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Review

Case Histories of Orogenic Gold Deposits

by
Franz Michael Meyer
Institute of Applied Mineralogy and Economic Geology, RWTH Aachen University, Wüllnerstrasse 2, 52056 Aachen, Germany
Minerals 2023, 13(3), 369; https://doi.org/10.3390/min13030369
Submission received: 16 January 2023 / Revised: 19 February 2023 / Accepted: 20 February 2023 / Published: 6 March 2023
(This article belongs to the Special Issue Economic Mineral Deposits: A Review)
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Abstract

:
This review compares genetic parameters of 12 orogenic gold deposits. The set of examples is considered to represent largely the variability of orogenic gold deposit (OGD) characteristics. The data are presented in tables and include following definitive parameters: regional geologic settings, nature of hosts rocks and mineralization, ore controlling structures, ages of host rocks and mineralization and timing of mineralization relative to metamorphism, hydrothermal alteration mineralogy and ore mineral assemblages, isotopic signatures, physical conditions of ore formation and proposed origin of ore fluids aa well as gold reserves, production, and grades. This allows comparison of deposits from different geologic terrains having different ages and formed under different P-T conditions. The data are further discussed before the background of the orogenic gold system and the crustal metamorphic models that provide different scenarios to explain the source of ore fluids. The orogenic gold system model advocates a metal and fluid source external to the terrain in which mineralization occurred, but the model applies only for 3 of the 12 deposits studied. All other deposits formed most likely from a crustal source, which would favor the crustal metamorphic model. However, the formation of hypozonal OGDs cannot be accounted for by the crustal metamorphic model or by the metamorphic devolatilization model. The data identify a set of coherent signatures in OGDs, but there seem to be no unified model for all possible environmental conditions and facets of ore formation and fluid sources, tectonic and lithologic setting, and scale of gold endowment.

1. Introduction

This paper reviews research on orogenic gold deposits (OGD) carried out by staff and students at the Institute of Mineralogy and Economic Geology, RWTH Aachen University. A large body of research is documented in formal publications as well as MSc and PhD theses so that this review paper can only showcase some of the many aspects related to the gold deposits that were studied.
The section on Materials and Methods is purposely lacking, as all information on regional and mine geology, host rock and ore sample descriptions, as well as analytical methods such as litho- and mineral chemical and petrological analyses, fluid inclusion studies, stable and radiogenic isotope analyses, and petrological modeling are provided in the references cited below.
Data for the review originate from research projects involving following mines and mining districts (in order of decreasing mineralization ages). References pertaining to individual ore deposits discussed are indicated in square brackets: New Consort, South Africa [1,2,3,4,5,6,7,8,9]: Cuiabá Brazil [10,11,12,13]: Ajjanahalli, India [14,15]; Renco, Zimbabwe [16,17,18,19,20]; Hutti [21,22,23,24,25,26,27], Hira Buddini [28,29], India; Pilgrim’s Rest [30,31,32,33], South Africa; Lega Dembi, Ethiopia [34]; Navachab, Namibia [35,36,37,38,39,40]; Mindyak [41,42,43], Kochkar [44,45,46], Russia; Awak Mas, Indonesia [47,48,49,50,51,52,53].
Processes of formation of orogenic gold systems have been well studied and the salient features are thoroughly documented [54,55,56,57,58]. With the recognition of diagnostic criteria and based on an adequate theoretical basis it is widely accepted that orogenic gold deposits represent structurally controlled hydrothermal deposits that formed by focused fluid flow in an orogenic setting during metamorphism and deformation [54,55,56,57,58,59]. The crustal continuum model has been one of the generally accepted descriptive models for OGDs formation in Archaean greenstone belts [56,57]. The model proposes that OGDs formed along crustal faults over an extended range of pressure and temperature conditions throughout a 20–25 km vertical profile, from temperatures ranging from below 180 °C to above 700 °C. Mineralization occurred synchronous with the peak of metamorphism extending from sub-greenschist to lower granulite facies conditions [56,57]. The crustal continuum model was mainly based on the observation of systematic variations of mineralization and alteration features in the vertical crustal profile. Accordingly, shallow-level gold deposits were classified as epizonal, intermediate-level deposits as mesozonal, and the deep-seated deposits as hypozonal [56]. Since hypozonal deposits commonly form in mid-amphibolite or lower-granulite terrains the crustal continuum model requires the existence of a deep source for the ore fluids [57]. The metamorphic devolatilization model was invoked to explain the fluid source in greenstone-hosted gold deposits [60]. In this model, ore fluids are generated by metamorphic devolatilization in an orogenic setting of hydrated and carbonated metabasic rocks across the greenschist–amphibolite facies boundary [60,61]. Further work on hypozonal orogenic gold deposits suggests that the crustal continuum model is not valid over the large spectrum of temperatures and pressures of OGD formation. Additionally, the metamorphic devolatilization model fails to explain the formation of OGDs at metamorphic conditions of the host terrane above the greenschist to amphibolite facies transition [1,62]. Alternative models consider external sources of fluids and gold such as devolatilization of a subducted oceanic slab with its overlying gold-bearing sulfide-rich sedimentary package, or release from mantle lithosphere that was metasomatized and fertilized during a subduction event, particularly adjacent to craton margins [59]. Enhanced asthenopsheric heat input, typical of an unstable or delaminated subcontinental lithospheric mantle, promotes the devolatilization of sedimentary and/or volcanic rocks that produce metamorphic fluids in well-endowed gold provinces [63,64,65].
This paper reviews distinctive signatures, on a deposit scale, of 12 selected OGDs. The data are presented in tables and include following definitive characteristics: regional geologic settings, nature of hosts rocks and mineralization controlling structures, ages of host rocks and mineralization and timing of mineralization relative to metamorphism, hydrothermal alteration mineralogy and ore mineral assemblages, isotopic signatures, physical conditions of ore formation and proposed origin of ore fluids as well as gold reserves, production, and grades. This allows comparison of deposits from different geologic terrains having different ages and formed under different P-T conditions.

