Mineralogical and metallurgical study of supergene ores of the Mike Cu-Au (-Zn) deposit, Carlin trend, Nevada

The undeveloped Mike Cu-Au(-Zn) deposit on the Carlin trend of Nevada, USA, currently held by Newmont Mining, hosts an unusual suite of oxide Cu and Au minerals. This study evaluates the possibilities for economic recovery by 1) comparing the Cu and Au extraction achieved from the supergene ores by six different lixiviants, and 2) identifying which minerals cause low recovery for each lixiviant and why. Gold is present as auricupride and electrum and copper exists as conichalcite (Ca-Cu arsenate), native Cu, cuprite, chrysocolla, and minor malachite, with locally significant copper in jarosite, hematite, and goethite. The reagents tested were sulfuric, sulfurous, and methanesulfonic acids to recover Cu, cyanide and thiourea to recover Au, and glycine to recover both. No reagent recovered both Cu and Au effectively, although sulfuric and methanesulfonic acids recovered most of the Cu and cyanide and thiourea recovered most of the Au. Glycine recovered nearly all of the Au and >50% of the Cu from the cuprite- and native-Cu-dominated ore type, but did not recover much Cu or Au from any of the other ore types. This supports the hypothesis that Cu2+ catalyzes Au dissolution in glycine, and thus effective Au leaching by glycine from oxide Cu-Au ores may be contingent upon the solubility of the Cu minerals in glycine. Post-leaching QEMSCAN analysis indicates that conichalcite and Cu-bearing Fe oxides failed to dissolve completely in all reagents and were the principal causes of low recovery. In addition, native Cu dissolved only partly in thiourea, and chrysocolla dissolved only partly in thiourea, glycine, or cyanide. Other observed mineralogical changes include the total loss of dolomite and partial loss of alunite and iron oxide from all samples, with apparent gains in alunite and jarosite.