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Sampling problems and hydraulic factors related to the dispersion of scheelite in drainage sediments, Clea property, Yukon Territory Saxby, Donald William


Multifractional analysis for scheelite (G=5.9-6.1), magnetite (G=5.2), heavies (G>3.3), mediums (3.3<G<2.9), and lights (G<2.9), in drainage sediments downstream of the Clea tungsten-bearing skarn deposit, Yukon Territory, revealed three interdependent problems that complicate interpretation of results of drainage surveys for W: (1) Scheelite concentrations in stream sediments reflect, in part, hydraulic sorting rather than source distribution. (2) Low numbers of scheelite grains in stream sediments cause high random sampling and subsampling errors and present analytical difficulties. (3) Scheelite/heavies or scheelite/mediums ratios may not be appreciably higher downstream of scheelite-bearing skarn than barren bedrock, because scheelite, heavies and mediums occur in increased amounts in skarns (i.e. they covary in source materials). Hydraulic effects result in systematic changes in mineral distributions between high and low energy environments (characterized by coarse gravel and fine gravel to sand, respectively). Scheelite, magnetite, heavies and mediums concentrations in minus 10-mesh sediments are greater in high than low energy environments, and this relative enrichment increases with grain size and density (up to one-hundredfold for scheelite). Using a regression method, developed in this study, hydraulically equivalent sizes of magnetite and heavies were empirically determined for finer (3.0-3.5 phi and 3.5-4.25 phi) scheelite sizes. Ratioing the weight of scheelite to that of a hydraulically equivalent mineral greatly reduces hydraulic variability. Resulting profiles of hydraulically equivalent scheelite concentrations more clearly delineate locations of scheelite input to the stream. The rare-grain problem can be reduced at the sampling level by sampling from high energy environments (thereby exploiting hydraulic effects) and/or by sampling for finer scheelite. Heavy mineral separation is the most effective means of reducing the rare grain problem at the subsampling and analytical levels. The problem of covariance in source materials can best be remedied by ratioing to an individual (hydraulically equivalent) heavy mineral which has a constant and ubiquitous distribution in the area of interest. In this study, magnetite shows promise, but more information on its source distribution is needed.

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