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1.
Plant species that accumulate high levels of metals in proportion to the metal content in the soil are of considerable interest in biogeochemical and biogeobotanical prospecting. This study was aimed at investigating copper and nickel accumulation in the plants Helichrysum candolleanum and Blepharis diversispina, to assess their potential use as mineral indicators in biogeochemical prospecting. Soils and plants were collected from copper–nickel mineralised areas in Botswana. Analyses of the soils and the respective plant parts (roots, stem, leaves and flowers) were carried out using ultrasonic slurry sampling electrothermal atomic absorption spectrometry (ETAAS), which allowed rapid determination of copper and nickel in small amounts of the samples.The metal concentration in the soil was in the range ≈ 40 μg/g–4% (w/w) for Cu and ≈ 60 μg/g–0.3% (w/w) for Ni. The concentration ranges of the elements in the plant parts were ≈ 6 μg/g–0.2% Cu and ≈ 3–210 μg/g Ni. At high soil metal content (greater than 2.5% (w/w) Cu and 0.1% (w/w) Ni), high levels of both nickel and copper were found in the shoots (leaves and flowers) of H. candolleanum. Concentrations as high as 0.2% (w/w) Cu were found in the leaves and flowers of H. candolleanum, indicating hyperaccumulation for this plant. For B. diversispina, the metal concentrations did not exceed 100 μg/g for any plant part, for both metals. Both plant species tolerate high concentrations of metals and should therefore be categorized as metallophytes. In order to evaluate metal translocation from the soil to the shoots, metal leaf transfer coefficients (ratio of metal concentration in the leaf to metal concentration in the soil) were calculated. Our data suggest that the two plant species have different metal uptake and transport mechanisms, which needs to be investigated further. The present work also suggests that H. candolleanum may be used as a copper/nickel indicator plant in biogeochemical or biogeobotanical prospecting.  相似文献   

2.
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   

3.
Vein-type gold deposits in the Atud area are related to the metagabbro–diorite complex that occurred in Gabal Atud in the Central Eastern Desert of Egypt. This gold mineralization is located within quartz veins and intense hydrothermal alteration haloes along the NW–SE brittle–ductile shear zone, as well as along the contacts between them. By using the mass balance calculations, this work is to determine the mass/volume gains and losses of the chemical components during the hydrothermal alteration processes in the studied deposits. In addition, we report new data on the mineral chemistry of the alteration minerals to define the condition of the gold deposition and the mineralizing fluid based on the convenient geothermometers. Two generations of quartz veins include the mineralized grayish-to-white old vein (trending NW–SE), and the younger, non-mineralized milky white vein (trending NE–SW). The ore minerals associated with gold are essentially arsenopyrite and pyrite, with chalcopyrite, sphalerite, enargite, and goethite forming during three phases of mineralization; first, second (main ore), and third (supergene) phases. Three main hydrothermal alteration zones of mineral assemblages were identified (zones 1–3), placed around mineralized and non-mineralized quartz veins in the underground levels. The concentrations of Au, Ag, and Cu are different from zone to zone having 25–790 ppb, 0.7–69.6 ppm, and 6–93.8 ppm; 48.6–176.1 ppb, 0.9–12.3 ppm, and 39.6–118.2 ppm; and 53.9–155.4 ppb, 0.7–3.4 ppm, and 0.2–79 ppm for zones 1, 2, and 3, respectively.The mass balance calculations and isocon diagrams (calculated using the GEOISO-Windows program) revealed the gold to be highly associated with the main mineralized zone as well as sericitization/kaolinitization and muscovitization in zone 1 more than in zones 2 and 3. The sericite had a higher muscovite component in all analyzed flakes (average XMs = 0.89), with 0.10%–0.55% phengite content in wall rocks and 0.13%–0.29% phengite content in mineralized quartz veins. Wall rocks had higher calcite (CaCO3) contents and lower MgCO3 and FeCO3 contents than the quartz veins. The chlorite flakes in the altered wall rocks were composed of pycnochlorite and ripidolite, with estimated formation temperatures of 289–295 °C and 301–312 °C, respectively. Albite has higher albite content (95.08%–99.20%) which occurs with chlorite in zone 3.  相似文献   

