Diversity of Gold Deposits, Geodynamics and Conditions of Formation: A Perspective View

Gold occurs in a wide range of deposit types and settings. In the last decade, significant progress has been made in the definition, classification, characterization, thereby aiding understanding of the main gold deposit types. The present work aims to provide an update on the current state of knowledge on the different types of gold deposits models, geodynamics, their mode of formation and the condition suitable for their formation Several subsets of gold deposits are distinguished from one another on the bases of and their main geological models and their mode of formation described. Gold deposits of magmatic-hydrothermal origin are classified into Porphyry, Epithermal, Skarn, Iron Oxide-Copper-Gold and Intrusion related deposits; those of hydrothermal origin are Orogenic, Volcanogenic Massive Sulphide deposits, and Carlin-type; while those of Sedimentary Origin are placers. In terms of the major Period of gold deposit formation, the Mesoarchean was the largest gold period. Other gold peaks followed, particularly in the Neoarchean, Paleoproterozoic and Paleozoic while numerous and diverse gold deposit types were formed during the Cenozoic era.Wide varieties of geodynamic contexts in which each of the gold deposits are formed being explained while the conditions favourable for its formation are also being summarized. With the recent rise in the price of gold, mining companies and research centers continue to provide lighting of the key geology features of then ore-forming environments and the key geologic manifestations of the different deposit types.


Introduction
Gold, a symbol of wealth, has been known and used by Man for at least 5000 years. Its usage spans various important fields, justifying the enthusiasm displayed by Man for this metal. It is a transition metal with atomic number 79, it is very dense (density 19.3 unit required) and malleable. In its natural state, gold occurs as the single stable isotope 197 Au. Its most common oxidation state is zero (0). Though, it can vary from (−I) at (+V), with (+I) (called aurous ion) and (+III) (Auric ion) predominating. Gold is a relatively chemically stable metal, it does not form oxides or ions in aqueous solution, but it can form complexes with CN, Cl, OH or HS. In its natural state, it occurs as native and form alloys with other elements like silver or tellurium, or as traces in the crystalline lattice of some minerals.
In spite of gold's relevance, prospecting for the metal is a laborious and complex venture due to its erratic distribution. Depletion of most of the world's shallow gold deposit cause a decrease in the world's gold supply and an increase in the international price of gold; in 2011, it reached a peak of 1800 USD per ounce. The consequence of this was a revival not only in the gold exploration and mining companies, but also on gold exploration research.
In a bid to curtail this problem and discover newer resources, researches have been undertaken in conjunction with mining companies to adequately study, characterise and classify gold deposits in order to ease exploration for newer deposits, and to improve exploitation of pre-existing ones. There has been many published research on gold deposit in the last decade, some of these research have led to significant improvement in the understanding of some gold deposits models; the definition of new types or sub-types of these deposits; and the introduction of new terms. However, significant uncertainty still remains regarding the specific distinction between some types of deposits and specific giant deposits are ascribed to different deposit types by different authors.
Gold is a siderophile element, it is part of the highly siderophile elements (HSE). Goldshmidt geochemical classification of elements defines Siderophile elements as those that have affinity for iron and migrated to the earth's core during the differentiation of the various crustal layers (Figure 1). A comparison of gold concentration between the silicate crust and chondrites suggests that 98% of terrestrial gold is contained in the core [1] [2] [3]. This siderophile character combined with the low abundance of gold in the chondritic earth and explains why it is found in small amounts in the earth's crust, making it a rare metal. The average concentration of gold is only 4 ppb in the crust with the continental crust having 1.3 ppb [4]. Due to its chemical stability, gold could sometimes concentrate locally in ore bodies and indicate a spike that can reach 10 4 times the background concentration in the rest of the crust [3]. The factors that control its concentration at certain sites to deposit level in the Earth's crust and the ultimate source of the gold in the various deposits have remained topics of debate [3] [5] [6].  Many classification schemes have been developed for subsets of gold deposits.
These many classification schemes has provided additional points of view and expanded the nomenclature surrounding the problem of gold deposit classification. Examples of such gold deposits subset developed are, intrusion-related gold deposits [7], bulk mineable gold deposit [8], the epithermal gold deposits [9], or the epigenetic Archean gold deposits [10] [11] stated that a rational geological classification of the commonly recognized lode gold deposits is not feasible if it is not based on the geological settings of the deposits, host rocks, nature of mineralization and geochemical signature.
The present work aims to provide an update on the current state of knowledge on the different types of gold deposits models and their mode of formation. Section one 1) introduce us to the paper; section two 2) describe the major Periods of Gold formation; while sectionthree 3) and four 4) describe the main types of gold deposits and shows that the deposits can be formed in a wide variety of geodynamic contexts with each deposit type formed in a very particular context that meets all the conditions favourable for its formation; and section five 5) is the conclusion part of this paper.

