Formation of Lode Gold Deposits

Formation of Lode Gold Deposits (written by Brian Heike). Ores bearing native gold consist of grains or microscopic particles of metallic gold embedded in rock, often in association with veins of quartz or sulfide minerals like pyrite. These are called “lode” deposits. Native gold is also found in the form of free flakes, grains or larger nuggets that have been eroded from rocks and end up in alluvial deposits (called placer deposits). Such free gold is always richer at the surface of gold-bearing veins owing to the oxidation of accompanying minerals followed by weathering, and washing of the dust into streams and rivers, where it collects and can be welded by water action to form nuggets. Gold occurs mainly in pyrite- and polymetallic sulfide–quartz vein/veinlet stockworks.

Fluid inclusions in the deposit are divided into three main types, namely CO2–H2O, H2O–CO2 ± CH4 and aqueous ones. A popular misconception is that small veins of gold or silver ore in a mining district are necessarily branches of a single rich and massive mother lode deep in the ground. This idea is contrary to modern theories of ore deposits. The term is also used metaphorically to refer to the origin of something valuable or in great abundance. The clustered distribution of low displacement faults and associated mesothermal Au deposits within fault systems can be governed by static stress changes (Dsf) associated with large rupture events on high displacement components of the fault systems. The low displacement faults, which have localized gold deposition and attendant fluid flow, are interpreted as being formed and repeatedly reactivated during aftershock sequences following numerous, high displacement rupture events. The distribution of aftershock activity and associated permeability enhancement is particularly influenced by the location of rupture arrest on main shock faults. The formation of lode gold deposits requires a history of repeated fault slip and fluid flow events. Accordingly, an important factor controlling the distribution of aftershock fault arrays is the development of long-lived structures which repeatedly inhibit rupture propagation and accumulation of fault slip on high displacement faults. Using two case studies, we demonstrate that both dilatant and contractional jogs on high displacement faults can be effective barriers to rupture propagation. Modeling of Dsf associated with large slip events demonstrates that the distribution of Au deposits on low displacement faults in both the Mount Pleasant and St Ives goldfields in the Archaean Yilgarn Craton (Western Australia) is well-matched by the domains of positive Dsf and enhanced aftershock probability. In geology a lode is the metalliferous ore that fills a fissure in a rock or a vein of ore deposited between layers of rock.

Mother lode is a principal vein or zone of veins of gold ore. In the United States, Mother lode is most famously the name given to the long alignment of hard rock gold deposits stretching northwest to southeast in the Sierra Nevada of California. The zone contains hundreds of mines and prospects, including some of the best-known historic mines of the gold-rush era. Individual gold deposits within the Mother Lode are gold-bearing quartz veins up to 50 feet thick and a few thousand feet long. The California Mother Lode was one of the most productive gold-producing districts in the United States, but is now given over to tourism. The Carlin mine near Carlin, Nev., USA, is producing gold from a large low-grade deposit that was opened in 1965 after intensive scientific and technical work had been completed. Similar investigations have led to the more recent discovery of a Carlin-type gold deposit in Jerritt Canyon, Nev. Many placer districts in California have been mined on a large scale as recently as the mid-1950’s. Streams draining the rich Mother Lode region–the Feather, Mokelumne, American, Cosumnes, Calaveras, and Yuba Rivers–and the Trinity River in northern California have concentrated considerable quantities of gold in gravels. California Gold Region 6 was second only to the Mother Lode in California gold production.

California Gold Region 6 includes Susanville, Greenville, Westwood, Shasta, Weed, Red Bluff, Redding, Enterprise, Yreka, Weaverville, French Gulch, Alturas, Happy Camp, Orleans and Crescent City. Both lode and placer mining have been done in this region, which is adjacent to and south of the Oregon/California state line. California Gold Region 6 has gold deposit sites ranging eastward from Crescent City on the Pacific Ocean to Modoc National Forest northeast of Alturas. The gold sites range southward from the Oregon state line to latitude 40 degrees, north, which is five miles north of Quincy. The Klamath Mountains region in northwestern California is the second-most gold-productive province in California. The principal gold districts are in Shasta, Siskiyou, and Trinity Counties. Although there are several important lode-gold districts, the placer deposits have been the largest sources of gold. In Australia, the Mosquito Creek belt has been the largest source of metasediment-hosted lode Au in the southeast Archean Pilbara Craton. The city of Johannesburg located in South Africa was founded as a result of the Witwatersrand Gold Rush which resulted in the discovery of some of the largest gold deposits the world has ever seen. Gold fields located within the basin in the Free State and Gauteng provinces are extensive in strike and dip requiring some of the world’s deepest mines, with the Savuka and TauTona mines being currently the world’s deepest gold mine at 3,777 m. Other major producers are United States, Australia, China, Russia and Peru. Mines in South Dakota and Nevada supply two-thirds of gold used in the United States. In South America, the controversial project Pascua Lama aims at exploitation of rich fields in the high mountains of Atacama Desert, at the border between Chile and Argentina. Today about one-quarter of the world gold output is estimated to originate from artisanal or small scale mining. The Rushan gold deposit in the Jiaodong Peninsula is currently the largest lode gold in China. You may also wish to read Lode or Placer: What Difference Does It Make? by Keith Bowen

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