English

Abstract

The Summer Camp Pit (SCP) is situated on the Placer Dome owned Getchell Property in Northern Nevada. The deposit is a typical Carlin type partly oxidized disseminated sulfide-micron gold deposit in strongly deformed and altered Paleozoic metasediments. The pit was mined by a former site owner from March 1990 to December 1991. Although the pit was mostly dry during operations, flows increased as mining deepened the pit, forcing the operators to begin periodic pumping of a small sump in the SW corner of the pit. A small sump developed which due to sulfide oxidation showed low pH and elevated metals and sulfate. To buffer this chemistry the pit was partly filled by water from the underground mining operations approximately 1 mile to the north of the pit and the water level maintained above the oxide-sulfide boundary. The pit lake was monitored at the site for approximately 10 years and during this time the pit was used as part of the site water management strategy. For operational reasons, water was removed from the pit in 2002 and this resulted in exposure of sulfides in the pit wallrock, causing further oxidation and acid generation to occur. The chemistry of contact water created by exposure to these materials exhibited low pH with elevated metals and sulfate concentrations. Getchell Gold Corporation evaluated various options for closure of the pit. Draining and partial backfill provided the most suitable closure alternative for this pit, because it would eliminate the pit lake, reducing the potential for future groundwater impacts and risks to terrestrial and avian wildlife. In order to evaluate the pit backfilling alternative, it was necessary to 1) identify appropriate and available backfill materials, and 2) ensure that those materials do not present an equal or greater risk to groundwater than the existing in-pit materials. The evaluation of potential backfill materials in the vicinity of SCP was based on environmental risk assessment, geochemical testwork and engineering considerations.