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1.
At the inside corner between the South Mid‐Atlantic Ridge (SMAR) and the Cardno fracture zone, the 14°S hydrothermal field, with its abundant silica‐rich sulfides, was identified as a volcanic massif, which provided the first opportunity to study inside corner‐related hydrothermal mineralization. S, Pb, and O isotopes were measured in hydrothermal sulfide samples from the two sites (volcanic top and slope sites) in the 14°S field. The homogeneous Pb isotope values (207Pb/204Pb = 15.466 ? 15.472; 206Pb/204Pb = 18.242 ? 18.252) and the δ34S values (top site, +6.3 to +6.85‰; slope site, +2.37 to +3.36‰) suggest that the source of metals and most of the sulfur is the upper crust, whereas some sulfur is sourced from downward‐penetrating seawater, especially in the top site. The calculated oxygen isotope equilibration temperatures from quartz and seawater are between 94 and 144°C for the top site and between 179 and 196°C for the slope site. These isotopic proxies, together with geological background survey data, have an important implication: Discrepancies between the sites may result from variations in crust permeability, which also affect the mixing level between downward‐penetrating seawater and upward hydrothermal end‐member fluids and determine the various mineralization types.  相似文献   

2.
The Baizhangyan skarn‐porphyry type W–Mo deposit is located in a newly defined Mo–W–Pb–Zn metallogenic belt, which is in the south of Middle‐Lower Yangtze Valley Cu–Fe–Au polymetallic metallogenic belt in SE China. The W–Mo orebodies occur mainly within the contact zone between fine‐grained granite and Sinian limestone strata. There are two types of W–Mo mineralization: major skarn W–Mo mineralization and minor granite‐hosted disseminated Mo mineralization which was traced by drilling at depth. Eight molybdenite samples from Mo‐bearing ores yield Re–Os dates that overlap within analytical error, with a weighted average age of 134.1 ± 2.2 Ma. These dates are in close agreement with SIMS U–Pb concordant zircon age for fine‐grained granite at 133.3 ± 1.3 Ma, indicating that crystallization of the granite and hydrothermal molybdenite formation were coeval and likely cogenetic. The Baizhangyan W–Mo deposit formed in the Early Cretaceous extensional tectonic setting at the Middle‐Lower Yangtze Valley metallogenic belt and the Jaingnan Ancient Continent. Based on mineral compositions and crosscutting relationships of veinlets, hydrothermal alteration and mineralization, the ore mineral paragenesis of the Baizhangyan deposit is divided into four stages: skarn stage (I), oxide stage (II), sulfide stage (III), and carbonate stage (IV). Fluid inclusions in garnet, scheelite, quartz and calcite from W–Mo ores are mainly aqueous‐rich (L + V) type inclusions. Following garnet deposition at stage I, the high‐temperature fluids gave way to progressively cooler, more dilute fluids associated with tungsten–molybdenite–base metal sulfide deposition (stage II and stage III) (162–360°C, 2.7–13.2 wt % NaCl equivalent) and carbonate deposition (stage IV) (137–190°C, 0.9–5 wt % NaCl equiv.). Hydrogen‐oxygen isotope data from minerals of different stages suggest that the ore‐forming fluids consisted of magmatic water, mixed in various proportions with meteoric water. From stage I to stage IV, there is a systematic decrease in the homogenization temperature of the fluid‐inclusion fluids and calculated δ18O values of the fluids. These suggest that increasing involvement of formation water or meteoric water during the fluid ascent resulted in successive deposition of scheelite and molybdenite at Baizhangyan.  相似文献   

3.
The Liushanyan deposit is an important volcanic‐host massive sulfide (VMS) Cu–Zn deposit in the Qinling‐Tongbai‐Dabie orogenic belt, central China, with reserve of 2.38 Mt Cu and 16.11 Mt Zn. Orebodies occur in the meta‐quartz keratophyre of the Liushanyan formation. In this paper, we present textural features and laser ablation ICP‐MS U–Pb dating results of zircons from the ore‐bearing mylonitized meta‐quartz keratophyre. The hydrothermal zircons are distinct from metamorphic zircons in this rock, showing low cathodoluminescence (CL) response and hydrothermal rims (black in CL images). They have relatively flat light rare earth element patterns and high La content and low (Sm/La)N and Ce/Ce* values. These features are typical of hydrothermal zircons. The cores of metamorphic zircons yield a weighted mean 206Pb/238U age of 900 ± 26 Ma, interpreted as the volcanic and related VMS mineralizing age. Two much younger events are also recorded by zircons in this rock: (i) the Early Silurian amphibolites–greenschist facies metamorphism at 435 ± 26 Ma; and (ii) the growth of hydrothermal zircons at ca. 241 ± 1 Ma, associated with the ductile shear deformation. The Silurian metamorphic event is probably associated with the arc–continent collision, while the Triassic ductile deformation event formed in the final continent–continent collision setting.  相似文献   

