Au-quartz Veins Research Articles

Evaluation of non-destructive tools for preliminary environmental risk assessment during mining exploration

Environmental assessment is generally performed when a mining project is at an advanced development stage, such as the prefeasibility study. It involves several laboratory tests such as detailed characterisation and geochemical mobility prediction tests, which require many samples considered representative of the deposit. Having early characterisation information could be useful to select the most relevant samples for the overall environmental assessment. The present study proposes a selection of non destructive and portable tools for improving the early environmental assessment of a mineral deposit. Three tools (x-ray fluorescence (XRF), mineral spectrometer, carbonate staining) were identified to form a preliminary environmental assessment methodology that can be performed directly on the exploration site. This methodology was applied to two different mining projects: i) a well developed mine, LaRonde zone 5, with available geochemical data in a volcanic-associated massive sulfide (VMS) context, and ii) an exploration project, O’Brien, with ongoing geochemical acquisition in an Au-quartz vein context. Both mine sites are in the south of the Blake River Group within the Abitibi greenstone belt, Quebec, Canada. The first site was used to validate the developed methodology, whereas the second acted as a test site to apply the methodology as a preliminary environmental assessment. The results on the first site confirmed that the selected tools provide geochemical and mineralogical data sets comparable to those of traditional testing methods. The developed methodology then enabled the classification of preliminary geo-environmental domains for the second site. Standard static and kinetic tests were performed on the second site samples to confront the preliminary assessment protocol. The proposed preliminary protocol made it possible to discriminate low-risk samples versus those with a potentially higher risk for the selection of subsequent environmental assessments.

Monazite-(Gd), a new Gd-dominant mineral of the monazite group from the Zimná Voda REE–U–Au quartz vein, Prakovce, Western Carpathians, Slovakia

AbstractMonazite-(Gd), ideally GdPO4, is a new mineral of the monazite group. It was discovered near Prakovce-Zimná Voda, ~23 km WNW of Košice, Western Carpathians, Slovakia. It forms anhedral domains (≤100 μm, mostly 10–50 μm in size), in close association with monazite-(Sm), Gd-bearing xenotime-(Y), Gd-bearing hingganite-(Y), fluorapatite and uraninite. All these minerals are hosted in a REE–U–Au quartz–muscovite vein, hosted in phyllites in an exocontact to granites. The density calculated using the average empirical formula and unit-cell parameters is 5.55 g/cm3. The average chemical composition measured by means of electron microprobe is as follows (wt.%): P2O5 29.68, As2O5 0.15, SiO2 0.07, ThO2 0.01, UO2 0.04, Y2O3 1.30, La2O3 3.19, Ce2O3 6.93, Pr2O3 1.12, Nd2O3 10.56, Sm2O3 17.36, Eu2O3 1.49, Gd2O3 22.84, Tb2O3 1.57, Dy2O3 2.27, CaO 0.21, total 99.67. The corresponding empirical formula calculated on the basis of 4 oxygen atoms is: (Gd0.30Sm0.24Nd0.15Ce0.10La0.05Dy0.03Y0.03Tb0.02Eu0.02Pr0.02Ca0.01)0.98P1.01O4. The ideal formula is GdPO4. The monazite-type structure has been confirmed by micro-Raman spectroscopy and selected-area electron diffraction. Monazite-(Gd) is monoclinic, space group P21/n, a = 6.703(1) Å, b = 6.914(1) Å, c = 6.383(1) Å, β = 103.8(1)°, V = 287.3(1) Å3 and Z = 4. The middle REE enrichment of monazite-(Gd) is shared with the associated Gd-bearing xenotime-(Y) to ‘xenotime-(Gd)’ and Gd-bearing hingganite-(Y). This exotic REE signature and precipitation of Gd-bearing mineral assemblage is a product of selective complexing and enrichment in middle REE in low-temperature hydrothermal fluids by alteration of primary uraninite, brannerite and fluorapatite on a micro-scale. The new mineral is named as an analogue of monazite-(La), monazite-(Ce), monazite-(Nd) and monazite-(Sm) but with Gd dominant among the REE.

Magnetic Signature of Gold Deposits: Example from Um Garayat Region, South Eastern Desert, Egypt

Magnetic methods of exploration have proved to be efficient and have potential in the gold mineralization industry. New magnetic processing technologies aid in improving the process of interpretation and gold opportunity identification. In this work, we show the possible application of combined digital magnetic filters to explore new gold mineralization localities with application to a well-known Au mineralization zone. Um Garayat (UG) region, southeastern desert, Egypt, is an ancient example of a potential area for gold mining. Modern analysis showed that other types of mineral concentrations are present. So, exploiting the magnetic signature of the area for future investment is of great interest. The old UG gold mine is characterized by volcanic and tectonic features such as faulting and folding that affect the arc sedimentary rock sequence of repeated deformation stages. A thorough geophysical effort has been carried out around the old gold mine in the UG area to explore the extension of mineralized ore deposits. A detailed geophysical survey using magnetics was carried out in this study together with the available aeromagnetic data. Field data sets on appropriate sites were measured, processed, and evaluated by suitable software. High magnetic anomalies were detected based on grid filter analysis and contact occurrence maps as marked as possible ore deposits after satisfying the geologic conditions for gold formation. An integrated understanding of attained results revealed that the new possible ore deposits are related directly to fault and fracture zones in the shape of lenses of variable thickness in this zone. Results show that newly detected mineral occurrences in the UG area are also controlled by major faults and hydrothermal solution enrichment along fault zones at a depth ranging from 20 to 70 m. Favorable fault/joint mineralized places were located. The relationship between the Au-quartz vein's strike direction compared with magnetic anomaly lineament analysis was studied. Search for new sources of Au and other mineral deposits in addition to quartz veins is needed as pockets of accumulated mineral-rich rock fragments are deposited in drainage wadis and fault/joint zones because of hydrothermal solution enrichment.