Major Gold Formation Periods
Gold deposits were not consistently formed over geological time ( Figure 2). The Mesoarchean was the largest gold period (representing approximately 90,000 tonnes of gold) from of a single deposit: paleoplacers of the Witwatersrand in South Africa. Other gold peaks followed, particularly in the Neoarchean, Paleoproterozoic and Paleozoic. These are dominated by orogenic gold deposits that were put in place during the major orogenies [13]. Numerous and diverse gold deposit types formed during the Cenozoic era include epithermal, porphyry, skarn and Carlin-type deposits. However, this distribution is based on the duration of preservation of certain types of deposits. In effect, the surficial deposits (e.g. porphyry and epithermal deposits) are more susceptible to erosion and eventually disappear. Thus, the gold period observed in the Cenozoic is artificially amplified with respect to older periods which are minimized.
The study of the spatial distribution of known gold deposits has led to the as-  showed that an external supply was not needed to explain the gold contents of the mantle [19]. The Mesoarchean gold event can also be explained by a change in the thermal properties of the mantle during this period. It has been shown that the temperature of the mantle peaked (250˚C higher than the current temperature) at 3 Ga [20]. This temperature peak favoured the formation of mantle plumes which acted as vectors of transport of gold from the core-mantle discontinuity where it is enriched to the lithosphere [3].

Types of Gold Deposits and Their Formation Conditions
There are several types of gold deposits, with each type differing in geometry;

Deposits of Magmatic-Hydrothermal Origin
Interactions between igneous and hydrothermal processes play an important role in the formation of certain types of ore deposits, and more particularly in the sub-surface context.

Porphyry Deposits
Porphyry deposits are hydrothermal deposits associated with felsic to intermediate magma intrusions and they make up the largest gold reservoirs deposit in the Earth's crust ([1] [12]). This type of deposit is characterized by low to medium metal contents, but very high tonnages ([7] [21]). Porphyries occur at shallow depth (1 to 5 km), around calc-alkalic intrusions related to subduction zones, island arcs or continental Cordilleras [22]. Main metals in this type of deposit are copper, molybdenum and gold. The formation of porphyry deposit depends on the formation of hybrid magma (of mantle and crustal sources) during subduction. The plunging plate is dehydrated, which causes partial melting of the overlapping plate. The magma goes up with the balance of the overlying crust and causes in its turn a fusion of its base. The calc-alkaline magma ascends to the surface through the crust via dykes to supply batholiths at lesser depth. Two phenomena will then intervene, contributing to the separation of aqueous fluid phase of the magma. Firstly, the ascent of the magma along the dykes is accompanied by a pressure drop that will decrease the solubility of water. On the other hand, the gradual crystallization of the magma will increase the water content of the residual liquid until it reaches threshold solubility [23]. The combination of these two processes will be at the origin of the different phases. Thus, the liberated fluid phase causes a hydraulic fracturing of the host rock, within the stockworks leading to mineralisation.

Epithermal Deposits
The epithermal deposits occur in the same tectonic settings relative to porphyry deposits, which are volcano-plutonic arcs, island arcs and Cordillera arcs associated with subduction zones. They are mainly present along the pacific ring and the alpine chain in Europe. They are very superficial deposits since they are deposited between 2 km depth and surface [24]. They are therefore very sensitive to erosion, which explains why most of the deposits still existing today are post Jurassic ( Figure 2).
There are two types of epithermal deposits which are distinguished primarily by the difference in oxidation state of sulphur in the related ore-forming fluids [25]: we have "high sulfidation" deposits (also called acid epithermal) and "low sulfidation" deposits (neutral epithermal). The "high sulfidation" epithermal deposits form at temperatures between 150˚C and about 300˚C [24] from a very acidic mineralizing fluid. The mineralization is characterized by copper minerals, in particular the chalcopyrite, enargite and luzonite. This type of deposit is also very rich in pyrite; it's the most common sulphide. These deposits are hosted in volcanic andesitic to dacitic rocks belonging to the calc-alkaline series.
Unlike the acidic epithermal deposits, "low sulfidation" deposits have the par- There is evidence showing the existence of a genetic and spatial link between porphyry and high sulfidation epithermal deposits, these latter constitutes the apex of porphyry systems [25]. The link between porphyry and low sulfidation epithermal deposits is less obvious. Indeed, unlike acid epithermal, low sulphide deposits do not occur in the immediate vicinity of a volcanic system but rather within geothermal systems. The formation acid epithermal deposits results from the release of vapours and magmatic fluids carrying metals during hydraulic fracturing around the porphyry, which will migrate to the surface [27]. The deposition of the mineralization will be caused either by a phenomenon of ebullition at the surface, a mixture and a dilution of the magmatic fluid by meteoric fluids, or a combination of these two phenomena ([27] [28]).
Hydrothermal fluids related to low sulphide epithermal deposits are a mixture between magmatic fluids and meteoric waters, with the latter being dominant.
These fluids are less salty, thus poor in chlorine but particularly rich in gases (H 2 S and CO 2 ). Under surface conditions, the fluids will enter into ebullition, phenomenon that will be accompanied by the loss of steam containing CO 2 and H 2 S [24]. This loss of sulphur will decrease the solubility of gold in the fluid provoking its precipitation along the fluid's circulation path.

