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1.
Gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km. The deposits are interpreted to have formed during a craton-wide metallogenic event during the Eburnean orogeny. In the Syama goldfield, gold mineralisation in 9 deposits is hosted in the hanging-wall of the Syama-Bananso Shear Zone in basalt, greywacke, argillite, lamprophyre, and black shale. Gold is currently mined primarily from the oxidised-weathered zone of the ore bodies. In the Syama deposit, mineralisation hosted in altered basalt is associated with an intense ankerite–quartz–pyrite stockwork vein systems, whereas disseminated style mineralisation is also present in greywackes. In contrast, the Tellem deposit is hosted in quartz–porphyry rocks.In the Tabakoroni goldfield, gold mineralisation is hosted in quartz veins in tertiary splay shears of the Syama-Bananso Shear Zone. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolite) veins, with pyrite and lesser amounts of arsenopyrite. There are four main styles of gold mineralisation including silica-sulphide lodes in carbonaceous fault zones, stylolitic quartz reefs in fault zones, quartz–Fe–carbonate–sulphide lodes in mafic volcanics, and quartz–sulphide stockwork veins in silicified sediments and porphyry dykes. The several deposit styles in the goldfield thus present a number of potential exploration targets spatially associated with the regional Syama-Bananso Shear Zone and generally classified as orogenic shear-hosted gold deposits. 相似文献
2.
The Macraes orogenic gold deposit is hosted by a graphitic micaceous schist containing auriferous porphyroblastic sulphides. The host rock resembles zones of unmineralised micaceous graphitic pyritic schists, derived from argillaceous protoliths, that occur locally in background pelitic Otago Schist metasediments. This study was aimed at determining the relationship between these similar rock types, and whether the relationship had implications for ore formation. Argillites in the protolith turbidites of the Otago Schist metamorphic belt contain minor amounts of detrital organic matter (<0.1 wt.%) and diagenetic pyrite (<0.3 wt.% S). The detrital organic carbon was mobilised by metamorphic–hydrothermal fluids and redeposited as graphite in low-grade metaturbidites (pumpellyite–actinolite and greenschist facies). This carbon mobility occurred through >50 million years of evolution of the metamorphic belt, from development of sheared argillite in the Jurassic, to postmetamorphic ductile extension in the Cretaceous. Introduced graphite is structurally controlled and occurs with metamorphic muscovite and chlorite as veins and slicken-sided shears, with some veins having >50% noncarbonate carbon. Graphitic foliation seams in low-grade micaceous schist and metamorphic quartz veins contain equant graphite porphyroblasts up to 2 mm across that are composed of crystallographically homogeneous graphite crystals. Graphite reflectance is anisotropic and ranges from ~1% to ~8% (green light). Texturally similar porphyroblastic pyrite has grown in micaceous schist (up to 10 wt.% S), metamorphic quartz veins and associated muscovite-rich shears. These pyritic schists are weakly enriched in arsenic (up to 60 ppm). The low-grade metamorphic mobility and concentration of graphite in micaceous schists is interpreted to be a precursor process that structurally and geochemically prepared parts of the Otago Schist belt for later (more restricted) gold mineralisation. Economic amounts of gold, and associated arsenic, were subsequently introduced to carbonaceous sulphidic schists in the Macraes gold deposit by a separate metamorphic fluid derived from high-grade metaturbidites. Fluid flow at all stages in these processes occurred at metamorphic rates (mm/year), and fluids were broadly in equilibrium with the rocks through which they were passing. 相似文献
3.
The West African craton is known for its structurally hosted Au deposits in Ghana, Burkina Faso, Côte d'Ivoire, Mali and Niger. The Essakane goldfield in northeast Burkina Faso has produced 1 606,000 oz of gold since 2010 from the Essakane Main Zone. The Essakane goldfield is made up of several exploration and artisanal sites that include; Essakane Main Zone, Gossey, Falagountou, Sokadie, Tin Zoubratan, Essakane North and South, Korizéna, Bom Kodjélé, Tin Taradat, Tassiri, Gaigou, and Takabangou. Gold mineralisation in sheeted and stockworks quartz–carbonate and tourmaline veins occurs with pyrite, arsenopyrite, and traces of pyrrhotite, galena and hematite. It is hosted in sheared, folded and contact metamorphosed volcanic, volcanoclastic and sedimentary Birimian Supergroup sequences. The maximum age of gold mineralisation in the Essakane goldfield is syn-deformational and formed during the Eburnean Orogeny (D2) at 2130–1980 Ma. 相似文献
4.
