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Orogenic gold deposits are 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.[1] 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 focussing deposition to structurally controlled locations.[2]

General 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.[3] 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.[2] These discontinuities provide pathways and channel fluid flow, not only of ore-bearing fluids, but also of fluids transporting metallic elements such as Ag, As, Hg and Sb and gases, as well as melts.[4] 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.[3]

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.[5] The term mesothermal refers to temperatures between 175–300°C and a formation depth of 1.2–3.6 km. In 1993, the therm 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.[6] Orogenic gold deposits were only formed in certain time slices of the Earth's history.[7][8] 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 1.80–0.75 Ga,[7] referred to as a period of general minimum ore-forming activity.[8] The same temporal occurrence is documented for conglomerate-hosted deposits.[9] 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.[7] Therefore, identification of rocks that have been formed or deformed during one of these three epochs, provides a basic tool for exploration targeting.

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.[2] For this reason, a discussion of the gold deposit formation in a universal genetic model is most difficult and several models have been considered (e.g. Groves et al., 2003).

Fluid Source

In magmatic systems, the ores and host rocks are derived from the same fluid.[10] In the case of hydrothermal fluids, host rocks are younger 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.[11] 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.[12] and points to orogenic gold deposits being generally epigenetic, i.e. mineralized after the rock sequence was formed.[3] These temporal gaps suggest an overall genetic independence of the fluid formation and that of local lithologies.[13]

Mineralogy and Geochemistry

Geochemical studies on quartz veins are important to determine temperature, pressure, at which the veins were generated, and the chemical signature of fluids. Obviously, quartz is the dominant mineral in the veins. Ore bodies of orogenic gold deposits are generally defined by ≤ 3–5% sulfide minerals, most commonly arsonopyrite in metasedimentary hostrocks and pyrit/pyrrothite in meta-igneous rocks, and ≤ 5–15% carbonate minerals, such as ankerite, dolomite and calcite.[14] A common characteristic of almost all orogenic gold lodes is the presence of widespread carbonate alteration zones, notably ankerite, ferroan dolomite, siderite and calcite.[15] The tendency of gold to be preferentially transported as a sulphide 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 sulphur.[16]

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.[16] High salinity fluids can result from dehydration of evaporite sequences, containing high Na and Cl concentrations and above mentioned base metal complexes.[16] 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.[3] Whereby CO2-rich fluids may indicate high fluid production temperatures > 500 °C.[17]

Genetic Models

There are two main categories of models that summarize the knowledge about mineral deposits: Descriptive models and genetic models. Descriptive models involve the description of features of known deposits, while genetic models involve an understanding of how gold deposits form and what characteristics provide a platform to develop and rank parameters during exploration. When comparing genetic models, diverse geoscience information on geology, mineral occurrences, geophysics, and geochemistry must be. 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.[2] 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.

Magmatic-hydrothermal fluid source

A magmatic-hydrothermal source from which felsic-intermediate magmas release fluids as they crystallize (Tomkins, 2013). 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.[18] But a main constraint is, that in many gold provinces, gold mineralization and granitic intrusion, which indicate magmatic activity, show no age relationship.[3] 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 source is a much more common scenario.

Tectonics and Gold Formation

Although efforts have been made to define a specific deformation structure associated with the formation of orogenic gold deposits,[18] no specific structure could be identified. Rather, there are various types of faults hosting gold deposits.[2] Nevertheless, orogenic gold deposits have a number of repetitive structural geometries that control ore-fluid formation, transport, and precipitation.

Geodynamik 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.[19] 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.[20] 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.[21] Lamprophyre dykes are not the source of the ore fluid itself but indicate a deep lithospheric connection for fluid conduits.[22]

Orogenic gold deposits show a spatial relationship to structural discontinuities, including faults, fractures, dilatation zones and shear zones.[2] The ore- hosting structures are subsidiary faults or shear zones (mostly D3–D4 in a D1 to D4 structural sequence), which are always related to a major regional-scale deformation structures, such as lithospheric boundaries and suture zones.[16] The deformation structures hosting the gold deposits are typically discordant with respect to the stratigraphic layering of the host rocks. The mineralised structures indicate syn- to post-mineralisation displacements, such as slicken-sides formed under hydrothermal conditions. The geometry of vein systems is primarily influenced by a combination of dynamic stress changes and fluid pressure variations.[23]