Osmium isotopes fingerprint mantle controls on the genesis of an epithermal gold province

Abstract The formation of crustal blocks enriched with gold (Au) deposits above subduction zones is intimately bound to the genesis and evolution of magmatic-hydrothermal systems. A long-standing question, however, is whether the metal fertility of these systems stems from distinct sources that are anomalously enriched in Au or from subsequent processes occurring during crustal magma emplacement and hydrothermal activity. The Deseado Massif auriferous province in southern Patagonia (Argentina) is a unique place to test these contrasting hypotheses because Au-bearing mantle xenoliths indicate the presence of an underlying Au-rich lithospheric mantle reservoir. However, direct geochemical links between the Au-rich mantle source and the formation of the Deseado Massif auriferous province in the overlying crust remain to be established. To address this prominent gap in knowledge, we used sulfide Re-Os geochronology to identify the source of Au at Cerro Vanguardia, the largest low-sulfidation epithermal Au-Ag deposit in the Deseado Massif. Pyrite from high-grade Au quartz veins yielded an isochron age of 147.4 ± 2.9 Ma (mean square of weighted deviates = 1.04, n = 8) and an initial 187Os/188Os ratio of 0.26 ± 0.01, fingerprinting a dominant mantle control for the source of Os and, by inference, the source of Au. Our data provide a unique geochemical linkage between an Au-rich subcontinental lithospheric mantle source and the genesis of epithermal Au deposits, supporting the hypothesis that pre-enriched mantle domains may be a critical factor underpinning the global-scale localization of Au provinces.

Characteristic of gold mineralization associated with granites at Hamash old gold mine, South Eastern Desert, Egypt

Well-known controversy over the genesis of orogenic gold mineralization in granites has been a subject of research worldwide. Hamash gold mine is considered one of the pharaohs mines in the South Eastern Desert of Egypt, where gold mineralization is hosted by granites. In this study, we utilized field, ore microscopy, and geochemical data to determine the genesis of granites and the associated gold deposits. We found that the granites, intruded into island-arc metavolcanics and metavolcanoclastic assemblages, are characterized by variable major and trace elements, calc-alkaline, and I-type granitoids that reflect a fractional crystallization. Their trace and rare earth elements reflect magma that is related to subduction tectonic setting. The gold mineralization is confined to quartz veins that cut through granitic rocks, extend in NW-SE and N-S directions, and are formed during shear-extensional tectonics. The gold is found with sulfides in quartz veins and wallrock alteration. Silicification and sericitization are the most hydrothermal alteration zones associated the ore formation, comprise an inner quartz-sericite-carbonate-chlorite-sulfide assemblage. The results of chemical analysis reveal an increase of SiO2, Fe2O3, S, and L.O.I. concentrations and moderate depletion in K2O, Na2O, and LILE suggesting that they were affected by mineral replacement reactions and sulfidation in the altered wallrocks close to Au-quartz veins. The gold is represented by tiny inclusions, with minor chalcopyrite, galena, and arsenopyrite that mainly fill the microfracture in quartz and pyrite grains. Furthermore, a general model of gold mineralization suggests that the gold was contemporaneously formed during syn-orogenic stage, transpressional deformation assigned to D2 deformation, and subduction-related thermal processes followed by gold-bearing fluids that fill the fractures, forming veins.

Gold mineralization assisted by crustal melting: the case of the Maevatanana mesothermal gold deposit, Madagascar

The genesis of mesothermal Au-quartz vein deposits, which are among the more economically important type of Au deposits, is still under debate. There are two main theories, proposed in the literature, for the subsequent formation of these deposits in suitable structure involved: (1) Au-bearing ore fluid was derived from magmatic intrusions; or (2) Au was extracted from a greenstone belt by metamorphic fluids. In this paper, we report geochemical data from the mesothermal Maevatanana Au deposit in Madagascar, from which we deduce, based on synthesis of these data with field observations and with data from previous studies, that Au-quartz veins and silicified gneisses in the footwall of the granite dike were derived from both greenstone rocks and magmatic intrusions. From this interpretation, we propose a two-stage fertilization model for the mesothermal Maevatanana Au deposit. The first stage involved recycling of metal-fertilized lower crust. The second stage involved emplacement of granitic magma into the greenstone belt, resulting in increase in temperature of the host rock, and subsequent low-degree partial melting of the crust produced volatile-rich high-silica melts. Subsequently, Au-rich fluid was extracted from the granitic magma and convection in the magma reservoir caused by ascent of the less-dense volatile-rich high-silica melt and settlement of crystals (e.g., plagioclase) resulted in the formation of ore-forming fluids at the top of the magma reservoir. Finally, the Au-rich fluids were carried by magmatic dikes and transported from the magma reservoir to shallow host structures to form the mesothermal Maevatanana Au deposit.