4.
The extensive Changba-Lijiagou Pb-Zn deposit is located in the north of the Xihe–Chengxian ore cluster in West Qinling. The ore bodies are mainly hosted in the marble, dolomitic marble and biotite-calcite-quartz schist of the Middle Devonian Anjiacha Formation, and are structurally controlled by the fault and anticline. The ore-forming process can be divided into three main stages, based on field geological features and mineral assemblages. The mineral assemblages of hydrothermal stage I are pale-yellow coarse grain, low Fe sphalerite, pyrite with pits, barite and biotite. The mineral assemblages of hydrothermal stage II are black-brown cryptocrystalline, high Fe shalerite, pyrite without pits, marcasite or arsenopyrite replace the pyrite with pits, K-feldspar. The features of hydrothermal stage III are calcite-quartz-sulfide vein cutting the laminated, banded ore body. Forty-two sulfur isotope analyses, twenty-five lead isotope analyses and nineteen carbon and oxygen isotope analyses were determined on sphalerite, pyrite, galena and calcite. The δ34 S values of stage I(20.3 to 29.0‰) are consistent with the δ34 S of sulfate(barite) in the stratum. Combined with geological feature, inclusion characteristics and EPMA data, we propose that TSR has played a key role in the formation of the sulfides in stage I. The δ34 S values of stage II sphalerite and pyrite(15.1 to 23.0‰) are between sulfides in the host rock, magmatic sulfur and the sulfate(barite) in the stratum. This result suggests that multiple S reservoirs were the sources for S2-in stage II. The δ34 S values of stage III(13.1 to 22‰) combined with the structure of the geological and mineral features suggest a magmatic hydrothermal origin of the mineralization. The lead isotope compositions of the sulfides have 206 Pb/204 Pb ranging from 17.9480 to 17.9782, 207 Pb/204 Pb ranging from 15.611 to 15.622, and 208 Pb/204 Pb ranging from 38.1368 to 38.1691 in the three ore-forming stages. The narrow and symmetric distributions of the lead isotope values reflect homogenization of granite and mantle sources before the Pb-Zn mineralization. The δ13 CPDB and δ18 OSMOW values of stage I range from-0.1 to 2.4‰ and from 18.8 to 21.7‰. The values and inclusion data indicate that the source of fluids in stage I was the dissolution of marine carbonate. The δ13 CPDB and δ18 OSMOW values of stage II range from-4 to 1‰ and from 12.3 to 20.3‰, suggesting multiple C-O reservoirs in the Changba deposit and the addition of mantle-source fluid to the system. The values in stage III are-3.1‰ and 19.7‰, respectively. We infer that the process of mineralization involved evaporitic salt and sedimentary organic-bearing units interacting through thermochemical sulfate reduction through the isotopic, mineralogy and inclusion evidences. Subsequently, the geology feature, mineral assemblages, EPMA data and isotopic values support the conclusion that the ore-forming hydrothermal fluids were mixed with magmatic hydrothermal fluids and forming the massive dark sphalerite, then yielding the calcite-quartz-sulfide vein ore type at the last stage. The genesis of this ore deposit was epigenetic rather than the previously-proposed sedimentary-exhalative(SEDEX) type.  相似文献   

5.
The Koktokay No. 3 pegmatite is the largest Li–Be–Nb–Ta–Cs pegmatitic rare‐metal deposit of the Chinese Altai orogenic belt, and is famous for its concentric ring zonation pattern (nine internal zones). However, the formation age and evolution time span have been controversial. Here, we present the results of LA‐ICP–MS zircon U–Pb dating and muscovite 40Ar–39Ar dating. Four groups of zircon U–Pb ages (～210 Ma, ～193–198 Ma, ～186–187 Ma and ～172 Ma) for Zones II, V, VI, VII, and VIII, and a weighed mean 206Pb/238U age of 965 ± 11 Ma for Zone IV are identified. Also, Zones II, IV, and VI have muscovite 40Ar–39Ar plateau ages of 179.7 ± 1.1 Ma, 182.1 ± 1.0 Ma, and 181.8 ± 1.1 Ma, respectively. Considering previous U–Pb age studies (Zones I, V, and VII), the ages of emplacement, Li mineralization peak, hydrothermal stage of the No. 3 pegmatite are in ranges of 193–198 Ma, 184–187 Ma and 172–175 Ma, with weighted mean 206Pb–238U ages of 194.8 ± 2.3 Ma, 186.6 ± 1.3 Ma and 173.1 ± 3.9 Ma, respectively. The No. 3 pegmatite formed in the early Jurassic. The results of xenocrysts suggest that there is another pegmatite forming event of around 210 Ma in the mining district and the old zircon U–Pb ages imply that Neoproterozoic crustal rocks pertain to sources of the No. 3 pegmatite. Including the previous muscovite 40Ar–39Ar age studies (Zones I and V), a cooling age range of 177–182 Ma is considered as the time of hydrothermal stage and end of formation. The evolution process of the No. 3 pegmatite lasted 16 Ma. Therein, the magmatic stage continued for 9–11 Myr and the magmatic–hydrothermal transition and hydrothermal stages were sustained at 5–7 Ma. These time spans are long because of huge scale, cupola shape, large formation depth, and complex internal zoning patterns and formation processes. Considering some pegmatite dikes in the Chinese Altai, there is an early Jurassic pegmatite forming event.  相似文献   

