Tuesday, February 14, 2012

Waihi Gold Mine

Paraphrased from ‘Waihi’s Gold’ brochure by the Newmont Gold Co, Waihi, NZ.  A leading gold producer with operations on 5 continents.Gold was discovered at Martha Hill in 1978 by John McCombie and Robert Lee.  The first stamper battery started operating in 1882.  The Martha mine was closed in 1952 after producing 5.6 million oz of gold and 38.4 million oz of silver.  In the 70s rising gold prices led to the reopening of the mine.
The Martha open pit and the Favona reef system underground mines are currently being operated.  In recent decades the Martha mine has been producing an average of 100,000 oz gold and 700,000 oz silver/yr. 
The Waihi gold deposits sit within an andesitic pluton created millions of years ago.  Earthquakes cracked the pluton and hot water rose up through the cracks and left veins or reefs of quartz and trace minerals like gold and silver.  Erosion has since exposed the pluton.  The Martha Lode is huge by world standards - 1.6km long, 600m deep, and up to 30 m wide.  This has mostly been worked out now and smaller veins are being exploited.  Rock with as little as one gram of gold per tonne can be mined economically using open pit modern technology.  The open pit operates 12 hrs/day, 5.5 days/wk.  The Favona underground mine operates 24/7.
The area was explored in the 70s and 80s by drilling 92 45degree holes up to 450m deep.  Other exploration methods include trenching, aerial surveys, stream and surface sediment sampling, and geological mapping. 
Mining of ore and waste rock occur separately.  Rock is loosened by blasting and removed, and the blasted areas are bolted, meshed, and sprayed with shotcrete.  Since 2006 most activity in the open pit has been rehabilitation work.
Blasted ore is run through a primary crusher, than old mining debris (steel, wood, etc) is removed.  The crushed ore is delivered to the mill on a long conveyer belt.  Then it is ground to a fine powder in a mill with hardened steel balls, lime, and water.  This is placed in large tanks where a weak cyanide solution dissolves the gold and silver, which is then adsorbed onto activated carbon granules.  The carbon is screened out and washed with super-heated water to remove the precious metals.  The metals are removed from solution in electro winning cells where they adhere to stainless steel cathodes.  The muddy sludge off the cathodes is dried and put in a furnaces with fluxes.  The result is a slag of impurities and 99% pure gold/silver mix. 
The slurry of tailings is stored until the solids settle out.  UV lights breaks down the residual cyanide into carbon and nitrogen.  Underdrains and perimeter drains surround the tanks to isolate seepage from groundwater.  Waste rock embankments are gradually rehabilitated into pasture and wetland areas.  According to Newmont, waste water pumped out of the mines and from tailing pools is carefully monitored for cleanliness before it is released into a nearby river.
An eighteen-member mine rescue team of Newmont employees is training in firefighting, hazmat, rope and confined space rescue, first aid, and use of closed-circuit long duration breathing apparatus for underground rescue.  The team is available to assist local emergency services in non-mine events. 
The long-term rehabilitation plan for the mine involves transfer of the area to a charitabl. 
Modern uses of gold include treatment of arthritis, some cancers, chronic ulcers, patching damaged blood vessels, nerves, bones, and membranes, dental work, electronics, solar radiation protection in the space program, insulation and glare reduction in hi rises, and more.  Uses of silver include xray film, burn creams, purifying water, production of some textiles, batteries, photographic film, and solar panels.