4.
We studied primary ore samples from Kalmakyr, a giant Cu–Au–Mo porphyry deposit in eastern Uzbekistan. Disseminated and stockwork-type high-grade Cu–Au–Mo mineralization showed average concentrations of 55 ppb Pd, 5.5 ppb Pt, 0.95 ppb Rh, 0.49 ppb Ir, and 4.1 ppm Au (n = 8). This type of mineralization is characterized by the presence of pyrite, chalcopyrite, molybdenite, and gold. A peak Pd content of 292 ppb was determined in a base-metal-rich quartz vein in granodiorite porphyry, which contains galena, sphalerite, chalcopyrite, tetrahedrite, and gold. Palladium correlates with Cu, Ag, Se, and S. Mineralogical and laser ablation ICP-MS study confirmed that Pd is homogeneously distributed in chalcopyrite, which contains up to 110 ppm Pd, and tetrahedrite, containing up to 20 ppm Pd. An assessment of the Pd and Pt budget at Kalmakyr showed the potential of approximately 17 t of Pd and 1.7 t of Pt.  相似文献   

5.
The local and geometrical structure around gold (III) e.g., Au3+ ions in aqueous solution with different OH−/Cl− molar ratios, has been investigated by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) spectra of [AuCln(OH)4−n]− solutions have been calculated and the multiple-scattering spectral features have been attributed to Cl d-states, axial water molecules and the replacement of Cl− ligands by OH− ligands. A square–planar geometry for [AuCln(OH)4−n]− with two axial water molecules has been identified. Moreover, a spectral correlation between XANES features and the type of planar atoms has been identified. By extended X-ray absorption fine structure spectra (EXAFS), the planar Au bond distances in the solutions have also been determined, e.g., 2.28 Å for Au–Cl and 1.98 Å for Au–O, respectively. The same EXAFS analysis provides evidence that the peak at about 4.0 Å in solutions with the lowest OH−/Cl− molar ratio arises from collinear Cl–Au–Cl multiple-scattering contributions. For the first time, a complete detailed reconstruction of the hydration structure of an Au ion at different pH values has been achieved.  相似文献   

6.
广西巴马料屯金矿床是较为典型的卡林型金矿床,金矿体与燕山晚期石英斑岩脉空间上相关。矿床勘查成果和野外露头观察均表明石英斑岩脉切割金矿体,说明岩浆岩形成于矿体之后。成矿元素地球化学测量结果表明,Au、As元素沿石英斑岩脉走向出现明显的分异,表现为在导矿和容矿断层区段Au、As质量分数高,局部形成矿（化）体;远离矿体的砂泥岩和灰岩围岩地段,则逐渐降低至正常背景值,说明石英斑岩侵位过程中切割早期的矿体或含矿断层,萃取了矿（化）体中成矿元素Au、As,从而在这些断层之间形成一定宽度的高值带。成矿元素地球化学测量支持岩浆岩形成于矿体之后的结论。石英斑岩脉白云母斑晶的40Ar/39Ar坪年龄为（95.5±0.7） Ma,代表了岩浆侵位年龄,由于岩脉形成于矿体之后,此年龄可以作为成矿年代的上限年龄。本矿床及滇黔桂"金三角"其他卡林型金矿的确切成矿年代,尚需更多高精度直接成矿年龄数据的约束。  相似文献   

7.
The Red Lake greenstone belt is one of the foremost Au mining camps in Canada and hosts the world-class Campbell-Red Lake Au deposit. Belt-scale hydrothermal alteration is characterized by proximal ferroan dolomite zones associated with Au mineralization surrounded by distal calcite zones, both being accompanied by potassic alterations (sericite, muscovite, and biotite). At the Campbell-Red Lake and Cochenour deposits Au mineralization (in particular high-grade ore) is associated with silica and sulfides (especially arsenopyrite) that replace carbonate ± quartz veins and the host rocks. The prevalence of carbonic fluid inclusions and rare occurrence of aqueous-bearing inclusions in carbonate–quartz–Au veins in the Campbell-Red Lake deposit, and the consistency of homogenization temperatures of carbonic inclusions within individual fluid inclusion assemblages (FIA), have been interpreted to indicate that H2O-poor, CO2-dominated fluids were responsible for the carbonate veining and Au mineralization. Further studies of fluid inclusions in carbonate–quartz veins within and outside the deformation zone hosting the Campbell-Red Lake deposit (the Red Lake Mine trend) including the Cochenour Au deposit, the Redcon Au prospect, and outcrops in the distal calcite zone also reveal the dominance of carbonic fluid inclusions in vein minerals. These studies indicate that CO2-dominated fluids were flowing through fractures during carbonate vein formation and Au mineralization both within and outside major structures. The carbonic fluid may have been initially undersaturated with water, or it may have resulted from phase separation of an H2O–CO2–NaCl fluid. In the latter case, phase separation modeling indicates that the initial fluid likely had XCO2XCO2 values larger than 0.8. Calculations based on hydrothermal mineral assemblages indicate XCO2XCO2 values in the host rocks from 0.025 to 0.85, reflecting a change from CO2-dominated fluids in the fractures (veins) to H2O-dominated fluids in the host rocks away from the fractures. The CO2-dominated fluids were likely advected from granulite facies in the deeper crust, whereas the H2O-dominated fluids were derived from the ambient host rocks of amphibole to greenschist facies. Calculations based on CO2 requirements and source constraints indicate that the mineralizing fluids were likely two orders of magnitude more enriched in Au than the commonly assumed values of a few μg/L, which may explain why the Campbell-Red Lake deposit has a very high-grade of Au (average 21 g/t for the whole deposit and 81 g/t for the Goldcorp High-Grade zone). Fluid inclusion data suggest that the carbonate veining and Au mineralization likely took place at depths from 7 to 14 km. The development of crustiform–colloform structures in the carbonate ± quartz veins, which was previously interpreted to indicate relatively shallow environments, may alternatively have been related to extremely high fluid pressures and the CO2-dominated nature of the fluids, which could have enhanced the brittle properties of the rocks due to their high wetting angles.  相似文献   

