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1.
The vein system in the Arinem area is a gold‐silver‐base metal deposit of Late Miocene (8.8–9.4 Ma) age located in the southwestern part of Java Island, Indonesia. The mineralization in the area is represented by the Arinem vein with a total length of about 5900 m, with a vertical extent up to 575 m, with other associated veins such as Bantarhuni and Halimun. The Arinem vein is hosted by andesitic tuff, breccia, and lava of the Oligocene–Middle Miocene Jampang Formation (23–11.6 Ma) and overlain unconformably by Pliocene–Pleistocene volcanic rocks composed of andesitic‐basaltic tuff, tuff breccia and lavas. The inferred reserve is approximately 2 million tons at 5.7 g t?1 gold and 41.5 g t?1 silver at a cut‐off of 4 g t?1 Au, which equates to approximately 12.5t of Au and 91.4t of Ag. The ore mineral assemblage of the Arinem vein consists of sphalerite, galena, chalcopyrite, pyrite, marcasite, and arsenopyrite with small amounts of pyrrhotite, argentite, electrum, bornite, hessite, tetradymite, altaite, petzite, stutzite, hematite, enargite, tennantite, chalcocite, and covellite. These ore minerals occur in quartz with colloform, crustiform, comb, vuggy, massive, brecciated, bladed and calcedonic textures and sulfide veins. A pervasive quartz–illite–pyrite alteration zone encloses the quartz and sulfide veins and is associated with veinlets of quartz–calcite–pyrite. This alteration zone is enveloped by smectite–illite–kaolinite–quartz–pyrite alteration, which grades into a chlorite–smectite–kaolinite–calcite–pyrite zone. Early stage mineralization (stage I) of vuggy–massive–banded crystalline quartz‐sulfide was followed by middle stage (stage II) of banded–brecciated–massive sulfide‐quartz and then by last stage (stage III) of massive‐crystalline barren quartz. The temperature of the mineralization, estimated from fluid inclusion microthermometry in quartz ranges from 157 to 325°C, whereas the temperatures indicated by fluid inclusions from sphalerite and calcite range from 153 to 218 and 140 to 217°C, respectively. The mineralizing fluid is dilute, with a salinity <4.3 wt% NaCl equiv. The ore‐mineral assemblage and paragenesis of the Arinem vein is characteristically of a low sulfidation epithermal system with indication of high sulfidation overprinted at stage II. Boiling is probably the main control for the gold solubility and precipitation of gold occurred during cooling in stage I mineralization.  相似文献   

2.
Abstract: Characterization of Neogene magmatism in the Ryuo mine area in the Kitami metallogenic province was carried out on the basis of K-Ar data for felsic–to–mafic terrestrial extrusive and intrusive volcanism from Late Miocene to Early Pliocene. The Ryuo epithermal gold-silver deposit occurs primarily in the felsic volcaniclastic rocks of the Ikutahara Formation and in Ryuo Rhyolite. The Ryuo mineralization age of 7. 7 – 8. 1 Ma coincides well with the hydrothermal alteration age (7. 7 Ma) of Ryuo Rhyolite hosting ore veins. It is concluded that the Ryuo mineralization was essentially accompanied by felsic volcanic activity during the sedimentation of the Ikutahara Formation, and was closely related both temporally and spatially to the intrusive activity of Ryuo Rhyolite. Hydrothermal alteration related to the epithermal gold-silver mineralization of the Ryuo deposit is primarily characterized by early regional and vein-related alterations, and late steam-heated alteration. Early regional alteration consists of a smectite halo (smectite+pyrite±quartz±opal–CT±mordenite°Clinoptilolite–heulandite series mineral). Early vein-related alteration is primarily marked by potassic alteration. This alteration halo can be subdivided into a K-feldspar halo (quartz+adular–ia+pyrite±illite±interstratified illite/smectite±smectite), an illite halo (quartz+illite + chlorite + pyrite ± interstratified illite/smec–tite±smectite) and an interstratified illite/smectite halo (quartz + interstratified illite/smectite+pyrite±smectite). Late steam-heated alteration characterized by kaolinite or alunite locally overprints the early K-feldspar halo. The style of the Ryuo gold-silver deposit is a low-sulfidation epithermal type. The gold–silver–bearing quartz vein precipitates during boiling of ore fluid. The origin of the ore fluid might be meteoric water. The temperature and sulfur fugacity conditions during precipitation of electrum and acanthite are estimated to be 206°– 238°C and 10-13.5 – 10-11.6 atm, respectively.  相似文献   

