Mining Methods And Operation Gold Mines Bald Mountain

      The Bald Mountain mine is one of largest gold mines in the world. The Bald Mountain mine lies within the Southern Ruby Mountains of northeastern Nevada, approximately 110 kilometers northwest of Ely, Nevada and 110 kilometers southeast of Elko, Nevada. Bald Mountain is operated by Barrick and ore is sourced from multiple open pits over an estimated 600 km2 property with processing at multiple conventional heap leaching facilities. Bald Mountain is the largest mine site by area in the U.S. It stretches 40 km north to south and 16 km east to west with the north and south areas being 19 km apart.

History Bald Mountain mine 
      The Bald Mountain mine lies on the western flanks of Little Bald Mountain and Big Bald Mountain. Rocks exposed in the area comprise a conformable, generally SSE dipping sequence of Cambrian to Silurian limestones, dolomites, shales, quartzites and siltstones. These sediments have been intruded by the Jurassic (159 Ma) Bald Mountain quartz-monzonite stock, and related felsic dykes, along a major north-west structural trend. Intense pre- and post-mineral faulting has produced a complex structural regime at Bald Mountain, controlled by the intersection of deep crustal NW and NNE trending fractures which have localised and controlled the location of gold deposits. Overall , the pluton, associated dykes and Au mineralisation are all controlled by the crustal scale, NW trending Bida Trend structure. Every rock exposed in that part of the district, from the lower Cambrian Hamburg Dolomite to the Silurian Laketown Dolomite has hosted some ore (Placer Dome, 1995).
      Mining has occurred in the Bald Mountain Area since the late 1800s. Minerals recovered include copper, antimony, silver, and gold. Most of the early mining occurred next to a small granitic intrusion south of Big Bald Mountain. From 1869 to 1877 minor amounts of silver chloride, antimony, lead and copper were production, whereas tiny quantities of gold were taken from a nearby creek. From 1876, when the Copper Basin mine was established, operations followed sporadically until the 1930's. In the 1940's and 1950's tungsten was mined from skarn deposits nearby. Significant exploration was commenced in the 1960's, with an emphasis on gold. Several 'deposits' were discovered, with a pilot scale heap leach operation being started in 1983. This was upgraded to a full production facility in 1985, with the development of the Top Area, followed in 1988 by the RBM Area orebody (Placer Dome, 1995).Operation Bald Mountain mine is by open pit at a rate of about 1 Mt of ore+waste per month (1995). In the early 1990's Bald Mountain produced 1.5 to 1.8 t Au and 0.15 to 0.18 t Ag per year from conventional heap leach methods, comprising agglomeration, leaching, carbon recovery, stripping, electrowinning and dore production (Placer Dome, 1995). In August 1994 the nearby Alligator Ridge mine was purchased by Placer Dome US Inc and amalgamated with the Bald Mountain operation (Placer Dome, 1995).

Mining Methods And Operation Bald Mountain Mine

      The Bald Mountain Mine (BMM) is an open pit mining method, run - of - mine, heap leach gold mine located in northeastern Nevada, United States. BMM owns approximately 9,010 claims within the Bald Mountain Mining District and this area covers approximately 154,440 acres. The total land position is approximately 30 miles long north to south by 11 miles wide east to west. Bald Mountain Mine consists of open pit mines, waste rock dumps, process leach pads, associated process ponds, and process buildings. Power is provided via a 17 mile 69 KVA power line from the Alligator Ridge Mine substation. Water is supplied by wells located in the mine property. Facilities have been designed, constructed, and are operated, for zero discharge to the environment
      Prior to 2011 gold recovery was conducted at processing facilities at Process in Mooney Basin (now identified as Mooney Basin North). Since the 2010 Recertification Audit the construction of new gold recovery facilities were completed south of Mooney Basin. These new facilities identified as Mooney Basin South were under construction during the 2010 Recertification Audit but were not commissioned for operation until July 2011. These facilities include a heap leach pad, pregnant solution tank (constructed adjacent to a process/barren solution pond), an events pond, and a CIC processing facility that includes an off load facility for cyanide. Except for the addition of the Mooney Basin South facilities no changes or modifications were made to the Bald Mountain Mine facilities since the 2010 Recertification Audit.

      An operational change in February 2013 incorporated the discontinuation of stripping and refining at Process. Loaded carbon from the three carbon plants is now sent to Barrick’s Goldstrike operation at Carlin, NV for further gold recovery. Mining is conducted at multiple pits located on the property and run - of - mine ore is hauled to heap leach pads at process, Mooney Basin North, and Mooney Basin South. The Process Mooney Basin North heap leach pads incorporate composite liners with solution collection systems. The liners consistofan 80 - mil high - density polyethylene (HDPE) primary liner placed over a prepared 12 - inch thick, imported low hydraulic conductivity soil layer. Under drain systems of perforated piping are placed over the 80 - mil HDPE liners for solution collection and reduction of head on the liner systems. The heap leach pad solution collection channels consist of HDPE lined channels and leak detection, collection, and removal systems to contain any process solution leaks from the collection channels.
      The Mooney Basin South heap leach pad also incorporates a composite liner with a solution collection system. However, the liner consists of an 80 - mil high density polyethylene (HDPE) primary liner placed over a prepared 12 -inch thick, imported low hydraulic conductivity soil layer or over a Geosynthetic Clay Liner (GCL) with equivalent or lower maximum permeability placed on a prepared subbase. An under drain system of perforated piping is placed over the 80 - mil HDPE liner for solution collection and reduction of head on the liner system. Pregnant solutions from the leaching operations at Process and Mooney Basin North pads report to the pregnant ponds from where it is pumped to the adsorption, desorption and recovery (ADR) plants at the respective facilities. The pregnant ponds are designed and constructed with 80 - mil HDPE primary liners and 40 - mil HDPE secondary liners placed over prepared native soil subgrade. A HDPE geonet is sandwiched between the primary and secondary liners to act as a leak detection system. The Mooney Basin South pregnant solution is collected within the under drain piping system that conveys it by gravity to a collection header pipe running in channel adjacent to the pad, which serves as secondary containment. Under normal operating conditions, the pregnant solution reports directly to the Process Building. Alternatively, in the case of a temporary shutdown of the plant for maintenance, the pregnant solution may be diverted to the Solution Pond and later pumped to the plant once it has restarted. The Solution Pond is designed and constructed with an 80 - mil HDPE primary liner and a 60 - mil HDPE secondary liner placed over a combination of prepared native soil and engineered compacted till. Sandwiched between the primary and secondary liners is an HDPE geonet for collection and transfer of fugitive process solution, via gravity, to the leak detection sump. The pregnant solutions are pumped to the carbon columns where it gravity flows, countercurrent to the activated carbon for precious metal recovery. Barren solutions report to the barren tanks where the pH is adjusted, if necessary, and sodium cyanide is added prior to pumping to the heaps. The loaded carbon from the last columns is removed and transported to the Goldstrike Mine for stripping and refining.
      In early 2015, the BLM is expected to release a draft environmental impact statement on Barrick’s plan to expand the Bald Mountain mine. The proposed expansion to Bald Mountain’s north operations would increase the area from 38 km2 to 55 km2 and add four new heap leach pads. The south operations expansion would increase the disturbance area from 4 km2 to 14 km2. The project would also install an electric transmission line and build a road linking the two mining areas. Bald Mountain produced 161,000 ounces of gold in 2014 at all-in sustaining costs of $1,070 per ounce. For 2015, Barrick is forecasting production of 170,000 – 195,000 ounces of gold.