Evolution of fluorite-mica-feldspar veins: Evidences of a fossil geothermal system in the são Gabriel terrane and consequences for Pb-Zn-Cu metallogeny

Fluorite-mica-feldspar hydrothermal veins and stockworks host Pb-Zn-Cu sulphides in the São Gabriel Terrane, Southern Brazil. The veins fill second- and third-order structures in Neoproterozoic calcitic marbles on the Cambaizinho Complex. Veins are mostly symmetrically zoned showing, from borders to the centre, fluorite and sericite, muscovite, adularia and albite (minor quartz, calcite and iron oxides). Galena, sphalerite, chalcopyrite, pyrite and minor cassiterite are disseminated in the veins and concentrated in ore pockets. Vein petrography, mineral chemistry and microthermometric measurements in fluorite fluid inclusions indicate that the hydrothermal veins were formed in a shallow crust environment from epithermal fluids near 140 °C. Their origin is attributed to a fossil geothermal system related to the post-collisional alkaline volcanism and caldera system of the Acampamento Velho Formation (574-549 Ma). This hydrothermal system is interpreted as genetically associated to fluorite and Pb-Zn-Cu occurrences on rhyolites and miarolitic cavities of related alkaline granitoids, Pb-Zn-Cu anomalies on stream sediment and soil analyses and Au quartz veins hosted by alkaline granitoids accompanied by pyrite, sericite and fluorite alteration halos in the same region. Although there is no mineral deposit discovered so far, the studied veins linked to the other base metal occurrences would indicate that the northwestern São Gabriel Terrane may host a poorly preserved mineralized epithermal system.

Early uranium mobilization in late Variscan strike-slip shear zones affecting leucogranites of central western Spain

Mineralization related to the post-collisional reorganization in Iberia occurs along strike-slip shear zones (Sn, Au, ± U), in extremely differentiated leucogranites and pegmatites (Sn, Nb, Ta, Be, Li), in skarns (W, ± Au, ± U), and in quartz-veins with greisen (W, Sn, ± Au, ± U). Episyenites/albitites with uraninite mineralization in strike-slip shear zones yield U–Pb ages of 304 ± 6 Ma (2σ) that are identical within error with the 40Ar/39Ar age of micas grown during fabric development (306 ± 3 Ma, 2σ), suggesting shearing and formation of episyenite/albitite and uraninite mineralization are directly related. Mineral chemistry of uraninite from episyenite and deformed muscovite-rich leucogranite host are indistinguishable and has immobile element signatures typical for uraninite that formed from a hydrothermal or metamorphic fluid. The mineral chemistry of uraninite hosted in the deformed muscovite-rich leucogranite, together with the large time span between granite emplacement (347.6 ± 1.9 Ma, U–Pb zircon) and uraninite (chemical age 300 ± 3 Ma) excludes a magmatic origin of uraninite. Mass balance indicates that leaching of rock-forming minerals (especially muscovite and monazite) of the muscovite-rich leucogranite during extensive metasomatism (e.g. albitite or episyenite formation) could provide most uraninite-hosted U. The precipitation of the uraninite-pyrite association with carbonaceous matter in episyenite/albitite was related to local reducing conditions and/or fluid unmixing processes, analogue to hydrothermal uraninite from retrograde scheelite skarn and W ± Au quartz veins in the Iberian Massif. Chemical ages of uraninite from episyenite and muscovite-rich deformed granite demonstrate that episyenitization is not a late-stage Variscan process, but a post-orogenic one. Metal mobilization is structurally controlled. Chemical dating of uraninite may provide a first-order age constraint in unravelling the post-Variscan tectonic history.

The Zaderzhninskoe gold deposit: Mineral composition, fluid inclusions, and age (South Verkhoyansk region)

Based on results of study of the regional position, chemical composition of ores, fluid inclusions, and age relationships between mineralization and igneous rocks, we propose a geological and genetic model for the formation of gold mineralization of the Zaderzhninskoe deposit. Mineralization is located in the tectonic node of the intersection of two regional structures in the supraintrusive zone of a latent granitoid pluton among the terrigenous rocks of the Verkhoyansk complex, which are regionally metamorphosed to the greenschist facies. The sequential deposition of three types of mineralization—Au-quartz (including early low-gold Au-As and late productive Au-Pb-Zn mineralization), Au-rare-metal, and Au-silver—has been established. The Au-Bi (Te) assemblage contains native bismuth, bismuthinite, hedleyite, Bi sulfotellurides, gustavite group minerals, and secondary minerals—Bi oxides and Bi tellurites with low-grade gold. Hg-containing electrum and kustelite, Ag-Sb and Ag-Pb-Sb sulfosalts, stutzite, Te-Pb-containing canfieldite, freibergite, and Au and Ag sulfides are indicator minerals of the Au-Ag (Sb) assemblage. Ore formation occurred at temperatures from 90 to 340 °C, with the participation of lowly and moderately concentrated solutions with CO2 ± CH4 ± N2 gas phase. A decrease in temperature from Au-quartz mineralization (200-220 °C) to the late epithermal one (160 °C) and a slight increase in the concentration of solutions (up to 10 wt.% NaCl equiv.) have been established. The deposit resulted from the intricate multistage geodynamic evolution of the South Verkhoyansk region. Ore-forming processes are associated with the evolution of magmatic objects. Dating of igneous rocks yields the following ages: diorites—130-137 Ma (Rb-Sr), spessartites—126 ± 3 Ma (Rb-Sr), and kersantites—115 ± 1.7 Ma (Ar/Ar). Early concordant Au-quartz (Au-As) mineralization of the deposit is comparable with metamorphic-related Au-quartz veins of the Yur-Bular type, and its age is taken as >137 Ma. The time of formation of Au-quartz (Au-Pb-Zn) mineralization is estimated at 123.5 ± 1.6 Ma (Ar/Ar) and is coeval with the time of intrusion of the Early Cretaceous granitoids of the South Verkhoyansk region. The imposed low-temperature mineralization undoubtedly has a younger age. Its formation was followed by the successive deposition of Au-rare-metal mineralization at the final stage of formation of granitoid plutons (~120 Ma) and Au-Ag mineralization in the period 100 ± 5 Ma, i.e., the time of formation of late-stage granodiorite-granite intrusions.