6.
The Weiquan Ag-polymetallic deposit is located on the southern margin of the Central Asian Orogenic Belt and in the western segment of the Aqishan-Yamansu arc belt in East Tianshan,northwestern China. Its orebodies, controlled by faults, occur in the lower Carboniferous volcanosedimentary rocks of the Yamansu Formation as irregular veins and lenses. Four stages of mineralization have been recognized on the basis of mineral assemblages, ore fabrics, and crosscutting relationships among the ore veins. Stage I is the skarn stage(garnet + pyroxene), Stage Ⅱ is the retrograde alteration stage(epidote + chlorite + magnetite ± hematite 士 actinolite ± quartz),Stage Ⅲ is the sulfide stage(Ag and Bi minerals + pyrite + chalcopyrite + galena + sphalerite + quartz ± calcite ± tetrahedrite),and Stage IV is the carbonate stage(quartz + calcite ± pyrite). Skarnization,silicification, carbonatization,epidotization,chloritization, sericitization, and actinolitization are the principal types of hydrothermal alteration. LAICP-MS U-Pb dating yielded ages of 326.5±4.5 and 298.5±1.5 Ma for zircons from the tuff and diorite porphyry, respectively. Given that the tuff is wall rock and that the orebodies are cut by a late diorite porphyry dike, the ages of the tuff and the diorite porphyry provide lower and upper time limits on the age of ore formation. The δ~(13)C values of the calcite samples range from-2.5‰ to 2.3‰, the δ~(18)O_(H2 O) and δD_(VSMOW) values of the sulfide stage(Stage Ⅲ) vary from 1.1‰ to 5.2‰ and-111.7‰ to-66.1‰, respectively,and the δ~(13)C, δ~(18)O_(H2 O) and δD_(V-SMOW) values of calcite in one Stage IV sample are 1.5‰,-0.3‰, and-115.6‰, respectively. Carbon, hydrogen, and oxygen isotopic compositions indicate that the ore-forming fluids evolved gradually from magmatic to meteoric sources. The δ~(34)S_(V-CDT) values of the sulfides have a large range from-6.9‰ to 1.4‰, with an average of-2.2‰, indicating a magmatic source, possibly with sedimentary contributions. The ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb ratios of the sulfides are 17.9848-18.2785,15.5188-15.6536, and 37.8125-38.4650, respectively, and one whole-rock sample at Weiquan yields~(206)Pb/~(204)Pb,~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb ratios of 18.2060, 15.5674, and 38.0511,respectively. Lead isotopic systems suggest that the ore-forming materials of the Weiquan deposit were derived from a mixed source involving mantle and crustal components. Based on geological features, zircon U-Pb dating, and C-H-OS-Pb isotopic data, it can be concluded that the Weiquan polymetallic deposit is a skarn type that formed in a tectonic setting spanning a period from subduction to post-collision. The ore materials were sourced from magmatic ore-forming fluids that mixed with components derived from host rocks during their ascent, and a gradual mixing with meteoric water took place in the later stages.  相似文献   

7.
The Pb–Zn deposit at Jebel Ghozlane, in the Nappe zone (northern Tunisia), is hosted by Triassic dolostones and Eocene limestones and is located along faults and a thrust‐sheet boundary. The sulfide mineralization of the deposit consists mainly of galena and sphalerite and occurs as vein, stockwork, breccia, dissemination and replacement ores. Three hydrothermal stages are involved in the formation of the ores: stage I is dominated by celestite‐barite, hydrothermal dolomite DII, colloform sphalerite, and galena I; stage II consist of galena II; and stage III contains calcite. Galena in the deposit yielded average 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of 18.705, 15.667 and 38.734, respectively, suggesting a single upper crustal source reservoir for metals. Trace element data indicate the presence of Zn‐ and As‐free galena and As‐rich galena (with 0.2–0.5% As). Sphalerite contains 0.4% As, 0.7–0.9% Cd and 0.1–1.5% Fe. Microthermometric analysis of fluid inclusions in celestite shows that the deposit formed from fluids composed of heterogeneous mixtures of saline (19.5 ± 1 wt% NaCl eq.) aqueous solutions sourced from basinal brines, and gaseous CO2‐rich phases bearing low amounts of CH4, N2 and/or H2S, at temperatures of 172 ± 5°C.  相似文献   

8.
The Haobugao deposit, located in the southern segment of the Great Xing'an Range, is a famous skarn‐related Pb‐Zn‐(Cu)‐(Fe) deposit in northern China. The results of our fluid inclusion research indicate that garnets of the early stage (I skarn stage) contain three types of fluid inclusions (consistent with the Mesozoic granites): vapor‐rich inclusions (type LV, with VH2O/(VH2O + LH2O) < 50 vol %, and the majority are 5–25 vol %), liquid‐rich two‐phase aqueous inclusions (type VL, with VH2O/(VH2O + LH2O) > 50 vol %, the majority are 60–80 vol %), and halite‐bearing multiphase inclusions (type SL). These different types of fluid inclusions are totally homogenized at similar temperatures (around 320–420°C), indicating that the ore‐forming fluids of the early mineralization stage may belong to a boiling fluid system. The hydrothermal fluids of the middle mineralization stage (II, magnetite‐quartz) are characterized by liquid‐rich two‐phase aqueous inclusions (type VL, homogenization temperatures of 309–439°C and salinities of 9.5–14.9 wt % NaCl eqv.) that coexist with vapor‐rich inclusions (type LV, homogenization temperatures of 284–365°C and salinities of 5.2–10.4 wt % NaCl eqv.). Minerals of the late mineralization stage (III sulfide‐quartz stage and IV sulfide‐calcite stage) only contain liquid‐rich aqueous inclusions (type VL). These inclusions are totally homogenized at temperatures of 145–240°C, and the calculated salinities range from 2.0 to 12.6 wt % NaCl eqv. Therefore, the ore‐forming fluids of the late stage are NaCl‐H2O‐type hydrothermal solutions of low to medium temperature and low salinity. The δD values and calculated δ18OSMOW values of ore‐forming fluids of the deposit are in the range of ?4.8 to 2.65‰ and ?127.3‰ to ?144.1‰, respectively, indicating that ore‐forming fluids of the Haobugao deposit originated from the mixing of magmatic fluid and meteoric water. The S‐Pb isotopic compositions of sulfides indicate that the ore‐forming materials are mainly derived from underlying magma. Zircon grains from the mineralization‐related granite in the mining area yield a weighted 206Pb/238U mean age of 144.8 ±0.8 Ma, which is consistent with a molybdenite Re‐Os model age (140.3 ±3.4 Ma). Therefore, the Haobugao deposit formed in the Early Cretaceous, and it is the product of a magmatic hydrothermal system.  相似文献   