The Hillgrove gold–antimony deposit is hosted in late Palaeozoic, biotite-grade metasedimentary rocks and Permo-Carboniferous granitoid intrusions of the New England Orogen. Mineralisation occurred at a range of structural levels during rapid uplift in the orogen at 255–245 Ma. Hydrothermal fluids were controlled by extensional faults in a regional-scale sinistral strike-slip fault system. Principal faults in this system were developed in, and possibly evolved from, mylonite zones which were active during Late Permian tectonics. Earliest mineralisation formed scheelite-bearing quartz veins, and these were followed by auriferous arsenopyrite–pyrite–quartz–carbonate veins with minor base metal sulphides. This latter type was accompanied by sericitisation and carbonation of the host rock, with addition of sulphur, arsenic and gold, in zones up to 20 m from veins. Quartz–stibnite veins with electrum, gold, aurostibite, and arsenopyrite form a prominent and economically important hydrothermal type, with little wall-rock alteration but extensive hydrothermal breccia formation and local open-space filling textures. Below a mining depth of 300–500 m, this type passes over a short distance downwards into stibnite-poor gold-bearing veins. Late-stage carbonate–stibnite veins with gold and silver sulphosalts cut all earlier veins, and have open-space filling textures. Aspects of the Hillgrove deposit have similarities to many other orogenic gold deposits in the SW Pacific which have been formed at different structural levels. Hillgrove is distinctive in having evidence for mineralisation at this wide range of structural levels in the one deposit, formed progressively during syn-orogenic uplift.
Editorial handling: N. White 相似文献
5.
Orogenic gold-bearing quartz veins in the middle Tertiary Bullendale Fault Zone, New Zealand were mined historically for coarse gold in a narrow zone (ca. 5 m thick). However, recent drilling has revealed a broad hydrothermal alteration zone extending into the host schist, in which disseminated sulphide and gold mineralisation has occurred. The evidence of alteration is first seen over 150 m across strike from the fault zone, and the best-developed alteration halo is about 50 m wide. The extent and intensity of alteration is strongly controlled by local structures that developed during regional Tertiary kink folding of the pervasively foliated and fissile metasedimentary schist host. The earliest structures are foliation-parallel microshears (micron to millimeter scale) formed during flexural-slip folding. Later, but related, structures are predominantly normal faults and associated shear zones that have formed extensional sites during the regional folding event. All these structures facilitated hydrothermal fluid penetration and rock alteration, with localised vein formation and brecciation. Where fluid has followed structures, metamorphic chlorite, phengite, and titanite have been altered to hydrothermal ankerite, rutile, and muscovite or kaolinite. Ankerite with Fe/(Fe + Mg) < 0.4 formed in host rocks with Fe/(Fe + Mg) of 0.6, and iron released by ankerite alteration possibly formed pyrite and arsenopyrite that host disseminated gold. Fault zones were extensively silicified and veined with quartz, albite, sulphides, and gold. Host rocks have wide compositional variations because of centimeter-scale metamorphic segregation. However, the alteration halo is characterised by elevated CO2 and S, as measured by loss-on-ignition (doubled to ca. 6 wt.%), elevated As (100–10,000 ppm), and weakly elevated Sb (up to 14 ppm). Strontium is elevated and Ba depleted in many altered rocks, so Sr/Ba ratio increases from < 1 (host rocks) to > 3 in the most altered and silicified rocks. Many altered and mineralised rocks have low Sr/Ba (< 0.5) as well. The subtle geochemical signature is not useful as a vector to ore because of the strong microstructural control on alteration. Likewise, there is no evidence for spatial mineralogical zonation across the alteration halo, although the most intense alteration is centred on the main fault zone, and intensity of alteration is controlled by microstructures at all scales. As documented in previous studies, hydrothermal alteration haloes enlarge the exploration target for some orogenic gold deposits, and may include disseminated gold, as in this Bullendale example. 相似文献
6.