Whole-Rock Geochemistry of Host Rocks and K/Ar Age of Hydrothermal Mineral of the Co-O Epithermal Gold Deposit, Mindanao, Philippines

Whole-rock chemical composition of host rocks and recently acquired K/Ar age of hydrothermal mineral of the Co-O epithermal gold deposit in Mindanao Island of the Philippines are herein reported. Located along a Pliocene-Quaternary calc-alkaline magmatic zone at eastern Mindanao region, the Co-O gold deposit is of intermediate sulfidation epithermal Au (+Ag ± Cu ± Pb ± Zn) quartz vein type. Geological units in the area are probable Eocene to Oligocene basaltic-andesitic to andesitic volcanic flows and volcaniclastic rocks, Oligocene andesitic to dioritic stocks and dikes, a diatreme-maar complex, and an overlying sedimentary sequence. The mineralized quartz ± calcite veins are mainly hosted in the intrusive rocks and surrounding volcanic rocks that are hydrothermally altered generally to K-feldspar, chlorite and other clay minerals. Discrimination diagram using immobile elements such as Zr/TiO2 vs. Nb/Y indicates that these rocks belong to sub-alkaline andesite and basaltic-andesite to basalt and alkali basalt in composition. The volcanic rocks plot in the island arc tholeiite and calc-alkaline fields of generally basaltic andesite and andesite composition, with a few in basalt and dacite. In the “alteration box plot”, the samples mostly plot within the least altered intermediate volcanic host rocks in the hydrothermal alteration field. Plots of each rock unit show a general chlorite-carbonate (-pyrite) alteration trend, with plots of volcanic rocks more dispersed and a few fallen outside the least altered box. K/Ar dating of hydrothermal minerals from the andesite porphyry and polymictic diatreme breccia samples yielded ages of 28.6 ± 0.9 Ma (Late Oligocene) and 31.7 ± 1.9 Ma (Early Oligocene), respectively. Age dating of these hydrothermally formed minerals gives the age of the hydrothermal activity associated with the mineralization. This suggests that the hydrothermal activity associated with the Co-O epithermal vein system transpired immediately after or during the Oligocene magmatism in a tectonic setting that produced island arc tholeiitic to calc-alkaline magmas, prior to drifting to its present location and accretion with the central and western parts of the Mindanao Island; in contrast to more prominent Miocene and Pliocene to Pleistocene mineralization ages along the Philippine archipelago.

Granitoid-associated gold mineralization in Egypt: a case study from the Atalla mine

Gold-bearing sulfide-quartz veins cutting mainly through the Atalla monzogranite intrusion in the Eastern Desert of Egypt are controlled by subparallel NE-trending brittle shear zones. These veins are associated with pervasive sericite-altered, silicified, and ferruginated rocks. The hosting shear zones are presumed as high-order structures of the Najd-style faults in the Central Eastern Desert (~ 615–585 Ma). Ore minerals include an early pyrite-arsenopyrite (±pyrrhotite) mineralization, partly replaced by a late pyrite-galena-sphalerite-chalcopyrite (±gold/electrum ± tetrahedrite ± hessite) assemblage. Gold occurs as small inclusions in pyrite and arsenopyrite, or more commonly as intergrowths with galena and sphalerite/tetrahedrite in microfractures. Arsenopyrite geothermometry suggests formation of the early Fe-As-sulfide mineralization at 380–340 °C, while conditions of deposition of the late base metal-gold assemblage are assumed to be below 300 °C. Rare hessite, electrum, and Bi-galena are associated with sphalerite and gold in the late assemblage. The early and late sulfide minerals show consistently a narrow range of δ34S ‰ (3.4–6.5) that overlaps with sulfur isotopic values in ophiolitic rocks. The Au-quartz veins are characterized by abundant CO2 and H2O ± CO2 ± NaCl inclusions, where three-dimensional clusters of inclusions show variable aqueous/carbonic proportions and broad range of total (bimodal) homogenization temperatures. Heterogeneous entrapment of immiscible fluids is interpreted to be caused by unmixing of an originally homogenous, low salinity (~ 2 eq. mass % NaCl) aqueous-carbonic fluid, during transition from lithostatic to hydrostatic conditions. Gold deposition occurred generally under mesothermal conditions, i.e., 1.3 kbar and ~ 280 °C, and continued during system cooling to < 200 °C and pressure decrease to ~ 0.1 kbar. Based on the vein textures, sulfur isotope values, composition of ore fluids, and conditions of ore formation, we suggest that the Atalla monzogranite intrusion acted only as a competent structural host for ore deposition from shear-related, metal-rich fluids migrated up from depth. This model is also presumed for most granitoid-associated Au deposits in the region, considering the similarity in their structural control, alteration pattern and mineralogy, and chemistry of the ore fluids.

