated, quantitative XRD techniques.

Keywords: Economic geology, Host rock alteration, Uranium deposits, Mineralization, Athabasca Group, Midwest Lake.