9.
The Chatree deposit is located in the Loei‐Phetchabun‐Nakhon Nayok volcanic belt that extends from Laos in the north through central and eastern Thailand into Cambodia. Gold‐bearing quartz veins at the Q prospect of the Chatree deposit are hosted within polymictic andesitic breccia and volcanic sedimentary breccia. The orebodies of the Chatree deposit consist of veins, veinlets and stockwork. Gold‐bearing quartz veins are composed mainly of quartz, calcite and illite with small amounts of adularia, chlorite and sulfide minerals. The gold‐bearing quartz veins were divided into five stages based on the cross‐cutting relationship and mineral assemblage. Intense gold mineralization occurred in Stages I and IV. The mineral assemblage of Stages I and IV is characterized by quartz–calcite–illite–laumontite–adularia–chlorite–sulfide minerals and electrum. Quartz textures of Stages I and IV are also characterized by microcrystalline and flamboyant textures, respectively. Coexistence of laumontite, illite and chlorite in the gold‐bearing quartz vein of Stage IV suggests that the gold‐bearing quartz veins were formed at approximately 200°C. The flamboyant and brecciated textures of the gold‐bearing quartz vein of Stage IV suggest that gold precipitated with silica minerals from a hydrothermal solution that was supersaturated by boiling. The δ18O values of quartz in Stages I to V range from +10.4 to +11.6‰ except for the δ18O value of quartz in Stage IV (+15.0‰). The increase in δ18O values of quartz at Stage IV is explained by boiling. PH2O is estimated to be 16 bars at 200°C. The fCO2 value is estimated to be 1 bar based on the presence of calcite in the mineral assemblage of Stage IV. The total pressure of the hydrothermal solution is approximately 20 bars at 200°C, suggesting that the gold‐bearing quartz veins of the Q prospect formed about 200 m below the paleosurface.  相似文献   

10.
This report presents mineralogical, geochemical and isotopic data on samples obtained using the Benthic Multi‐coring System (BMS) to drill a submarine hydrothermal deposit developed in a caldera on the summit of the Suiyo Seamount in the Izu–Bonin Island Arc, south of Japan. This deposit is regarded as the first example of Kuroko‐type sulfide mineralization on a volcano at the volcanic front of an island arc. The mineralization and hydrothermal alteration below the 300 × 150‐m area of active venting was investigated to depths of 2–9 m below the sea floor. Drilling beneath the area of active venting recovered a sequence of altered volcanic rocks (dacite lavas, pyroclastic rocks of dacite–rhyolite compositions and pumice) associated with sulfide veining and patches/veins of anhydrite. No massive sulfide was found, however, and the subsea‐floor mineralization to 10 m depth is dominated by anhydrite and clay minerals with some sulfides. Sulfide‐bearing samples contained high Au (up to 42 ppm), Ag (up to 263 ppm), As (up to 1550 ppm), Hg (up to 55 ppm), Sb (up to 772 ppm), and Se (up to 24 ppm). Electron probe microanalyzer indicated that realgar, orpiment, and mimetite were major As‐bearing minerals. The sulfides were also characterized by high Zn (>10%) compared to Cu (<6.3%) and Pb (<0.6%). The δ202Hg/198Hg, δ202Hg/199Hg and δ202Hg/200Hg of the sulfide‐bearing dacite samples and a sulfide chimney decreased with increasing Hg/Zn concentration ratio. The variation of the δ202Hg/198Hg ranged from ?2.8 to +0.5‰ to relative to S‐HG02027. The large range of these δ202Hg/198Hg was greater than might be expected for such a heavy element and may be due to a predominance of kinetic effects. The variation of δ202Hg/198Hg of sulfide‐bearing dacite samples suggested that light Hg isotope in the vapor mixed with oxygenated seawater near sea floor during mineralization. Lead isotope ratios of the sulfide were very similar to those of the dacite lava, suggesting that lead is of magmatic origin. The 87Sr/86Sr ratio (0.70872) of anhydrite was different from that of the dacite lava, and suggests an Sr derivation predominantly from seawater. Hydrothermal alteration of the dacite in the Suiyo hydrothermal field was characterized by Fe‐sulfides, anhydrite, barite, montmorillonite, chlorite/montmorillonite mixed‐layer minerals, mica, and chlorite with little or no feldspar or cristobalite. Hydrothermal clay minerals changed with depth from montmorillonite to chlorite/montmorillonite mixed‐layer minerals to chlorite and mica. Hydrogen isotope ratios of chlorite/montmorillonite and mixed‐layer, mica‐chlorite composites obtained below the active venting sites ranged from ?49 to ?24‰, suggesting seawater as the dominant fluid causing alteration. Oxygen isotope ratios of anhydrite ranged from 9.2 to 10.4‰ and anhydrite formation temperatures were calculated to be 188–207°C. Oxygen isotope ratios ranged from +5.2 to +9.2‰ for montmorillonite, +3.2 to +4.5‰ for chlorite/montmorillonite mixed‐layer minerals, and +2.8 to +3.8‰ in mixtures of chlorite and mica. The formation temperatures of montmorillonite and of the chlorite–mica mixture were 160–250°C and 230–270°C, respectively. The isotope temperatures for clay minerals (220–270°C) and anhydrite (188°C) were significantly lower than the borehole temperature (308.3°C) measured just after the drilling, suggesting that temperature at this site is now higher than when clay minerals and anhydrite were formed.  相似文献   

