Occurrence and distribution of invisible gold in a Carlin-type gold deposit in China

From the results of EPMA on various minerals from the ore and from SEM observations and chemical dissolution experiments on pyrite in a Carlin-type gold deposit (Banqi gold deposit, Guizhou, China), the following conclusions are drawn: Pyrite is the most important Au-bearing mineral. Most of the pyrite grains occur as anhedral granules consisting of earlier formed, euhedral crystal cores containing little or no Au and As and later formed rims that are rich in Au and As. The average tenor of Au is257.6 ppm in pyrite, about 600 ppm in rims of anhedral grains, and less than 82 ppm rn cores. There is a positive correlation between the concentrations of Au and As, so the distribution of Au is similar to that of As in SEM images and EPMA data. EPMA data show highly variable Au concentrations at different spots in a Au-rich rim within a pyrite grain. Moreover, in chemical dissolution tests on pyrite, most of the Au is not in the supernatant HNO. solution but remains in the undissolved residues and is available for leaching into KI + I, AuJeaching solution. All these facts illustrate that Au in pyrite occurs as ultramicrometer native gold inclusions, not as isomorphous substitutron. It is shown that more than 600/o of the Au given by chemical assay of clay minerals occurs in submicrometer-size pyrite inclusions in which Au is remarkably enriched. The Au contents in other minerals, such as arsenopyrite, quartz, calcite, or barite, are all below the detection limit of EPMA. Considering the results of chemical assays of these mineral separates, it is concluded that these minerals are not significantly Au bearing. Knowledge of the mode of occurrence and distribution characteristics of invisible Au in Carlin-type deposits is beneficial to metalluryical process design for Au recovery. In this case, sufficient oxidation and leaching of the pyrite surface layer will produce satisfactory recovery at reduced cost in time, energy, and reagents.