3.
The gold deposits at Kalgoorlie in the 2.7-Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, occur adjacent to the D2 Golden Mile Fault over a strike of 8 km within a district-scale zone marked by porphyry dykes and chloritic alteration. The late Golden Pike Fault separates the older (D2) shear zone system of the Golden Mile (1,500 t Au) in the southeast from the younger (D4) quartz vein stockworks at Mt Charlotte (126 t Au) in the northwest. Both deposits occur in the Golden Mile Dolerite sill and display inner sericite–ankerite alteration and early-stage gold–pyrite mineralization replacing the wall rocks. Late-stage tellurides account for 20 % of the total gold in the first, but for <1 % in the second deposit. In the Golden Mile, the main telluride assemblage is coloradoite?+?native gold (898–972 fine)?+?calaverite?+?petzite?±?krennerite. Telluride-rich ore (>30 g/t Au) is characterized by Au/Ag?=?2.54 and As/Sb?=?2.6–30, the latter ratio caused by arsenical pyrite. Golden Mile-type D2 lodes occur northwest of the Golden Pike Fault, but the Hidden Secret orebody, the only telluride bonanza mined (10,815 t at 44 g/t Au), was unusually rich in silver (Au/Ag?=?0.12–0.35) due to abundant hessite. We describe another array of silver-rich D2 shear zones which are part of the Golden Mile Fault exposed on the Mt Charlotte mine 22 level. They are filled with crack-seal and pinch-and-swell quartz–carbonate veins and are surrounded by early-stage pyrite?+?pyrrhotite disseminated in a sericite–ankerite zone more than 6 m wide. Gold grade (0.5–0.8 g/t) varies little across the zone, but Au/Ag (0.37–2.40) and As/Sb (1.54–13.9) increase away from the veins. Late-stage telluride mineralization (23 g/t Au) sampled in one vein has a much lower Au/Ag (0.13) and As/Sb (0.48) and comprises scheelite, pyrite, native gold (830–854 fine), hessite, and minor pyrrhotite, altaite, bournonite, and boulangerite. Assuming 250–300 °C, gold–hessite compositions indicate a fluid log f Te2 of ?11.5 to ?10, values well below the stability of calaverite. The absence of calaverite and the dominance of hessite in the D2 lodes of the Mt Charlotte area point to a kilometer-scale mineral and Au/Ag zonation along the Golden Mile master fault, which is attributed to a lateral decrease in peak tellurium fugacity of the late-stage hydrothermal fluid. The As/Sb ratio may be similarly zoned to lower values at the periphery. The D4 gold–quartz veins constituting the Mt Charlotte orebodies represent a younger hydrothermal system, which did not contribute to metal zonation in the older one.  相似文献   

4.
《Ore Geology Reviews》2009,35(4):580-596
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   

