Gazdasági Ismeretek | Közgazdaságtan » Robert Moritz - What Have We Learnt about Orogenic Lode Gold Deposits Over The Past 20 years

What have we learnt about orogenic lode gold deposits over the past 20 years? Robert Moritz Section des Sciences de la Terre, University of Geneva, Switzerland Introduction Gold deposits discussed in this paper have been called mesothermal gold, metamorphic gold, gold-only, lode gold, shear-zone hosted, structurally-controlled deposits, and also include the variety of greenstonehosted and turbidite-hosted deposits. Recently, the term orogenic gold deposits has been proposed (Groves et al., 1998), since the term mesothermal appears to be misused with respect to the original definition of Lindgren (1933). Why do we have this proliferation of nomenclature? It certainly reflects some confusion within the scientific community. Most certainly the confusion of the terminology does not stem from a lack of studies. Indeed, since around 1980, orogenic gold deposits have been studied in great detail. This research effort is reflected in a large number of scientific papers and reviews (e.g Foster,

1991; Macdonald, 1986; Keays et al., 1989; Ramsay et al., 1998) The reason about the confusion probably resides in the inherent nature of these gold deposits hosted by metamorphosed and intensely altered rocks, that are generally highly deformed and display a complex geologic evolution. The orthomagmatic model has been the most popular for many decades for the formation of the gold deposits under review (e.g Emmons, 1937) Syngenetic deposition of gold in submarine chemical sediments was invoked in the 1970s and 1980s (e.g Fripp, 1976; Hutchinson and Burlington, 1984), possibly followed by multistage remobilization of gold. Linkage to metamorphic processes as the driving mechanism for generating vein-type, shear-zone hosted gold deposits was envisaged by Fyfe and Henley (1973) with goldbearing quartz veins formed along shear zones that focused metamorphic fluids expelled from deeper structural levels in the crust. In his comprehensive review, Boyle (1979) suggested a model on a more

local scale, with lateral secretion processes resulting in gold enrichment in dilatant structures during regional metamorphism. The dramatic price increase of gold in the late 1970s, culminating at a high of U.S$ 620/oz in January 1980, was certainly a major trigger for the world-wide launched studies on vein-type and shear zone hosted gold deposits. The considerable development of different analytical techniques during the past 20 years has also allowed us to accumulate a vast data set and to improve our knowledge of these gold deposits. The combination of independent lines of investigation, including fieldwork, laboratory studies, experimental research and computer modelling, has set important constraints on the genetic models that are offered nowadays. However, how far have we progressed in our knowledge since the early 1980s? The following review shows that we have gained a fairly good knowledge about the descriptive aspects of orogenic gold deposits. It is recognised that they

constitute an independent class of deposits with recurring characteristics throughout the geologic record and in different geologic environments. Geochronology has provided important constraints for fitting confidently the deposits into a regional tectonic evolution. On the other hand, despite intense research effort in the past two decades, sources of fluids and solutes are still intensely debated. This is reflected by the disagreement about genetic models. Regional geologic features Orogenic gold deposits are present in metamorphic terranes of various ages, displaying variable degrees of deformation. The host geological environments include volcano-plutonic and clastic sedimentary terranes. The host rocks have been characteristically metamorphosed up to greenschist facies conditions, and locally to amphibolite or granulite facies conditions. The gold deposits typically occur within or in the vicinity of regional, crustal-scale deformation zones with a brittle to ductile type of

deformation. Some of them likely are the present-day expression of ancient major tectonic boundaries. The geologic structures generally indicate compressional to transpressional tectonic settings. The gold deposits can be hosted by any rock type. A wide diversity of morphologies Typically, there is a strong structural control of the gold deposits and orebodies at all scales. The morphology can be highly variable, including (1) brittle faults to ductile shear zones, (2) extensional fractures, stockworks and breccias, and (3) fold hinges (Hodgson, 1989; Robert, 1996). The orebodies can consist dominantly of altered host rock with disseminated mineralization or of fissure-filled mineralization, i.e veins, sensu stricto. The deposits can have a significant vertical extent, exceeding 1 km in many deposits, reaching even 3.2 km in the Kolar gold field in India. Mineralogy of ore & gangue minerals Veins in the orogenic gold deposits are dominated by quartz with subsidiary carbonate