8.
Geomicrobial and geochemical studies were carried out in Argentina (Patagonia, Chubut Province) on four Au and polymetallic sulfide vein-type deposits. A horizon soils were analyzed for Bacillus reacting to lecithin [Bacillus L.(+)], Au and 12 additional elements. In two of the four sampling sites, exhibiting known and relatively simple mineralized structures, Bacillus L.(+) populations are clearly related to Au, As, Pb, Zn, Cu-sulfide mineralization. In areas containing more complex mineralized structures, the spatial relationship between Bacillus L.(+) and metals in the A horizon is more difficult to interpret. Results of a factor analysis performed on all analytical data (n = 130) suggest a partial relationship between Bacillus L.(+) and Au-As-Y pedochemical associations located above known Au mineralization. Bacillus L.(+) was first analyzed in Argentina in December 1994 and re-analyzed in Belgium five to seven months later. Most of the Bacillus contents (85%) of the Belgian tests are higher than those determined in Argentina. The present results and data of a previous study in Mexico (Melchior et al., 1994a; Melchior et al., 1994b) suggest that this may be the result of temperature variations during sample storage between periods of microbial analysis. From a strictly analytical point of view, the geomicrobial method is not an accurate, reproducible technique. However, Bacillus L.(+) can be used as a microbiological indicator of Au and polymetallic mineralization at a reconnaissance-level regional survey. At a local scale, this microbiological tool should be combined with classical exploration techniques such as soil geochemistry. It is recommended that the collection of all A horizon samples (for microbial study) should be accompanied by B or C horizon soils (for potential geochemical study, after prioritizing targets) so that a second field sampling program does not have to be undertaken.  相似文献   

9.
The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt, where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks. The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dipping WNW-trending shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite, subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins.Petrographic and microthermometric studies aided recognition of CO2-rich, H2O-rich and mixed H2O–CO2 fluid inclusions in the gold-bearing quartz veins. The H2O–CO2 inclusions are dominant over the other two types and are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids. The generally light δ34S of pyrite and chalcopyrite may suggest a magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and 350–1100 bar, under conditions transitional between mesothermal and epithermal systems.The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?), fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and variations in pH and fO2.  相似文献   

10.
The quartz veins containing scheelite from Fonte Santa mine cut the Lower Ordovician quartzites. A muscovite–biotite granite (G1) and a muscovite granite (G2), both S-type, crop out close to the Fonte Santa mine and are related to the Moncorvo–Bemposta shear zone. The most altered samples of G2 show intense muscovitization and microclinization and contain chlorite, columbite–tantalite, wolframite, W-ixiolite and Fe-oxides. The tin-bearing granites contain 18 ppm (G1) and 73 ppm (G2) Sn. The most altered samples of G2 correspond to a tungsten granite. The quartz veins contain muscovite, chlorite, tourmaline, scheelite, pyrrhotite, pyrite, sphalerite, chalcopyrite, galena, arsenopyrite, iron oxides, Fe sulfates, phosphates of Pb, Fe and Al. The Fonte Santa mine area was exploited for W between 1942 and 1982. At the end of November 2006, a flood event damaged the tailings dam of Fonte Santa mine, releasing contaminated material and increasing contaminant levels in water within the area of influence of the mine. The waters related to the Fonte Santa mine are poorly mineralized, with electrical conductivity of < 965 µS/cm, and of a mixed type or HCO3− and SO42− types. Most pH values (5.0−8.5) indicate that there is no significant acidic drainage in the region, as found in other areas. More acidic values (pH = 3.4) were found in the mine's lagoon. Waters associated with mineralized veins and old mine activities have Fe and Mn concentrations that forbid their use for human consumption and agriculture. Natural Na, Mg and K water contents are associated with the alteration of albite, chlorite and muscovite of country rock, while Ca with the W-bearing quartz veins. Weathering agents are carbonic and sulphuric acids and the latter has a strong influence in areas draining fine-grained mine tailings.  相似文献   