2. Geologic Setting and Mineralization Characteristics

The geographical spread of OGDs studied is shown in Figure 1. Four deposits are situated in southern Africa, three in India, two in Russia, one in Ethiopia, Brazil and Indonesia. The geographic diversity also points to diverse geologic settings that host the OGDs as summarized in Table 1.
The nature of host formations is diverse on regional scales. New Consort, Cuiabá, Hutti, Ajjanahalli, Hira Buddini and Lega Dembi are situated in greenstone belts, Renco formed in a granulite facies mobile belt, Pilgrim’s Rest is hosted by Proterozoic platform carbonates, Navachab is in the Pan African Damara orogen, Mindyak and Kochkar formed as consequence of the Uralian orogeny, and Awak Mas is hosted by the Upper Cretaceous Latimojong Formation.
There is common acceptance that the formation of OGDs is structurally controlled [57]. Most deposits shown in Table 2 are located along major compressional to transtensional crustal-scale fault zones in deformed greenstone terranes. Gold mineralization, in general, is localized in lower order structural traps such as moderately to steeply dipping, compressional brittle-ductile shear zones, reverse faults, shallow-dipping extensional veins, and tubular sheath folds [56]. As an exception, Ajjanahalli is situated within a first-order shear zone and at Kochkar the ore-bearing shear zones are developed along granite-mafic dyke contacts in the Plast Granite. The Pilgrim’s Rest host structures are shallow dipping, bedding-parallel thrust faults located at the contact between interbedded carbonaceous shale layers and carbonate host rocks.
Important for triggering gold precipitation is the rheological behavior of the host-rock assemblage [58]. Failures of more competent rocks provide specific permeability for fluid migration and fluid-rock reaction. Hydraulic fracturing and pressure cycling between supralithostatic to hydrostatic pressure results in fluid immiscibility and gold precipitation. Fluid reaction with Fe-rich host rocks leads to gold deposition via sulfidation reactions as well as fluid reduction in carbon-rich sedimentary host rocks [63]. As shown in Table 3, gold precipitation does not require an association with one specific rock type. Ore-bearing hosts are mafic and ultramafic volcanic rocks, Fe-rich gabbroic dykes, carbonaceous siliclastic metasedimentary rocks and banded iron formation. Renco is an example of an unusual setting in that it is hosted by discrete mylonites developed in an Enderbite intrusion. Irrespective of the nature of host rocks, gold mineralization is predominantly confined to (often laminated) quartz-carbonate vein networks or may be present within Fe-rich sulfurized wall rock selvages or silicified and arsenopyrite-rich replacement zones.
The ages of host lithologies and related mineralization are summarized in Figure 2. Mineralization ages range from the Mesoarchean (New Consort) to Phanerozoic times (Awak Mas). The distribution of ages is not continuous as this data set reveals a gap over 1500 Ma between the Paleoproterozoic Pilgrim’s Rest district and the Neoproterozoic Lega Dembi and Navachab deposits. Most of the greenstone-hosted OGDs in Figure 2 are of Neoarchean age while Mindyak, Kochkar and Awak Mas formed during Phanerozoic times.
Deposits hosted by collisional or accretionary orogenic belts, commonly formed at peak to post-peak metamorphism (Table 4) at the waning stage of tectonism that produced the orogenic belt [59]. Mineralization at Cuiabá, Ajjanahalli, Renco, Hutti, Hira Buddini, Lega Dembi, Navachab, Mindyak and Kochkar is between 5 Ma (Navachab) and up to 90 Ma (Ajjanahalli) younger than the host rock. The time age gaps are even larger for New Consort (245 Ma), Pilgrim’s Rest (245 Ma), and Awak Mas (114 Ma).
In the case of New Consort, the tectonic processes responsible for gold mineralization, most likely, were external to the greenstone belt. Asthenospheric heat input resulted from instability or delamination of the subcontinental lithospheric mantle as consequence of crustal accretion and convergence along the edges of the Kaapvaal Craton [1,2,3,4,5,6,7,8,9]. This promoted a crustal architecture composed of a craton-wide period of granite plutonism and associated low-P, high-T metamorphism. The resulting thermal disequilibrium was causative for the devolatilization of sedimentary and/or volcanic rocks at deeper crustal levels, that produced the ore-bearing metamorphic fluids [1,2,3,4,5,6,7,8,9]. At Renco fluids were derived from thrusting of the granulite facies Northern Marginal Zone of the Limpopo Belt over the greenstone terrane of the Zimbabwe Craton, whereby the latter suffered prograde metamorphism and dehydration [16,17,18,19,20].
