Primary Gold Research Articles

Geology and Gold Mineralization in the Pan-African Rocks of the Adola Area, Southern Ethiopia

Primary and placer gold deposits are mined from the Pan-African Adola volcano-sedimentary sequence, in southern Ethiopia. Two major mineralized belts can be recognized: the Megado (‘Gold Belt’) and the Kenticha Belts. The Kenticha Belt is also known for its rare metal mineralization. Extensive exploration of the area resulted in two most important primary gold deposits of Lega Dembi and Sakaro. The primary gold deposits are classified into four classes based on their geological setting: - auriferous veins, lodes, stockworks and silicified zones disseminated in schistose rocks - gold associated with quartzite - gold mineralization confined to conglomerates and meta-arkoses - auriferous quartz veins in high grade gneiss rocks This classification provides a useful guide for future exploration programme

Mineralogy and fluid inclusion studies of the Au-Cu quartz veins in the Hamash area, South-Eastern Desert, Egypt

Field relations, petrographic observations and fluid inclusion data are used to characterize the mineralizing fluids of gold-copper bearing quartz veins, which are spatially associated with a granite-porphyry, metavolcanics and metagabbro in the Hamash area, South Eastern Desert of Egypt. Four generations of genetically related quartz veins occur in the Hamash mine area. Two types of alteration are developed in vicinity of quartz veins; i.e., sericite-quartz-pyrite and chlorite-epidote-pyrite-sericite alteration. Fe-Cu sulfides in the veins were precipitated in two stages, early pyrite (PI) and chalcopyrite were altered to secondary chalcocite, bornite and digenite and a second generation of pyrite (PII and PIII). PI pyrite and quartz contain inclusions of gold as well as remobilized gold along cracks and microfractures. Two types of fluid inclusions are distinguished: 1) primary H2O-CO2-CH4-NaCl inclusions (type I) and 2) primary and secondary aqueous inclusions (type II). Type II is further subdivided by the inclusions occurrence within different vein types. Type I inclusions entrapped the endmembers of an unmixed fluid which consists of an aqueous phase and a CO2-rich gas phase, respectively. The entrapment conditions of approximately 250°C and 200 bars were estimated by intersecting the isochores of the two coexisting aqueous and CO2-rich fluids and indicate a shallow crustal level. The salinity of type IIa inclusions is generally low (< 9 wt.% NaCleq), they homogenize above 234°C and included heterogeneous and homogeneous fluids over a wide pressure range. Homogenization temperatures of type IIb inclusions vary between 102° and 284 °C, their assumed entrapping temperatures are 200°C and 110°C, respectively. The four generations of quartz veins are related with different inclusion types. The metals including primary gold were probably transported as bisulfide complexes and precipitated due to wallrock sulfidation, fluid mixing and phase separation. The Hamash Au-Cu mineralization shows a combination of porphyry- and epithermal-deposits characteristics.

Adularia associated with epithermal gold veins in the Tapajós Mineral Province, Pará State, Northern Brazil

The potassium feldspar variety adularia was found in the Davi and Joel primary gold prospects located in the Tapajos Mineral Province, southwestern Para State, Brazil. In both areas adularia occupies fractures and vugs intimately associated with mineralized quartz veins hosted by hydrothermally altered Proterozoic subvolcanic mafic rocks. Adularia constitutes small (1-4 mm) red to pink crystals of pseudo-orthorhombic form. The Felsobanya habit was identified in specimens from the Joel prospect. Microprobe analyses show that adularia has a composition near to pure KalSi3O8. X-ray diffraction data indicate that the feldspars correspond to predominantly triclinic types with structural state near that of maximum microcline. This high degree of ordering is relatively rare in adularia and may be attributed to the extensive action of late fluids over originally metastable, disordered crystals whose primitive monoclinic form is still preserved. Fluid inclusion studies in quartz crystals adjacent to adularia show that the feldspar formed under temperatures of 240° -320°C at Davi and 220° - 340°C at Joel. The trapped fluids are poor in or devoid of CO2. Fluid salinity was low, averaging 1.15 at Davi and 0.43 at Joel, attaining exceptionally values up to 11 wt.% NaCl equivalent. Considering that the presence of adularia is indicative of boiling, it is possible to estimate a maximum depth of 1.5 km for the formation of adularia and the associated quartz veins. The presence of adularia in the Davi and Joel prospects is important in the definition of the typology of primary gold of the Tapajos Province as this feldspar is a typical mineral of epithermal deposits of the adularia-sericite category. Other features also point to the epithermal nature of the mentioned occurrences. These include the non-metamorphic character of the host rocks and their hydrothermal alteration pattern; the open-space filling textures of the veins and their mineral assemblage; and the brittle nature of the deformation imprinted in both veins and wallrocks. The composition of fluids in inclusions also lies within the limits admitted for adularia-sericite epithermal deposits. The temperatures of formation of the adularia and quartz veins, in turn, belong to a high temperature rank in epithermal deposits, suggesting that the Davi and Joel prospects correpond to exposures of the deeper, higher temperature sectors of their original hydrothermal systems. Two other aspects should be emphasized concerning the definition of an epithermal typology for gold occurrences in the Tapajos region. The Proterozoic age is an unusually old one for this type of deposit, as the majority of their representatives in the world are Cenozoic; and the definition is interesting for gold exploration in connection with the potential for bulk-tonnage deposits.