Iron-Oxide-Copper-Gold (IOCG) Deposits
IOCG deposits (or iron-oxide deposits) recently appeared in the classification of metal deposits and still remains quite poorly defined. IOCG deposits are characterized by their abundance in iron oxides (magnetite or hematite), phosphorus, fluorine, barium and rare earths, and their low titanium content ([34] [35]). These deposits are mainly exploited for copper and gold. Main IOCG deposits range from Archean to Mesozoic and are found in Australia (e.g. Olympic Dam, Prominent Hill) and South America. These deposits are formed by extensive context along major structures and their geometry tends to follow the local or regional deformations [34]. The origin of IOCG deposits is under debate, particularly concerning the nature of mineralizing fluids involved. Views differ between magmatic brine ([34] [35]) and external brine set in motion by intrusions [36] as mineralizing fluid. However, a study of IOCG in the Central Andes [37] favours a magmatic origin for fluids as well as the source of gold, although other types of fluids (metamorphic, sea or connate water) can intervene locally. In all cases, the fluids are oxidizing, have high temperature (up to 600˚C, [34]) and high salinity. The mineral deposition seems to depend on structural control. It occurs along faults or shear zones, lithostratiqraphic contacts, or around contacts between granitoids and their host rocks [35]. IOCG are not systematically formed close to intrusive bodies, however, several arguments indicate a genetic link between mineralization and magmas, particularly alkaline magmas.

Intrusion-Related Deposits
In the broad sense, deposits related to a magmatic intrusion ("intrusion-related deposits" or "intrusion-related systems") may include the four types of deposits described above ([34] [38]). In a more restricted definition, this term designates the deposits that form outside the areas of formation of porphyry, preferentially in continental domain [32]. These deposits are characterized by a mineralization spatially associated with reduced magmatic intrusions with metaluminous, granitic to granodioritic composition of crustal origin (

Deposits of Hydrothermal Origin
The term "hydrothermal deposits" refers to deposits whose formation depends These deposits may be associated with magmatic intrusions like the previous deposits, but in this case, the intrusion is only a heat engine of fluid circulations.

Orogenic Gold Deposits
Orogenic gold deposits, also called mesothermal gold deposits are of great economic importance as they represent about 30% of World gold production ( Figure 3). Unlike most deposits, orogenic gold deposits are formed late in the evolution of convergent margin environments during the major orogenic events ([13] [22] [41]), and are syn to post metamorphic. They formed in an episodic manner from mid-Archean to Proterozoic, then more evenly throughout the Phanerozoic (Figure 4) [13].
The formation of orogenic gold deposits is closely associated with the tectono-thermal events during orogenies. As such, these deposits are hosted in metamorphosed rocks, most often in the greenschist facies. Metamorphic fluids released by rocks during dehydration reactions during prograde metamorphism form regional hydrothermal systems ([13] [41]). It concerns aqueous-carbonic fluids of low salinity [42]. The origin of gold appears to be both due to these metamorphic fluids and it's leaching by rocks they circulate through [43]. These gold-bearing fluids are then channeled through major discontinuities along which the reaction of fluids with the host rocks is going to be at the origin of lateral zones of alteration that can extend over several meters [41].   (Figure 4).

