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.