and sulfide minerals, and less abundantly, albite, chlorite, white mica (fuchsite in ultramafic host rocks), tourmaline, and scheelite. Carbonate minerals consist of calcite, dolomite and ankerite. Gold occurs in the veins and in adjacent wallrocks. Gold is usually intimately associated with sulfide minerals, including pyrite, pyrrhotite chalcopyrite, galena, sphalerite, and arsenopyrite. In volcano-plutonic settings, pyrite and pyrrhotite are the most common sulfide minerals in greenschist and amphibolite grade host rocks, respectively, while arsenopyrite is the predominant sulfide mineral in ores hosted by sedimentary rocks. Tellurides can be significant ore minerals in some deposits. Gold to silver ratios typically range between 10:1 and 5:1, less commonly 1:1. Although the vein systems can be continuous for over 1 to 2 km vertical extent, there is generally little change in ore grade, and ore and gangue mineralogy with depth. Hydrothermal alteration Hydrothermal wallrock

alteration in orogenic gold deposits is developed in a zoned pattern with a progression from proximal to distal assemblages. The alteration intensity decreases with distance with respect to the orebodies. Scale, intensity and mineralogy of the alteration is a function of wallrock composition and crustal level (e.g McCuaig and Kerrich, 1998) Alteration is most intensely developed in intermediate to ultramafic igneous wallrocks. In clastic sedimentary rocks, alteration is restricted to narrow zones around the orebodies. The main alteration products of the wallrocks include (1) carbonate minerals (calcite, dolomite, ankerite, in some cases siderite and magnesite), (2) sulfide minerals (generally pyrite, pyrrhotite or arsenopyrite), (3) alkali-rich silicate minerals (sericite, fuchsite, albite, and less commonly, K-feldspar, biotite, paragonite), (4) chlorite and (5) quartz. Carbonatization, sulfidation and alkalimetasomatism of the wallrocks reflect the addition of variable amounts of

CO2, S, K, Na, H2O, and LILE during mineralization. Amphibole, diopside, plagioclase and garnet are recognized at deeper crustal levels under amphibolite and granulite metamorphic grades, where carbonate minerals become less abundant. Geometrical relationships & relative timing The structures hosting the gold deposits (shear zones, faults, extensional veins, breccia) are typically discordant with respect to the stratigraphic layering of the host rocks, but in some cases they can be parallel to bedding planes and fold hinges or intrusive contacts. This reflects the epigenetic nature of the gold deposits and the influence of strength anisotropy on their development. The mineralized structures display synto post-mineralization displacements, such as striated slip surfaces with hydrothermal slickenlines, and folding and boudinage. This indicates that the gold deposits are syn-tectonic. Veins and structures in the wallrocks typically record reverse-oblique movements along the

associated shear zones and faults. A combination of dynamic stress changes and fluid pressure variations is generally invoked to explain the geometry of the vein systems (Sibson et al., 1988; Sibson and Scott, 1998) In metamorphic greenschist grade settings, hydrothermal alteration assemblages are typically superimposed on the regional metamorphic mineralogy of the host rocks. Therefore, the gold deposits are interpreted as post-peak metamorphic. However, the relationship is more ambiguous in higher metamorphic grade environments where hydrothermal alteration is reported as syn-metamorphic (Neumayr et al., 1993), and in other cases as overprinted by deformation and regional metamorphism (Penczak and Mason, 1997). Geochronology Radiometric dating has certainly provided an absolute time frame for integrating orogenic gold deposits into the regional geologic evolution of their host environments. Indeed, 40Ar/39Ar, Sm-Nd, and conventional or SHRIMP U-Pb dating have allowed us to refine