Late-Hercynian intrusion-related gold deposits: An integrated model on the Tighza polymetallic district, central Morocco

Gold have been recently recognized in the Tighza (formerly Jebel Aouam) district, in the Hercynian belt of central Morocco. This district has long been known for its W mineralization, as well as major Pb–Ag–Zn, and minor Sb–Ba deposits, all geographically associated with late-Hercynian calc-alkaline magmatism. Gold mineralization in the district is mainly hosted by thick W–Au quartz veins located around the “Mine granite” small granitic plug. Within the veins, gold grade is highest (up to 70g/t) close to the granite but rapidly decreases going outward from the granite, defining a perigranitic zoning. Anomalous gold grades have also been measured in hydrothermal skarn layers close to two other granitic plugs (Kaolin granite and Mispickel granite), associated with disseminated As–Fe sulfides. The paragenetic sequence for the W–Au quartz veins shows three stages: (1) an early oxidized stage with wolframite-scheelite associated with early quartz (Q1), (2) an intermediate Bi–As–Te–Mo–Au sulfide stage with loellingite, bismuth minerals and native gold with a later quartz (Q2), restricted to a narrow distance from the granite, and (3) a late lower temperature As–Cu–Zn–(Pb) stage with abundant massive pyrrhotite, arsenopyrite and sphalerite, locally forming independent veins (“pyrrhotite vein”). Both Q1 hyaline and Q2 saccharoidal gold-bearing quartz display aqua-carbonic fluids with minor H2S and Cu and an homogeneous composition (81mole% H2O, 18mole% CO2 and about 1mole% NaCl). The trapping pressure is estimated to 1.5–2kbar with temperature ranging from 300 to 350°C. Q1 inclusions have exploded indicating an uplift of the Tighza block, that lead to saccharoidal Q2 quartz deposition with multiphase NaCl-saturated fluid inclusions. 40Ar/39Ar dating demonstrates that the “Mine granite”, tungsten skarnoid, scheelite-molybdenite veins, and very likely gold-bearing veins are coeval, emplaced at 286±1Ma. Multiple and widespread metal sources are indicated by radiogenic isotope studies. Nd and Sr isotope compositions of scheelite and granites suggest the participation of a juvenile component while lead isotopes demonstrate a major participation of the basement.Both gold mineralization and zoning suggest that the system developed at the end of the magmatic activity, accompanying a major transition in magmatic fluid composition. The morphology of the gold-bearing mineralization is dependent of the permeability and the reactivity of host-rocks: focus circulation of fluids through pre-existing tectonic corridors, reactivated by late-Hercynian intrusions favor the formation of large W-type gold veins, while infiltration of fluid within reactive stratigraphic layers gives rise to skarn mineralization. A 40Ar/39Ar date (W1 north vein: 291.8±0.3Ma) indicates that hydrothermal circulation predates gold and tungsten deposition in open fractures as well as Mine granite emplacement.The W–Au mineralization preceded the onset of a large convective hydrothermal cell around the intrusion that led to the formation of the Pb–Ag–Zn mined veins. The Tighza polymetallic district displays numerous similarities with the R-IRG model that was defined in the American Cordillera, such as thermal and zonation patterns, carbonic hydrothermal fluids and chronology of intrusion and related deposits, but also provides new insight to the R-IRG model such as wide Au-quartz veins instead of sheeted Au-veins, oxidation state of the magma, and Sr–Nd isotopic data. These results establish a major magmatic contribution and discard a direct genetic relationship between gold mineralization and major neighboring Pb–Ag–Zn veins. A large number of classic Pb–Zn district of the Western Hercynides belong to the same clan.

Metallogenesis of the Paleoproterozoic Piaba orogenic gold deposit, São Luís cratonic fragment, Brazil

Piaba is an orogenic gold deposit (~3.5Moz) of the São Luís cratonic fragment in north-northeastern Brazil. The deposit is epizonal–mesozonal and associated with the development of a subvertical strike–slip fault that cut across Rhyacian metavolcano-sedimentary rocks and granitoids. The metavolcano-sedimentary sequence comprises carbon-bearing schists, andesite/dacite, ultramafic rocks and felsic tuffs formed at 2240±5Ma to 2227±33Ma that were intruded at 2214±3Ma by fine-grained, subvolcanic granophyric granodiorite. Whole-rock geochemistry indicates that the hosting metavolcano-sedimentary sequence formed in a subduction-related (arc, back-arc?) setting, which has previously been interpreted as the early-arc stage of an accretionary orogen (2240–2214Ma) that was followed by voluminous subduction-related calc-alkaline arc magmatism (2168–2145Ma) and by a collisional phase (2100±15Ma) that produced several bodies of peraluminous granites. The hydrothermal alteration of the host rocks produced early, distal carbonization and hematitization and proximal, ore-related chlorite–sericite–carbonate–sulfide alteration. Two styles of mineralization, occurring in association, are present: (1) a network of thin quartz±sulfide veinlets and subordinate narrow breccia veins with refractory- to free-milling gold dissemination in hydrothermally-altered host rocks, and (2) subordinate low-sulfide Au-quartz veins with free-milling gold. Fluid inclusions and stable isotopes characterize the mineralizing solution as a relatively reduced (log fO2=−30 to −35), neutral to slightly alkaline (pH5 to 6.2), low-salinity (average 5% NaCl), aqueous-carbonic (XCO2=5 to 59mol%, XH2O=40 to 93mol%, XN2 and XCH4 <1mol%) metamorphic fluid (δ18OH2O=+5.5 to +8.9‰, δDH2O=−8 to −69‰), and suggest that gold mineralization occurred at 250° to 330°C and 1.25 to 2.8kbar in response to phase separation and fluid–rock interactions that produced sulfidation and minor carbonatization. Sulfur (δ34S=−2.7 to −3.8‰) is from an undetermined source, probably magmatic (oxidized) or sedimentary. Lead isotope compositions of gold, ore-related pyrite and K-feldspar from regional rocks suggest that collision-related granitoids of ca. 2100±15Ma are the most likely sources for Pb in the ore, although mantle and lower crustal sources are also observed. 40Ar/39Ar in hydrothermal sericite yielded a plateau age of 1980±13Ma, which is similar to the Pb–Pb model age of gold of 1959Ma. These are considered the minimum age for gold mineralization, whereas the maximum age of 2009±11Ma is given by a small gold-hosting intrusion in the same region.