Gold mineralization at Pilgrim’s Rest may also be related to an external process such as the intrusion of the Bushveld Igneous Complex into the sediments of the Transvaal Supergroup at 2069 Ma [30,31,32,33]. The regional metamorphic grade of the Malmani Subgroup is sub-greenshist facies, however, amphibole occurs close to the contact with the Bushveld Complex [30,31,32,33]. From the Awak Mass deposit, a spatial relationship is recorded with a granodiorite intrusion into the flysch-type metasediments of the Upper Cretaceous Latimojong Formation dated at 8–6 Ma, but a genetic relationship is unclear [47,48,49,50]. In some instances, it is difficult to determine whether magmatism has played a role in forming gold mineralization. OGDs and IRGDs (intrusion-related gold systems) form from similar ore fluids and have similar elemental or mineralogical characteristics except for Sn as the only metal that may be present in IRGDs but not in OGDs [66]. Tin minerals are not recorded from the above gold deposits. There is, however, at Pilgrim’s Rest and Awak Mas a possible spatial and temporal relationship with igneous intrusions that may have been causative for the thermal disequilibrium with intruded host rock and the activation of the ore and alteration system [30,31,32,33,47,48,49,50].
Relevant questions related to the formation of OGDs encompass timing of mineralization relative to deformation and metamorphism. These data are summarized in Table 5. Gold mineralization is generally syn- to late-deformation [58,63]. Mesozonal deposits are typically syn- to post-peak greenschist facies and hypozonal types formed syn-to late-peak amphibolite facies metamorphism [1,63]. New Consort, Hutti, Hira Buddini and Navachab display two distinct stages of gold mineralization. Deposits related to igneous Intrusions are not directly connected to regional metamorphism.
Pressure-temperature conditions of mineralization are shown in Figure 3. Pressure-temperature estimates are based on detailed petrographic analyses of mineral assemblages and textures of host rocks and alteration zones. Orogenic gold deposits form in thermal equilibrium with their host rocks under similar P-T conditions to the metamorphosed host [1,63]. P-T conditions were analyzed using several techniques. Conventional geothermometers and geobarometers were applied on suitable alteration mineral assemblages supported by phase-diagram modeling of TX pseudo-sections. Other P-T estimates were based on sulfide thermo-barometry, stable isotope thermometry, and pressure-corrected fluid inclusion data. P-T conditions record a range from 1 to 7 kbars and 215 to 680 °C. Most temperatures and pressures are in the range 1 to 3 kbars and 200 to 400 °C. The highest P-T conditions are observed for the first stage of mineralization at New Consort. The economic gold grade, however, is due to the second mineralization stage at 4.5 kbar and 590 °C. Two stages of gold mineralization are also recorded for Hutti and Hira Buddini. At both deposits the second mineralization stage with P-T conditions of 2 kbar and 350 °C is the most fertile gold producing stage.
Figure 4 shows that temperature conditions for mesozonal mineralization are below the temperature range of main metamorphic fluid production in mafic bulk compositions while hypozonal deposits plot to the right of this temperature field [60]. The two stages of mineralization at Hutti and Hira Buddini are separated into the hypozonal and mesozonal areas with the latter representing the most productive second stage of ore formation. The conditions for the first stage of mineralization at New Consort and Renco plot close to the wet solidus line for granite, but below the wet solidus for the enderbite host rock at Renco and the amphibolite host at New Consort [1]. However, pegmatite dykes, suggestive of crustal anatexis, are associated with the ore zones in both deposits. Other deposits occupying the hypozonal field are Hutti, Hira Buddini, and Navachab. At Hutti and Hira Buddini the external magmatic fluid source originates from crustal derived pegmatites and granites, as is the case with the contemporaneous Kavital Granite. At Navachab magmatic activity evidenced by lamprophyre, pegmatite and aplite dykes overlap with the mineralization, and regional scale plutonism was coeval with deformation. The high geothermal gradient and elevated fluid pressures indicate a hot, but relatively shallow-crustal hydrothermal system resulting from magmatic activity [35,36,37,38,39,40].
