Abstract Volcanic redbed copper (VRC) deposits occur as concordant and peneconcordant disseminated and crosscutting vein and fault-controlled copper sulphide and/or native copper deposits in predominantly subaerial volcanic sequences. They are characterized by relatively simple copper sulphide and/or native copper mineral assemblages, contain variable amounts of silver, and are distinct from submarine, polymetallic, volcanic-associated massive sulphide base metal deposits. The name ‘volcanic redbed copper’ is used to draw attention to the similarity with redbed- and Kupferschiefer-type deposits in sedimentary sequences and to the fact that oxidized flow tops and some red interlayered sedimentary rocks are probably essential in the genesis of these deposits. In mixed volcanic and sedimentary sequences, sedimentary and volcanic redbed copper deposits occur together and, in places, no clear distinction can be drawn between the two deposit types.

Abstract The Section 23 mine is one of about 18 large uranium mines opened in sandstones of the fluvial Westwater Canyon Member of the Jurassic Morrison Formation, in the Ambrosia Lake uranium district, during the early 1960s. Two distinct types of unoxidized ore occur in the mine. One type consists of uranium-rich authigenic organic matter that impregnates parts of the reduced sandstone host rocks. This type of ore occurs as peneconcordant layers which are typically elongate east-southeast, subparallel both to the sedimentary trends and to the regional strike of the host rock. A second type of ore is essentially devoid of organic matter and occurs in thick zones which conform to interfaces that separate oxidized from reduced parts of the host rocks. Genesis of the second type of ore is similar to that of roll-type deposits in Tertiary rocks of Wyoming and Texas. Organic matter in the primary ores was probably introduced into the host rock as humic acids that precipitated in the pores of the sandstone. This material is inferred first to have fixed uranium as urano-organic compounds but, with further diagenesis, to have released some of the uranium to form coffinite. Vanadium, molybdenum, and selenium are associated with primary ore and may also have been fixed by the organic matter. The secondary or roll-type ores contain uranium mostly in the form of coffinite and only rarely as uraninite. They also contain vanadium and selenium but are virtually devoid of molybdenum.