11.
Abstract: The Daejang mine is located within the Cretaceous Gyeongsang basin. Mineral paragenesis can be divided into four stages (stages I, II, III and IV) by major tectonic fracturing. Stages I, III and IV are economically barren. Stage II, at which the precipitation of major ore minerals occurred, is further divided into three substages with paragenetic time based on minor fractures and discernible mineral assemblages: substage IIa, marked by deposition of quartz and Fe–sulfides; substage IIb, by introduction of base-metal sulfides within carbonates and some quartz; substage IIc, by quartz and carbonates with various sulfosalts. Fluid inclusion data indicate a complex geochemical evolution of hydrothermal fluids. Both CO2–rich and H2O–rich fluids were trapped in fluid inclusions at stage I and substage IIa. It is suggested that a compositionally heterogeneous fluid was formed by fluid boiling and CO2 immiscibility at temperatures of about 400° to 300°C. Composite lodes of base-metal sul–fides, carbonates and quartz at substage IIb were deposited in open spaces created by fracturing. The fracturing event prompted rapid decreases in pressure and temperature of residual fluids and resulted in retrograde fluid boiling at about 200 bars and 300°–250°C during substage IIb. The progressive loss of CO2 by CO2 effervescence and retrograde boiling from substage IIa and IIb fluids resulted in pH increase and related increase in carbonate activity, causing deposition of abundant carbonates. The change in pH also caused the decrease of stability of hydrogen sulfide with Cu, Zn and Pb chloride complexes (as main transporting agents at Daejang) and resulted in the pricipitation of base-metal minerals. Deposition of Ag– and Sb-bearing sul–fides and sulfosalts of substage IIc occurred at temperatures of about 250° to 150°C from a dominantly aqueous fluid with low salinity (down to 3. 0 equiv. wt % NaCl). At this substage, aqueous fluid formed by mixing with cooler and less saline meteoric groundwater. There is a systematic decrease in caculated δ18Owater values with the mineralization stage (and decreasing temperature) in the Daejang hydrothermal system, from values of about 11% for stage I, through about 4% for stages II and III, to about –3 per mil for stage IV. The result of stable isotope and fluid inclusion studies are interpreted to indicate progressive less evolved and/or unexchanged meteoric water influx of an early hydrothermal system formed by highly evolved meteoric waters.  相似文献   

12.
This work presents data for the radiogenic Pb isotopic ratios (206Pb/207Pb and 208Pb/206Pb) in nine biogenic certified reference materials (NIST SRM 1515, 1566b, 1570a, 1573a, 1575a; BCR 100, BCR 101, BCR 670 and IAEA 359), which are suitable for analytical quality control in environmental research. The results were obtained using three different types of ICP‐based mass spectrometer (quadrupole‐based/magnetic sector field single‐collector ICP‐MS instruments and a multi‐collector ICP‐MS) and applying different mass bias correction procedures (calibrator‐sample bracketing and external Tl normalisation) with and without Pb separation from the matrix using ion exchange chromatography. In the majority of the samples, the measurements from all three of the ICP‐MS instruments were in agreement within ± 0.1%, despite the lower analytical precision of the single‐collector ICP‐MS instruments. We demonstrate that the presence of the sample matrix did not significantly influence the Pb isotopic ratios measured by magnetic sector field ICP‐MS, whereas the use of the two different mass bias corrections resulted in a systematic difference of 0.09% for the 208Pb/206Pb ratio.  相似文献   

13.
The Yonaguni Knoll IV hydrothermal vent field (24°51′N, 122°42′E) is located at water depths of 1370–1385 m near the western edge of the southern Okinawa Trough. During the YK03–05 and YK04–05 expeditions using the submersible Shinkai 6500, both hydrothermal precipitates (sulfide/sulfate/carbonate) and high temperature fluids (Tmax = 328°C) presently venting from chimney‐mound structures were extensively sampled. The collected venting fluids had a wide range of chemistry (Cl concentration 376–635 mmol kg?1), which is considered as evidence for sub‐seafloor phase separation. While the Cl‐enriched smoky black fluids were venting from two adjacent chimney‐mound structures in the hydrothermal center, the clear transparent fluids sometimes containing CO2 droplet were found in the peripheral area of the field. This distribution pattern could be explained by migration of the vapor‐rich hydrothermal fluid within a porous sediment layer after the sub‐seafloor phase separation. The collected hydrothermal precipitates demonstrated a diverse range of mineralization, which can be classified into five groups: (i) anhydrite‐rich chimneys, immature precipitates including sulfide disseminations in anhydrite; (ii) massive Zn‐Pb‐Cu sulfides, consisting of sphalerite, wurtzite, galena, chalcopyrite, pyrite, and marcasite; (iii) Ba‐As chimneys, composed of barite with sulfide disseminations, sometimes associated with realgar and orpiment overgrowth; (iv) Mn‐rich chimneys, consisting of carbonates (calcite and magnesite) and sulfides (sphalerite, galena, chalcopyrite, alabandite, and minor amount of tennantite and enargite); and (v) pavement, silicified sediment including abundant native sulfur or barite. Sulfide/sulfate mineralization (groups i–iii) was found in the chimney–mound structure associated with vapor‐loss (Cl‐enriched) fluid venting. In contrast, the sulfide/carbonate mineralization (group iv) was specifically found in the chimneys where vapor‐rich (Cl‐depleted) fluid venting is expected, and the pavement (group v) was associated with diffusive venting from the seafloor sediment. This correspondence strongly suggests that the subseafloor phase separation plays an important role in the diverse range of mineralization in the Yonaguni IV field. The observed sulfide mineral assemblage was consistent with the sulfur fugacity calculated from the FeS content in sphalerite/wurtzite and the fluid temperature for each site, which suggests that the shift of the sulfur fugacity due to participation of volatile species during phase separation is an important factor to induce diverse mineralization. In contrast, carbonate mineralization is attributed to the significant mixing of vapor‐rich hydrothermal fluid and seawater. A submarine hydrothermal system within a back‐arc basin in the continental margin may be considered as developed in a geologic setting favorable to a diverse range of mineralization, where relatively shallow water depth induces sub‐seafloor phase separation of hydrothermal fluid, and sediment accumulation could enhance migration of the vapor‐rich hydrothermal fluid.  相似文献   