An overview of main auriferous regions of Brazil

Considering auriferous regions of Brazil carrying production (1965-94) plus reserves over 101 metal and their geological settings, it may be advanced that the regions contain some primary gold deposit types normally represented in Precambrian shields, and some others, perhaps less commonly found elsewhere. Among the former are mesothermal lodes in greenstone belts and the like, disseminations in Paleoproterozoic orthoquartzitic conglomerates, and sediment-hosted ores, particularly in deformed Meso- to Neoproterozoic turbidite sequences. The latter includes deposits associated with medium to high metamorphic grade sequences as by- or co-product gold of strata-bound disseminated copper sulfides, or hosted by Paleoproterozoic graphite- and/or manganese-bearing metasediments, e.g. itabirites, or yet showing epizonal to epithermal features but located in intra-plate setting of Mesoproterozic age. A broad comparison between the different regions suggests that those exhibiting the greater variety of mineralization styles - that is to say, of productive geological settings - appear to be richer in gold. This is perhaps a significant aspect to be considered in trying to identify better-endowed gold provinces.

On the Mechanism of Transformation from Primary Ores into Oxidized Ores in Finely Disseminated Gold Deposits

Abstract: This paper, taking the finely disseminated gold deposits in southwestern Guizhou and northwestern Guangxi as examples, discusses the difference between the occurrence of primary ores and that of oxidized ores, analyzes the occurrence mechanism in terms of crystallochemistry and geochemistry, and especially studies the chemical model for supergene leaching transformation of clay minerals in the two types of ores. This study proposes a new idea for dressing and smelting technology for primary gold ores.

Granite-ore deposit relationships in Central Brazil

Ore deposits related to granitic rocks in Central Brazil are associated with different Paleo- to Mesoproterozoic granite types. Sn and Au are the economic ore elements with important indium concentrations in some tin deposits. Tin mineralization is related to an A-type granite province, in the State of Goiás, composed of two distinct granite groups of 1.77 Ga and 1.58 Ga ages. The most important tin deposits are closely related to the younger group, hosted in or near to Li-mica±topaz leucogranites. Despite the chemical differences, both groups are enriched in F, Sn, Rb, Y, Th, Nb, Ga and REE. Their Nb/Ta>1 and their high F/Li ratios allow their classification as a NYF fertile granite association. Recent studies reveal indium concentrations that might be recoverable as by-product of tin. Besides indium minerals and In-bearing phases, cassiterite is the most important indium carrier (0.2–0.4% In). Primary gold mineralization is hosted in oxidized I-type calc-alkaline plutons in the northern Mato Grosso State, with characteristics either of volcanic arc or post-collisional granites. Gold occurs in small high grade vein type deposits or is disseminated in widespread hydrothermal zones with alteration such as sericitization, feldspathization and pyritization. δ 34S values between +1.3 and +3.5‰ of associated sulfides are typical of magmatic deposits. The association of gold with oxidized I-type granites and the style of hydrothermal alteration are analogous to those associations present in world-class porphyry-style deposits. Although the tectono-magmagtic setting of these granites is not well understood, such an association constitutes an important target in the search for gold in the northern region of the State of Mato Grosso.

The Archaean Mount Gibson gold deposits, Yilgarn Craton, Western Australia: Products of combined synvolcanic and syntectonic alteration and mineralisation