Examples

Australia

  • Bendigo  
  • Kalgoorlie  

Canada

  • Timmins
  • Lamaque
  • Val d'Or camp

France

  • Salsigne

Ghana

  • Ashanti

Kazakhstan

  • Vasil'kovsk

Russia

  • Berezovsk

USA

  • Mother Lode Homestak


Vorlage:Navbox Ores Category:Mining templates

References

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  1. ↑ G. Phillips: Australian and global setting for gold in 2013. In: AusIMM Bulletin. World Gold 2013, 2013 (edu.au [abgerufen am 10. Februar 2021]).
  2. ↑ a b c d e f J. Vearncombe, M. Zelic: Structural paradigms for gold: do they help us find and mine? In: Applied Earth Science. Band 124, Nr. 1, 1. März 2015, ISSN 0371-7453, S. 2–19, doi:10.1179/1743275815Y.0000000003 (doi.org [abgerufen am 10. Februar 2021]).
  3. ↑ a b c d e f Richard J. Goldfarb, David I. Groves: Orogenic gold: Common or evolving fluid and metal sources through time. In: Lithos (= Geochemistry and Earth Systems – A Special Issue in Memory of Robert Kerrich). Band 233, 15. September 2015, ISSN 0024-4937, S. 2–26, doi:10.1016/j.lithos.2015.07.011 (sciencedirect.com [abgerufen am 10. Februar 2021]).
  4. ↑ Damien Gaboury: Parameters for the formation of orogenic gold deposits. In: Applied Earth Science. Band 128, Nr. 3, 3. Juli 2019, ISSN 2572-6838, S. 124–133, doi:10.1080/25726838.2019.1583310 (doi.org [abgerufen am 10. Februar 2021]).
  5. ↑ Waldemar Lindgren: Mineral deposits. McGraw-Hill Book Company, inc., 1913 (google.de [abgerufen am 10. Februar 2021]).
  6. ↑ M. E. Barley, D. I. Groves: Supercontinent cycles and the distribution of metal deposits through time. In: Geology. Band 20, Nr. 4, 1. April 1992, ISSN 0091-7613, S. 291–294, doi:10.1130/0091-7613(1992)0202.3.CO;2 (geoscienceworld.org [abgerufen am 11. Februar 2021]).
  7. ↑ a b c R. J Goldfarb, D. I Groves, S Gardoll: Orogenic gold and geologic time: a global synthesis. In: Ore Geology Reviews. Band 18, Nr. 1, 1. April 2001, ISSN 0169-1368, S. 1–75, doi:10.1016/S0169-1368(01)00016-6 (sciencedirect.com [abgerufen am 11. Februar 2021]).
  8. ↑ a b C Meyer: Ore Deposits as Guides to Geologic History of the Earth. In: Annual Review of Earth and Planetary Sciences. Band 16, Nr. 1, Mai 1988, ISSN 0084-6597, S. 147–171, doi:10.1146/annurev.ea.16.050188.001051 (annualreviews.org [abgerufen am 11. Februar 2021]).
  9. ↑ H. Frimmel, H. Frimmel: Episodic concentration of gold to ore grade through Earth's history. 2018, doi:10.1016/J.EARSCIREV.2018.03.011 (semanticscholar.org [abgerufen am 11. Februar 2021]).
  10. ↑ Stephen J. Barnes, Alexander R. Cruden, Nicholas Arndt, Benoit M. Saumur: The mineral system approach applied to magmatic Ni–Cu–PGE sulphide deposits. In: Ore Geology Reviews. Band 76, 1. Juli 2016, ISSN 0169-1368, S. 296–316, doi:10.1016/j.oregeorev.2015.06.012 (sciencedirect.com [abgerufen am 17. Februar 2021]).
  11. ↑ Derek A. Wyman, Kevin F. Cassidy, Peter Hollings: Orogenic gold and the mineral systems approach: Resolving fact, fiction and fantasy. In: Ore Geology Reviews. Band 78, Oktober 2016, ISSN 0169-1368, S. 322–335, doi:10.1016/j.oregeorev.2016.04.006 (doi.org [abgerufen am 17. Februar 2021]).
  12. ↑ S. F. Cox, M. A. Knackstedt, J. Braun: Principles of Structural Control on Permeability and Fluid Flow in Hydrothermal Systems. 1. Januar 2001, doi:10.5382/Rev.14.01 (geoscienceworld.orghttps [abgerufen am 17. Februar 2021]).
