Orogenic gold deposit
Amalgamation of disrupted continental masses to form new supercontinents, known as Wilson cycles, play a key role in the formation of deposits, by initiating major regional change of the geochemical, mineralogical and structural nature of the lithosphere.
[16] A common characteristic of almost all orogenic gold lodes is the presence of widespread carbonate alteration zones, notably ankerite, ferroan dolomite, siderite and calcite.
[18] In general, hydrothermal fluids are characterized by low salinities (up to 12 wt% NaCl equivalent), high H2O and CO2 contents (> 4 mol%), with lesser amounts of CH4 and N2 and near-neutral pH.
[20] Orogenic gold deposits formed in metamorphosed terranes of all ages that have little in common except for being sites of complexity and low mean stress.
The fundamental control of the chemical signature of orogenic gold fluids can most likely be found in the processes that take place in the source region.
[21] Fluids that exsolved from a granitic melt intrude into the upper or middle crust and are enriched in many elements, such as S, Cu, Mo, Sb, Bi, W, Pb, Zn, Te, Hg, As, and Ag.
[3] The generally low salinity of the hydrothermal fluids can be attributed to devolatilization of minerals associated with metamorphic phase reactions, involving dehydration and decarbonisation.
[26] These gold- and silica fluids migrated through fractures over long distances and were precipitated in deformation structures at the brittle-ductile transition and near the base of the seismogenic zone.
First, recent fluid-flow experiments confirm that serpentinite acts as a lubricant for the overlying subcontinental lithospheric mantle (SCLM) and, therefore, plays a major role in dynamic settings.
[30] Slab dewatering may start at depths less than 100 km and over-pressured fluids migrate into fault zones in the upper lithosphere and eventually form gold deposits.
[2] Nevertheless, orogenic gold deposits have a number of repetitive structural geometries that control ore-fluid formation, transport, and precipitation.
[38] Orogenic gold deposits are typically located in metamorphosed fore-arc and back-arc regions, as well as in the arc[3] and show a close spatial relationship to lamprophyres and associated felsic porphyry dikes and sills.
The geometry of vein systems is primarily influenced by a combination of dynamic stress changes and fluid pressure variations.
[42] The Witwatersrand placer gold deposit in South Africa is estimated to have been mineralized by orogenic processes at a similar time.
[42] In this time period, deposits formed in interior Australia, northwestern Africa, northern South America, Sveconfennia, and the Canadian shield.
[42] The break-up of Pangea in the mesozoic is the event which marks the final major global distribution of orogenic gold deposits.
[48] The cost of remediation for the environmental hazards of operating a mine at an orogenic gold deposit will impact its economic feasibility.
[49] Generally, ores of 5 parts per million (g/t) or greater grade will be extracted using underground mining and aim follow the gold bearing structure.
[51] These values will differ based on the fluctuating price of gold and the variable cost and capacity of, mining, milling and refining.
[54] This contamination can occur in many forms such as dam failures, unregulated drainage into rivers,[55] or leeching of toxic liquids through permeable soils.
[57] The energy consumption associated with operating a mine in an orogenic gold deposit also produces a large carbon footprint, which as a greenhouse gas contributes to climate change.
