Geology and Mineralization Related to Heap Leaching at the Rain Mine, Elko County Nevada

摘要

The Rain and SMZ mines from 1987 to 1994 produced a total of 22.01 of gold from a pre-mine reserve of 14.24 Mt of ore grading 2.13 g/t Au. Gold at Rain is hosted within the basal portion of the Mississippian Webb Formation. Mineralization is closely tied to the Rain Fault which is interpreted to be the main feeder for the deposit. Multiple collapse and hydrothermal breccia events provided permeability for the mineralizing fluids in the otherwise unreactive siltstones. Major alteration events include: decalcification and dolomitization of the Devils Gate Limestone; and silicification, baritization, argillization and oxidation of the Webb Fm. Six ore types based on XRD and visual rock chip logging have been identified: siliceous ore containing >40 percent quartz and <30 percent barite; siliceous-baritic ore containing mostly silica but >30 percent barite; baritic ore containing >40 percent barite; argillaceous ore containing >40 percent total clays (including alunite); calcareous ore containing mostly calcite and dolomite; and carbonaceous ore which has significant pyrite and organic carbon. Metallurgical testing of the oxide leach grade ores found that recoveries, on -1.3 cm crushed material, were greater than 70 percent. Results also found that Rain ore has strong grind dependency due to silica encapsulation. The Rain leach pad was commissioned in 1988, and to date 9.3 Mt at an average grade of 0.65 g/t Au have been placed. Life to date recovery, through December 1995, is approximately 51 percent, for a total of 3.871 of gold produced. Two major factors have contributed to the low leach recoveries at Rain when compared with the metallurgical testing: 1) More argillaceous ore than expected was placed on the pad especially late in the mine life and 2) approximately 50 percent of the tons placed were run-of-mine with no agglomeration.