Kochkar and Lega Dembi plot at an intermediate position in Figure 4. Gold mineralization at Kochkar formed at a mid-crustal level of a continental magmatic arc and post-dates active subduction and post orogenic plutonism. The lack of postcollisional collapse led to slow exhumation and a long-lasting cooling history, which triggered the fluid system responsible for gold mineralization [44,45,46]. Kochkar is not spatially associated with a transcrustal shear zone which differs markedly from other Phanerozoic orogenic gold settings [46]. At Lega Dembi, ore deposition occurred close to peak metamorphic conditions at lower-amphibolite metamorphic grades [34]. This is supported by the presence of kyanite along the margins of laminated quartz veins. The timing of metamorphism and fluid production was syn-to-late Pan-African orogeny and hence at the waning stages of the regional tectonism in the Adola granite-greenstone terrain [34]. The origin of ore fluids and age of formation at Awak Mas is contentious. A genetic relationship with a granodiorite intrusion at 8-6 was proposed by [47,48]. However, the deposit has also been classified as an epizonal OGD with a metamorphic fluid source [52]. Accordingly, rapid uplift after Miocene continental collision resulted in the metamorphic overprint of metasedimentary rocks during the retrogression stage accompanied by rock dehydration and ore fluid migration [52].
When considering typical hydrothermal alteration assemblages associated with orogenic gold mineralization a distinction is to be made between mesozonal and hypozonal types [56]. Mesozonal mineralization associated with greenschist facies metamorphism is generally accompanied by a proximal carbonate and sericite alteration and a distal zone characterized by chlorite. In contrast, hypozonal alteration can be more complex and varied in amphibolite facies terranes. Common assemblages associated with gold mineralization include garnet, biotite, amphibole, diopside, K-feldspar, plagioclase, calcite, and titanite. Alteration minerals seem to inherit their chemical signature from the host rocks and the alteration assemblages and mineralization are in general thermal equilibrium with the country rocks [1,63]. There is a distinct mineralogical difference between the first and second alteration stages at New Consort, Hutti, Hira Buddini and Navachab. The different modal compositions are either related to different mineral stabilities at the prevailing differences in P-T conditions or differences in the hydrothermal fluid composition.
Principal gangue minerals in the orogenic gold lodes studied are quartz and carbonates together with variable amounts of sericite, chlorite, amphibole, biotite, scheelite, and tourmaline (Table 5).
Main ore minerals observed include native gold, pyrite, pyrrhotite, and minor amounts of chalcopyrite, arsenopyrite, tellurides, and bismuth minerals (Table 6). Sulfides typically constitute less than 10% of the ore with generally very low base metal sulfides. There is no general difference in the ore assemblages with mineralization ages. However, the occurrence of galena is restricted to Phanerozoic deposits and the presence of arsenopyrite together with loellingite is conspicuous in hypozonal deposits where arsenopyrite is the main sulfide. Sphalerite is a common constituent in almost all deposits but was not recorded at New Consort and Awak Mas. Native bismuth and bismuth minerals appear to be restricted to the hypozonal deposits New Consort, Renco and Navachab. The ore mineral assemblages determine the metal inventory of Au, As, Ag, Sb, Cu, Bi, Mo, Pb, S, W, Zn, but only gold is economically exploitable (Table 6). At New Consort, the simple ore assemblage of the low Au-grade first stage of mineralization consists of loellingite and pyrrhotite and is overprinted by the second stage of mineralization at conditions of 4.5 kbars and up to 590 °C. The mineralogy of this higher-grade gold ore is more complex including antimony and bismuth minerals. Gold fineness values reported for Cuiabá are in the range 759–941, for Kochkar 860–890, and for Awak Mas 920–940.
Orogenic gold mineralization is typically associated with low salinity CO2-H2O-rich, near neutral pH hydrothermal fluids that are further characterized by the presence of H2S, CH4 and/or N2 [65]. This fluid composition is regarded typical for mesozonal OGDs. The problem, however, is with hypozonal deposits that formed at conditions in excess of the optimal temperature range for metamorphic fluid production (Figure 4).
There is evidence that fluids forming the hypozonal mineralization at New Consort [3], Hutti [21], Hira Buddini [29], and Navachab [40] originated not only from metamorphic fluids but also from fluids that were released from crustal derived pegmatites and granites.