11.
Copper mineralization in the Dir area of northern Pakistan is confined to the quartz veins and associated with hydrothermally altered metavolcanics. Chalcopyrite is the main copper-bearing phase with subordinate amounts of bornite, chalcocite, covellite, malachite and azurite. Both mineralized quartz veins and associated unmineralized (least altered) and mineralized (strongly altered) metavolcanics have been analyzed for Cu, Au, Ag, Tl, K, Rb, Ba and Sr. An increase of Cu, Au, Ag, Rb/Sr and Tl/Sr, and a decrease of Sr and K/Rb is observed in both mineralized metavolcanics and mineralized quartz veins. Thallium shows lithophile behaviour in the Dir metavolcanics and no chalcophile behaviour was observed. The Tl/Sr ratio might be an indirect guide for the exploration of volcanic-hosted hydrothermal copper deposits.  相似文献   

12.
The newly discovered Jiyuan Cu–Ag–(Pb–Zn–Au) deposit is located in the southern section of the eastern Tianshan orogenic belt, Xinjiang, northwestern China. It is the first documented deposit in the large Aqikekuduke Ag–Cu–Au belt in the eastern Tianshan orogen. Detailed field observations, parageneses, and fluid inclusion studies suggest an epithermal ore genesis for the main Cu–Ag mineralization, accompanied by a complicated hydrothermal alteration history most likely associated with the multi-stage tectonic evolution of the eastern Tianshan. The Jiyuan Cu–Ag ore bodies are located along the EW-striking, south-dipping Aqikekuduke fault and are hosted by Precambrian marble and intercalated siliceous rocks. Early-stage skarn alteration occurred along the contact zone between the marble layers and Early Carboniferous diorite–granodiorite and monzogranite intrusions; the skarns are characterized by diopside–tremolite–andradite–pyrite–(magnetite) assemblages. Local REE-enriched synchysite–rutile–arsenopyrite–(clinochlorite–microcline–albite) assemblages are related to K–Na alteration associated with the monzogranite intrusions and formed under conditions of high temperature (310°C) and high salinity (19.9 wt.% NaCl). Subsequent hydrothermal alteration produced a series of quartz and calcite veins that precipitated from medium- to low-temperature saline fluids. These include early ‘smoky’ quartz veins (190°C; 3.0 wt.% NaCl) that are commonly barren, coarse-grained Cu–Ag mineralized quartz veins (210°C; 2.4 wt.% NaCl), and late-stage unmineralized calcite veins (140°C; 1.1 wt.% NaCl). Tremolite and Ca-rich scapolite veins formed at an interval between early and mineralized quartz veins, indicating a high-temperature, high-salinity (>500°C; 9.5 wt.% NaCl) Ca alteration stage. Fluid mixing may have played an important role during Cu–Ag mineralization and an external low-temperature Ca-rich fluid is inferred to have evolved in the ore-forming system. The Jiyuan auriferous quartz veins possess fluid characteristics distinct from those of the Cu–Ag mineralized quartz veins. CO2-rich fluid inclusions, fluid boiling, and mixing all demonstrate that these auriferous quartz veins acted as hosts for the orogenic-type gold mineralization, a common feature in the Tianshan orogenic belt.  相似文献   

