User:BrettKuzmicz/Orogenic gold deposit

Mineralogy and geochemistry
Geochemical studies on gold bearing quartz-carbonate veins are important to determine temperature, pressure, at which the veins were generated, and the chemical signature of fluids. Quartz is generally the dominant mineral in the veins, but there are also gold bearing carbonate dominant veins in orogenic deposits. Ore bodies of orogenic gold deposits are generally defined by ≤ 3–5% sulfide minerals, most commonly arsenopyrite in metasedimentary host rocks and pyrite/pyrrhotite in meta-igneous rocks, and ≤ 5–15% carbonate minerals, such as ankerite, dolomite and calcite. A common characteristic of almost all orogenic gold lodes is the presence of widespread carbonate alteration zones, notably ankerite, ferroan dolomite, siderite and calcite. The tendency of gold to be preferentially transported as a sulfide complex also explain the near absence of base metals (Cu, Pb, Zn) in the same mineral systems, because these metals form complexes with chlor rather than sulfur.

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. High salinity fluids can result from dehydration of evaporite sequences, containing high Na and Cl concentrations and above mentioned base metal complexes. Although some authors suggest a specific range of CO2 of about 5–20%, there is a wide variety from almost pure CO2 to almost pure H2O. Whereby CO2-rich fluids may indicate high fluid production temperatures > 500 °C.

Economics
Orogenic gold deposits are responsible for approximately 75% of the world's gold production at over 1 billion ounces, when accounting that the origin of many gold placer deposits were orogenic in nature. The price of gold at a given time will have an impact on whether a deposit will be economically feasible. The economic viability of a deposit will also depend on the grade and tonnage of the reserves of a deposit, along with the costs associated with extracting the ore. Methods of deliniating reserves and of extracting gold ore are improving over time, increasing the possibility for production of more gold. On the other hand, the environmental impact of extracting gold from orogenic gold deposits, such as cyanidation, is coming more under consideration over time. The cost of remediation for the environmental hazards of operating a mine at an orogenic gold deposit will impact its economic feasibility.

The typical grade of unmineralized igneous, sedimentary and metamorphic rocks is on average between 0.5 to 5 parts per billion. 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. A gold mine can expect to extract ores of 1-2 parts per million (g/t) in an open pit mine due to the relatively lower operating costs of an open pit mine. These values will differ based on the fluctuating price of gold and the variable cost and capacity of, mining, milling and refining.

Timeline of Global Emplacement
Improved geochronological data on gold and paleo-reconstructions have given a better understanding of the emplacement of orogenic gold deposits over time. The oldest known orogenic gold deposits (>3 Ga) are in the Kaapvaal craton in the Barberton greenstone belt, the Ukranian shield and the Pilbara craton. The Witwatersrand placer gold deposit in South Africa is estimated to have been emplaced by orogenic processes at a similar time. The next period of time for favorable orogenic gold deposit formation was 2.8-2.55 Ga in the greenstone belts of the Yilgarn craton, Superior province, Dharwar craton, Zimbabwe craton, Wyoming craton and Baltic shield.

Proceeding the Archean, the next episode of orogenic gold deposit formation was from 2.1-1.8 Ga following the breakup of an Archean supercontinent and subsequent orogenic processes which ensued. In this time period, deposits formed in interior Australia, northwestern Africa, northern South America, Sveconfennia, and the Canadian shield. This is followed by a period of insignificant orogenic gold formation from 1.6 Ga to 0.8 Ga which is argued to either be due to worldwide major extension associated with anorogenic magmatism, or due to erosion of narrow continental margins in which the orogenic gold was emplaced.

The formation of Godwana in the Neoproterozoic by the process of collisions of cratons indicates the time which orogenic gold-vein formation became continuous and wide spread until present day. From the formation of Godwana until the beginning of the paleozoic, deposits formed in the Arabian-Nubian shield, western Africa, Brazil's Atlantic shield, in the Sao Francisco craton, and northwestern Australia. From the paleozoic until the beginning of the mesozoic, in conjunction with the various orogenies which contributed to the evolution of Pangea, orogenic gold deposits were emplaced in Australia, Westland in New Zealand, Victoria Land in Antarctica, southern South America, southern Europe, central Asia and northwest China.

The break-up of Pangea in the mesozoic is the event which marks the final major global distribution of orogenic gold deposits. This event created an immense range of subduction zones surrounding the Pacific ocean. To the east of the Pacific, the Cordilleran orogen resulted in many Middle Jurassic to mid-Cretaceous orogenic gold deposits. To the west of the Pacific, a similar contemporaneous orogenic event occured resulting in orogenic gold deposits emplacing in the Russian Far East and the North China craton during the Early Cretaceous.

Environmental Effects
The mining at orogenic gold deposits has significant negative environmental effects. Over 90% of ore extracted from orogenic gold deposits is treated by the process of cyanidation. The toxic waste created from this process is stored in tailings ponds, which presents a risk for contamination of soil and water in the event of accidents or negligence by those handling the toxic liquids. This contamination can occur in many forms such as dam failures, unregulated drainage into rivers, or leeching of toxic liquids through permeable soils. One such example of this type of environmental disaster is the August 19, 1995 Omai cyanide spill in which the tailings dam of the Canadian owned Omai Gold Mines Ltd failed, releasing over 440 000 cubic meters of cyanide-laced effluent into the Omai river, causing over 80 km of distaster zone downriver. 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. Furthermore, creating space for open pit mines, tailings ponds, and mine infrastructure requires clearing vast amount of land, leading to deforestation and the destruction of natural habitats.

Examples
Australia


 * Bendigo
 * Kalgoorlie

Brazil


 * Fazenda

Canada


 * Timmins
 * Lamaque
 * Val d'Or camp

France


 * Salsigne

Ghana


 * Ashanti

Burkina Faso


 * Poura

Kazakhstan


 * Vasil'kovsk

Russia


 * Berezovsk

USA


 * Mother Lode Homestake