An orogenic gold deposit is a type of hydrothermal mineral deposit. More than 75% of the gold recovered by humans through history belongs to the class of orogenic gold deposits. Rock structure is the primary control of orogenic gold mineralization at all scales, as it controls both the transport and deposition processes of the mineralized fluids, creating structural pathways of high permeability and focusing deposition to structurally controlled locations.
Overview Orogenic gold deposits are hosted by shear zones in orogenic belts, specifically in metamorphosed fore-arc and back-arc regions and were formed during syn- to late metamorphic stages of orogeny. Formation of orogenic gold deposits is related to structural evolution and structural geometry of lithospheric crust, as hydrothermal fluids migrate through pre-existing and active discontinuities (faults, shear zones, lithological boundaries) generated by tectonic processes. These discontinuities provide pathways and channel fluid flow, not only of ore-bearing fluids, but also of fluids transporting metallic elements such as silver, arsenic, mercury and antimony and gases, as well as melts. Gold-bearing fluids precipitate at an upper-crustal level between 3 and 15 km depth (possibly up to 20 km depth), forming vertically extensive quartz veins, typically below the transition of greenschist-to-amphibolite metamorphic facies.
Historical term Waldemar Lindgren made the first widely accepted classification of gold deposits and introduced the term "mesothermal" for mostly gold-only deposits in metamorphic terranes and greenstone belts. The term mesothermal refers to temperatures between 175 and 300 °C and a formation depth of 1.2–3.6 km. In 1993, the term orogenic gold deposits was introduced, as gold deposits of this type have a similar origin and gold mineralization is structurally controlled.
Temporal pattern
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. Orogenic gold deposits were only formed in certain time slices of the Earth's history. Orogenic gold deposits are mainly concentrated in three epochs of Earth history: (1) Neoarchean 2.8–2.5 Ga, (2) Paleoproterozoic 2.1–1.8 Ga and (3) Phanerozoic 0.500–0.05 Ga. With an absence in the period 1.80–0.8 Ga, referred to as a period of general minimum ore-forming activity. The same temporal occurrence is documented for conglomerate-hosted deposits. The time-bound nature of many mineral deposits reflects the break-up or formation of supercontinents, which most likely also applies for orogenic gold deposits.
Fluid source In magmatic systems, ores and host rocks are derived from the same fluid. In the case of hydrothermal fluids, host rocks are older than the predominantly aqueous fluids that carry and deposit metals and thus complicate defining a host rock associated with gold fluid formation. A number of rock types have been suggested as the source of orogenic gold, but due to the variability of host rocks in Earth's history and deposit-scale, their relation to Earth-scale gold formation processes is unclear. Furthermore, age dating of the deposits and their host rocks shows that there are large time gaps in their formation. Age dating indicates that mineralization took place 10 to 100 Ma after the formation of the host rocks. These temporal gaps suggest an overall genetic independence of the fluid formation and that of local lithologies.
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.
Genetic models 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. For this reason, a discussion of the gold deposit formation in a universal genetic model is most difficult and several models have been considered. 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. Therefore, the discussion about genetic models of orogenic gold deposits concentrates on the possible source of gold-bearing fluids.
Magmatic–hydrothermal fluid source A magmatic–hydrothermal source from which felsic-intermediate magmas release fluids as they crystallize. 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. But a main constraint is, that in many gold provinces, gold mineralization and granitic intrusion, which indicate magmatic activity, show no age relationship. In addition, the composition of granites are extremely variable and show no consistent temporal pattern through geological time. Even if some deposits clearly indicate a magmatic source, it must be considered that only due to overprinting mineralization with higher gold grades from other sources, these deposits became economic. A hybrid deposit with a combination of a magmatic and a metamorphic (mid- or sub-crustal) source is a much more common scenario.
Mid-crustal fluid source A model that fits most of the gold provinces and provides some of the major gold resources, entails a metamorphic fluid source. In this style of gold deposit, gold and other elements have been released into metamorphic fluids, from material accreted to a craton during subduction-related scenarios. Most likely, fluids have been produced under prograde greenschist-to-amphibolite-facies metamorphism (220–450 °C and 1–5 Kbar). The generally low salinity of the hydrothermal fluids can be attributed to devolatilization of minerals associated with metamorphic phase reactions, involving dehydration and decarbonisation. Composition of produced fluids vary, depending on the P–T conditions and rock chemistry and may be influenced by fluid rock–rock interactions along the pathway. Coupling between fluid flow and structural deformation plays a key role for mineralization. Gold formation occurs typically in the late phase of an orogeny, during changes in far-field stresses. Created and reactivated faults serve as pathways for hydrothermal solutions. 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. Gold deposits in this model are characterized by elevated S and As and only minor enrichment of other elements.
Sub-crustal fluid source The model of a sub-crustal source is similar to the middle-crustal model. In both cases fluids and metals formed from volcanic and sedimentary products in tectonic processes, but also show differences in the origin of the source and the processes involved. This model is associated with fluid ascent from devolatilization of a subducting slab and overlying sediment wedge. Oceanic mantle, crust and overlying sediments were subducted, and rapidly heated, and H-O-C-rich vapours released fluids during heating, at temperatures below 650 °C and depths of 100 km.
Serpentinization (slab mantle hydration) may play an important role for two reasons. 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. Secondly, serpentinization involves volume increase as large as 40% that enhances fracturing in peridotites and provides permeability for hydrothermal fluids. Serpentinite formed by hydrated oceanic mantle carries up to 13 wt. % water to the deep mantle. 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. However, fluid migration along faults might not be effective in a compressional stress field, thereby increasing the possibility that neutralstress planes control a vertical fluid supply in the fault zones. Under this assumption, the trigger to cause fluid release might be the end of subduction or a stalling of the slab during subduction, resulting in a delayed fluid migration and gold mineralization process. The sub-crustal fluid source model is more robust as it describes both a source and a mechanism, but also has limitations, as many Precambrian gold deposits do not have thick sedimentary successions.
Tectonics and gold formation Although efforts have been made to define a specific deformation structure associated with the formation of orogenic gold deposits, no specific structure could be identified. Rather, there are various types of faults hosting gold deposits. Nevertheless, orogenic gold deposits have a number of repetitive structural geometries that control ore-fluid formation, transport and precipitation.
Geodynamic setting and architecture Large-scale lithospheric deformation structures correlate with gold endowment, and active structural permeability in the crust is controlled by the prevailing tectonic stress field. There is an increasing body of evidence that the formation of orogenic gold deposits is tied to specific geodynamic settings, primary orogenic belts.
A variety of gold deposits are formed in accretionary orogens, including orogenic gold deposits. Orogenic gold deposits are typically located in metamorphosed fore-arc and back-arc regions, as well as in the arc and show a close spatial relationship to lamprophyres and associated felsic porphyry dikes and sills. Lamprophyre dykes are not the source of the ore fluid itself but indicate a deep lithospheric connection for fluid conduits.