In the majority of the OGDs studied here, the conditions are such that phase separation (immiscibility or boiling) led to the development of salt-rich aqueous fluids coexisting with a volatile-rich phase (Table 7). In these deposits boiling is considered a major Au-depositing process. At Hutti, fluctuations in the fluid pressure stimulated phase separation, which initiated the precipitation of gold. Some of the laminated quartz veins contain gold up to 1000 g/t with visible gold co-precipitated with quartz. At Navachab fluid-pressure cycling during extensional fracturing has also been proposed to be the main trigger for gold mineralization in these veins. Fluid mixing played a role in the mineralization of the sediment-hosted deposits Pilgrim’s Rest and Awak Mass.
Oxygen and deuterium isotope data are ambiguous as the data range of magmatic and metamorphic fluids overlap, and there is no simple distinction possible between both fluid sources [67]. This means that constraining the fluid origin on stable isotope data only could be erroneous, but the interpretations made from isotope data in this paper are supported by published data on geochemistry, mineralogy, petrography, and fluid inclusions. Some deposits studied are related to intrusions and overlap in age with magmatic events. In this case, a mixed magmatic–hydrothermal model may be most appropriate for explaining the source of metals and fluids.
This ambivalence related to the source is displayed in the data set in Table 7. The mesozonal OGDs Cuiabá, Lega Dembi, and Mindyak are assumed to have a metamorphic fluid source. The mixed metamorphic-magmatic model seems to be more appropriate for New Consort, Hutti, and Navachab. Pilgrim’s Rest and Awak Mas are hosted by thick low-grade sedimentary successions which may exclude a metamorphic contribution to the ore fluid from the host sedimentary pile. A scenario that invokes a magmatic heat source and a mixture of magmatic fluids and formation waters is regarded the best model.
Mean δD and δ18O values for ore-forming fluids calculated for Hutti, Pilgrim’s Rest and Navachab are shown in Figure 5.
The δ18O and δD compositions for Hutti, Pilgrim’s Rest and Navachab occupy the field of metamorphic fluids whereby the Hutti δ18O fluid value plots to the left of the primary magmatic fluid field. The Pilgrim’s Rest and Navachab δ18O fluid values are slightly heavier than magmatic fluids but their δD fluid data plot close to the lower boundary of the metamorphic fluid field. The isotope data are equivocal, but they do not militate against a mixed magmatic-metamorphic fluid origin as was proposed for Hutti. Pilgrim’s Rest, Navachab and Awak Mas are hosted by sediments like dolomite and carbonaceous shale and banded marble and biotite shists. Thus, it is likely that the fluids were magmatic in origin and exchanged their isotopic signature with that of the host successions. As for the remaining deposits δD data are not available, their mean δ18O fluid values are plotted on the δ18O axis in Figure 5. Mindyak has the heaviest mean δ18O fluid value, and the fluid is regarded of metamorphic origin. Awak Mas, Kochkar, Ajjanahalli and Renco straddle the δ18O range defined by magmatic, metamorphic and formation waters. While at Awak Mas the ore fluid is probably a mixture of formation waters and magmatic fluids, the data for Kochkar and Ajjanahalli suggest a deep magmatic and metamorphic fluid source. The ore fluid as Renco was externally derived from devolatilization of the overthrusted greenstone terrane of the Zimbabwe Craton.
Economic data such as gold reserves, production and grade are summarized in Table 8 The high demand for and price of gold and innovative mining technology has encouraged mining companies to engage in the exploitation of deposits that previously deemed too difficult or expensive. For example, the Navachab open cast operation produces gold from ore with an average grade of 1.29 g/t but the ore tonnage is of a magnitude like that of many high tonnage-low grade porphyry copper deposits. Because of the presence of native gold in some deposits like New Consort, Hutti, and Pilgrim’s Rest ore grades can be highly variable with up to 10 kg/t of gold locally while on a mining level mean economic grades of 5–19 g/t are obtained. However, none of the single deposits discussed here reaches the status of a world-class or giant deposit which according to the threshold proposed by [69] should have contained gold of at least 100 metric tons (3.2 Moz). Only the goldfields at Barberton (320 t), Pilgrim’s Rest (186 t), Kochkar (about 300 t) and Hutti (ca. 100 t) are in the league of giant gold producers. When Navachab is excluded from the calculation, a negative trend is observable between ore tonnage und grade. Ore reserves of producing mines are, however, dependent on exploration activities and represent dynamic figures that change with demand and commodity price.