Cu–Au vein mineralization related to the Talat n’Imjjad shear zone (western High Atlas, Morocco): geological setting, ore mineralogy, and geochemical evolution

The Cu–Au deposit of the Talat n’Imjjad occurrence, southwestern Azegour mine, western High Atlas of Morocco, is situated in a NW trending quartz vein along a shear zone in quartz diorite host rocks. The recently discovered shear zone, ~200-m long, NW–SE trending, so-called Talat n’Imjjad vein system of Cu–Au–Bi–Te–Se mineralization that crosscuts the Al Medinet quartz diorite, represents a landmark in regional metallogeny. This region has long been known for its polymetallic ore deposits, with the historical Mo–Cu–W mine of Azegour and Pb–Zn–Ag mine of Erdouz. The economic sulfide enrichment is often hosted by breccias and quartz veins in the ductile shear zone. Samples from the field surface and boreholes show macro- and microscopic evidence of shearing such as S-C fabrics and microshear planes. The quartz diorite was deformed and mylonitized, then the igneous texture was modified. On the basis of textures, mineral assemblages, and hydrothermal alteration, four stages were documented and could be distinguished: (1) quartz I–sericite–chlorite–epidote pre-ore stage; (2) chlorite–quartz II–calcite–pyrite ore stage; (3) native bismuth–bismuthinite–emplectite–wittichenite–(tsumoite–laitakarite)–chalcopyrite–sphalerite–native gold–calcite ore stage II; and (4) bismuthite–hematite–malachite–covellite post-ore stage. Bi-chalcogenides are considered as pathfinders for gold in Au–quartz veins. The association of gold with Bi-minerals is linked to the role of Bimelt as scavenger for gold. Tellurides and selenides are also the best indicators of the magmatic signature. The alteration types of the wall rocks include seritization, silicification, chloritization, carbonatization, and propylitization. The auriferous shear zone of Talat n’Imjjad represents a newly discovered occurrence in the western High Atlas characterized by its mineralogy, paragenetic sequence, hydrothermal alteration, and magmatic and structural control of the deposit of sulfide minerals.

Orogenic gold and rare metal deposits of the Upper Kolyma District, Northeastern Russia: Relation to igneous rocks, timing, and metal assemblages

The Upper Kolyma district is the most important part of the Yana–Kolyma gold province, administratively located in Magadan Oblast, Russia. To date, historic production of the region is about 90Moz of gold, mostly from placers, but with about 5% contribution from lode deposits. Two major types of gold deposits are distinguished in the region: (1) Au-quartz veins related to regionally metamorphosed rocks; and (2) granitoid-related Au. Besides gold, tin deposits are also wide-spread and can be distinguished as: (1) Sn-quartz veins, (2) Sn-silicate-sulfide veins, and (3) Sn-polymetallic veins. Quartz-molybdenite (porphyry Mo) and Ag-polymetallic (Ag-quartz) veins are less abundant. Metamorphic zones and major regional faults are the most important factors of control for the Au-quartz veins. Granitoid intrusions are the most important ore-controlling factor for other deposit types.Field relationships between Au-quartz vein, granitoid-related Au mineralization and intrusions, as well as the 40Ar/39Ar dating reveal several pulses in formation of the Upper Kolyma gold deposits. The majority of granitoid-related Au deposits and occurrences in the region are related to the oldest (150Ma) metaluminous granitoid plutons of the subduction nature. 40Ar/39Ar dates for Au-quartz mineralization in the Upper Kolyma district correspond mostly to 139–125Ma, which is a period of less intense igneous and metamorphic activity in the area. We interpret these ages to indicate vein formation by metamorphic fluids, boiling from uplift and decompression, following Early Cretaceous collision-induced metamorphism. Most Sn-quartz, Sn-silicate-sulfide, Sn-polymetallic veins, as well as Ag-polymetallic and U-REE mineralization relates to the ca. 80Ma plutons, emplaced in an extensional setting.The 40Ar/39Ar data, obtained for Au-quartz vein mineralization, were used as a basis to develop a genetic model that assumes a time interval (from 1 to 15m.y.) between the ore-mobilizing processes and the timing of ore deposition. Such an interval can be interpreted by virtue of the regional uplift and irregular drop in pressure of fluid systems as the main factors of ore deposition.Iron- and As-bearing sulfides are typical for all hardrock deposits of the Upper Kolyma district, and they reflect the event and/or remobilization of gold. Pyrite is the main sulfide in greenschist metaturbidities and igneous rocks. Metamorphism of turbidites resulted in pyrite–pyrrhotite transformations with sulfur, arsenic, and metal mobilization. Arsenopyrite assemblages, usually dominating in the ores of all deposit types of the district, are products of geochemical evolution of the above-mentioned mobilized sulfur and arsenic. The Au-quartz vein deposits are characterized by pyrite–arsenopyrite assemblage, and arsenopyrite has almost a stoichiometric composition; the granitoid-related Au- and Sn-quartz veins are characterized by lollingite–arsenopyrite assemblage and As/S>1 in arsenopyrite; the Sn silicate-sulfide and Sn-polymetallic veins are characterized by pyrite–pyrrhotite–arsenopyrite assemblage and stoichiometric arsenopyrite; the Ag-polymetallic and quartz molybdenite veins are characterized by pyrite–arsenopyrite assemblage and As/S<1 in arsenopyrite. However, some deposits do not agree with the above-mentioned criteria, which can be explained either by overprinting of several mineralization types in an individual deposit or by metamorphism of its ores. The results demonstrate high importance of re-mobilization of metals due to the later hydrothermal and thermal events.