13.
Based on previous studies and detailed field investigations of the Dexing porphyry copper deposit, the Yinshan Ag-Pb-Zn deposit and the Jinshan shear zone – hosted gold deposit in the Dele Jurassic volcanic basin, in the northeastern Jiangxi province, East China, we propose that the three deposits share spatial, temporal and genetic relationships and belong to the same metallogenic system. Dexing is a typical porphyry Cu–Au–Mo deposit in which both ore-forming fluid and metals are derived from the granite porphyry. The Yinshan deposit consists of a porphyry copper ore located in the cupola of a quartz porphyry stock, in the lower part, and Ag–Pb–Zn ore veins in the upper part. The hydrothermal fluids were mainly derived from the magma in the early stages of the mineralizing event and became mixed with meteoric waters in the late stages. Its ore metals are magma-derived. Both the Jinshan base metal veins and the Hamashi, Dongjie and Naikeng quartz vein-type gold deposit are hosted by brittle–ductile structures, which are distal in relation to the porphyry intrusions and were formed by mixed magmatic fluids and meteoric water, whereas the gold was mainly leached from the country rocks (Mesoproterozoic Shuangqiaoshan Group phyllite and schist). The deposits show a distinct spatial arrangement from porphyry Cu, to epithermal Ag–Pb–Zn and distal Au. We suggest a porphyry–epithermal–distal vein ore system model for this group of genetically related mineral deposits. They were formed in a back-arc setting in a Middle Jurassic active continental margin, with magmas derived from the subducted slab.  相似文献   

14.
Interpretation of various exploration data, in particular geochemical prospecting, offers a powerful and rapid assessment of grass-root projects in a green-field terrain. Here, we present an example of the Collins epithermal prospect in Aceh Province, Indonesia. In this area, the Au+ base-metal-bearing sheeted quartz veins (individually mostly 2–4 cm wide), which are controlled by a 250 m wide by 800 m long NNE-trending structural corridor within Paleogene sandstone and volcanic rocks, are the product of two main stages of deposition. Stage I formed veins with a sliver of cryptocrystalline quartz wall zone followed by an inner zone of comb quartz with interstitial rhombic adularia that terminates in open space. Stage I or main-stage sulfide mineralization consisting of early galena + sphalerite and later chalcopyrite occurs with the quartz + adularia. Small amounts of galena also occur in the wall zone. Stage II mineralization brecciated Stage I veins and overprinted them with silicification characterized by vuggy texture. Mineralization associated with this episode consists of earlier chalcopyrite + sphalerite + tennantite–tetrahedrite and later, vug-filling Au–Ag alloy (Ag0.37–0.41Au0.62–0.59). The above mineralized veins are successively flanked by silicic selvages, an illite + chlorite + pyrite ± kaolinite zone and a chlorite + epidote + carbonate + pyrite zone. Local supergene alteration induced replacement of galena by plumbogummite and anglesite and chalcopyrite by covellite. Data from fluid inclusion microthermometry in quartz indicated that the inner zone of Stage I veins formed from fluids with a 2.3 wt% salinity (0.5–3.3 wt% NaCl equivalent), at 174°C (155–211°C). Combining these physico-chemical parameters with the mineral assemblage, the mineralization occurred under a reduced environment. Rock and soil assays indicate that elevated Au concentrations (up to 16.5 ppm over 1 m) occur along northeast-trending zones and show a strong correlation with Pb, while Cu (up to 2.58% over 1 m), Zn, As, Sb, and Mo anomalies lie mostly at the periphery. The high-grade mineralized veins correlate with moderate to high resistivity and chargeability zones, and the pseudosections of such geophysical signals are interpreted as reflecting coalesced or enlarged veins at depth, or inclined veins in other localities. The intermediate sulfidation affinity for Collins points to potential mineralization at depth as well as preservation of Au-rich and sulfide-poor zones in the less eroded areas.  相似文献   

15.
Analyses of a sediment core from Little Pond, located in the town of Bolton, Massachusetts, provide new insights into the history of environmental and ecological changes in southern New England during the late Holocene. Declines in organic content and peaks in the abundance of Isoetes spores indicate reduced water depth at 2900–2600, 2200–1800, and 1200–800 calibrated years before present (cal yr BP), generally consistent with the timing of dry conditions in records from elsewhere in the northeastern United States. The Little Pond pollen record features little change over the last 3000 yr, indicating that the surrounding vegetation was relatively insensitive to these periods of drought. The 1200–800 cal yr BP dry interval, however, coincides with increased abundance of Castanea pollen, suggesting that the expansion of Castanea in southern New England may have been influenced by late-Holocene climatic variability.  相似文献   