the absolute time relationships among tectonic, metamorphic, magmatic, and ore formation events. The emplacement of orogenic gold deposits can occur immediately following or just a few million years after magmatic activity, regional metamorphism and deformation, to many tens of millions years after the intial orogenic events of the host rocks (Groves et al., 1998) Absolute radiometric dating has yielded diachronous ages for orogenic gold deposits, revealing that formation of these deposits is episodic and can occur during a prolonged time-span during the late orogenic evolution of a given tectonic province, in general as the host terrains are uplifted and progressively cooling. 40 Ar/39Ar and SHRIMP U-Pb For instance, geochronology in Australian turbidite-hosted gold deposits indicate episodic hydrothermal gold mineralization during a prolonged time-span in the midPaleozoic at 420-455 Ma synchronous with regional metamorphism and thrusting, and at 360-380 Ma overlapping with magmatic

activity dated at 360-410 Ma (Arne et al., 1998; Foster et al, 1998) In the northwestern Alps, 40Ar/39Ar dating on hydrothermal micas from gold lodes yields progressively younger ages from 31.6 Ma in the southwest to 106 Ma in the northeast. Thus, the hydrothermal systems depositing gold were recurrent for about 20 m.y across the Alpine belt (Pettke et al., 2000) While Periadriatic plutonism was coeval with the older deposits, there is no known magmatic activity contemporaneous with the younger deposits. Therefore Pettke et al (2000) concluded that the gold deposits are related to metamorphic devolatilization during uplift of the Alpine belt. By contrast, in other areas, as for example the Massif Central, France (Bouchot et al., 1997) or the Pataz province, Peru (Haeberlin et al., 1999), the mineralizing event apparently lasted only a short lapse of time, within just a few million years. Although radiogenic isotopes allow us to obtain absolute and precise ages, their interpretation

can be subject to debate. For instance, U-Pb, Sm-Nd and 40 Ar/39Ar ages of hydrothermal rutile, scheelite and muscovite, respectively, from gold deposits at Val d’Or, southern Abitibi greenstone belt, Canada, yield gold deposition ages about 2600 Ma according to Wong et al. (1991), Hanes et al (1992), and Anglin et al (1996) Such ages are younger by 70 to 100 m.y than any plutonic rocks and metamorphic events of the area. By contrast, Kerrich and Cassidy (1994) interpret the “late” 2600 Ma ages as resetting of isotopic systems below their conventional blocking temperatures due to later fluid migration associated with reactivation of the structures hosting the gold deposits. Kerrich and Cassidy (1994) argue that SHRIMP U-Pb ages between 2680 and 2690 Ma of hydrothermal zircons from the same gold deposits are more consistent with the known tectonic, structural and metamorphic relationships of the gold deposits and the local geologic environment. Such a debate shows that care must

be taken in the choice of the minerals to be dated, and that the radiometric ages must be critically assessed with respect to the regional geologic evolution of the environment hosting the gold deposits. Moreover, attention must be paid to possible thermal and secondary fluid migration overprints on the gold deposits. Recent developments in Re-Os geochronology may provide an alternative for dating directly ore minerals in orogenic gold deposits. For instance, Re-Os dating of vein pyrite at the Sigma Mine from the southern southern Abitibi greenstone belt yields a mean age of 2679 Ma (Stein et al., 1999), in good agreement with the 2680-2690 Ma SHRIMP ages of the zircons (Kerrich and Cassidy; 1994). Geodynamic setting Phanerozoic orogenic gold deposits are spatially associated with convergent plate boundaries (Nesbitt, 1991; Kerrich and Cassidy, 1994; Goldfarb et al., 1998). The tectonic setting of the Precambrian deposits remained more ambiguous until the late 1980s. Based on abundant

geological similarities between the gold deposits from Phanerozoic orogenic belts and Archean greenstone belts, a plate convergent setting is also suggested for Archean deposits (Barley et al., 1989; Hodgson and Hamilton, 1989; Kerrich and Wyman, 1990). For instance, the Archean Superior Province of Canada is characterized by the diachronous accretion from north to south of diverse plutonic, volcanoplutonic, and metasedimentary subprovinces between 2710 and 2670 Ma (Card, 1990). Lode gold deposits are linked in time and space to the diachronous accretion events, and typically occur after and during the later magmatic, metamorphic and deformation stages of each accretion event (Poulsen et al., 1992) Goldfarb et al. (2000) recognize a broad correlation of formation of orogenic gold deposits with thermal events associated to the growth of new continental crust during the evolution of our planet. Formation of orogenic gold deposits on a global-wide scale was episodic at about 3100 Ma,