5.
巴达铜金矿位于藏东富碱斑岩带南段,是藏东地区近年来新发现的大型铜金矿。虽然对巴达铜金矿开展了大量勘查工作,但对该矿床的成因尚未取得共识。本文基于详细的野外调研、岩心与坑道编录及系统的镜下鉴定,对巴达铜金矿床地质特征进行研究。巴达矿床主要产于石英二长斑岩中,局部产于斑岩和砂岩地层的接触带内。矿床发育的围岩蚀变主要为青磐岩化、钾化、绢英岩化,高岭土化、蛋白石化、蒙脱石化次之,蚀变分带从内向外依次为钾硅酸盐化带、绢英岩化带、青磐岩化带、高岭土化带,铜金矿体主要赋存于钾硅酸盐化和绢英岩化带内,铜矿化主要以黄铜矿形式产出,金矿化主要以银金矿形式产于白云石±石英+细粒黄铁矿±黄铜矿脉中,铜矿化与金矿化呈正相关,矿体的产出受北西向逆冲断层的控制。与典型斑岩和浅成低温热液矿床不同,巴达铜金矿化主要产于白云石±石英+黄铁矿脉中;矿床内既发育碳酸盐、伊利石、绢云母和黄铁矿、黄铜矿、方铅矿、黝铜矿、低FeS闪锌矿等一套中硫型浅成低温热液矿床的蚀变矿物组合,又发育符合碱性斑岩系统的特征矿物赤铁矿。基于以上特征判断,巴达铜金矿矿床成因类型应为与富碱斑岩有关的浅成低温热液矿床,巴达铜金矿矿床成因的厘定,为下一步找矿提供了理论指导。  相似文献   

6.
Middle–Late Jurassic bimodal volcanism, typical of a retroarc setting, developed during widespread extensional tectonism within the Deseado Massif, southern Argentina. This geologic environment led to the formation of numerous low-sulfidation epithermal deposits that are spatially and temporally related to the volcanic activity. The lack of significant high-sulfidation epithermal deposits may be because the tectonic and volcanic settings do not favor the formation of these types of deposits. El Dorado-Monserrat is a low-sulfidation epithermal prospect located near the southern boundary of the Deseado Massif. Mineralization is genetically linked to the Late Jurassic Chon Aike Formation and hosted by volcanic rocks of the middle Late Jurassic Bajo Pobre Formation. Two different mineralization areas have been identified. The Monserrat area is the most important, with veins hosted in a north-striking, left-lateral shear zone. The average thickness is 0.85 m, and the average metal content is 6.2 ppm gold and 153 ppm silver. The El Dorado area has discontinuous echelon veins within a right-lateral shear zone with low gold and silver grades. Hydrothermal alteration of the host rocks includes an inner zone of quartz-adularia and illite alteration and an outer zone of propylitic alteration. The main gangue mineral is quartz, which formed in successive pulses, plus adularia, pyrite, hematite, magnetite, and barite. Precious metals occur as zoned electrum. Ore mineral precipitation took place between 200 and 280 °C from low salinity fluids due to boiling.  相似文献   

7.
Orogenic or mesothermal quartz lodes in lower Palaeozoic Greenland Group metasedimentary rocks of the Reefton area have produced 67 tonnes (t) of gold prior to 1951, and recent exploration has identified new gold resources in several deposits, including the largest past producers, Blackwater and Globe-Progress. The metasedimentary rocks consist of alternating sandstone and mudstone beds that were metamorphosed to lower greenschist facies prior to being hydrothermally altered adjacent to the quartz lodes. The sandstones are feldspathic litharenites averaging Q65-F10-R25, with detrital grains of quartz, rock fragments, muscovite, and plagioclase and biotite that were altered to albite and chlorite, respectively, during metamorphism. Accessory minerals are graphite, apatite, zircon, tourmaline and titanite. Hydrothermal alteration of the sandstones has developed a mineral assemblage of K-mica, carbonate (dolomite, ankerite, ferroan magnesite and magnesian siderite), chlorite, pyrite and arsenopyrite. The abundance of hydrothermal chlorite is greater at Blackwater than at the other prospects studied. Hydrothermal alteration associated with the quartz lodes is marked by bleaching, magnesian siderite spots, disseminated arsenopyrite and pyrite and thin carbonate, quartz and sulphide veins. These trends are accompanied by increasing concentrations of S, As and Sb and decreasing Na, and a decrease of Fe and Mg in K-mica. The alkali alteration indices 3K/Al (representing K-mica) and Na/Al (representing albite) generally show antipathetic trends, with 3K/Al increasing near the lodes and Na/Al decreasing. These trends reflect the replacement of albite by K-mica. Carbonate alteration indices CO2/(Ca + Mg +Fe) and CO2/[Ca + Mg + Fe -0.5(S + As)] quantify the abundance of hydrothermal carbonates, but they show variable correlation with the lodes. They increase the width of the alteration halo in the hanging wall of the lodes at the Globe-Progress and General Gordon prospects, but the peak values are as far as 150 m from the lodes. By contrast, peak values of the carbonate alteration indices are within 10 and 2 m of the lodes, respectively, at the Merrijigs and Blackwater deposits. Data show that for deposits with wide hydrothermal alteration halos, such as at the Globe-Progress and General Gordon prospects, the use of a suite of geochemical indicators can assist exploration by indicating trends in hydrothermal alteration that provide vectors to mineralisation. They also increase the size of the exploration target. By contrast, the alteration halo of the Blackwater deposit is restricted to within less than 5 m of the quartz lode and, therefore, the geochemical indicators are of more limited assistance to exploration.  相似文献   