The 43 t (1.4 Moz) of gold in the Woodcutters goldfield 50 km north of Kalgoorlie has wide geological significance in terms of gold in Archaean granite, as well as its local commercial and exploration significance. Woodcutters is already one of the largest Archaean gold systems in granite, and is unusual in being so far laterally from the nearest greenstone belt. Gold in the Federal zone, one of the deposits making up the Woodcutters goldfield, is hosted in hornblende‐biotite granodiorite,6 km from the mapped contact with greenstone. In Federal open pit, the granodiorite is coarse‐grained in the northern half, and a fine‐grained granodiorite in the south, with both hosting gold. These two types of granodiorite are rather similar in both mineralogy and geochemistry. There is also a subordinate fine‐grained monzodiorite. The Federal gold mineralisation is in a northwest‐striking, northeast‐dipping (315° strike/60°E dip) shear zone in the Scotia granite. Variation in grainsize of the host rocks might have affected the style of deformation with more brittle fabrics in the coarse‐grained phase and more ductile fabrics prominent in the fine‐grained granodiorite. Hydrothermal alteration is extensively developed around the Federal deposit and is a useful vector towards gold mineralisation. Distal epidote alteration surrounds a proximal muscovite‐biotite alteration zone that contains quartz‐sulfide veins. The alteration shares some of the common alteration characteristics of Archaean greenstone‐hosted gold, but differs in that carbonate‐chlorite alteration is only weakly developed. This difference is readily explained in terms of host‐rock composition and lower concentrations of Fe, Mg and Ca in the granite compared with greenstone. Fluid‐inclusion studies demonstrate that the fluids associated with the hydrothermal alteration at Woodcutters shared the common characteristics of fluids in Archaean greenstone gold, namely low‐salinity and dominant H2O–CO2. Fluid inclusions with moderate salinity were found in one fresh sample away from mineralisation, and are inferred to represent possible magmatic fluid. There is no evidence of a granite‐derived fluid being responsible for gold mineralisation. The granodiorite host rock had cooled, crystallised and had at least started to undergo deformation prior to gold introduction. The distribution of gold mineralisation in the Woodcutters goldfield has the style, shape and orientation comparable with greenstone‐hosted gold deposits in the same region. The northwest trend, the quartz veining and simple pyrite mineralogy are all features common to other greenstone‐hosted gold deposits near Kalgoorlie such as Mt Pleasant. The alteration fluid appears to have penetrated the granite on the scale of many hundreds of metres, causing large‐scale alteration. Woodcutters gold mineralisation resulted from the same metamorphic fluid processes that led to formation of greenstone gold deposits. In this metamorphic model, granitic rocks are predicted to be less‐favourable gold hosts than mafic rocks for two reasons. Granitic rocks do not generally fracture during regional deformation in such a way as to create large‐scale dilation. Furthermore, with less iron and no carbon, granitic rocks have lower potential to precipitate gold from solution by wall‐rock reaction. The metamorphic model predicts that those granite types with higher Fe should host better gold deposits, all other factors being equal. Accordingly, tonalite‐trondhjemite and hornblende‐bearing granodiorite should provide better environments for major gold deposits compared with monzogranite, and granite sensu stricto, as borne out by Woodcutters, but mafic rocks should be better hosts than any of these felsic to intermediate rocks. 相似文献
7.
The Hatu, Qi-III, and Qi-V gold deposits in the Hatu–Baobei volcanic–sedimentary basin (west Junggar, Xinjiang) represent the proximal, middle, and distal parts of the Hatu gold district, respectively. Orebodies of these deposits mainly consist of Au-bearing quartz veins and altered host rocks with disseminated sulfide minerals. Six types of pyrite in these mines are studied here to illustrate ore-formation processes. Sedimentary pyrite, including framboidal and fine-grained pyrite, occurs in mudstone-bearing sedimentary rocks or altered volcanic–sedimentary rocks. Framboidal pyrite formed during redox changes in sedimentary layers. Hydrothermal pyrite contains five subgroups, from Py1 to Py5. Porous Py1 formed prior to gold mineralization, and is overgrown by Py2 that contains inclusions of sulfide minerals and native gold. Coarser Py3 coexists with arsenopyrite and native gold, and contains the greatest As concentrations. Gold and antimony are also preferentially concentrated in arsenian Py2 and Py3. The Au–As-deficient Py4 and Py5 formed during the post-ore process. There is a negative correlation between the As and S contents in Py1, Py2, and Py3, implying the substitution of sulfur by arsenic. Gold precipitated under relatively reducing condition in framboid- and graphite-bearing tuffaceous rocks. Cesium, Rb, Sr, La, Ce, Au, As, Sb, Cu, and Pb are concentrated in altered host rocks. The Au-bearing quartz veins and disseminated sulfide mineral orebodies were formed via a co-genetic hydrothermal fluid and formed during different stages. The variation of fO2 during fluid/rock interactions, and crystallization of arsenian pyrite were major factors that controlled gold precipitation. 相似文献
8.