14.
The Heijianshan Fe–Cu (–Au) deposit, located in the Aqishan-Yamansu belt of the Eastern Tianshan (NW China), is hosted in the mafic–intermediate volcanic and mafic–felsic volcaniclastic rocks of the Upper Carboniferous Matoutan Formation. Based on the pervasive alteration, mineral assemblages and crosscutting relationships of veins, six magmatic–hydrothermal stages have been established, including epidote alteration (Stage I), magnetite mineralization (Stage II), pyrite alteration (Stage III), Cu (–Au) mineralization (Stage IV), late veins (Stage V) and supergene alteration (Stage VI). The Stage I epidote–calcite–tourmaline–sericite alteration assemblage indicates a pre-mineralization Ca–Mg alteration event. Stage II Fe and Stage IV Cu (–Au) mineralization stages at Heijianshan can be clearly distinguished from alteration, mineral assemblages, and nature and sources of ore-forming fluids.Homogenization temperatures of primary fluid inclusions in quartz and calcite from Stage I (189–370 °C), II (301–536 °C), III (119–262 °C) and V (46–198 °C) suggest that fluid incursion and mixing probably occurred during Stage I to II and Stage V, respectively. The Stage II magmatic–hydrothermal-derived Fe mineralization fluids were characterized by high temperature (>300 °C), medium–high salinity (21.2–56.0 wt% NaCl equiv.) and being Na–Ca–Mg–Fe-dominated. These fluids were overprinted by the external low temperature (<300 °C), medium–high salinity (19.0–34.7 wt% NaCl equiv.) and Ca–Mg-dominated basinal brines that were responsible for the subsequent pyrite alteration and Cu (–Au) mineralization, as supported by quartz CL images and H–O isotopes. Furthermore, in-situ sulfur isotopes also indicate that the sulfur sources vary in different stages, viz., Stage II (magmatic–hydrothermal), III (basinal brine-related) and IV (magmatic–hydrothermal). Stage II disseminated pyrite has δ34Sfluid values of 1.7–4.3‰, comparable with sulfur from magmatic reservoirs. δ34Sfluid values (24.3–29.3‰) of Stage III Type A pyrite (coexists with hematite) probably indicate external basinal brine involvement, consistent with the analytical results of fluid inclusions. With the basinal brines further interacting with volcanic/volcaniclastic rocks of the Carboniferous Matoutan Formation, Stage III Type B pyrite–chalcopyrite–pyrrhotite assemblage (with low δ34Sfluid values of 4.6–10.0‰) may have formed at low fO2 and temperature (119–262 °C). The continuous basinal brine–volcanic/volcaniclastic rock interactions during the basin inversion (∼325–300 Ma) may have leached sulfur and copper from the rocks, yielding magmatic-like δ34Sfluid values (1.5–4.1‰). Such fluids may have altered pyrite and precipitated chalcopyrite with minor Au in Stage IV. Eventually, the Stage V low temperature (∼160 °C) and low salinity meteoric water may have percolated into the ore-forming fluid system and formed late-hydrothermal veins.The similar alteration and mineralization paragenetic sequences, ore-forming fluid sources and evolution, and tectonic settings of the Heijianshan deposit to the Mesozoic Central Andean IOCG deposits indicate that the former is probably the first identified Paleozoic IOCG-like deposit in the Central Asian Orogenic Belt.  相似文献   

15.
This paper presents a review of hydrothermal alteration and K–Ar age data from the Toyoha‐Muine area (TMA), where the Toyoha polymetallic (Ag–Pb–Zn–Cu–In) deposit is located near the Pliocene andesitic volcano that formed Mt Muine. Systematic prospect‐scale mapping, sampling, X‐ray analysis and microscopic observation show that hydrothermal alteration is divided into two groups: acid‐pH and neutral‐pH alteration types. The former is further divided into mineral assemblages I, II and III, while the latter into mineral assemblages IV and V. Different mineralogical features in five mineral assemblages are summarized as follows: (I) Quartz (silicified rock); (II) Pyrophyllite or dickite; (III) Kaolinite or halloysite ± alunite; (IV) Sericite or K‐feldspar; and (V) Interstratified minerals (illite/smectite and chlorite/smectite) and/or smectite. K–Ar radiometric ages determined on twenty‐eight K‐bearing samples (whole volcanic rocks and separated hydrothermal minerals) mainly fall into one of three periods: Early Miocene (24.6–21.4 Ma), Middle–Late Miocene (12.5–8.4 Ma) and Pliocene–Pleistocene (3.2–0 Ma). These three periods are characterized as follows. Early Miocene: A minor hydrothermal activity, which might be genetically related to the intermediate or felsic magmatic activities, formed mineral assemblage IV at 24.6 Ma in the northern part of the TMA. Middle to Late Miocene: The basaltic intrusion, andesitic eruption, and granodiorite intrusions induced hydrothermal activities between 12.5 and 8.4 Ma, resulting in the formation of a mineral assemblage IV with some base metal mineralization. Pliocene–Pleistocene: An andesitic eruption formed Mt Muine between 3.2 and 2.9 Ma. The andesitic activity was associated with acid‐pH mineral assemblages I, II and III locally around the volcano. Latent magmatic intrusions subsequent to the andesitic eruption generated hydrothermal activities that formed mineral assemblages IV and V between 1.9 and 0 Ma in the southern and southeastern parts of Toyoha deposit at depth, overprinting the Middle to Late Miocene alteration. The hydrothermal activities also formed mineral assemblages I, II and III along the Yunosawa fault (east of the Toyoha deposit) and assemblage III in the south and southeast of the Toyoha deposit near the surface.  相似文献   