8.
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   

9.
The Kay Tanda epithermal Au deposit in Lobo, Batangas is one of the Au deposits situated in the Batangas Mineral District in southern Luzon, Philippines. This study aims to document the geological, alteration, and mineralization characteristics and to determine the age of the mineralization, the mechanism of ore deposition, and the hydrothermal fluid characteristics of the Kay Tanda deposit. The geology of Kay Tanda consists of (i) the Talahib Volcanic Sequence, a Middle Miocene dacitic to andesitic volcaniclastic sequence that served as the host rock of the mineralization; (ii) the Balibago Diorite Complex, a cogenetic intrusive complex intruding the Talahib Volcanic Sequence; (iii) the Calatagan Formation, a Late Miocene to Early Pliocene volcanosedimentary formation unconformably overlying the Talahib Volcanic Sequence; (iv) the Dacite Porphyry Intrusives, which intruded the older lithological units; and (v) the Balibago Andesite, a Pliocene postmineralization volcaniclastic unit. K‐Ar dating on illite collected from the alteration haloes around quartz veins demonstrated that the age of mineralization is around 5.9 ± 0.2 to 5.5 ± 0.2 Ma (Late Miocene). Two main styles of mineralization are identified in Kay Tanda. The first style is an early‐stage extensive epithermal mineralization characterized by stratabound Au‐Ag‐bearing quartz stockworks hosted at the shallower levels of the Talahib Volcanic Sequence. The second style is a late‐stage base metal (Zn, Pb, and Cu) epithermal mineralization with local bonanza‐grade Au mineralization hosted in veins and hydrothermal breccias that are intersected at deeper levels of the Talahib Volcanic Sequence and at the shallower levels of the Balibago Intrusive Complex. Paragenetic studies on the mineralization in Kay Tanda defined six stages of mineralization; the first two belong to the first mineralization style, while the last four belong to the second mineralization style. Stage 1 is composed of quartz ± pyrophyllite ± dickite/kaolinite ± diaspore alteration, which is cut by quartz veins. Stage 2 is composed of Au‐Ag‐bearing quartz stockworks associated with pervasive illite ± quartz ± smectite ± kaolinite alteration. Stage 3 is composed of carbonate veins with minor base metal sulfides. Stage 4 is composed of quartz ± adularia ± calcite veins and hydrothermal breccias, hosting the main base metal and bonanza‐grade Au mineralization, and is associated with chlorite‐illite‐quartz alteration. Stage 5 is composed of epidote‐carbonate veins associated with epidote‐calcite‐chlorite alteration. Stage 6 is composed of anhydrite‐gypsum veins with minor base metal mineralization. The alteration assemblage of the deposit evolved from an acidic mineral assemblage caused by the condensation of magmatic volatiles from the Balibago Intrusive Complex into the groundwater to a slightly acidic mineral assemblage caused by the interaction of the host rocks and the circulating hydrothermal waters being heated up by the Dacite Porphyry Intrusives to a near‐neutral pH toward the later parts of the mineralization. Fluid inclusion microthermometry indicates that the temperature of the system started to increase during Stage 1 (T = 220–250°C) and remained at high temperatures (T = 250–290°C) toward Stage 6 due to the continuous intrusion of Dacite Porphyry Intrusives at depth. Salinity slightly decreased toward the later stages due to the contribution of more meteoric waters into the hydrothermal system. Boiling is considered the main mechanism of ore deposition based on the occurrence of rhombic adularia, the heterogeneous trapping of fluid inclusions of variable liquid–vapor ratios, the distribution of homogenization temperatures, and the gas ratios obtained from the quantitative fluid inclusion gas analysis of quartz. Ore mineral assemblage and sulfur fugacity determined from the FeS content of sphalerite at temperatures estimated by fluid inclusion microthermometry indicate that the base metal mineralization at Kay Tanda evolved from a high sulfidation to an intermediate sulfidation condition.  相似文献   