  13. ↑ Structural geometry of orogenic gold deposits: Implications for exploration of world-class and giant deposits. In: Geoscience Frontiers. Band 9, Nr. 4, 1. Juli 2018, ISSN 1674-9871, S. 1163–1177, doi:10.1016/j.gsf.2018.01.006 (sciencedirect.com [abgerufen am 12. Februar 2021]).
  14. ↑ D. I Groves, R. J Goldfarb, M Gebre-Mariam, S. G Hagemann, F Robert: Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types. In: Ore Geology Reviews. Band 13, Nr. 1, 1. April 1998, ISSN 0169-1368, S. 7–27, doi:10.1016/S0169-1368(97)00012-7 (sciencedirect.com [abgerufen am 15. Februar 2021]).
  15. ↑ David I Groves, Richard J Goldfarb, Carl M Knox-Robinson, Juhani Ojala, Stephen Gardoll: Late-kinematic timing of orogenic gold deposits and significance for computer-based exploration techniques with emphasis on the Yilgarn Block, Western Australia. In: Ore Geology Reviews. Band 17, Nr. 1, 1. September 2000, ISSN 0169-1368, S. 1–38, doi:10.1016/S0169-1368(00)00002-0 (sciencedirect.com [abgerufen am 15. Februar 2021]).
  16. ↑ a b c d Franco Pirajno: Hydrothermal Processes and Mineral Systems. 2009, doi:10.1007/978-1-4020-8613-7 (springer.com [abgerufen am 15. Februar 2021]).
  17. ↑ F. L. ELMER, R. W. WHITE, R. POWELL: Devolatilization of metabasic rocks during greenschist-amphibolite facies metamorphism. In: Journal of Metamorphic Geology. Band 24, Nr. 6, August 2006, ISSN 0263-4929, S. 497–513, doi:10.1111/j.1525-1314.2006.00650.x (doi.org [abgerufen am 17. Februar 2021]).
  18. ↑ a b Richard J. Goldfarb, Timothy Baker, Benoît Dubé, David I. Groves, Craig J. R. Hart: Distribution, Character, and Genesis of Gold Deposits in Metamorphic Terran. 1. Januar 2005, doi:10.5382/AV100.14 (geoscienceworld.orghttps [abgerufen am 12. Februar 2021]).
  19. ↑ Colleen A. Barton, Mark D. Zoback, Daniel Moos: Fluid flow along potentially active faults in crystalline rock. In: Geology. Band 23, Nr. 8, 1. August 1995, ISSN 0091-7613, S. 683–686, doi:10.1130/0091-7613(1995)0232.3.CO;2 (geoscienceworld.org [abgerufen am 15. Februar 2021]).
  20. ↑ Frank P. Bierlein, David I. Groves, Peter A. Cawood: Metallogeny of accretionary orogens — The connection between lithospheric processes and metal endowment. In: Ore Geology Reviews. Band 36, Nr. 4, 1. Dezember 2009, ISSN 0169-1368, S. 282–292, doi:10.1016/j.oregeorev.2009.04.002 (sciencedirect.com [abgerufen am 15. Februar 2021]).
  21. ↑ Nicholas M. S. Rock: The nature and origin of lamprophyres: an overview. In: Geological Society, London, Special Publications. Band 30, Nr. 1, 1. Januar 1987, ISSN 0305-8719, S. 191–226, doi:10.1144/GSL.SP.1987.030.01.09 (lyellcollection.org [abgerufen am 15. Februar 2021]).
  22. ↑ Nicholas M. S. Rock, David I. Groves, Caroline S. Perring, Suzanne D. Golding: Gold, Lamprophyres, and Porphyries: What Does Their Association Mean? 1. Januar 1989, doi:10.5382/Mono.06.47 (geoscienceworld.orghttps [abgerufen am 15. Februar 2021]).
  23. ↑ Richard H. Sibson, Francois Robert, K. Howard Poulsen: High-angle reverse faults, fluid-pressure cycling, and mesothermal gold-quartz deposits. In: Geology. Band 16, Nr. 6, 1. Juni 1988, ISSN 0091-7613, S. 551–555, doi:10.1130/0091-7613(1988)0162.3.CO;2 (geoscienceworld.org [abgerufen am 15. Februar 2021]).
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