The Mount Gibson gold deposits are sited at the southern tip of the Archaean Yalgoo–Singleton greenstone belt, in the southern portion of the Murchison Province, Yilgarn Craton, Western Australia. Primary gold mineralisation is hosted within an eastward facing, dominantly tholeiitic metabasalt sequence which has been metamorphosed to amphibolite facies conditions. Alteration and mineralisation occurs within the north to north-northeast striking, steeply east-dipping, Mount Gibson shear zone, paralleling primary layering within the tholeiitic mine sequence. Two main styles of wallrock alteration and associated sulphide mineralisation are present. The first of these is represented by garnet (spessartine–almandine)–gahnite and cordierite–muscovite-bearing schists. These lithologies are unusual for lode-gold mineralisation, and, despite their apparently pelitic nature, have mafic precursors and reflect pre-metamorphic alteration of tholeiitic basalts. The garnet-bearing schists form a coherent stratiform horizon which overlies the main ore zone at the Orion Two deposit and are associated with recrystallised, metamorphosed, polymetallic sphalerite–galena–pyrite–(pyrrhotite–chalcopyrite–tetrahedrite) mineralisation. A more typical (for Archaean lode-gold), pervasive, syn-peak metamorphic, syn-shearing quartz–biotite±sulphide alteration envelope is associated with pyrrhotite–pyrite±chalcopyrite mineralisation at Mount Gibson. Although free gold is rare, there is a strong empirical relationship between enhanced gold grades and the pyrrhotite–pyrite±chalcopyrite assemblage, suggesting that gold was introduced as part of this event. Mass balance calculations at Orion Two show enrichment of Au, Ag, K, Rb, Ba and W and depletion of Na, Ca and Sr over mineralised zones, which is typical of lode-gold mineralisation. However, the deposit also exhibits strong base-metal enrichment, particularly within the garnet-bearing schists, and zonation from the Au–Cu enrichment of the main ore zone to the overlying, stratiform Pb–Zn–Ag–(Mn) enrichment, which is dominant in the garnet-bearing schist horizon. These features are typical of VHMS mineralisation. Hornet exhibits similar overall elemental enrichment and depletion to Orion Two. Lead-isotope data for sulphides from Orion Two and Hornet provide evidence of two discrete mineralising events at Mount Gibson, separated by approximately 300 m.y. A two-stage model best explains the features of the Mount Gibson gold deposits. Metamorphism of the aluminous alteration and weak mineralisation associated with an early synvolcanic event resulted in the formation of cordierite–muscovite and garnet (spessartine–almandine)–gahnite bearing assemblages, respectively, which were less competent than the surrounding tholeiitic metabasalts and consequently tended to localise shearing. Shear-hosted lode-gold mineralisation was synchronous with peak metamorphism and shearing. Like the shearing, hydrothermal alteration was mostly concentrated in and around the less competent rocks which were produced by metamorphism of synvolcanically altered precursors, resulting in a fundamental stratigraphic control on gold mineralisation at Mount Gibson.

Turbidite-hosted gold deposits of Central Victoria, Australia: their regional setting, mineralising styles, and some genetic constraints

Victoria is a gold province of international stature with primary gold mineralisation richly developed in the pre-Carboniferous rocks of the Lachlan Fold Belt. Total recorded production is in the order of 2,460,000 kg gold of which some 60% was sourced from extensive alluvial deposits, both of deep and shallow lead origin. Eleven mining centres each produced over 30 tonnes of gold with the major producer being Bendigo (684,300 kg). The Palaeozoic sequence in central Victoria is characterised by Cambrian metabasites or greenstones with associated volcanogenic and pelagic sequences followed by a Cambro–Ordovician to Early Devonian sequence of silici-clastic sediments. This succession is characterised by north–south trending open to tight folds and west-dipping thrust faults. Extensive S- and I-type granites, were emplaced from Late Silurian to Late Devonian, with the latter group, in part, associated with acid volcanic complexes. Mineralising styles include mesothermal deposits, either turbidite-hosted or dyke-affiliated, and these account for the bulk of primary gold mineralisation in the State. Less significant epithermal prospects and deposits are associated with either Cambrian calc-alkaline volcanics or sediments, typically of Siluro–Devonian age. Other styles of minor importance include Cambrian volcanogenic exhalative Cu–Zn mineralisation with gold as a minor credit (0.15–0.6 g/t Au), granite-hosted gold, and a potentially important group comprising disseminated gold in sediments. Examples of this latter group may exhibit spatially-related coticule development and by analogy with Nova Scotian disseminated intrametallic deposits, may have implications for a hitherto unrecognised style of gold mineralisation in Victoria. A gradation in lode-Au mineralising styles occurs across central Victoria. To the west in the Stawell metallogenic province, mineralisation was emplaced structurally and stratigraphically lowest in the crust. Mesothermal mineralisation is shear-hosted in Cambrian meta-sediments and greenstones exhibiting repeated deformation, multiple foliations, and extensive shearing. Host sediments were dominated by greenschist grade regional metamorphic conditions. Within the Ballarat metallogenic province, mesothermal mineralisation is hosted by dilational-site's style, in Ordovician sediments which were subjected to upper prehnite–pumpellyite facies conditions to the west and zeolite facies conditions to the east adjacent to the Melbourne metallogenic province. Ore fluids were of the low salinity C–O–H type with 1–10 wt% NaCl equivalent. CO 2 and CH 4 contents of fluid inclusions tend to be low with variable CO 2/CH 4 ratios. Depth of deposition (based on evidence from Wattle Gully mine) was in the order of 7 km and isotopic data suggest that ore fluids were derived either from an evolved igneous or from a devolatilised metamorphic source. Towards the eastern margin of the Ballarat metallogenic province and within the adjoining Siluro–Devonian Melbourne metallogenic province, epithermal style Au mineralisation is commonly emplaced in structurally and stratigraphically higher rocks. Trapping temperatures are lower than in deposits to the west and ore fluids are of low salinity C–O–H–N type with N 2>15 vol%. Gangue pyrite has elevated Sb contents and ore fluids were dominated by evolved meteoric water. δ 13 C values from vein carbonates suggest their derivation from oxidised organic carbon from within the Siluro–Devonian marine sediments. Alteration styles in gold deposits across central Victoria include sericitisation, carbonatisation, sulphidation, chloritisation, and (de-) silicification. Owing to the primary mineral assemblages and the physico-chemical characteristics of the ore-bearing fluids, visible evidence for wall rock alteration is commonly limited to 5–10 m wide zones of discolouration containing disseminated arsenopyrite and pyrite porphyroblasts and carbonate spotting. However, current petrographical and geochemical studies have demonstrated that the effects of pervasive wall rock alteration are more pronounced and extensive than previously recognised. Structural deformation within the Stawell and Ballarat metallogenic provinces, both from field evidence and 40 Ar / 39 Ar dating of cleavage phyllosilicates, indicates an Early Silurian event which migrated progressively eastwards with time. This Silurian event probably dates accretion of the proto Lachlan Fold Belt with the Kanmantoo Group of foreland Australia. Preliminary dating of hydrothermal sericites and felsic dykes associated with mineralisation, suggests episodic mineralising events 20–70 million years after accretion. These events demonstrate a spatial and temporal relationship with low-pressure regional metamorphism and/or granite plutonism; the latter being concentrated in two main pulses (400 Ma and 365 Ma) in central Victoria. The source of gold is equivocal with both igneous and devolatilised metamorphic sources being suggested. One potential reservoir source is the basement Cambrian greenstones of both MORB and boninitic affinity with associated sulphidic interflow sediments. Initial geochemical data indicate mean values of 65 ppb Au in these sulphidic interflow sediments which could have reacted with CO 2-rich fluids developed during Siluro–Devonian regional metamorphism and magmatism. Subsequent deposition of gold from such fluids could have occurred in structurally higher level dilational traps.