3. Discussion and Conclusions

If OGDs define a coherent group of mineralization types, the same general processes should always have been in operation for the formation of OGDs of all ages and geologic setting. On a deposit scale, the set of OGDs presented here reveal distinct differences with respect to the time-lag between age of host rock and gold mineralization, geologic setting, nature of host rocks, pressure-temperature conditions of ore formation, source of fluids, ore- and alteration mineralogy and metal association. The common feature among the deposits discussed here is that gold mineralization is predominantly confined to often laminated quartz-carbonate veins and that they are gold-only deposits because only gold is economically exploitable. Other similarities include the chemistry of ore fluids which does not significantly differ with mineralization age or geologic setting and the syn- to post-peak metamorphic timing of mineralization.
The formation of giant orogenic gold mineral systems is explained by a model where gold is sourced from devolatilization of pyritic sediments above a subducting slab of oceanic crust [70,71,72]. In this model, fluids and metals are sourced from below the craton or released from mantle lithosphere that was metasomatized and fertilized during a subduction event. In this context, the crustal metamorphic model which proposes that orogenic gold formed late in regional metamorphism from deep-crustal metamorphic fluids is regarded less likely. In the crustal metamorphic model, the source rocks would be part of the craton where mineralization occurred. The problem with this model is that hypozonal gold deposits formed under amphibolite to lower granulite facies conditions would have to be considered anomalous exceptions [70,71]. Furthermore, magmatic hydrothermal models also seem to fail because of the commonly observed lack of temporal and spatially associated granitic intrusions [70,71,72].
This orogenic gold system model defines useful parameters for the search for world-class OGDs or well-endowed mineral provinces [63,65,72,73]. However, most of the deposits discussed here are situated in less-endowed orogenic gold provinces or in areas where they are the only gold know gold deposit like Renco, Navachab, or Ajjanahalli. This may explain the deviation of their genetic signatures from the model parameters proposed for giant OGDs. For example, a two-stage mineralization event by different fluids is not considered by the crustal metamorphic model. However, two stages of gold-depositional processes were significant at New Consort, Hutti, Hira Buddini, and Navachab with the second stage fluids being the principal gold carriers. Furthermore, there is evidence, in some cases from boron and strontium isotopes, supporting the involvement of a mixture of metamorphic and magmatic sourced ore fluids such as at Ajjanahalli, Hutti, Hira Buddini, Pilgrim’s Rest. Navachab, and Kochkar. At Navachab the intrusion of lamprophyre, granite pegmatite and aplite dykes are coeval with mineralization [36]. Pegmatites and aplites pointing to a crustal source, but lamprophyre dikes may reflect the mantle connectivity of controlling first-order structures [71].
The ore mineral assemblages recognized determine the metal inventory of Au, As, Ag, Sb, Cu, Bi, Mo, Pb, S, W, and Zn. The occurrence of galena is restricted to Phanerozoic deposits and the presence of arsenopyrite together with loellingite is conspicuous in hypozonal deposits where arsenopyrite is the main sulfide. Sphalerite is a common constituent in almost all deposits but was not recorded at New Consort and Awak Mas. Native bismuth and bismuth minerals appear to be restricted to the hypozonal deposits New Consort, Renco and Navachab.
The salinity of metamorphic OGD ore fluids is generally low, with 3–7 wt.% NaCl equiv. [70]. This is a distinct characteristic of gold-only deposits, because low salinity fluids facilitate the decoupling of Au from base metals by the partitioning of Au into the auriferous fluid, without the base metals Cu, Pb and Zn [74]. However, experimental results [75] provide indirect support for a magmatic component in fluids forming OGDs. The experiments [75] showed that CO2-rich magmatic fluids that exsolve early or at greater depth from ascending hydrous magmas suppress fluid salinity and thus facilitate the formation of base-metal-poor but gold-rich fluids, as observed in OGD formation associated with late granitoid intrusions. The salinity values recorded for Hutti, Renco, Mindyak and Awak Mass fall in the range of most OGDs and a mixed methamorphic-magmatic fluid source appears feasible. The fact, that Hira Buddini, Pilgrim’s Rest, Lega Dembi, Navachab and Kochkar have ore fluid salinities that are outside the common range of OCDs does not contradict a mixed magmatic-metamorphic fluid source. This opens the question if the gold-base metal decoupling process advocated by [74] is the only possible explanation for gold-only deposits or if fluid sources and base metal availability equally play a role. For example, experimental work and modeling carried out by [75] demontrate that in melts that begin degassing at a late stage of differentiation decoupling of copper and chlorine is due to the relatively higher fluid/melt partition coefficient for copper compared to chlorine. Additionally, the presence of reduced sulphur in the melt may promote sulphide saturation that inhibits copper enrichment in the residual melt.
The orogenic gold system model [70,71,72] advocates a metal and fluid source external to the terrain in which mineralization occurred. Here, externally sourced ore fluids are suggested only for New Consort, Renco and possibly Navachab. All other deposits most likely formed from a crustal source, which would favor the crustal metamorphic model.
However, the formation of hypozonal mineralization cannot be accounted for by the crustal metamorphic model. It has been proposed [1] that they formed in evolved accretionary or collisional orogens, where nappe stacking or extensional unroofing generated P-T gradients and structures that promoted fluid migration. The ore fluid was sourced either by metamorphic devolatilization at lower metamorphic grades or represent mixtures of high-temperature metamorphic and magmatic fluids, the latter being derived from anatectic granites and pegmatites [1].
Obviously, neither the orogenic gold system model nor the crustal metamorphic model can provide adequate explanations for all facets of OGDs formation. Although OGDs do have to certain extent a set of coherent signatures but there seem to be no unified model for all possible environmental conditions and facets of ore formation and fluid source, tectonic and lithologic setting, and scale of gold endowment.

Funding

This research received no external funding.

Data Availability Statement

Data are available in the refrences cited.

Acknowledgments

This manuscript grew out of research that the author conducted with colleagues and PhD students on a large number of orogenic gold deposits in various parts of the world. I would like to extend particular thanks to Annika Dziggel, Alex Kisters and Jochen Kolb for their invaluable contributions, knowledge and insights into this complex type of gold deposits. The manuscript benefited from critical reviews by three anonymous reviewers. Financial support by the Deutsche Forschungsgemeinschaft (DFG grant Me 1425/1-1 to ME 1425/13-1) and the German Academic Exchange Service (DAAD) is gratefully acknowledged.

Conflicts of Interest

The author declares no conflict of interest.