16.
Structurally hosted lode gold-bearing quartz vein systems in metamorphic terranes possess many characteristics in common, spatially and through time; they constitute a single class of epigenetic precious metal deposit, formed during accretionary tectonics or delamination. The ore and alteration paragenesis encode numerous intensive and extensive variables that constrain the pressure—temperature—time—deformation—fluid (P—T—t—d—f) evolution of the host terrane and hence the origin of the deposits. The majority of lode gold deposits formed proximal to regional translithospheric terrane—boundary structures that acted as vertically extensive hydrothermal plumbing systems; the structures record variably thrust, and transpressional—transtensional displacements. Major mining camps are sited near deflections, strike slip or thrust duplexes, or dilational jogs on the major structures. In detail most deposits are sited in second or third order splays, or fault intersections, that define domains of low mean stress and correspondingly high fluid fluxes. Accordingly, the mineralization and associated alteration is most intense in these flanking domains. The largest lode gold mining camps are in terranes at greenschist facies; they possess greenschist facies hydrothermal alteration assemblages developed in cyclic ductile to brittle deformation that reflects interseismic—coseismic cycles. Interseismic episodes involve the development of ductile S—C shear zone fabrics that lead to strain softening. Pressure solution and dislocation glide microstructures signify low differential stress, slow strain rates of ≤ 10−13 s−1, relatively high confining stress, and suprahydrostatic fluid pressures. Seismic episodes are induced by buildup of fluid pressures to supralithostatic levels that induce hydraulic fracturing with enhanced hydraulic conductivity, accompanied by massive fluid flow that in turn generates mineralized quartz veins. Hydrothermal cementing of ductile fabrics creates ‘hardening’, lowers hydraulic conductivity, and hence promotes fault valve behaviour. Repeated interseismic (fault valve closed), coseismic (valve open) cycles results in banded and/or progressively deformed veins. Alteration during both interseismic and coseismic episodes typically involves the hydrolysis of metamorphic feldspars and Fe, Mg, Ca-silicates to a muscovite/paragonite—chlorite ± albite/K-feldspars assemblage; carbonization of the metamorphic minerals to Ca, Fe, Mg-carbonates; and sulphidation of Fe-silicates and oxides to sulphides. Geochemically this is expressed as additions of K, Rb, Ba, Cs, and the volatiles H2O, CO2, CH4, H2S in envelopes of meter to kilometer scale. K/Rb and K/Ba ratios are close to average crustal values, potentially ruling out late stage magmatic fluids where K/Rb and K/Ba are respectively lower and higher than crustal values. Smaller deposits are present in subgreenschist, and amphibolite to granulite facies terranes. The former are characterized by subgreenschist facies alteration assemblages, vein stockworks, brittle fracturing and cataclastic microstructures, whereas the latter feature amphibolite to granulite facies alteration assemblages, ductile shear zones, ductilely deformed veins, and microstructures indicative of dislocation climb during interseismic episodes. Hence the lode gold deposits constitute a crustal continuum of deposits from subgreenschist to granulite facies, that all formed synkinematically in broad thermal and rheological equilibrium with their host terranes. These characteristics, combined with the low variance of alteration assemblages in the higher temperature deposits, rules out those being metamorphosed counterparts of greenschist facies deposits. Deposits at all grades have a comparable metal inventory with high concentrations of Au and Ag, where Au/Ag averages 5, with enrichments of a suite of rare metals and semi-metals (As, Sb, ± Se, Te, Bi, W, Mo and B), but low enrichments of the base (Cu, Pb, Zn, Cd) and other transition (Cr, Ni, Co, V, PGE, Sc) metals relative to average crust. The hydrothermal ore-forming fluids were dilute, aqueous, carbonic fluids, with salinities generally ≤ 3 wt.% NaCl equivalent, and X(CO2 ± CH4) 10–24 wt.%. They possess low Cl but relatively high S, possibly reflecting the fact that metamorphic fluids are generated in crust with ∼ 200 ppm Cl, but ∼ 1 wt.%S. Primary fluid inclusions are: (1) H2OCO2, (2) CO2-rich with variable CH4 and small amounts of H2O, and (3) 2-phase H2O (liquid-vapor) inclusions. Inclusion types 2 and 3 represent immiscibility of the type 1 original ore fluid. Immiscibility was triggered by fluid pressure drop during the coseismic events and possibly by shock nucleation, leading to highly variably homogenization temperatures in an isothermal system. A thermodynamic evaluation of alteration assemblages constrains the ore fluid pH to 5–6; redox controlled by the HSO4/H2S and CO2/CH4 buffers; and XCO2 that varies. The higher temperature deposits formed under marginally more oxidizing conditions. Stable isotope systematics of the ore and gangue minerals yields temperatures of 200–420°C, consistent with the crustal spectrum of the deposits, very high fluid rock ratios, and disequilibrium of the externally derived ore fluids with wall rocks. The ore fluid δD and δ18O overlap the metamorphic and magmatic ranges, but the total dataset for all deposits is consistent only with dominantly metamorphic fluids. Carbon isotope compositions of carbonates span −11 to +2% and show provinciality: this is consistent with variable proportions of reduced C (low δ13C) and oxidized C (higher δ13C) in the source regions contributing CO2 and CH4 to the ore fluids. In some instances, C appears to have been derived dominantly from proximal to the deposits, as in the Meguma terrane (δ13C ∼ − 22%). Sulphur isotope compositions range from 0 to +9‰, and are consistent with magmatic S, dissolution or desulphidation of magmatic sulphides, or average crustal sulphides. 34S-depleted sulphides occur in ore bodies such as Hemlo where fluid immiscibility led to loss of H2S and consequent fluid oxidation. Gold is probably transported as an Au(HS)−2 complex. Relatively high S but low Cl in the hydrothermal fluid may explain the high Au slow base metal characteristic of the deposits. Gold precipitated in ore bodies due to loss of S from the ore fluid by sulphidation of wall rock, or immiscibility of H2S; and by oxidation or reduction of the fluid, or by chemisorption, or some combination of these processes. Most lode gold deposits have been brittly reactivated during uplift of host terranes, with secondary brines or meteoric water advecting through the structures. These secondary fluids may remobilize gold, generate retrograde stable isotope shifts, reset mineral geochronometers, and leave trails of secondary fluid inclusions. Data on disturbed minerals has led to invalid models for lode gold deposits. The sum of alteration data leads to a model for lode gold deposits involving a clockwise P—T—t evolution and synkinematic and synmetamorphic mineralization of the ‘deep later’ type. During terrane accretion oceanic crust and sediments are subcreted beneath the terrane boundary. Thermal equilibration generates metamorphic fluids that advect up the terrane structure, at lithostatic fluid pressure, into the seismogenic zone where the majority of deposits form. Thus many lode gold deposits are on intrinsic part of the development of subduction—accretion complexes of the high-T, low-P type.  相似文献   