Oxidised gold deposits: Relationships between oxidation and relative position of the water‐table

Many oxidised gold deposits exhibit features which strongly suggest an interplay between geo‐chemical processes and movement of the water‐table. Geochemical processes involving the dissolution of gold‐silver and gold‐tellurium alloys, and precipitation of high‐fineness gold (low Ag) in transition zones, neck‐oxide and lateral oxide zones can take place both above and below the water‐table. In general the lower availability of oxygen below water‐tables ensures that these processes occur at a slower rate in the saturated zone than in the overlying water‐undersaturated weathering profile. Mass‐balance calculations on the Golden Web deposit, Coolgardie, indicate that dissolution and reprecipitation occur above the water‐table. Transfer of gold downwards from the leached zone created to the transition zone located just above the water‐table, is quantitative. Variations in the rate of downward movement of the water‐table(s), relative to the rate of oxidation of the primary gold and sulfides contained in the system, can result in up to eight different types of oxidised gold deposits. Three of these occur in relict profiles (R1‐R3), three are related to erosional profiles (E1‐E3), and two are representative of depositional profiles (D1, D2). Type R1, relict profile with one (high) water‐table, is representative of new systems in which oxidation and downward leaching are incipient. Type R2, relict profile with recently receded water‐table, is representative of a more mature system in which there has been late‐stage stream capture or climate change resulting in water‐table recession, whereas type R3, relict profiles with multiple stillstands, occurs in yet more mature profiles with a succession of such events. When such relict profiles are eroded, three related profiles are developed with very different surface expression: type E1, eroded profile with recently receded water‐table; type E2, eroded profile with receding water‐table and very active weathering; and type E3, eroded profile with stillstands deep in the profile. In depositional profiles two distinct types can be recognised: type D1, depositional profiles with lateral flow; and type D2, depositional profiles with no lateral flow (for which salt‐lake environments provide a classic example). Examples for all but the first type are known in Western Australia. Gradual, rather than episodic reductions in water‐table can result in secondary gold in oxide remnants in the upper parts of weathered profiles. These styles of oxidation have considerable impact on exploration in two ways: they affect the methods of exploration, and the economics of mining the deposit. Deeply leached profiles clearly have large amounts of overburden; enriched transition zones can have a positive impact on profitability. Knowledge of the oxidation processes and styles of gold oxidation can be used to direct exploration to the optimum strategy.

