The vein-type barite mineralization of the Draïssa ore field, Ougarta; SW- Algeria: mineralogy, trace elements and halogens
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Quartz vein-type gold deposit is the most important type of gold deposits in the world. However, the lack of minerals suitable for most conventional isotopic dating methods constrains the direct and precise dating. Recent development in mass spectrometry makes it possible to determine the age of auriferous quartz veins by U-Pb dating of zircons from quartz veins. Unfortunately, hydrothermal zircons that grow directly from mineralizing fluids and inherited magmatic or metamorphic zircons from wall rocks may coexist in the same vein. Such complexity poses significant problem while interpreting the U-Pb data. Thus, the key for zircon U-Pb dating of quartz vein-type gold deposit will be to distinguish hydrothermal zircon precipitated from ore-forming fluids from the inherited zircons. Combined studies of zircon morphology, internal texture, trace elemental geochemistry (including rare earth element), compositions of mineral and fluid inclusions will permit identification of hydrothermal zircons, which then can be precisely dated by SHRIMP or LA-ICP-MS methods to provide reliable age constraints of quartz vein-type gold deposits.
Geochronology
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Microthermomemc techniques were applied to samples from mantos and veins in the Hualgayoc District, Peru. Mineralisation in the district is hosted mainly in rocks from the Hualgayoc group (Mesozoic marine sedimentary and volcanic rocks). Primary and secondary inclusions in sphaleriteand gangue minerals were examined petrographically and their homogenisation and freezing temperatures measured. Three stages of sphalerite, with intermittent deposition of quartz, calcite and rhodocrosite are recognized. The results indicate that mineralisation took place at temperatures from about to above 350°C; the lowest values corresponding to vein mineralisation and the highest to the mantos. The highest apparent salinity values were observed in the manto samples (more than 16% NaCl) in contrast to lower salinities (4.3% NaCl) measured in one sphalerite sample from the San Filipe Vein. From the Mantos area to the north and the east, a trend of decreasing temperature and salinity exists. No evidence of boiling was observed in the sphalerite samples; however, boiling may have occurred during deposition of quartz and in the veins.
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Abstract: The Bamianshan fluorite deposit is a super‐large one recently discovered in Zhejiang Province of China. This paper presents an analysis of its geological background, orebody and ore characteristics, petrochemical characteristics of host rocks, rare earth elements (REE) of rocks and ores, fluid inclusions in fluorite and Sm‐Nd isotopic features in an effort to study its sedimentary mineralization. The result shows that the super‐large Bamianshan fluorite deposit is of hydrothermal sedimentation genesis, deformed by the later hydrothermal fluid. Integrated with host rocks and orebody characteristics, it is inferred that the deposit originates from the Cambrian sedimentary rocks. And the later magmatic activities deformed some orebodies in different degrees, forming steeply dipping vein orebodies in the tectonic belts regionally.
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Ore genesis
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Arsenopyrite
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Abstract. Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of vein quartz in the Cu-mineralised, Shuteen Complex (South Gobi, Mongolia) has revealed a complex history of crystal growth, dissolution and microfracture healing, associated with several hydrothermal events that could not be detected using other observational techniques (e.g. transmitted/reflected light microscopy, back-scattered electron imaging, or secondary electron imaging). The quartz initially grew as CL-bright/grey crystals in a 345±30d̀C liquid reservoir, as inferred by the analysis of primary liquid fluid inclusions (average Th of 343d̀C; 6.6∼7.7 wt% NaCleq). Quartz precipitation occurred at the edge of the crystals as reservoir fluids cooled to 260±25d̀C, as indicated by micron-scale CL-dark/CL-bright quartz growth bands containing abundant fluid inclusions (with an average Th values of 261d̀C). Pressure fluctuations were the likely cause of dissolution, as SEM-CL imaging reveals the quartz have corroded or rounded crystal edges, and precipitation of later quartz into open space. SEM-CL imaging shows the quartz contains healed microfractures that trapped low salinity fluids (3.9 wt% NaC1eq) with Th values of 173±15d̀C. SEM-CL imaging provides a means of deciphering the thermal and chemical evolution of the fossil Shuteen hydrothermal system, and the nature of hydrothermal quartz vein-forming processes, by facilitating the correlation of distinct fluid inclusion populations and their relative chronology, with specific hydrothermal events.