The Tasiast gold deposits are hosted within Mesoarchean rocks of the Aouéouat greenstone belt, Mauritania. The Tasiast Mine consists of two deposits hosted within distinctly different rock types, both situated within the hanging wall of the west-vergent Tasiast thrust. The Piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite-quartz-pyrrhotite ± actinolite ± garnet ± biotite. Gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units. The West Branch deposit is hosted within meta-igneous rocks, mainly diorites and quartz diorites that lie stratigraphically below host rocks of the Piment deposits. Most of the gold mineralisation at West Branch is hosted by quartz–carbonate veins within the sheared and hydrothermally altered meta-diorites that constitute the Greenschist Zone. At Tasiast, gold mineralisation has been defined over a strike length > 10 km and to vertical depths of 740 m. All of the significant mineralised bodies defined to date dip moderately to steeply (45° to 70°) to the east and have a south–southeasterly plunge. Gold deposits on the Tasiast trend are associated with second order shear zones that are splays cutting the hanging wall block of the Tasiast thrust. An age of 2839 ± 36 Ma obtained from the hydrothermal overgrowth on zircons from a quartz vein is interpreted to represent the age of mineralisation. 相似文献
9.
M. A. Hough F. P. Bierlein L. Ailleres S. McKnight 《Australian Journal of Earth Sciences》2013,60(7):969-992
The Walhalla-Woods Point Goldfield in southeast Australia is characterised by large gold deposits associated with a Late Devonian dyke swarm. The setting of this goldfield is unique because unlike the major gold deposits in Victoria, it occurs close to the eastern margin of the Western Lachlan Orogen, and highlights the disparities between the evolving phases of orogenic gold mineralisation in the Western Lachlan Orogen, and the contrasts between sediment hosted, dyke-associated and dyke-hosted gold mineralisation. This study integrates existing and new data from renewed mapping of the geology and geochemistry of three gold deposits near the township of Walhalla, in the historically important yet under-explored and under-researched Walhalla-Woods Point Goldfield. The ten highest yielding deposits within the goldfield are either hosted within, or adjacent to, intrusions of the Woods Point Dyke Swarm. This is due to the greater chemical reactivity of the calc-alkaline dykes, and the greater rheological contrast between the dykes and surrounding low-grade metasedimentary units, which allowed for the formation of dyke-hosted quartz breccia veins that are consistently favourable sites for gold mineralisation in the Walhalla Goldfield. This is in contrast to historical production, which concentrated on visible gold within the shear zone-hosted laminated quartz veins. Gold and As assay results have highlighted the increased levels of invisible gold disseminated along dyke margins in proximity to shear zones and quartz reefs. The high-yielding gold deposits hosted wholly by the dyke intrusions of the Woods Point Dyke Swarm are orogenic gold deposits, as they are not associated with elevated levels of Bi, W, As, Mb, Te and Sb, typical of intrusion-related gold deposits. 相似文献
10.
Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold deposit, northwest Iran 总被引：1，自引：0，他引：1
Gold mineralisation at Zarshuran, northwestern Iran, is hosted by Precambrian carbonate and black shale formations which
have been intruded by a weakly mineralised granitoid. Granitoid intrusion fractured the sedimentary rocks, thereby improving
conditions for hydrothermal alteration and mineralisation. Silicification is the principal hydrothermal alteration along with
decalcification and argillisation. Three hydrothermal sulphide mineral assemblages have been identified: an early assemblage
of pyrrhotite, pyrite and chalcopyrite; then widespread base metal sulphides, lead-sulphosalts and zoned euhedral arsenical
pyrite; and finally late network arsenical pyrite, massive and colloform arsenical pyrite, colloform sphalerite, coloradoite,
and arsenic–antimony–mercury–thallium-bearing sulphides including orpiment, realgar, stibnite, getchellite, cinnabar, lorandite
and a Tl-mineral, probably christite. Most of the gold at Zarshuran is detectable only by quantitative electron microprobe
and bulk chemical analyses. Gold occurs mainly in arsenical pyrite and colloform sphalerite as solid solution or as nanometre-sized
native gold. Metallic gold is found rarely in hydrothermal quartz and orpiment. Pure microcrystalline orpiment, carbon-rich
shale, silicified shale with visible pyrite grains and arsenic minerals contain the highest concentrations of gold. In many
ways Zarshuran appears to be similar to the classic Carlin-type sediment-hosted disseminated gold deposits. However, relatively
high concentrations of tellurium at Zarshuran, evidenced by the occurrence of coloradoite (HgTe), imply a greater magmatic
contribution in the mineralising hydrothermal solutions than is typical of Carlin-type gold deposits.
Received: 13 May 1999 / Accepted: 2 February 2000 相似文献
11.
The Ar Rjum goldfield is an example of late Neoproterozoic Au mineralization that is hosted by submarine arc assemblage and syn-anorogenic intrusive rocks. Apart from ancient workings, recent exploration in the goldfield defined three main targets along 3 km N–S corridor (Um Na'am, Ghazal and Wasema), and indicated that Wasema alone hosts 11.8 Mt @ 2.5 g/t Au. The majority of gold and sulfide mineralization is confined to diorite, where gold content increases with shearing, pyrite–sericite–carbonate alteration and development stockworks of quartz–carbonate–pyrite veins and stringers. Generally, the concentration of gold increases in the diorite samples that experienced variable degrees of hydrothermal alterations near local shear zones. Anomalous gold content (up to 11.76 g/t) in some metachert is the result of the remobilization of volcanogenic lattice-bound (refractory) Au into free Au due to post-metamorphic hydrothermal alterations. The chemistry of pyrite from the mineralized veins and stringers indicates considerable amounts of gold that reaches ~ 0.3 wt.%.Chlorite that co-exists with pyrite in the hydrothermally altered metavolcanics is mostly sheridanite with up to ~ 25 wt.% FeOt and minor amounts of ripidolite. Chlorite geothermometry suggests that two temperature ranges affecting the area. The first temperature range (290–334 °C) is consistent with regional greenschist facies metamorphism, and the second (306–355 °C) is interpreted to be related to recrystallization-submarine hydrothermal alteration related to the gold mineralization. Stable isotope (δ34S, δ18O and δ13C) data suggest an original volcanogenic arc signature that has been slightly modified by low-grade metamorphism, and finally by the late interaction of hydrothermal fluids. Ore evolution model for the Ar Rjum goldfield includes seafloor sulfide alteration, several deformation episodes and intrusive effects, and in this context the ore resulted from the reduction of seawater sulfates. The gold-rich veins interpreted as orogenic lode deposits are confined to localized shear zones in a syn-orogenic diorite. 相似文献
12.
The Bepkong gold deposit is located in the Wa–Lawra belt of the Paleoproterozoic Baoulé-Mossi domain of the West African Craton, in NW Ghana. It occurs in pelitic and volcano-sedimentary rocks, metamorphosed to greenschist facies, in genetic association with zones of shear interpreted to form during the regional D3 deformational event, denominated DB1 at the deposit scale. The ore zone forms a corridor-like body composed of multiple quartz ± carbonate veins surrounded by an alteration envelope, characterized by the presence of chlorite, calcite, sericite, quartz and disseminated pyrite, arsenopyrite plus subordinate pyrrhotite and chalcopyrite. The veins contain only small proportions of pyrite, whereas most of the sulphides, particularly arsenopyrite, occur in the altered host rock, next to the veins. Pyrite is also common outside of the ore zone. Gold is found in arsenopyrite, where it occurs as invisible gold and as visible – albeit micron-size – grains in its rims, and as free gold within fractures cross-cutting this sulphide. More rarely, free gold also occurs in the veins, in fractured quartz. In the ore zone, pyrite forms euhedral crystals surrounding arsenopyrite, but does not contain gold, suggesting that it formed at a late stage, from a gold-free hydrothermal fluid. 相似文献
13.