A metamorphosed, stratabound-epigenetic origin for a gruneritic Archean gold deposit, Barberton, South Africa

The Lily mine, located some 24 km northeast of Barberton in South Africa, occurs stratabound within carbonaceous Fe- and Mn-rich metapelites containing abundant grunerite. It is interpreted to be a metamorphosed epigenetic Archean gold deposit. The host rocks form part of a graywacke-shale sequence close to a major regional structure known as the Lily Fault. They are intensely deformed close to the fault with strain partitioned into finely laminated, carbonaceous phyllosilicate-rich units. The bulk geochemistry of the host rocks is dominated by pelitic, ferruginous and quartz-rich fractions mixed in varying proportions. Mineralization is concentrated in rocks composed largely of the pelitic and ferruginous fractions that have prograde metamorphic almandine, biotite, grunerite, magnetite and Fe carbonates. Magnetite-rich rocks were produced by progressive metamorphism of greenalite and/or siderite and do not reflect `oxide facies iron formation'. Much of the quartz-rich fraction occurs in deformed and metamorphosed veins associated with the mineralization. Gold is concentrated in the `Main Reef Zone' immediately adjacent to the Lily Fault. Native gold occurs with arsenopyrite, pyrrhotite, minor chalcopyrite, sphalerite, galena and stibnite in association with abundant deformed and metamorphosed veins composed of quartz–albite–grunerite–ankerite–sulfides in graphite-depleted, biotite–grunerite–tourmaline schists and massive grunerite–ankerite rocks. Ultramafic schists in the footwall of the Lily Fault exhibit intense K–CO 2 alteration (phlogopite±dolomite±calcite) and quartz veining but are essentially barren of gold. Primary gold concentration probably occurred by destabilization of bisulfide complexes through reactions with reduced iron silicates and carbonaceous matter during channelized fluid influx along the Lily Fault. Subsequent metamorphism occurred at T 500–550°C and P<3.5 kbar (transitional greenschist–amphibolite), possibly in association with the emplacement of the Nelspruit batholith at ≈3105 Ma.

Technological characterization of riacho dos machados gold ore for cyanide leaching and study of the utilization of oxidising agents

This work evaluates the behavior in the cyanide leaching of primary gold ore from Riacho dos Machados, central Brazil. The parameters studied were: extraction of gold/silver in different ore sizes to determine the exposition of precious metals; rate of extraction in bottle roll tests with crushed and ground ores; and the effect of addition of oxidising agents (air, oxygen and hydrogen peroxide) on gold/silver leaching. The run of mine ore contains about 7 ppm of gold and 2 ppm of silver. The gold/silver particles are of microscopic and submicroscopic sizes and 90% of the gold was extractable. The use of oxygen and hydrogen peroxide mixed with air increased the rate of gold dissolution.

The geology and gold deposits of the Victorian gold province

The Palaeozoic succession of Victoria represents a major world gold province with a total production of 2500 t of gold (i.e. 78 million oz). On a global scale, central Victoria represents one of the most gold mineralized areas outside the Witwatersrand of South Africa, and remains the prime example of a ‘slate belt’ gold province (also known as ‘turbidite-hosted’, or ‘shale-greywacke’ gold province). Production came from quartz veins (1000 t), modern placers (1200 t) and palaeoplacers (300 t). Up to 7000 mines produced gold from quartz veins; however, the 168 mines of this type which produced over 1 t Au contributed 68% of the primary gold production. Twelve goldfields have each produced 30 t Au (approximately one million ounces) from all sources, with Bendigo (697 t), Ballarat (408 t) and Castlemaine (173 t) being the largest: these twelve goldfields have contributed 71% of the combined primary and secondary production. Gold mining commenced in 1851, declined dramatically around 1914, and has not increased significantly since. The Palaeozoic succession of Victoria is part of an extensive fold belt along much of eastern Australia (i.e. the Tasman Orogenic Belt). In Victoria, the succession is dominated by Cambrian to Early Devonian clastic metasedimentary rocks that have undergone major upright deformation and low-grade metamorphism, culminating in the Middle Devonian (Tabberabberan) Orogeny. North-south inliers of Cambrian metavolcanic- and metasedimentary rocks associated with regional fault zones divide the province into several geologically distinct zones. In central Victoria, there are particularly well-developed acid volcanic complexes of Late Devonian age, and synchronous peraluminous granites which represent high-temperature crustal melting. Tertiary to Quaternary basalts have covered some of the Palaeozoic gold deposits, and have also covered several rich gold placer deposits of Cainozoic age. Larger goldfields, including Bendigo and Ballarat, occur in Ordovician slate and greywacke sequences, but a wide range of metamorphosed sedimentary and igneous rock types are host to goldfields. Most, if not all, primary deposits demonstrate structural control, although the controlling structural features differ between deposits. Wallrock alteration is strongly influenced by host rock: in metasedimentary rocks, it is limited and subtle, but it is more pervasive in mafic and felsic igneous rocks. Carbonates, muscovite and pyrite are the most widespread alteration minerals in these rocks, representing CO 2, K and S addition. Enrichment of As within gold deposits is common, with Sb important in some small deposits, whereas base metals are only abundant locally, and Bi, W, Mo and Te show strong spatial association with granites and are rarely associated with economic gold deposits. Granites are an important component of the Palaeozoic succession in Victoria. No large deposits are within granites, but a few goldfields, including Maldon (65 t Au), are within contact aureoles of either S or I type granites of differing degrees of fractionation and silica content. A single-stage, but protracted and possibly diachronous, period of gold introduction near the time of the Middle Devonian (Tabberabberan) deformation can explain geological relationships at many gold deposits. The possibility that this event was slightly older (i.e. Silurian or Early Devonian) in the west of the state cannot be precluded from the available evidence. The mineralizing event appears linked to the same thermal event as diorite (‘lamprophyre’) dykes and graite intrusions, acidic and basaltic volcanism, deformation and regional metamorphism. The limited amount of fluid chemistry data from Victorian gold deposits suggests low-salinity fluids which contained CO 2, and variable methane, with homogenization temperatures from 150 to 300°C. The sulphur isotopic compositions of pyrite are mostly +1 to +5‰, with some values slightly above + 10‰ Mineralizing temperatures are inferred to be near 300°C, and pressure around 1–2 kbar. Sulphide-rich gold deposits in the east of Victoria, many of which contain significant base metals, contrast with the rest of the province (which contains mostly gold deposits with lower sulphide content), and this eastern area should possibly be viewed as separate from the Victorian gold province, per se. The benefits to Victorian gold mining coming from the 1980's global gold boom have been insignificant compared to the historical importance of the Victorian gold province. There seems little justification for assuming that all the large deposits have been discovered, and there is reason to believe new deposit styles may emerge to complement the limited current exploration activity which is focussed on the search for extensions and repeats of known deposit types. A major advance in scientific understanding of these oft neglected deposits could kindle a major exploration and production revival.