A biogeochemical orientation survey in the Moisan gold-mineralized area, Haenam district in Korea

A biogeochemical orientation survey was carried out in the vicinity of an epithermal Au deposit in the Moisan Au–Ag mineralized area, Haenam district in Korea. The Au–Ag bearing quartz veins of the mine occur as narrow open-space fillings within Cretaceous silicic pyroclastics. The vein minerals consist mainly of quartz, sericite, pyrite, chalcopyrite, and galena, with some electrum and argentite. The main objectives of this study were to study the geochemical characteristics of rocks, soils and plants in this area, to investigate the spatial relationship between Au and associated elements in rock–soil–plant system, and to evaluate the applicability of biogeochemical prospecting for Au vein occurrences in Korea. Samples of rocks and soils, and leaves of three plant species (Japanese red pine — P. densiflora, oriental white oak — Q. aliena, Japanese mallotus — M. japonicus) were collected from the target mineralized area and control barren locations, and analyzed for trace elements by instrumental neutron activation analysis. Sampling lines were composed of one slope line which is almost parallel to the mineralized quartz-veins, and four transect lines spaced 100 m apart across the veins at 20 m sampling intervals. From the multi-element data of rock samples (n = 9), high values of Au (maximum 2030 ppb) are spatially related to Au–quartz veins. Soil samples (n = 61) collected from five sampling lines show higher values of Au (24–825 ppb) whereas soil samples from the control locations have lower values of Au (below 25 ppb). Many plant species collected from the vicinity of the veins have high Au contents compared with those at the control locations, but the ranges of Au values are variable among plant species. In a total of 128 samples of plant leaves, Q. aliena yielded Au values of 0.4 to 6.9 ppb, and M. japonicus 0.9 to 4.1 ppb. Gold contents in P. densiflora ranged from 0.1 to 5.6 ppb. Plant leaves from control areas show less than 1.6 ppb Au. The biological absorption coefficient (BAC) of Au in plants decreases in the order of Q. aliena > M. japonicus > P. densiflora. Based on the results of the study, Q. aliena appeared to be the best sampling media for biogeochemical prospecting of Au in the study area.

Geology of the epithermal Ag–Au Huevos Verdes vein system and San José district, Deseado massif, Patagonia, Argentina

The San Jose district is located in the northwest part of the Deseado massif and hosts a number of epithermal Ag–Au quartz veins of intermediate sulfidation style, including the Huevos Verdes vein system. Veins are hosted by andesitic rocks of the Bajo Pobre Formation and locally by rhyodacitic pyroclastic rocks of the Chon Aike Formation. New 40Ar/39Ar constraints on the age of host rocks and mineralization define Late Jurassic ages of 151.3 ± 0.7 Ma to 144.7 ± 0.1 Ma for volcanic rocks of the Bajo Pobre Formation and of 147.6 ± 1.1 Ma for the Chon Aike Formation. Illite ages of the Huevos Verdes vein system of 140.8 ± 0.2 and 140.5 ± 0.3 Ma are 4 m.y. younger than the volcanic host rock unit. These age dates are among the youngest reported for Jurassic volcanism in the Deseado massif and correlate well with the regional context of magmatic and hydrothermal activity. The Huevos Verdes vein system has a strike length of 2,000 m, with several ore shoots along strike. The vein consists of a pre-ore stage and three main ore stages. Early barren quartz and chalcedony are followed by a mottled quartz stage of coarse saccharoidal quartz with irregular streaks and discontinuous bands of sulfide-rich material. The banded quartz–sulfide stage consists of sulfide-rich bands alternating with bands of quartz and bands of chlorite ± illite. Late-stage sulfide-rich veinlets are associated with kaolinite gangue. Ore minerals are argentite and electrum, together with pyrite, sphalerite, galena, chalcopyrite, minor bornite, covellite, and ruby silver. Wall rock alteration is characterized by narrow ( 220°C. Kaolinite occurring with the late sulfide-rich veinlet stage indicates pH 315°, whereas strike directions of <315° are predicted with an induced dextral strike–slip movement. The components of the structural model appear to be present on a regional scale and are not restricted to the San Jose district.