Volcanogenic Massive Sulfide (VMS) Deposits
Hydrothermal circulation in the ocean crust can be at the origin of the formation of ore deposits: volcanogenic massive sulphide (VMS). VMS are stratiform ore bodies formed from hydrothermal fluids in the seafloor in association with volcanic rocks [44]. This type of deposit is formed in varied tectonic settings

Carlin-Type Deposits
The Carlin-type gold deposits make up the largest hydrothermal gold deposits in the world, they take their name from the Carlin deposit discovered in Nevada in the 1960s. They share many similarities with the orogenic gold deposits but, unlike the latter, they are formed during extensive regimes that follow the subduction processes [49]. They are deposits hosted in sedimentary rocks, with the disseminated gold mineralization localised in arsenic pyrites, and characterized by enrichment of As, Sb, Hg and Tl, and low base metal content [50]. They have high tonnage which enables exploitation even at low grades. These deposits were put in place within structural traps (faults, folds...) or lithological traps such as the interface between a carbonate platform and siliciclastic rocks [51], at a depth of 1 to 4 km [52].
The model of formation of Carlin type deposits is not well understood, and the source of metals remains controversial as well as the nature of hydrothermal fluids that carry them [53]. Meteoric, magmatic, metamorphic fluids and basins were in turn proposed. Following the study of the isotopic composition of sulphur in a deposit in Nevada, [53] deduced that the mineralizing fluids are essentially of magmatic origin. However, this hypothesis seems hardly compatible with the general characteristics of the fluid inclusions indicating low salinity (1 to 8 wt% eq. NaCl) and low temperature (150˚C to 250˚C, [54]). On the other hand, a magmatic origin of fluids cannot be considered for the districts devoid of intrusions as is the case in China [54]. The tendency would therefore be to consider meteoric fluids as mineralizing fluids for this type of deposit or a mixture of different types of fluids.
The mineralization of the Carlin-type deposits is represented by sub-micrometric gold grains found usually in the Crystal structure of disseminated pyrite accompanied by arseno-pyrite, or in arsenic-enriched pyrite overgrowths ( [49] [51]). Hydrothermal circulation at the origin of the mineralization is accompanied by alteration of country rocks consisting of silicification, a decarbonation or dissolution of calcite and dolomite [51]. These transformations have consequently increased porosity and permeability of the rock, which promotes the migration of hydrothermal fluids. These fluid-rock alterations, accompanied or not by a dilution of fluids by mixing, which cause the destabilization of the sulphide complexes and therefore the deposit of gold [35].

Deposits of Sedimentary origin: Placers
Hydrothermal processes (whether or not related to magmatic processes) are at the origin of the formation of the majority of gold deposits on earth. However, some sedimentary processes can also lead to deposits of economic importance such as placer gold. Placers are typically secondary deposits that require remobilization from a source reservoir, transportation and then re-sedimentation at a concentration site [55].  [67]). One of the main problems in the study of paleoplacers is to identify the source of the gold. The source of some placers located in the Pacific belt [68], Russia [69] or Australia [13] has been attributed to existing orogenic gold deposits, but in most cases the source of deposits have disappeared, which further complicates their identification.

Geodynamics and Formation of Gold Deposits
There is a wide variety of gold deposits that differ by their geology and also by their mode of formation. The description of the main types of gold deposits in the preceding paragraphs shows that the gold deposits can be formed in a wide variety of geodynamic contexts. In addition, each type of deposit is formed in a very particular context ( Figure 5) and meets all the conditions favourable for its formation. Gold deposits are thus markers of the geological events that led to their formation, and other than its purely economic aspect, their study enables the study of these events ( Figure 5).
It is noted that among the different contexts that can lead to the formation of a gold deposit, active margins, and in particular the subduction zones, are the most effective as they host the largest epigenetic gold deposits which include:  The geodynamic processes controlling the formation of a deposit will also affect the depth of emplacement. Thus, sedimentary deposits such as Placers systematically form at the surface while hydrothermal and magmatic deposits can be emplaced at different levels within the crust ( Figure 6). Most of these deposits occur within 5 km of depth, except orogenic golddeposits which occur further (from 5 to more than 25 km of depth). This difference explains the better preservation of orogenic deposits over geological time ( Figure 6).
The largest gold deposits (except Placers) thus formed during periods of crustal growth as shown in Figure 7  The study and understanding of the formation of gold deposits therefore indirectly enable the study of these exchanges (Figure 7).

Conclusions
Earlier it has been argued that gold deposits, by their diversity, are markers of   The grey bands represent the distribution of the supercontinents after [73].
To date the literature noted that among the different contexts that can lead to the formation of a gold deposit, active margins, and in particular the subduction zones, are the most effective as they host the largest epigenetic gold deposits While this study does not offer a conclusive answer, the research raises important questions about the state of knowledge on the different types of gold deposits models, geodynamics, their mode of formation and the condition suitable for their formation.
As a result of conducting this research, I propose that, if policymakers were to take this study seriously, they might be better equipped than ever to face the increasingly difficult challenge of finding gold.