Placer gold occurrences along the major rivers of Nepal Himalaya and their possible primary sources

Recent exploration in Lungri Khola area, Rolpa district, midwestern Nepal helped to delineate a discontinuous 1.5 to 40 m thick and about 30 km long primary gold mineralization zone in the Precambrian greenschists and Lower Paleozoic micaceous marbles of the Lesser Himalaya. The gold content in these lodes vary from 0.01 to 6.7 ppm.&#x0D; In eastern Nepal occurrences of primary gold is recorded in quartz-biotiteschist, amphibolite and pegmatite bodies in Sunmai and Bering Khola of Ilam district. Fine flakes of gold also occur in the pyritiferous quartz beds/lenses in chlorite-sericite phyllite and quartzite of Bamangaon polymetallic prospect, Dadeldhura district, far western Nepal. In this prospect the gold content is from 0.2 to 0.8 ppm, and one sample showed up to 14 ppm. Primary gold occurrences are also detected in few irregular quartz-pyrite veins and iron-copper sulphide bearing quartzite lying close to the amphibolite bodies. Some pyrite bearing radioactive quartzite beds in Banku Quartzite of Purchauni Crystalline Complex exposed at Boregad, Bangabagar, Baggoth and Jamari Gad area in Darchula and Baitadi districts, far western Nepal also contain gold. The gold content in the radioactive quartzite varies from 0.2 to 1.2 ppm and in some pyritiferous radioactive quartzite floats it reaches up to 5.06 ppm.&#x0D; The primary gold appears to be of synsedimentary, hydrothermal and possibly volcanogenic in origin. Placer gold is derived from primary sources and deposited at favourable locations along the river flood plains. Further investigations in similar geological terrain may help to identify economically viable primary as well as placer gold deposits in the Nepal Himalaya.

Metallogenic characteristics and ore-prospecting future of pre-cambrian gold deposits in Mufu-Jiuling district

Mufu-Jiuling district is an important placer gold producing land since the ancient time. In recent years, some primary gold deposits and mineralizations, e. g., Huangjindong and Tulongshan gold deposits, have been found. But the metallogenic conditions and regularities as well as the ore-prospecting future remain to be recognized. Through the study on the metallogenic background and condition, the auriferous source bed and the mineralizing characteristics, three types of primary gold deposits are confirmed to be prospective in this district and some key problems about their prospecting and evaluating are put forward.

Gold mineralization in the Ashanti Belt of Ghana; genetic constraints of the stable isotope geochemistry