10.
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   

11.
Detailed hydrothermal alteration investigations, including petrography, infrared reflectance spectroscopy (IRS) and XRD of the low sulfidation epithermal Co–O mine, located in Eastern Mindanao (Philippines) revealed that both distal and intermediate hydrothermal alteration zones contain dominantly illite and chlorite, whereas the proximal alteration zone comprises mainly illite, chalcopyrite and pyrite. The gold-bearing veins and the proximal hydrothermal alteration zone display a distinct absence of K-rich hydrothermal alteration minerals such as K-feldspar (adularia).Gold mineralization in the Co–O mine is controlled by an extensive quartz-breccia vein system, which is characterized by three distinct stages of vein (incl. breccias) formation. Gold is mainly observed in stages 2 and 3 veins. Stage 1 veins appear as fragments in stage 2 veins and display boiling textures such as quartz pseudomophs after bladed calcite. These veins further display colloform to crustiform banding and contain pyrite, chalcopyrite and minor gold located in the colloform bands and between bladed quartz pseudomorphs. Stage 2 veins comprise mostly banded to massive quartz and contains sulfides parallel to bands or disseminated. These veins are fine-grained with mosaic/jigsaw quartz and contain calcite blebs and/or fragments of stage 1 veins. Gold is in textural equilibrium with chalcopyrite, sphalerite, and locally pyrite. Stage 3 veins consist of quartz and carbonate (locally Mn-rich), and display irregular banded and comb textures. In auriferous veins of this stage gold is in textural equilibrium with chalcopyrite and pyrite (with local abundance of sphalerite). Other sulfide minerals observed with gold in stages 2 and 3 are galena, acanthite and locally jalpaite.The XRD and IRS provide inconsistent results regarding the abundance of K-rich clays (e.g., illite) associated with auriferous veins. Illite, with possibly interlayered swelling clays, such as Al-smectite, was identified in auriferous vein stages 2 and 3 using IRS, but could not be confirmed by XRD. Comparative analysis of the results of these techniques with respect to the ordering of micaceous minerals, suggest less ordered white mica proximal to the veins.Vein textures such as banded quartz, the absence of K-feldspar and the abundance of illite (interlayered Al-smectite) suggest relatively low temperatures of formation of the hydrothermal alteration system and point to a potential boiling horizon located deeper or marginal to the currently exploited levels of the Co–O mine. The absence of K-feldspar may also be related to relatively low temperatures of the hydrothermal fluid, the medium potassium-rich magma series of the host rocks, and/or a relatively low oxidation state of the hydrothermal fluid.  相似文献   