2700-2500 Ma, 2100-1700 Ma, and the Phanerozoic. Orogenic gold deposits are found in both accretionary orogens (e.g western North and South America) and collisional orogens (e.g Variscan, Appalachian and Alpine orogens), also called peripheral and internal orogens, respectively (Groves et al., 1998) Isotopic characteristics: attempts to constrain the sources of fluids & solutes Isotopic studies of a variety of minerals and of fluid inclusions have been carried out to determine the source of solutes and fluids, as well as the mechanisms of ore deposition. Since gold is monoisotopic (197Au), information about the source rocks of gold are inferred from Sr, Pb and stable isotope systematics. There is certainly no global consensus about the interpretation of the enormous data set acquired so far. The ambiguities attached to the interpretations are reflected in the wide range of genetic models proposed for the orogenic gold deposits. Hydrogen and oxygen isotopes have been popular in the

attempt at constraining the source of the fluids involved in the formation of orogenic gold deposits. δ18O values of the fluids generally range between +6 and +11‰ (SMOW), and δD fluid values between –30 and –80‰ (SMOW). Both metamorphic (eg McCuaig and Kerrich, 1998) and magmatic fluids (So and Yun, 1997; de Ronde et al., 2000) reasonably fit the observed values. δ18O and δD fluid values from a number of deposits fall outside the abovementionned “typical” ranges. In Archean lode-gold deposits from Western Australia such values are interpreted as incursion of surface-derived waters (Hagemann et al., 1994). Low δD values of –160 to -80‰ in Phanerozoic North American Cordilleran and Korean deposits are interpreted as reflecting the deep circulation of meteoric waters during formation of the orogenic gold deposits (Nesbitt et al., 1986; Shelton et al 1988) By contrast, other authors attribute such low δD values to fluid reaction with organic matter and reduced gas

species (Peters et al.; 1990; Goldfarb et al, 1997) Goldfarb et al. (1991) mention that such low δD values obtained by analysis of bulk extraction of fluid inclusions are due to late generations of secondary inclusions trapping meteoric water unrelated to gold deposition. Carbon and sufur isotope data are variable for orogenic gold deposits of all ages and from different districts (e.g Nesbitt, 1991; McCuaig and Kerrich 1998) The large range in isotopic data may reflect interactions between different rock types and the ore forming fluids along their channelways from their source to the gold deposition site, and variable chemical conditions before and during ore formation. Similarly, Pb and Sr isotope systematics of different minerals from orogenic gold deposits have not allowed us to define a unique rock reservoir as the source of gold. In contrast, they indicate heterogeneous sources, and yield a general picture of a large-scale reservoir with a variety of rocks providing the

components contained in the orogenic gold deposits (Nesbitt, 1991; McCuaig and Kerrich, 1998). Moreover, there is no certainty that gold originates from the same source rocks as Pb and Sr, since the radiogenic isotopic compositions may only reflect interaction with different rocks during fluid migration, thus altering the original isotopic signature of the fluid (e.g Moritz et al, 1990; Pettke et al., 2000) He isotopes can be used to discriminate among major fluid reservoirs such as mantle, meteoric and crust (Pettke et al., 2000) Fluid inclusion characteristics Fluid inclusion microthermometry has been abundantly used to define fluid compositions, ore forming processes and pressure-temperature conditions. Problems associated to fluid inclusion studies in syntectonic, orogenic gold deposits are their possible reequilibration due to overprinting deformation and uplift/exhumation following formation of the deposits. Uncertainties in reconstructing the pressuretemperature conditions

