Marigold Mine is a run of mine heap leach operation that commenced production in 1988. The mine was transformed from a modest, conventional milling operation into a large and extremely efficient heap leach mine. Exploration led to the discovery of the Millennium ore body in the southern reaches of the property which then led to the Millennium Expansion project.
Goldcorp has a 66.67% interest in the Marigold mine and Barrick holds the remaining 33.33% interest. In 2009, Goldcorp'share of gold production from the Marigold mine amounted to 98,000 ounces of gold.
The Marigold mine is situated in Humboldt County, Nevada at the north end of the Battle Mountain-Eureka Trend that extends through central Nevada. The region is characterized a high desert with low humidity, clear skies and large daytime variations in temperature. The annual average daily temperature is about 9 degrees C. Most precipitation (20-25 cm per year) falls as snow during winter months. The remainder of the year is dry with isolated rainfall occurring in thunderstorms during the summer months
The property consists of 28.9 square miles, including 13 square miles of fee mineral interest. The remaining 15.9 square miles are unpatented mining claims, 6.1 square of which are not subject to royalties.
Geology and Mineralization
The gold mineralization at Marigold is best described as distally disseminated. This means that the gold is finely disseminated into the host rock as very small particles and that it is far from its origin, meaning that the gold at Marigold was transported in from greater depths and along zones of hot water flow. This occurred along faults and fractures in the rock acting as fluid conduits. Ore deposition at Marigold is controlled by both favorable stratigraphy and or structurally prepared rock types near these conduits to deeper within the earth. The rock formations exposed or mined into at Marigold consist of the Valmy, Antler, and Havallah; all three formations have members that act as host rock. These sedimentary and meta-sedimentary rocks are Paleozoic in age with the Havallah formation as the upper plate of the Golconda thrust fault. The great majority of mineralization occurs in or below the Golconda thrust fault which has acted as a type of seal, causing the deposition of gold into favorable host rock.
The various open pit mines at Marigold contain examples of the variety of rock hosting gold mineralization; these include limestone, siltstone, breccias, meta-basalts and quartzite. To-date little in the way of sulfides has been encountered as exploration has concentrated on discovering and defining the oxide resource. This oxide ore is fractured enough that run of mine gold recovery is over 70%, making Marigold an excellent deposit type for bulk open pit heap leach mining methods and recovery. High angle north?south trending faults have facilitated the mineralizing hydrothermal fluid flow and these faults run the length of the property. Much of the property is masked by a cover of Tertiary tuff and Quaternary alluvium so drilling is very important in the unraveling of Marigold's geologic secrets.
Mining & Operations
The Marigold operation comprises 11 open pits, eight waste rock stockpiles, three leach pads with 16 cells, two carbon-in-leach processing facilities, and a carbon processing and refining facility.
Mining consists of a standard truck and shovel fleet using 190 and 300 tonne haul trucks and large hydraulic shovels. Approximately 30-40 million tonnes of waste rock and 10-15 million tonnes of oxide ore are mined per year.