Origin of the CO 2-only fluid inclusions in the Palaeoproterozoic Carará vein-quartz gold deposit, Ipitinga Auriferous District, SE-Guiana Shield, Brazil: Implications for orogenic gold mineralisation

The Carará gold deposit, located in the Ipitinga Auriferous District, south-eastern portion of the Guiana Shield, northern Brazil, is a typical orogenic, greenstone-hosted, auriferous quartz vein. Mineralisation was post-metamorphic and syn-tectonic in relation to the host Palaeoproterozoic (ca. 2.03 Ga) shear zone developed close to the tectonic boundary between a Palaeoproterozoic continental arc and an Archaean block. The deposit style is very simple, consisting of a quartz vein and its hydrothermal envelope, which is composed of muscovite and tourmaline; sulphides are rare. Muscovite and tourmaline, in addition to gold, fill small fractures in the quartz vein. The fluid inclusion assemblage trapped in high- and low-grade portions of the Au–quartz vein is rather enigmatic, consisting of one-phase CO 2 inclusions with no visible water at room or sub-zero temperatures, although small amounts of water have been detected by micro-Raman analysis. In this aspect Carará differs from the other gold showings in the same district, which are characterized by abundant aqueous-carbonic fluid inclusions. The carbonic fluid is composed predominantly of CO 2 in addition to < 2 mol.% N 2 and traces of CH 4 and C 2H 6. The carbonic fluid show very variable densities, which is interpreted to result from post-entrapment re-equilibration. Inclusions in the high-grade quartz are the densest (0.89 to 1.07 g/cm 3) and with less effects of re-equilibration. These inclusions approximate the physico-chemical characteristics of the parental fluid that started to be trapped at least around the amphibolite facies metamorphic conditions and then followed a retrograde path. Most of the inclusions appear to have been trapped and/or re-equilibrated at 350 to 475 °C and 1.8 to 3.6 kbar, which implies a 7 to 12 km depth of vein formation and gold mineralisation. Both phase separation of a carbonic-aqueous fluid ( XCO 2 > 0.8) and the existence of an originally CO 2-dominated fluid could account for the observed fluid inclusion properties and the absence of H 2O-bearing inclusions in the mineralised vein. The fluid inclusion characteristics, combined with published geological and isotopic information, indicate a deep-seated source for CO 2 that could be mantle, magmatic or metamorphic in origin. We suggest that the likely sources are fluid produced by the 2.07 Ga-old charnockites that occur in the region and/or the coeval high grade metamorphism that is widespread in the Guiana Shield.

Antimony quartz and antimony–gold quartz veins from northern Portugal

Antimony- and Pb–Sb-quartz veins from the Bragança district, Portugal, are mainly hosted by Silurian phyllites. Antimony–Au-quartz veins from the Dúrico–Beirã region are mainly hosted by a Cambrian schist–metagraywacke complex, as well as Ordovician phyllites and quartzites. The deposits were mostly exploited in the late 19th Century. Mineralogical characteristics and chemical compositions of individual ore minerals are similar in the two areas. First and second generations of arsenopyrite precipitated at 390 and 300 °C, respectively. Berthierite and stibnite are the most abundant Sb-bearing minerals and precipitated between 225 and 128 °C, native antimony at < 200 °C. Drastic fluid cooling is the main cause of mineral precipitation. The Pb isotope compositions of stibnite suggest a homogeneous crustal source of lead, from the metasedimentary sequences, for Sb, Pb–Sb and Sb–Au deposits in both areas, which is consistent with the findings for comparable mineralizations elsewhere in Europe. Remobilization of Pb is related to Variscan metamorphism and deformation.

Characteristics of the Bumo Gold Field, Hainan Island, China and Gold Exploration Through Soil Geochemical Surveys

AbstractThe Bumo gold deposit is a low‐sulfide quartz vein type deposit, situated in the west of Hainan Island, approximately 25 km southeast of Dongfang City (termed Basuo in alias). Tectonically, the SW‐NE‐oriented Gezhen Ductile‐Shearing Zone was largely attenuated in Bumo. It is hosted by gneisses and schists of Proterozoic age (locally called the Baoban Group), but the radiometric age of ores mainly is Jurassic–Cretaceous, being coeval to the regional metallogenesis along the Eurasian margin. The affinity with the Phanerozoic orogenic gold deposits is indicated by low salinity (2.4 wt% NaCl–10.3 wt% NaCl), intermediate temperature (150–300°C) and appropriate δ18O value (approximately 7–11‰). The gold metal tonnage, proved previously, within an area smaller than 1.0 km2 in the gold belt II amounted to 11.59 t, with the average gold grades for the major Au quartz veins (7 and 8) in this belt of 52.7 and 71.1 g/t, respectively. This shows significant enrichment of gold and an unusual “nugget effect”. A ring structure, mainly represented by ridges, valleys, dikes, big bull quartz veins and lithological borders or pertinent faults, marks the gold field, and its origin was linked to a possible magmatic diapir. It was viewed as macroscopic evidence that the dip angles of strata tend to decline gradually from deeper ones in the center area to nearly horizontal in the outskirts, and certain intrusion‐affiliated elements such as Bi, As and W are considerably abundant in soil samples. Two areas, namely the area A (3.06 km2) in the outskirts of the ring structure or in northeastern Bumo and the area B (1.32 km2) in the field center or to the west of the gold belts I and II, were selected for the geochemical survey in a 50 × 50‐m sampling grid. The analytical results of the samples from horizon C and in a −80 mesh size showed that the average contents of Bi, As and W amounted to 0.55, 19.3 and 3.67 ppm in the area A (1225 samples) and to 0.79, 68.3 and 4.14 ppm in the area B (529 samples), respectively. This geochemical exploration led to the discovery of several Au quartz veins in both areas A and B, which also manifested an Au nugget effect (the average Au grades between 50 and 100 g/t) in spite of their limited scales (from a couple of centimeters to 50 cm wide and from several meters to 15 m long). A mylonite zone in area A was also found by this study. An intimate spatial correlation between a lithological border (gneiss and schist or pegmatite) or the mylonite zone and Au anomalies indicates an important prospect for further gold exploration.