17.
The Palaeo-Mesoproterozoic metasiliciclastic rocks of the southern Serra do Espinhaço, Minas Gerais, Brazil, are host to historically important alluvial deposits of diamonds and gold. Detrital gold grains often comprise Au–Pd–Pt intermetallic compounds, with low Ag contents, which contain inclusions of tourmaline and titaniferous hematite (up to ~ 6 wt.% TiO2). The latter minerals connect the alluvial mineralisation to the rutile–hematite–quartz veins and tourmalinisation observed in the quartzitic country rocks of the alluvial gravel. The quartzite (Sopa-Brumadinho Formation of lacustrine to fan-deltaic origin) is affected by pervasive B metasomatism with F-bearing tourmaline replacing the recrystallised quartz fabric. The tourmaline belongs to the alkali group, with Mg/(Mg + Fe) and X/(X + Na) ratios in the ranges from 0.5 to 0.7 and 0.18 to 0.29, respectively, where X represents vacancies in the X site. Boron-isotopic values of tourmaline vary from ~ 1 to − 10.4‰ δ11B. The B-isotope range, in conjunction with the Na–Mg-rich tourmaline composition, and the widespread occurrence of tourmalinite in the Sopa-Brumadinho Formation suggest a derivation from non-marine evaporitic brines. Brines are capable of transporting otherwise immobile Ti and explain, under oxidising conditions, the fractionation of Ag from Pd to precipitate palladiferous gold with extremely high Pd/Ag ratios. Zirconium-in-rutile and Ti-in-quartz temperatures for a variety of hematite-rich veins suggest episodic vein emplacement over a temperature range from around 500 °C to ~ 350 °C. Cross-cutting relationships and episodic vein emplacement indicate a late-Brasiliano age.  相似文献   