(isochore calculations) are also attached to small, often undetectable dissolved gas species. Despite these problems, fluid inclusion studies have revealed the presence of four main types of fluids in the orogenic gold deposits: (1) two- to three-phase H2O-CO2 fluids with variable quantities of CH4, N2 and dissolved salts; (2) CO2-rich inclusions with variable contents of CH4, N2 and H2S and low to undetectable H2O contents; (3) low to medium saline two-phase H2O fluids with or without CO2; and (4) very saline H2O inclusions with a solid salt phase in some cases. The presence of CO2 in the fluids is entirely consistent with the carbonatization of the host rocks of orogenic gold deposits. This CO2-rich nature distinguishes the fluids associated to the orogenic gold deposits with respect to fluids encountered in other major classes of ore types. This feature is cited as evidence for a deep origin of the ore-forming fluids, like granulitization of the lower crust (Fyon et al., 1989) or

progressive fluid release under amphibolite facies conditions (Phillips and Powell, 1993). However, other authors contend that igneous intrusions can also produce CO2-rich fluids (McCoy et al., 1997; Sillitoe and Thompson, 1998) The fluid groups 2 and 3 described above are typically interpreted as resulting from unmixing of fluid 1. Phase separation is often cited as the trigger of gold deposition. However unmixing is not systematically associated with gold deposition (e.g Diamond, 1990), and other processes such as fluid-rock interaction or fluid cooling have been invoked. Although less popular, fluid mixing has been favoured in other cases between CO2-rich fluids and brines to explain gold deposition (e.g Anderson et al, 1992) Reported salinities of fluids associated to gold deposition are typically below 10 wt% NaCl equiv. (Groves et al., 1998; McCuaig and Kerrich, 1998) The interpretation of more saline fluids with up to 35 wt% NaCl equiv. (group 4 above), with daughter minerals in

some cases (e.g halite), is more controversial In some cases, such saline inclusions are interpreted as late brines unrelated to the gold deposits. In other deposits they have also been considered as resulting from phase separation of H2O-CO2 fluids. High fluid salinities are not only diagnostic of magmatic and basinal fluids, but can also be produced during retrograde metamorphism (Bennett and Barker, 1992). In Alpine and Variscan deposits, halogen systematics based on bulk fluid extraction has been interpreted by Yardley et al. (1993) and Boiron et al (1996) as reflecting deep circulation of meteoric water during gold ore formation. However, in the case of the Alpine deposits the meteoric fluid interpretation based on halogen systematics (Yardley et al., 1993) contrasts with He-Ar isotope systematics ruling out meteoric water during ore formation (Pettke et al., 2000) Insufficient database and knowledge of halogen systematics may explain such diverging interpretations (McCuaig and

Kerrich, 1998). In addition, analyses of bulk extracted fluid inclusions should be interpreted with caution because of possible mixing of various fluid inclusion populations. Some of these fluids maybe totally unrelated to gold deposition. In the future, single fluid inclusion analyses (e.g LA-ICP-MS) may give us more confidence about the nature of the fluids genetically related to orogenic gold deposits. The combination of the fluid inclusion and other data suggest a pressure-temperature range of ore formation of 1-3 kb and 250-350°C. Some shallow deposits may have formed as low as 0.5 kb and 150°C, and deeper deposits under higher pressures up to 5 kb and higher temperatures of 700°C (Hagemann and Brown, 1996). Modelling and experimental studies In recent years, various types of modelling have been applied to orogenic gold deposits. The models are developed and applied as a working tool, they provide information about the complexity of the system, and guidance on the type of

future useful measurements or observations. For instance, the improvement of internally consistent thermodynamic databases for mineral and volatile phases has allowed us to refine the modelling of physico-chemical conditions of the oreforming fluids. Based on phase equilibria, and fluid inclusion, isotope and thermodynamic data, Mikucki and Ridley (1993), and Mikucki (1998) have calculated that the orogenic gold deposits were in general formed from moderately reduced fluids with a nearly neutral to weakly alkaline pH at all crustal levels. The fluids maintained approximate thermal equilibrium with the rocks through which they circulated, but their chemical composition was progressively modified through fluidwallrock interaction and/or mineral precipitation during their ascent. Such thermodynamic calculations lend support to models advocating a crustal continuum of gold deposition, with a single fluid moving from deep to shallow crustal levels (Groves, 1993). Modelling of metamorphic

processes, fluid production and fluid circulation may explain conflicting temporal relationships between gold-bearing quartz veins and peak-metamorphism (Stüwe et al., 1993; Stüwe, 1998) In Barrovian-type metamorphic terrains, lower crustal levels experience prograde metamorphism later than shallow crustal levels, with a time difference of up to 50 m.y Such calculations show that veining in upper crustal levels can be coeval with metamorphism at deeper crustal levels, therefore explaining the postpeak metamorphic setting of orogenic gold deposits, and the relatively long-time gap in certain cases between peak-metamorphism of the immediate host rocks and the gold-bearing quartz veins. Similar calculations for terrains where magmatic underplating or magmatic intrusions are the cause of metamorphism indicate that quartz veining can be both pre- and postmetamorphic. There is a consensus that the main complex responsible for gold transport in orogenic gold deposits is Au(HS)2- (Seward,