Dave Craw Phaedra Upton Bing-Sheng Yu Travis Horton Yue-Gau Chen 《Mineralium Deposita》2010,45(7):631-646
Gold-bearing vein systems in the high mountains of Taiwan are part of the youngest tectonic-hydrothermal system on Earth.
Tectonic collision initiated in the Pliocene has stacked Eocene–Miocene marine sedimentary rocks to form steep mountains nearly
4 km high. Thinner portions of the sedimentary pile (∼5 km) are currently producing hydrocarbons in a fold and thrust belt,
and orogenic gold occurs in quartz veins in thicker parts of the pile (∼10 km) in the Slate Belt that underlies the mountains.
Metamorphic fluids (2–5 wt.% NaCl equivalent) are rising from the active greenschist facies metamorphic zone and transporting
gold released during rock recrystallisation. Metamorphic fluid flow at the Pingfengshan historic gold mine was focussed in
well-defined (4 km3) fracture zones with networks of quartz veins, whereas large surrounding volumes of rock are largely unveined. Gold and arsenopyrite
occur in several superimposed vein generations, with ankeritic alteration of host rocks superimposed on chlorite–calcite alteration
zones as fluids cooled and became out of equilibrium with the host rocks. Mineralising fluids had δ18O near +10‰, δ13C was between −1‰ and −6‰ and these fluids were in isotopic equilibrium with host rocks at ∼350°C. Ankeritic veins were emplaced
in extensional sites in kink fold axial surfaces, formed as the rock mass was transported laterally from compressional to
extensional regimes in the orogen. Rapid exhumation (>2 mm/year) of the Slate Belt is causing a widespread shallow conductive
thermal anomaly without igneous intrusions. Meteoric water is penetrating into the conductive thermal anomaly to contribute
to crustal fluid flow and generate shallow boiling fluids (∼250°C) with fluid temperature greater than rock temperature. The
meteoric-hydrothermal system impinges on, but causes only minor dilution of, the gold mineralisation system at depth. 相似文献
14.
笔者应用岩石学、构造学、矿床学、地球化学等手段和方法研究了该区金矿床的特点和形成条件,结果表明青龙河金矿的容矿岩石是一套成分以火山(碎屑)岩岩屑为主的浊积岩,其含金量高达2.87g/t,且金主要与硫化物共生,浊积岩是金的矿源层。矿区内有细脉浸染型和含金石英脉型矿床。金矿的形成受浊流作用、火山作用和后生热液作用的控制。 相似文献
15.
The Macraes deposit (> 10 Moz resource) is a Cretaceous orogenic system hosted in the Hyde-Macraes Shear Zone (HMSZ) which was mineralised under lower greenschist facies during later stages of lower greenschist facies metamorphism of host metasedimentary schists. Gold is encapsulated primarily in sulphides that have replaced silicates in ductile shears that are focussed in micaceous rocks. The shears anastomose around structurally competent lenses, and were enhanced by hydrothermal graphite deposition and alteration of albite to muscovite. In contrast, scheelite with minor auriferous sulphides occurs in multigenerational quartz veins that filled fractures in competent lithologies. Hence, scheelite was deposited coevally with gold, from the same hydrothermal fluid, but in different structural settings from most gold at all scales from millimetres to hundreds of metres. Consequentially, there is weak correlation between Au and W at all scales in the deposit. Multigenerational gold and scheelite mineralisation occurred during progressive deformation in the shear zone in two contrasting structural and mineralogical styles in syn-deformationally weakening gold-bearing micaceous shears, and in syn-deformationally hardened competent rocks that became silicified and veined with quartz and scheelite. Hydrothermal fluid flow in the gold-bearing shears occurred at the grain boundary, microshear, and microfracture scales, and was slow (< 1 m/year), continuous, and pervasive. In contrast, vein formation in more competent lithologies was episodic, locally rapid (> hundreds of m/year), and was controlled by fracture permeability. The Au and W enrichment in the Macraes deposit resulted from regional scale metal mobility, driven by coeval recrystallisation in higher-grade (upper greenschist to amphibolite facies) metamorphism that persisted structurally below the Macraes deposit for at least 10 Ma after mineralisation ceased. 相似文献
16.