12.
Middle Miocene (11.18–10.65 Ma) low sulfidation‐type epithermal gold mineralization occurred in the Cibaliung area, southwestern part of Java Island, Indonesia. It is hosted by andesitic to basaltic andesitic lavas of the Middle Miocene Honje Formation (11.4 Ma) and is covered by Pliocene Cibaliung tuff (4.9 Ma). The exploration estimates mineral resource of approximately 1.3 million tonnes at 10.42 g/t gold and 60.7 g/t silver at a 3 g/t Au cut‐off. This equates to approximately 435,000 ounces of gold and 2.54 million ounces of silver. That resource resulted from two ore shoots: Cibitung and Cikoneng. Studies on ore mineralogy, hydrothermal alteration, geology, fluid inclusion, stable isotopes and age dating were conducted in order to characterize the deposit and to understand a possible mechanism of preservation of the deposit. The ore mineral assemblage of the deposit consists of electrum, naumannite, Ag‐Se‐Te sulfide minerals, chalcopyrite, pyrite, sphalerite and galena. Those ore minerals occur in quartz veins showing colloform–crustiform texture. They are enveloped by mixed layer clay illite/smectite zone, which grades into smectite zone outward. The temperature of mineralization revealed by fluid inclusion study on quartz in the veins ranges from 170 and 220°C at shallow and deep level, respectively. The temperature range is in agreement with the temperature deduced from the hydrothermal alteration mineral assemblage including mixed layered illite/smectite and laumontite. The mineralizing fluid is dilute, with a salinity <1 wt% NaCl equivalent and has stable isotopes of oxygen and hydrogen composition indicating a meteoric water origin. Although the deposit is old enough that it would have been eroded in a tropical island arc setting, the coverage by younger volcanic deposits such as the Citeluk tuff and the Cibaliung tuff most probably prevented this erosion.  相似文献   

13.
The Mount Lyell copper deposits are located in the middle Cambrian Mount Read volcanic belt of western Tasmania and consist of more than 24 separate copper–gold–silver orebodies. The dominant copper mineralisation style is disseminated pyrite–chalcopyrite subvertical pipes with subordinate chalcopyrite–bornite ± other copper phases, massive pyrite and base metal sulfides. A zonation in mineralisation style within the pipes is defined from chalcopyrite–magnetite at depth to chalcopyrite–pyrite at intermediate levels, to chalcopyrite–bornite at the shallowest level. Alteration is developed broadly symmetrically around the ore zones and zoned from quartz–chlorite–phengite ± biotite at depth to quartz–muscovite at intermediate levels, and a quartz–muscovite–pyrophyllite–zunyite assemblage at the shallowest levels. This is interpreted to be a result of a fluid that evolved from hot, reduced and neutral conditions at depth to cool, oxidised and acidic conditions at the shallowest level. The chalcopyrite–bornite deposits occur at the top of the hydrothermal system and are associated with intensely silicified rock and muscovite/pyrophyllite alteration. The close relationship of these deposits with the top of the pipes suggests they are part of a single mineralising event. Where the chalcopyrite–bornite deposits are juxtaposed with the Owen Group, rather than a simple chalcopyrite–bornite mineralogy, there are numerous other copper phases, which represent higher oxidation states and collectively suggest variable and fluctuating fluid conditions during deposition. It is proposed that these deposits are formed by an interaction of the reduced hydrothermal fluid with an oxidised fluid generated at very shallow levels within and during deposition of the Owen Group. Mineralisation within the middle Owen Group sandstones and clasts of altered rock within the middle and upper Owen Group sediments marks the end of the hydrothermal system. Around the entire edge of the Mt Lyell field, there is a variation in the white mica composition from proximal muscovite to distal phengite that represents the neutralisation of the hydrothermal fluid by fluid–wall rock interaction.  相似文献   