The Ashanti belt of Ghana is the key district of gold mineralization in the Paleoproterozoic terrane of West Africa. The area considered in southwest Ghana is covered by lithologies of the volcanic-sedimentary Birimian Supergroup and the overlying elastic sedimentary Tarkwaian Group which were jointly folded and metamor- phosed under greenschist facies conditions during the Eburnean teetonothermal event at about 9..1 Ga. Regional fotiation md subparallel shear zones hosting mesothermal gold mineralization developed during deformation coeval with metamorphism. Four major tsoes of primary gold mineralization are present in the Ashanti belt: (1) mesothermal, generally steeply dipping quartz veins in shear zones mainly in Birimian sedimentary rocks, (9,) sulfide ores with auriferous arsenopyrite and pyrite, spatialty dosely associated with the quartz veins, (3) sulfide disseminations and stockworks in granitoids, and (4) paleoplaeers of the Tarkwaian Group. This study concentrates on types (1) and (9,) of the hydrotherlnal gold mineralization. Stable isotope analyses of host-rock and ore components were performed with the ailn of obtaining parameters relevant to the origin and evolution of the fluids that produced gold mineralization. Carbonaeeous matter in the Birimian metasediments displays 93C values ranging from -11.4 to -9,8.3 per mil relative to PDB, indicating an organogenie origin. Carbonates display a unimodal distribution of 9aC values ranging from -9.9 to -17.0 per rail relative to PDB. COz extracted from fluid inclusions in the auriferous quartz veins has 93C values ranging from -9.5 to -15.7 per mil relative to PDB. It is proposed that these carbon isotope compositions of carbonates and COz reflect extensive interaction of the CO,2-rieh hydrothermal fluids with reduced carbon in Birimian sediments in the deeper parts of the hydrothermal systems. Carbonates and auriferous vein quartz have 6So values ranging from 19,.9 to 9,9,.9, and 19,.8 to 15.6 per mil relative to SMOW, respectively. Carbonates and quartz were deposited in near isotopic equilibrium with respect to 6So, indicating fluid-dominated conditions during ore formation, from fluids of metamorphic or magmatie origin. Such an origin is corroborated bySD values of water extracted from fluid inclusions in vein quartz (-37 to -53% relative to SMOXV). Pyrite of synsedimentary-diagenetie origin in Birimian schists displays sulfur isotope compositions ranging from +7.3 to -9,0.9 per rail (median ca. -10% relative to CDT). Similar compositions and wide ranges are usually attributed to sulfide generation by bacterial sulfate reduction from seawater. Arsenopyrite and eogenetie pyrite from the sulfide ores generally have 534S values in the range -5.3 to -10.9, per rail relative to CDT. The tight unimodal distribution of 534S values indicates a large, homogeneous fluid reservoir. The low (534S values are interpreted as source-inherited, not related to unusual pH, Eh, temperature, or depositional conditions. Sulfides in Birimian sediments represent the most likely sulfur reservoir tapped by the fluid systems. The C, O, H, and S isotope compositions of ore-related hydrothermal minerals and fluid inclusion compo- nents indicate that the mineralizing fluids interacted extensively with the Paleoproterozoie rocks, especially Birimian sediments, at deeper crustal levels and at high temperatures. The isotopic compositions are most compatible with the formation of fluids from devolatilization reactions invoMng Birimian strata during pro- grade metamorphism at depth (metamorphic fluids).

Characteristics of gold mineralization in volcanogenic massive sulphide deposits of the Notre Dame Bay area, central Newfoundland

A reconnaissance study of 19 volcanogenic massive sulphide deposits in the Notre Dame Bay area indicates Au concentrations of up to 30 ppm (Betts Cove), and elevated gold contents (&gt;1 ppm Au) have been found in samples from 10 additional past producers and developed prospects. Systematic trends in the occurrence of gold are observed in two principal sulphide assemblages: polymetallic, pyrite–sphalerite–chalcopyrite–galena ± arsenopyrite assemblages (type I) and pyrite–chalcopyrite ± sphalerite ± pyrrhotite assemblages (type II). Type I assemblages occur in deposits with dominantly felsic host rocks, whereas type II assemblages are restricted to deposits in mafic-dominated ophiolite sequences. Free gold grains were observed in samples from eight different deposits in both type I and type II assemblages. X-ray emission spectra and electron microprobe analyses of the gold indicate that most grains are electrum, although a Au-bearing telluride occurs at Point Leamington. Ion microprobe analyses indicate that as much as 50% of bulk gold may be present as "invisible gold" locked in pyrite or arsenopyrite (up to 140 ppm Au at Point Leamington). Well-preserved primary depositional features in gold-bearing sulphides from several deposits suggest that the gold in type I assemblages is syngenetic. A strong correlation between gold and a polymetallic suite of Zn, Ag, Pb, As, and Sb, similar to that observed in Kuroko-type massive sulphides and in modern seafloor sulphides, supports a primary origin for gold in type I assemblages. In type II assemblages primary depositional features have been largely destroyed by deformation and annealing of sulphide grains. Gold is locally enriched in Zn-rich sulphides, and sulphides containing abundant pyrrhotite have the lowest gold contents. However, consistent geochemical associations with other elements are not observed, and this may reflect the strong remobilization of gold during structural deformation. The abundant free gold in some type II assemblages is a product of recrystallization during deformation and was derived locally from primary gold originally present in the host sulphides.

Evolutionary trends of gold and tin deposits

The time distribution of the most important goldbearing conglomerates (Fig. 1) and the most important primary gold deposits (Fig. 2) reveals a major gold hiatus - a gap in gold mineralization between 1800 m.y. and 700-600 m.y. Unlike gold, the biggest concentration of which occurred in the Precambrian, the greatest concentration of tin took place in the Phanerozoic. Periodicity in the orogenic processes and granitization produced a certain time and space pattern of the primary gold and tin deposition. The origin of the space and time separation of gold and tin lies probably in the early inhomogeneities of the mantle and in the formation of the granitic parts of the primitive crust.