16.
The Xiuwenghala gold deposit is located in the Beishan Orogen of the southern Central Asian Orogenic Belt. The vein/lenticular gold orebodies are controlled by Northeast‐trending faults and are hosted mainly in the brecciated/altered tuff and rhyolite porphyry of the Lower Carboniferous Baishan Formation. Metallic minerals include mainly pyrite and minor chalcopyrite, arsenopyrite, galena, and sphalerite, whilst nonmetallic minerals include quartz, chalcedony, sericite, chlorite, and calcite. Hydrothermal alterations consist of silicic, sericite, chlorite, and carbonate. Alteration/mineralization processes comprise three stages: pre‐ore silicic alteration (Stage I), syn‐ore quartz‐chalcedony‐polymetallic sulfide mineralization (Stage II), and post‐ore quartz‐calcite veining (Stage III). Fluid inclusions (FIs) in quartz and calcite are dominated by L‐type with minor V‐type and lack any daughter mineral‐bearing or CO2‐rich/‐bearing inclusions. From Stages I to III, the FIs homogenized at 240–260°C, 220–250°C, and 150–190°C, with corresponding salinities of 2.9–10.9, 3.2–11.1, and 2.9–11.9 wt.% NaCl eqv., respectively. The mineralization depth at Xiuwenghala is estimated to be relatively shallow (<1 km). FI results indicate that the ore‐forming fluids belong to a low to medium‐temperature, low‐salinity, and low‐density NaCl‐H2O system. The values decrease from Stage I to III (3.7‰, 1.7–2.4‰, and ?1.7 to 0.9‰, respectively), and a similar trend is found for their values (?104 to ?90‰, ?126 to ?86‰, and ?130 to ?106‰, respectively). This indicates that the fluid source gradually evolved from magmatic to meteoric. δ34S values of the hydrothermal pyrites (?3.0 to 0.0‰; avg. ?1.1‰) resemble those of typical magmatic/mantle‐derived sulfides. Pyrite Pb isotopic compositions (206Pb/204Pb = 18.409–18.767, 207Pb/204Pb = 15.600–15.715, 208Pb/204Pb = 38.173–38.654) are similar to those of the (sub)volcanic ore host, indicating that the origin of ore‐forming material was mainly the upper crustal (sub)volcanic rocks. Integrating evidence from geology, FIs, and H–O–S–Pb isotopes, we suggest that Xiuwenghala is best classified as a low‐sulfidation epithermal gold deposit.  相似文献   

17.
The Hujiayu Cu deposit,representative of the "HuBi-type" Cu deposits in the Zhongtiao Mountains district in the southern edge of the North China Craton,is primarily hosted in graphitebearing schists and carbonate rocks.The ore minerals comprise mainly chalcopyrite,with minor sphalerite,siegenite[(Co,Ni)_3S_4],and clausthalite[Pb(S,Se)].The gangue minerals are mainly quartz and dolomite,with minor albite.Four fluid inclusion types were recognized in the chalcopyrite-pyrite-dolomite-quartz veins,including CO_2-rich inclusions(type Ⅰ),low-salinity,liquid-dominated,biphase aqueous inclusions(type Ⅱ),solid-bearing aqueous inclusions(type Ⅲ),and solid-bearing aqueous-carbonic inclusions(type Ⅳ).Type I inclusion can be further divided into two sub-types,i.e.,monophase CO_2 inclusions(type Ⅰa) and biphase CO_2-rich inclusions(with a visible aqueous phase),and type Ⅲ inclusion is divided into a subtype with a halite daughter mineral(type Ⅲa) and a subtype with multiple solids(type Ⅲb).Various fluid inclusion assemblages(FIAs) were identified through petrographic observations,and were classified into four groups.The group-1 FIA,consisting of monophase CO_2 inclusions(type Ⅰa),homogenized into the liquid phase in a large range of temperatures from-1 to 28℃,suggesting post-entrapment modification.The group-2 FIA consists of type Ⅰb,Ⅲb and Ⅳ inclusions,and is interpreted to reflect fluid immiscibility.The group-3 FIA comprises type Ⅱ and Ⅲa inclusions,and the group-4FIA consists of type Ⅱ inclusions with consistent phase ratios.The group-1 and group-2 FIAs are interpreted to be entrapped during mineralization,whereas group-3 and group-4 FIAs probably represent the post-mineralization fluids.The solid CO_2 melting temperatures range from-60.6 to56.6℃ and from-66.0 to-63.4℃ for type Ⅰa and type Ⅳ inclusions,respectively.The homogenization temperatures for type Ⅱ inclusions range from 132 to 170℃ for group-3 FIAs and115 to 219℃ for group-4 FIAs.The halite melting temperatures range from 530 to 562℃ for typeⅢ b and Ⅳ inclusions,whereas those for type Ⅲa inclusions range from 198 to 398℃.Laser Raman and SEM-EDS results show that the gas species in fluid inclusions are mainly CO_2 with minor CH_4,and the solids are dominated by calcite and halite.The calcite in the hosting marble and dolomite in the hydrothermal veins have δ~(13)C_(V-pdb) values of-0.2 to 1.2‰ and-1.2 to-6.3‰,and δ~(18)O_(v-smow) values of 14.0 to 20.8 ‰ and 13.2 to 14.3‰,respectively.The fluid inclusion and carbon-oxygen isotope data suggest that the ore-forming fluids were probably derived from metamorphic fluids,which had reacted with organic matter in sedimentary rocks or graphite and undergone phase separation at 1.4-1.8 kbar and 230-240℃,after peak metamorphism.It is proposed that the Hujiayu Cu deposit consists of two mineralization stages.The early stage mineralization,characterized by disseminated and veinlet copper sulfides,probably took place in an environment similar to sediment-hosted stratiform copper mineralization.Ore minerals formed in this precursor mineralization stage were remobilized and enriched in the late metamorphic hydrothermal stage,leading to the formation of thick quartz-dolomite-sulfides veins.  相似文献   