The Bagassi gold deposits are situated on the West African craton and hosted in Palaeoproterozoic rocks of the Houndé greenstone belt, southwest Burkina Faso. High-grade gold mineralisation is hosted in quartz–gold ± pyrite veins-lodes (V1A), in dilational zones and narrow shears in the Bagassi granitoid, and forms the majority of the resource–reserve portfolio in the Bagassi exploration permits, with gold grades of 18–21 g/t. Shear hosted gold-pyrite mineralisation in quartz veins in dilational jogs (V1B) occurs along narrow discontinuous shear zones that trend north-northwest in Birimian-aged metabasaltic units, and forms a secondary gold resource. Gold mineralisation is restricted to formation in the late Eburnean Orogeny and formed during a change from east-west to transcurrent compression and shearing. The Bagassi deposits demonstrate that granitoids emplaced prior to onset of the Eburnean Orogeny represent viable gold mineralisation in host rocks that are increasingly seen to be associated with significant gold resources. 相似文献
17.
The Tarcoola goldfield is located in the Gawler Craton in northwestern Eyre Peninsula, South Australia. The gold deposits are hosted in the Middle Proterozoic Tarcoola Formation, comprised of the fluviatile Peela Coglomerate Member, the shallow marine Fabian Quartzite Member, and the marine Sullivan Shale Member. Mineralization in the goldfield consists of north-northeast to north-northwest trending gold-bearing quartz veins with associated hematite, pyrite, arsenopyrite, sphalerite, chalcopyrite, galena, electrum and gold. Adamellite in contact with the Tarcoola Formation has previously been included in the Middle Proterozoic Hiltaba Suite granitoids, on the basis of an apparent intrusive relationship with the Tarcoola Formation, and the gold-quartz veins were interpreted as being genetically related to the cooling pluton. However, detailed field and petrographic studies have demonstrated that the contact between the Tarcoola Formation and adamellite is a nonconformity. Hence, there is no genetic relationship between the mineralization and the adamellite. Oxygen isotope data indicate that an oreforming fluid, derived from convective circulation of meteoric or seawater, or from formation water, underwent isotope exchange with sediments of the Tarcoola Formation. A magmatic heat source for the hydrothermal system is suggested by the presence of intrusive igneous rocks, including dykes of aplite, quartz monzonite and microdiorite. Sulphur isotope characteristics of the mineralization can be explained by reduction of seawater sulphate or dissolution of disseminated sulphides in the sedimentary sequence. Metals were probably derived from rocks of the Tarcoola Formation. A complex paragenetic sequence involved deposition of minerals in several stages separated by episodes of fracturing. Fluid inclusions in quartz and fluorite show that deposition took place over a temperature range of about 340° to 110°C from a low salinity fluid. Analyses of chlorite coexisting with sulphide minerals suggest deposition of sulphides from about 300° to 170°C. Gold was transported as Au(HS)
2
–
and deposition appears to have coincided with a sudden decrease in fO2 at around 260° to 250 °C. 相似文献
18.
The geology of the Republic of Djibouti, in the SE Afar Triangle, is characterized by intense tectonic and bimodal volcanic activity that began as early as 25–30 Ma. Each magmatic event was accompanied by hydrothermal activity. Mineralization generally occurs as gold–silver bearing chalcedony veins and is associated with felsic volcanism. Eighty samples from mineralized hydrothermal chalcedony, quartz ± carbonate veins and breccias were studied from ten sites representing four major volcanic events that range in age from early Miocene to the present. The most recent veins are controlled by fractures at the edges of grabens established during the last 4 Myr. Gold in excess of 200 ppb is present in 30% of the samples, with values up to 16 ppm. Mineralogical compositions allowed us to identify different types of mineralization corresponding to different depths in the hydrothermal system: (1) surface and subsurface mineralization characterized by carbonate chimneys, gypsum, silica cap and quartz ± carbonate veins that are depleted in metals and Au; (2) shallow banded chalcedony ± adularia veins related to boiling that contain up to 16 ppm Au, occurring as native gold and electrum with pyrite, and tetradymite; (3) quartz veins with sulfides, and (4) epidote alteration in the deepest hydrothermal zones. Samples in which pyrite is enriched in As tend to have a high Au content. The association with bimodal volcanism, the occurrence of adularia and the native Au and electrum in banded chalcedony veins are typical of epithermal systems and confirm that this type of mineralization can occur in a young intracontinental rift system. 相似文献
19.