18.
Carbon isotope composition (δ13C) in tree-rings has become routinely used in palaeoclimatic research for the assessment of changes in plant water availability in seasonally dry climates. However, the distribution of long tree-ring records around the world is very limited. Alternatively, the original climate signal of wood δ13C is well preserved in fossil charcoal and, accordingly, charcoal δ13C can be used to quantify past changes in water availability (e.g. precipitation). We report a case study on spatial palaeoclimate reconstruction which aims to characterize the transition between Bronze and Iron Ages, the so-called Iron Age Cold Epoch (ca. 900–300 BCE), using charcoals of Quercus ilex/coccifera from a set of 11 contemporary archaeological sites of eastern Spain. Climatic inferences were obtained after calibrating a linear model predicting seasonal precipitation from δ13C of Q. ilex wood samples obtained across a rainfall gradient. The best regression model corresponded to September–December (autumn) precipitation (Paut), in agreement with the fact that Q. ilex is able to exploit previous-year water reserves thanks to very effective water uptake. Subsequently, we estimated Paut from the δ13C of fossil charcoal to infer spatial patterns in water availability. Overall, estimated past Paut was about 19% higher (296 mm) than present-time values averaged across archaeological sites (249 mm). However, a clear geographic pattern of differences in precipitation could be observed in which the inner continental regions of eastern Spain were characterized by more humid conditions in the past, whereas the coastal strip of the Mediterranean Sea barely differed in past and present Paut values. The quite uniform distribution of archaeological sites over eastern Spain allowed development of contour maps of absolute and relative (to present) past Paut using gridded interpolation methods implemented in a GIS, highlighting the potential of this approach for reconstructing high-resolution spatial patterns of past climate.  相似文献   

19.
The aim of this study is to identify geochemical anomalies using power spectrum–area (S–A) method based on the grade values of Cu, Mo and Au in 2709 soil samples collected from Kahang porphyry-type Cu deposit, Central Iran. S–A log–log plots indicated that there are three stages of Cu, Mo and Au enrichment. The third enrichment was considered as the main stage for the presence of Cu, Mo and Au at the concentrations above 416 ppm, 23 ppm and 71 ppb, respectively. Elemental anomalies are positively associated with monzo–granite–diorite and breccias units which are in the central and western parts of the deposit. The anomalies are located within the potassic, phyllic and argillic alteration types and also there is the positive correlation between the anomalies and nearing faults in the studied area. The results obtained via fractal model were interpreted accordingly to incorporate the information for the mineralized areas including detailed geological map, structural analysis and alterations. The results show that S–A multifractal modeling is applicable for anomalies delineation based on soil data.  相似文献   

20.
Fluid inclusions hosted in quartz and specular hematite from auriferous (jacutinga) and barren veins in the Quadrilátero Ferrífero (QF) have been studied using conventional and near infrared microscopy, respectively. The mineralization consists of veins that cross-cut metamorphosed iron formation (itabirite) of the Paleoproterozoic Itabira Group. The sample suite comprises hematite from veins from the low-strain domain in the W and SW of the study area, as well as hematite samples from the eastern high-strain domain in the central and NE parts of the QF. Halogen ratios of fluid inclusions in quartz and hematite from all studied deposits are consistent with a fluid evolved from dissolving and reprecipitating halite that was subsequently diluted. Fluid inclusions hosted in quartz and hematite are characterized by consistent Na/K ratios and considerable SO4 contents, and suggest similar formation conditions and, perhaps, fluid origin from a common source. Na/K and Na/Li fluid mineral geothermometers indicate water–rock interaction at approximately 340±40°C. Hematites from the high-strain domain contain fluid inclusion assemblages of high-temperature aqueous-carbonic and multiphase high-salinity, high-temperature aqueous inclusions probably due to fluid immiscibility in the system H2O–NaCl–CO2. Fluid inclusions hosted in hematite from barren veins in the low-strain domain, as well as in hematite from jacutinga-type mineralization from the central part of the QF, only host multiphase aqueous fluid inclusions all showing narrow ranges of salinity (7.2–11.7 wt.% NaCl equiv.) and homogenization temperatures (148 to 229°C). Lower homogenization temperatures and the absence of CO2-rich inclusions in specular hematite from these occurrences are attributed to carbonate precipitation and/or CO2 escape due to cooling during fluid migration from the high- to the low-strain domain. Pb–Pb and U–Pb systematics of gold, hematite and hematite-hosted fluid inclusions in combination with geochemical evidence indicate distinct sources for Pd, Au, and Pb. The formation of specular hematite veins may be related to retrograde metamorphic fluids being released during the Brazilian orogenic cycle (600–700 Ma). The Pb isotopic characteristics of all samples are readily reconciled in a simple model that involves two different Paleoproterozoic or Archean source lithologies for lead and reflects contrasting depths of fluid percolation during the Brasiliano orogeny.  相似文献   

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