References

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Minerals 13 00369 g001 550
Figure 1. Geographic distribution of gold deposits studied.
Figure 1. Geographic distribution of gold deposits studied.
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Figure 2. Ages of host rocks and mineralization.
Figure 2. Ages of host rocks and mineralization.
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Figure 3. Pressure-temperature conditions of mineralization. Error bars indicate level of uncertainty. New Consort (NC1, NC2: 1st and 2nd stages of mineralization), Cuiabá, (CB) Ajjanahalli (AH), Renco (RC), Hutti (HT1, HT2), Hira Buddini (HB1, HB2), Lega Dembi (LD), Navachab (NB), Mindyak (MD), Kochkar (KK), Awak Mas (AW).
Figure 3. Pressure-temperature conditions of mineralization. Error bars indicate level of uncertainty. New Consort (NC1, NC2: 1st and 2nd stages of mineralization), Cuiabá, (CB) Ajjanahalli (AH), Renco (RC), Hutti (HT1, HT2), Hira Buddini (HB1, HB2), Lega Dembi (LD), Navachab (NB), Mindyak (MD), Kochkar (KK), Awak Mas (AW).
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Figure 4. Pressure-temperature conditions of mineralization (modified from [1]). The fields for hypozonal and mesozonal orogenic gold deposits are delineated. Also included are geothermal gradients as well as wet solidi for granite, pelite, and gabbro. Errors are the same as in Figure 3.
Figure 4. Pressure-temperature conditions of mineralization (modified from [1]). The fields for hypozonal and mesozonal orogenic gold deposits are delineated. Also included are geothermal gradients as well as wet solidi for granite, pelite, and gabbro. Errors are the same as in Figure 3.
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Figure 5. Plot of δD versus δ18O fluid data compared to fluids of different origin. Modified from [68]. (HT-Hutti; PR-Pilgrim’s Rest; NB-Navachab, AM-Awak Mas; KK-Kochkar; AH-Ajjanahalli; RC-Renco; MD-Mindyak).
Figure 5. Plot of δD versus δ18O fluid data compared to fluids of different origin. Modified from [68]. (HT-Hutti; PR-Pilgrim’s Rest; NB-Navachab, AM-Awak Mas; KK-Kochkar; AH-Ajjanahalli; RC-Renco; MD-Mindyak).
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Table
Table 1. Regional geologic settings of hosts to the OGDs.
Table 1. Regional geologic settings of hosts to the OGDs.
DepositRegional Geologic Settings
New ConsortBarberton Greenstone Belt
CuiabáRio das Velhas Supergroup (Nova Lima Group)
AjjannahalliWest Dhawar Craton (Chitradurga Greenstone Belt)
RencoLimpobo Belt (Northern Marginal Zone)
HuttiEast Dhawar Craton (Hutti-Maski Greenstone Belt)
Hira BuddiniEast Dhawar Craton (Hutti-Maski Greenstone Belt)
Pilgrim’s RestTransvaal Supergoup (Malmani Subgroup)
Lega DembiMegado belt (Adola Granite-Greenstone Terrane)
NavachabDamara Belt (Southern Central Zone)
MindyakMain Uralian Fault Zone (Magnitogorsk Megazone)
KochkarEast Uralian Zone (Plast Granite Massif)
Awak MasLatimojong Formation
Table
Table 2. Mineralization controlling structures.
Table 2. Mineralization controlling structures.
DepositHost Structure
New ConsortConsort Bar shear zone
CuiabáCuiabá tubular sheath fold
AjjanahalliChitradurga boundary shear zone
RencoNorth Limpopo thrust zone (high-T mylonite zone)
HuttiHutti-Maske shear zone (high-angle shear zone system)
Hira BuddiniReverse shear zone
Pilgrims’s RestShallow dipping thrust faults
Lega DembiLega-Dembi-Aflata shear zone
NavachabMon Repos thrust zone
MindyakStrike-slip fault zones (Main Uralian Fault)
KochkarShear zones at mafic dike-granitoid contacts
Awak MasOblique normal faults, extensional shears
Table
Table 3. Host rocks to the gold mineralization.
Table 3. Host rocks to the gold mineralization.
DepositHost Rock Lithology
New ConsortMafic-ultramafic volcanics
CuiabáAlgoma-type BIF, mafic volcanics
AjjanahalliOxide-, Carbonate-facies BIF
RencoEnderbite
HuttiAmphibolite, felsic schist
Hira BuddiniMetabasalt, metagabbro, metadacite
Pilgrim’s RestCarbonaceous shale, gabbroic sills
Lega DembiGraphitic meta-sediments
NavachabMarble, calc-silicates, biotite shist
MindyakFe-rich diabase, carbonaceous shale
KochkarMafic dykes, granitoid
Awak MasCarbonaceous phyllites and shists
Table
Table 4. Timing of mineralization relative to metamorphism.
Table 4. Timing of mineralization relative to metamorphism.
DepositTiming Relative to Metamorphism
New Consort2 stages, post-peak metamorphism
Cuiabásyn- to late-peak metamorphism
Ajjanahallipost-peak metamorphism
Rencopost-peak metamorphism
Hutti2 stages, post peak metamorphism
Hira Buddini2 stages, post peak metamorphism