18.
The Xinlu Sn‐polymetallic ore field is located in the western Nanling Polymetallic Belt in northeastern Guangxi, South China, where a number of typical skarn‐, hydrothermal vein‐type tin deposits have developed. There are two types of Sn deposits: skarn‐type and sulfide‐quartz vein‐type. The tin mineralizations mainly occur on the south side of the Guposhan granitic complex pluton and within its outer contact zone. To constrain the Sn mineralization age and further understand its genetic links to the Guposhan granitic complex, a series of geochronological works has been conducted at the Liuheao deposit of the ore field using high‐precision zircon SHRIMP U‐Pb, molybdenite Re‐Os, and muscovite Ar‐Ar dating methods. The results show that the biotite‐monzogranite, which is part of the Xinlu intrusive unit of the Guposhan complex pluton, has a SHRIMP U‐Pb zircon age of 161.0 ± 1.5 Ma. The skarn‐type ore has a 40Ar‐39Ar muscovite plateau age of 160 ± 2 Ma (same as its isochron age), and the sulfide‐quartz vein‐type ore yields an Re‐Os molybdenite isochron age of 154.4 ± 3.5 Ma. The magmatic‐hydrothermal geochronological sequence demonstrated that the hydrothermal mineralization took place immediately following the emplacement of the monzogranite, with the skarn metasomatic mineralization stage predating the sulfide mineralization stage. Geochronologically, we have compared this ore field with 26 typical Sn deposits distributed along the Nanling Polymetallic Belt, leading to the suggestion of the magmatic‐metallogenic processes in the Xinlu ore field (ca. 161–154 Ma) as a component of the Early Yanshanian large‐scale Sn‐polymetallic mineralization event (peaked at 160–150 Ma) in the Nanling Range of South China. Petrogenesis of Sn‐producing granite and Sn‐polymetallic mineralization were probably caused by crust–mantle interaction as a result of significant lithospheric extension and thinning in South China in the Late Jurassic.  相似文献   

19.
Polyphase metamorphic paragneisses from the drill core of the continental deep drilling project (KTB; NW Bohemian Massif) are characterized by peak pressures of about 8 kbar (medium‐P metamorphism) followed by strain accumulation at T >650 °C, initially by dislocation creep and subsequently by diffusion creep. U–Pb monazite ages and Rb–Sr whole‐rock data vary in the dm‐scale, indicating Ordovician and Mid‐Devonian metamorphic events. Such age variations are closely interconnected with dm‐scale domainal variations of microfabrics that indicate different predominant deformation mechanisms. U–Pb monazite age variations dependent on microfabric domains exceed grain‐size‐dependent age variations. In ‘mylonitic domains’ recording high magnitudes of plastic strain, dislocation creep and minor static annealing, monazite yields concordant and near concordant Lower Ordovician U–Pb ages, and the Rb–Sr whole‐rock system shows isotopic disequilibrium at an mm‐scale. In ‘mineral growth/mobilisate domains’, in which diffusive mass transfer was a major strain‐producing mechanism promoting diffusion creep of quartz and feldspar, and in which static recrystallization (annealing) reduced the internal free energy of the strained mineral aggregates, concordant U–Pb ages are Mid‐Devonian. Locally, in such domains, Rb–Sr dates among mm3‐sized whole‐rock slabs reflect post‐Ordovician resetting. In ‘transitional domains’, the U–Pb‐ages are discordant. We conclude that medium‐P metamorphism occurred at 484±2 Ma, and a second metamorphic event at 380–370 Ma (Mid‐Devonian) caused progressive strain in the rocks. Dislocation creep at high rates, even at high temperatures, does not reset the Rb–Sr whole‐rock system, while diffusion creep at low rates and stresses (i.e. low ε/Deff ratios), static annealing and the presence of intergranular fluids locally assist resetting. At temperatures above 650 °C, diffusive Pb loss did not reset Ordovician U–Pb monazite ages, and in domains of overall high imposed strain rates and stresses, resetting was not assisted by dynamic recrystallization/crystal plasticity. However, during diffusion creep at low rates, Pb loss by dissolution and precipitation (‘recrystallization’) of monazite produces discordance and Devonian‐concordant U–Pb monazite ages. Hence, resetting of these isotope systems reflects neither changes of temperature nor, directly, the presence or absence of strain.  相似文献   

20.
The Shabaosi deposit is the only large lode gold deposit in the northern Great Xing'an Range. The gold ore bodies are hosted by sandstone and siltstone of the Middle Jurassic Ershi'erzhan Formation, and are controlled by three N–S‐trending altered fracture zones. The gold ore bodies are composed of auriferous quartz veinlets and altered rocks. Fluid inclusion studies indicate that the ore‐forming fluids belong to a H2O–NaCl–CO2–CH4 system, with salinities between 0.83 and 8.28 wt% NaCl eq., and homogenization temperatures ranging from 180 to 320 °C. The δ34S values of sulphides show a large variation from −16.9‰ to 8.5‰. The Pb isotope compositions of sulphides are characterized by a narrow range of ratios: 18.289 to 18.517 for 206Pb/204Pb, 15.548 to 15.625 for 207Pb/204Pb, and 38.149 to 38.509 for 208Pb/204Pb. The μ values range from 9.36 to 9.51. These results suggest that the ore‐forming fluids/materials were mainly of magmatic hydrothermal origin, derived from magmas produced by partial melting of the lower crust. The 40Ar/39Ar age of auriferous quartz veinlets from the Shabaosi gold deposit is about 130 Ma. The Shabaosi gold deposit has counterparts in similar orogenic gold deposits, and was formed during the post‐collisional setting of the Mongolia–Okhotsk Orogen. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

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