1984; Mikucki, 1998). Au(HS)0 may become predominant under low pH conditions (Benning and Seward, 1996). Gold-chloride complexes are generally considered to be negligible under the physico-chemical conditions prevailing in orogenic gold deposits. However, based on thermodynamic calculations, Mikucki (1998) suggests that AuCl2- will eventually supersede Au(HS)2- as the dominant goldcomplex at temperatures above 550°C, even in lowsalinity fluids. Experimental data are certainly necessary to understand the stability of gold-complexes under amphibolite and granulite facies conditions. Conclusion: no consensus about genetic models Because of the difficulties in defining a unique source of gold and the ore-forming fluids, a number of genetic models have been proposed. The main models are: (1) granulitization of the lower crust due to CO2-enriched fluids from the mantle accompanied by felsic magmatism (Fyon et al., 1989; Hodgson and Hamilton, 1989); (2) magmatic fluids exsolved from

tonalite trondjemite - granodioritic intrusions (Burrows and Spooner, 1987); (3) fluids produced by metamorphic processes (e.g Kontak et al, 1990; Phillips and Powell, 1993; Kerrich and Cassidy, 1994); and (4) deep circulation of meteoric water (Nesbitt et al., 1986; Boiron et al., 1996) Lamprophyres have also been suggested as a gold and fluid source, but their spatial association with orogenic gold deposits is downplayed to a merely structural relationship (Rock et al., 1989) There has been a tendency in the late 1980s and early 1990s to classify gold deposits located within deformation zones under a single category named mesothermal, with the understanding that they were formed by the same process regardless of their age and geographic location (Kerrich and Wyman, 1990; Nesbitt, 1991). It is clear that with such a unifying classification, the origin of some deposits has remained contentious, in particular in higher grade metamorphic settings. Some authors have ascribed a deep origin

to such deposits, suggesting a syn-metamorphic origin (e.g Neumayr et al, 1993), therefore supporting a crustal continuum model for the orogenic gold deposits (Groves, 1993; Groves et al., 1998) In contrast, other authors favour a shallow origin for such deposits, subsequently overprinted by deformation and regional metamorphism at deeper structural levels (e.g Morasse et al., 1995; Penzcak and Mason, 1997) Orogenic gold deposits hosted by upper amphibolite to granulite metamorphic grade rocks have only been recognised in Archean terrains, and seem to be absent in Phanerozoic orogenic belts (Groves et al., 1998) Presently, the genetic models ascribed to a number of gold deposits hosted by deformed host rocks are reexamined, in particular those with styles of alteration, ore assemblages and metal associations departing from the features typical of orogenic gold deposits cited above. Such deposits are reinterpreted using intrusionrelated, porphyry-like Au-Cu, Au-rich volcanogenic

massive-sulfide or low sulfidation epithermal models (Robert and Poulsen, 1997; Robert, 2000). This approach maybe an alternative to the crustal continuum model proposed by Groves (1993). In particular, the intrusion-related model has recently gained new attention. Sillitoe and Thompson (1998) and Thompson et al. (1999) identify various types of metal associations with gold in intrusion-related systems (e.g: Au-W-Sn, Au-Fe-Cu, Au-As-Bi-Sb, etc.) Gold deposits can be hosted by the genetically associated intrusion or be located in more distal settings (1-3 km). According to Sillitoe and Thompson (1998) and Groves et al. (1998), both intrusion-related and orogenic gold deposits are spatially associated in the same geodynamic setting. Therefore, a convergence of regional geological features is likely. Assigning systematically a metamorphic or meteoric model to gold deposits located in deformation zones is certainly an oversimplification. On the other hand, caution should be applied in

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