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Abstract The Tagun‐Khin‐Dan gold deposit in the Mogok‐Mandalay‐Mergui Belt, Central Myanmar, is characterized by an array of quartz‐veins hosted in mudstone of the Kogwe Formation of the Carboniferous Mergui Group. Two major deformational stages were recorded in the area; (1) N‐S shortening and (2) uplifting and emplacement of various dykes and quartz veinlets. The N‐S shortening within the area lead the development of km‐scale faults, determined largely by the presence of a zone of major WNW‐ESE trending dextral strike‐slip faulting. Quartz veins in the deposit include: (1) type‐A quartz veins, parallel to the dextral NW‐SE trending major fault; and (2) type‐B quartz veins which occur as isolated parallel veins. Gold in the type‐A quartz vein is present as native gold and electrum locked within pyrite and associated with pyrite and galena and in the type‐B quartz veins as electrum associated with sulfide minerals such as pyrite, chalcopyrite, galena and sphalerite. The mineralization stages can be classified into the type‐A quartz vein stage and the type‐B quartz vein stage. Two type of fluid inclusions; liquid‐rich aqueous inclusions (L‐type) and vapor‐rich aqueous inclusions (V‐type) are identified in the type‐A quartz veins. The homogenization temperature of L‐type fluid inclusions of the type‐A quartz veins ranges from 203 to 321°C and salinity of the fluid inclusions varies from 0.4 to 1.6 wt% NaCl equiv. The homogenization temperature of V‐type fluid inclusions of type‐A quartz veins ranges from 290 to 340°C with a salinity ranging from 0.4 to 1.9 wt% NaCl equivalent. In the type‐B quartz veins, only liquid‐rich aqueous inclusions (L‐type) are identified. The type‐B quartz veins yielded low homogenization temperatures from 160 to 220°C, with low salinities from 0.2 to 1.9 wt% NaCl equiv. compared with those of the type‐A veins. The depth range of ore formation is estimated to be a shallow depth of less than 0.2 km based on fluid inclusion microthermometry. Fluid boiling is evident during the type‐A quartz vein stage, and fluid cooling and mixing in the later type‐B quartz vein stage. Precipitation of pyrite in the ore zone occurred as four recognized types: arsenic‐rich pyrite‐1, 2, 3 in the type‐A quartz veins and pyrite‐4 in the type‐B quartz veins. A positive relation between Au and As contents of pyrites suggests that the gold is present together with arsenic in the structure of pyrites of the type‐A quartz veins as solid solution in addition to as nanoparticle inclusions. The high Co and Ni contents of pyrites of both the type‐A and the type‐B quartz veins, with no evidence of CO 2 in the system indicate that the ore‐forming fluids were epizonal magmatic‐hydrothermal fluids rather than metamorphic fluid. The hydrothermal fluids of the Tagun‐Khin‐Dan deposit were driven by faulting to form the mudstone‐hosted epithermal gold mineralization and related to continuing northwards movement of the Indian Plate that initiated the displacement on the strike‐slip Sagaing Fault.
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The Koryu gold-silver mine is located approximately at 141°19' of east longitude and 42°51' of north latitude in southwestern Hokkaido. The ore deposits are of epithermal gold-silver-quartz vein type, and they are composed of the No. 1-No. 7 veins hosted by the Miocene black mudstone. In this study, we have determined homogenization and ice melting temperatures of fluid inclusions in quartz, and chemical compositions of some ore minerals by microprobe analyses for the interpretation of mechanism of gold-silver deposition in the two veins (No. 1 and No. 3). Three stages of mineralization are discriminated on the basis of brecciation and cross-cutting relations in the vein, and gold-silver deposition occurred predominantly during the middle stage. The ores ordinarily exhibit banded structure composed of quartz, ±adularia, ±clay minerals, ±carbonates, and gold-silver and other metallic minerals (ginguro). The ore minerals are electrum (44-74 atom.%Ag), aguilarite (Se/(S+Se) atomic ratio : 0.25-0.51), pyrargyrite-proustite, pearceite-polybasite, miargyrite, Ag-bearing tetrahedrite (22-24 wt.%Ag) , galena, sphalerite (0.6-1.2 mole% FeS), chalcopyrite and pyrite etc. Ag content of electrum tends to decrease slightly toward the upper levels in the No. 3 vein, whereas FeS content of sphalerite is invariable between the upper and lower levels. Fluid inclusions observed in quartz of the middle stage are two phase and liquidrich type. Homogenization temperatures for the No. 1 vein are mainly 220°-270°C with high frequency in the range 250°-260°C. The temperatures for the No. 3 vein are about the same as those for the No. 1 vein, but show higher values (250°-280°C) at 30 mL. Ice melting temperatures of inclusion fluids for the No. 1 and No. 3 veins are from −0.4°C to −1.3°C, and from −0.5°C to −1.4°C, respectively. Spatial variations of these homogenization and ice melting temperatures, together with the data previously reported, would indicate that the relatively CO2-rich ore fluids had boiled on the way to form ore shoots.
Tetrahedrite
Gangue
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