Robert John Chapman James Keith Mortensen William P. LeBarge 《Mineralium Deposita》2011,46(8):881-903
Between 1978 and 2009, approximately 430,000 oz of placer gold were obtained from the Indian River and Black Hills Creek,
which equates to roughly 20% of the production for the entire Yukon Territory during that period. The area is unglaciated,
exposure is poor, and there are few known lode gold occurrences present. The technique of microchemical characterization of
placer gold grains has been applied to illuminate the style(s) of source mineralization and their relationship to placer gold
from the Klondike gold district immediately to the north. A total of 2,613 placer gold grains from 22 localities were characterised
in terms of the Au, Ag, Cu, and Hg content of their alloy and associated suite of opaque mineral inclusions. A combination
of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic
gold in the Klondike. Gold type 3b (8–25% Ag) is the main component of the placers in lower Dominion Creek but is augmented
and eventually replaced by type 3a gold (10–40% Ag) in placers in the main Indian River valley, probably through erosion of
gold-bearing veins in the valley floor. Type 4 gold exhibits highly variable Ag which may contain Hg to a maximum of 11 wt.%.
This gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides, tellurides, sulfotellurides,
and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization. Placer gold in drainages
radiating from Eureka Dome exhibits various proportions of types 3 and 4 gold depending on location, but type 3 gold forms
the major component in Black Hills Creek and northerly flowing tributaries of the Indian River with the exception of Eureka
and Montana creeks. Type 5 gold is found only in placers in the middle and lower Indian River. It is distinguished by slightly
elevated (0.05–0.17%) Cu in the gold alloy, together with low (5–9%) Ag contents. Inclusions of Bi minerals, Cr-bearing magnetite
and molybdenite within type 5 gold suggest derivation from an intrusion-related source. Candidates for such a source include
undiscovered lode occurrences associated with Cretaceous age intrusions to the south of the Indian River, or deformed Cu-Au
(−Mo) porphyry occurrences which are known to be present in the same area. This analysis of placer gold has indicated that
the contribution of low sulfidation epithermal gold from Eureka Creek to the larger placers of the Indian River is minor.
Consequently, the placer gold inventory of the Indian River is primarily orogenic in origin. Similarly, the characterization
of placer gold in Blackhills Creek strongly suggests an orogenic source. This study has demonstrated for the first time that
orogenic lode gold mineralization extends a considerable distance to the south of the southern Klondike goldfield. This information
contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration. 相似文献
20.
Geochemical signature of orogenic hydrothermal activity in an active tectonic intersection zone,Alpine Fault,New Zealand 总被引：1，自引：0，他引：1
A highly faulted and fractured rock mass has developed at the intersection of the Alpine and Hope faults, two major active faults in the South Island, New Zealand. The Alpine Fault is an oblique dextral reverse fault at the late Cenozoic-Recent Pacific-Australian plate boundary. The Hope Fault is a strike-slip fault parallel to the plate convergence vector. Hydrothermal fluids driven by the active tectonic processes have passed through the fractured rock mass, causing localised rock alteration and vein formation. Mylonites in the Alpine Fault zone are crosscut by cm-scale veins of quartz and/or ankerite with minor sulphides, with cemented breccias in dilational jogs. Breccia clasts and immediate (cm-scale) host rocks have been variably impregnated with carbonates and quartz. This generation of veins, breccias and altered rocks is post-dated by cataclasite and fault gouge zones which have been cemented by calcite, illite, smectite and chamosite. Ankerite and calcite have 18O between +10 and +30, and 13C between 0 and –8. These minerals are inferred to have formed from water with variable components of both meteoric and crustally exchanged fluid. Rock alteration associated with ankerite–quartz veins has added arsenic (up to 200 ppm As), strontium, and some Y to the rocks. Host-rock mylonites (<2 ppm As) have been depleted in arsenic compared to their precursors (5–15 ppm As). This depletion of arsenic in the middle crust provides the source for arsenic in shallower-level vein systems.Editorial handling: N. White 相似文献