Abstract

Carbonate content was determined for 888 samples from the Salt Wash member of the Morrison formation [Late Jurassic] in the Slick Rock and Uravan mining districts in southwestern Colorado. The carbonate content of most samples was determined semi-quantitatively by calculating the amount of calcite equivalent to the mass of carbon dioxide evolved when the samples were treated with 3 normal hydrochloric acid. The content of some samples was determined in the course of standard chemical assays, and the content of others was visually estimated. Samples were assigned to categories or "rock types" on the basis of gross lithology, V-U content, degree of epigenetic alteration, and degree of oxidation through weathering. The average carbonate content was determined for each rock type, and the distribution of carbonate around oxidized and unoxidized ore was investigated. Results indicate that sandstone in the uppermost (ore-bearing) part of the Salt Wash member contains 2.5 to 3.0% carbonate, whereas sandstone in the lower (generally barren) part of the Salt Wash member contains about 13% carbonate. Altered mudstone in the Salt Wash member contains about 4% carbonate, and unaltered mudstone about 7%. Carbonate-rich zones in sandstone adjacent to contacts with mudstone may be of syngenetic or early diagenetic origin, whereas carbonate-rich zones associated with ore bodies may be genetically related to the ore deposits. Where the sandstone has been subjected to weathering, the overall distribution of carbonate does not seem to have been greatly affected.

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