Pilgrim’s Restpost metamorphism
Lega Dembisyn- to late-peak metamorphism
Navachab2 stages, syn-peak metamorphism
Mindyakpost-peak metamorphism
Kochkarpeak-metamorphism
Awak Maspost metamorphism
Table
Table 5. Hydrothermal alteration mineralogy.
Table 5. Hydrothermal alteration mineralogy.
DepositAlteration Assemblage
New Consort1st stage: garnet, diopside, hornblende, K-feldspar, quartz, calcite, biotite
2nd stage: hornblende, plagioclase, K-feldspar, biotite, quartz
Cuiabáchlorite, carbonate, sericite, quartz, zoisite/clinozoisite
Ajjanahallichlorite, stilpnomelane, minnesotaite, sericite, ankerite
Rencogarnet, biotite, K-feldspar, quartz
Hutti1st stage: biotite, chlorite, plagioclase.
2nd stage: chlorite, K-feldspar
Hira Buddini1st stage: biotite, K-feldspar, albite, actinolite, tourmaline, calcite
2nd stage: muscovite, epidote, chlorite, quartz, calcite
Pilgrims’s Restferruginous carbonates, quartz, chlorite, sericite, rutile
Lega Dembiactinolite, biotite chlorite, epidote, calcite, sericite, kyanite
Navachab 1st stage: garnet, diopside, quartz, K- feldspar.
2nd stage: garnet, biotite
Mindyakquartz, albite, sericite, chlorite, carbonates
Kochkarbiotite, actinolite, albite, K- feldspar, quartz, epidote, tourmaline
Awak Massericite, calcite, epidote
Table
Table 6. Ore mineral assemblages.
Table 6. Ore mineral assemblages.
DepositOre Mineralogy
New Consort1st stage: arsenopyrite, pyrrhotite, loellingite
2nd stage: arsenopyrite, loellingite, pyrrhotite, chalcopyrite, pyrite, gold, ullmannite, stibnite, native antimony, bismuth, maldonite
Cuiabápyrite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, gold
Ajjanahallipyrite, pyrrhotite, marcasite, arsenopyrite, chalcopyrite, sphalerite, gold
Rencopryrite, pyrrhotite, chalcopyrite, sphalerite molybdenite, cubanite, magnetite ilmenite rutile, native bismuth, bismuth alloys, gold
Hutti1st stage: pyrite, arsenopyrite, gold
2nd stage: arsenopyrite, pyrrhotite, chalcopyrite, gold, scheelite
Hira Buddinipyrite, chalcopyrite, magnetite, apatite, sphalerite, gold
Pilgrims’s Restpyrite, arsenopyrite, fahlores, chalcopyrite, bismuthinite, native bismuth, gold
Lega Dembipyrite, pyrrhotite, arsenopyrite, chalcopyrite, galena, gersdorffite, sphalerite, tellurides, niccolite, bournonite, silvertetrahedrite, molybdenite, gold
Navachab1st and 2nd stage: pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, gold, bismuth, bismuthinite, bismuth tellurides, molybdenite, graphite
Mindyakpyrite, arsenopyrite, chalcopyrite, sphalerite, galena
Kochkarpyrite, arsenopyrite, chalcopyrite, sphalerite, tetradymite, gold, galena
Awak Maspyrite, galena, chalcopyrite, gold
Table
Table 7. Isotopic signatures, physical conditions, and proposed origin of ore fluids.
Table 7. Isotopic signatures, physical conditions, and proposed origin of ore fluids.
δ34S (‰)
Pyrite		δ18O (‰)
Quartz		δD (‰)
Quartz		Fluid Salinity
(wt. % NaCleq.)		Fluid Phase
Separation		Fluid Origin
New Consort--- magmatic/externally derived
Cuiabá--- metamorphic
Ajjanahalli+2.1–+2.713.6–14.487Sr/86Sr (Ro)
0.7068–0.7078		 mixed magmatic/metamorphic
Renco -7.8–9.2-2–10+magmatic/externally derived
Hutti-12.1–12.2−51–−611–5+mixed magmatic/metamorphic
Hira Buddini--tourmaline δ11B =
−4–+9 ‰		0–21+mixed magmatic/metamorphic
Pilgrims’s Rest−2.8–+3.114.1–16.1−51–−615–23+mixed contact metamorphism/basinal brines
Lega Dembi--87Sr/86Sr (R0)
= 0.771		3–16+metamorphic
Navachab+1.0–+8.312.2–17.9−50–−55 ‰4–16+mixed magmatic/metamorphic
Mindyak+1.0–+4.118.2–21.5calcite δ13C
= −5.31–−7.63,		3–7+metamorphic
Kochkar 10.5–11.887Sr/86Sr (R0)
= 0.7093		11–14+Mixed magmatic/metamorphic
Awak Mas−6.6–+12.910.2–12.7-2–6+metamophic
Table
Table 8. Gold reserve, production, and grade.
Table 8. Gold reserve, production, and grade.
DepositOre Reserves (Mt) (Proven & Probable)Historic Au Production (t)Annual Au Production (kg)Au Grade (g/t)
New Consort 1.4159.54340.29.4
Cuiabá7.89 4.68
Ajjanahalli7.34(1995–2002)
0.963		 1.43
Renco2.90 6.03
Hutti9.2589.59 5.62
Hira Buddini0.51 4.16
Pilgrim’s Rest45.5(1872–1972)
168		(1999–2014)
284		4.17
Lega Dembi
Closed 2018		20.877 3.71
Navachab 45.335.29 15011.29
Mindyak closed12.230.1 2.5
Kochkar10.7300 4.9
Awak Mas20.2 1.58
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Meyer, F.M. Case Histories of Orogenic Gold Deposits. Minerals 2023, 13, 369. https://doi.org/10.3390/min13030369

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Meyer, Franz Michael. 2023. "Case Histories of Orogenic Gold Deposits" Minerals 13, no. 3: 369. https://doi.org/10.3390/min13030369

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