Tectonic Controls on Dispersal of Gold into a Foreland Basin: An Example from the Western Canada Foreland Basin

ABSTRACT The Western Canada foreland basin and overlying post-Eocene succession preserve a nearly continuous succession of sediment derived from the rising Canadian Cordillera from the Jurassic to now. Initially, sediment was derived from the middle of the uplifting Omineca Belt (central Cordillera), exposing progressively deeper rocks including plutono-metamorphic complexes. With continued uplift, the drainage divide stepped eastward and only shallower-level eastern Omineca material was shed to the foreland basin. Laramide uplift, beginning in the Late Cretaceous, exposed more igneous material to be shed into the foreland basin, but an eastward shift of the drainage divide to the Rocky Mountain Belt in the early Tertiary cut off the supply of Omineca debris. Gold is present in four intervals f the foreland-basin succession: gravels of modern rivers draining the Rocky Mountain Belt; middle to upper Tertiary conglomerates derived from the Rocky Mountains; Upper Cretaceous to lower Tertiary sandstones and conglomerates; and a distinctive Lower Cretaceous igneous-clast conglomerate in the Blairmore Group. The latter two occurrences contain up to 90% igneous clasts derived from the gold-bearing Omineca Belt. Gold in the former two occurrences was derived from quartz veins in upper Proterozoic to Lower Cambrian low-grade (greenschist-subgreenschist) metasediments. These quartz veins were formed and exposed during Cretaceous-Tertiary uplift of the Rocky Mountains. Windows of gold deposition exist in a foreland-basin succession between exposure of primary gold deposits due to uplift and cutoff of source by lateral stepping of the drainage divide. The Western Canada foreland basin contains three such gold deposition windows, the last of which is still open.

Supergene ore and hypogene nonore mineralization at the Nagambie sediment-hosted gold deposit, Victoria, Australia

The Nagambie gold deposit, central Victoria, Australia, consists of approximately 7 million metric tons of ore averaging 1.2 g/t gold. Mineralization is confined largely to east-west- and N 50 degrees E-oriented shears and faults contained in a doubly plunging anticlinal dome of undifferentiated Silurian and Early Devonian siltstones and shales. Ore is associated with an east-west-trending anticlinal axis and is particularly concentrated in two east-northeast-trending structural zones: the Central and 303 shear zones. Five stages of protore vein genesis have been identified. The first two (A, B) are barren crenulated massive quartz. Primary gold mineralization occurs in stibnite-bearing quartz stockwork stringers which belong to the third (C) and fourth (D) quartz veining stages, whereas the fifth consists of unmineralized small veins (E). Fluid inclusion studies on the two gold mineralization stages C and D yield trapping temperatures and pressures of 280 degrees + or - 25 degrees C and 1,000 to 1,200 bars, and 200 degrees + or - 30 degrees C and 700 to 1,000 bars, respectively. The mineralization fluids contain H 2 O, CO 2 (0-69 mole %), CH 4 (0-21 mole %), and trace N 2 , with wt percent NaCl equiv values in the range of 0 to 10.2 wt percent. Calculated delta 18 O values (15.0-6.8%) and measured delta D data (-88 to -99%) of the hypogene fluids indicate a meteoric origin. These fluids underwent extreme 18 O enrichment by reacting with country rocks (metamorphic or igneous). Genesis of the ore, as opposed to the protore, is as secondary supergene enrichment accompanied by varying degrees of surficial limonite alteration. Ore mineralization occurs almost exclusively in limonitized fractures up to 60 m below the surface in and adjacent to major shears. Based on features such as the spatial distribution, high purity (999.26 fine), and the form of the gold and its close association with limonite, it is proposed that the gold is mobilized by surficial leaching and concentrated near the water table. Vertical movement of the water table, perhaps due to seasonal changes as well as erosion, produces rapid dissolution of hypogene gold and/or aurostibite from above or at the water table.

Deposition and transport of gold by thiosulphates, Veitsch, Austria

AbstractTwo types of gold can be distinguished in tetrahedrite from the Veitsch magnesite deposit, Austria. Primary gold present in unfractured massive tetrahedrite, has a grain size up to 18 μm and contains, on average, Cu 8.73, Ag 7.01, Au 78.63 and Hg 2.35 (wt.%). Secondary gold is present in fractures and is directly intergrown with digenite, covellite, Cu-thiosulphate, cuprite and chrysocolla but not with malachite or azurite. The secondary gold is up to 200 µm in size with an average composition of Cu 3.06, Ag 6.82, Au 86.41 and Hg 3.51 (wt.%). It is usually closely intergrown with thiosulphates containing up to 0.21 wt.% Au. This, together with the presence of ‘dirty’ gold with cloudy thiosulphate inclusions, directly indicates the transport and deposition of Au by a thiosulphate ligand. We believe this is the first reported direct evidence of gold transport and deposition by thiosulphate complexes in a natural environment.