14.
The Sawuershan region, one of the important gold metallogenic belts of Xinjiang, is located in the western part of the Kalatongke island arc zone of north Xinjiang, NW China. There are two gold deposits in mining, namely the Kuoerzhenkuola and the Buerkesidai deposits. Gold ores at the Kuoerzhenkuola deposit occur within Carboniferous andesite and volcanic breccias in the form of gold‐bearing quartz–pyrite veins and veinlet groups containing native gold, electrum, pyrite, pyrrhotite and chalcopyrite. Gold ores at the Buerkesidai deposit occur within Carboniferous tuffaceous siltstones in the form of gold‐bearing quartz veinlet groups and altered rocks, with electrum, pyrite and arsenopyrite as major metallic minerals. Both gold deposits are hosted by structurally controlled faults associated with intense hydrothermal alteration. The typical alteration assemblage is sericite + chlorite + calcite + quartz, with an inner pyrite–sericite zone and an outer chlorite–calcite–epidote zone between orebodies and wall rocks. δ34S values (0.3–1.3‰) of pyrite of ores from Kuoerzhenkuola deposit are similar to those (0.4–2.9‰) of pyrite of ores from Buerkesidai deposit. δ34S values (1.1–2.8‰) of pyrite from altered rocks are similar to δ34S values of magmatic or igneous sulfide sulfur, but higher than those from ores. 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb data of sulfide from ores range within 17.72–18.56, 15.34–15.61, and 37.21–38.28, respectively. These sulfur and lead isotope compositions imply that ore‐forming materials might originate from multiple, mainly deep sources. He and Ar isotope study on fluid inclusions of pyrites from ores of Kuoerzhenkuola and Buerkesidai gold deposits produces 40Ar/36Ar and 3He/4He ratios in the range of 282–525 and 0.6–9.4 R/Ra, respectively, indicating a mixed source of deep‐seated magmatic water (mantle fluid) and shallower meteoric water. In terms of tectonic setting, the gold deposits in the Sawuershan region can be interpreted as epithermal. These formations resulted from a combination of protracted volcanic activity, hydrothermal fluid mixing, and a structural setting favoring gold deposition. Fluid mixing was possibly the key factor resulting in Au deposition in the gold deposits in Sawuershan region.  相似文献   

15.
Gold mineralization in the Velvet District occurs in an eastward dipping sequence of late Tertiary rhyolitic ash-flow tuffs, flows, and tuffaceous sediments in northwestern Nevada. Minor gold and silver concentrations are associated with irregular zones of brecciation, argillic alteration, and quartz veining along north-northeast trending normal faults. Reaction of mineralizing fluids with wallrock produced an argillic alteration assemblage of illite, mixed-layer clays, smectite, and kaolinite. Illite alteration and highest gold concentrations appear to be associated with zones of high water/rock ratios. Kaolinite, smectite, alunite, and opal are postulated to have formed during a steam-dominated episode of alteration.Fluid inclusion studies indicate that the quartz veins were deposited in the temperature range 230 to 280°C from fluids which had salinities equivalent to 0.2–0.8 weight percent NaCl. δ 18O of quartz veins varies from ?2.5 to +6.7 ‰ and indicates that the ore fluid must have been Tertiary meteroric water. Stable isotope data appear to define a zone of concentrated fluid flow and potential subsurface mineralization in the southeastern part of the district. Fluid inclusion and isotope studies can be used in combination with more standard geochemical, geophysical, and geological information to provide site-specific targets for epithermal metal concentrations.  相似文献   

16.
Abstract. The Nena Cu‐Au deposit, located in the Frieda River mineral district of northwestern mainland Papua New Guinea, is a composite structurally‐lithologically controlled high sulfidation (HS) system. Its hydrothermal alteration and Cu‐Au mineralization are presented in this paper. Initially propylitized andesitic volcanics veined by epithermal quartz were pervasively superimposed by zoned HS alteration. The zonation grades from vuggy silica core to sulfur‐rich, pyritic silica‐alunite halo followed by pyrophyllite‐dickite‐kaolinite interval and finally to thin illite‐smectite margin, suggesting progressive decrease in temperature and increase in pH. This zonation is enveloped by chlorite‐epidote‐calcite‐gypsum alteration. The acid altered rocks were then invaded by multiple phases of pyrite, subsequently crosscut by quartz, vein alunite and barite. Then sequential deposition of bladed covellite, enargite, luzonite and stibioluzonite occurred from the NW to the SE portions of the deposit, forming a zonation suggestive of progressive decrease in temperature, sulfur fugacity and sulfidation stage. Most ore mineralization occurs in the vuggy silica core. Gold mineralization commenced from the transition of enargite to luzonite and continued throughout the stibioluzonite stage. Associated with gold deposition are Au‐rich pyrite, tennantite‐tetrahedrite, chalcopyrite‐bornite, native tellurium, electrum, calaverite, bismuthinite and galena. Native sulfur occupied the remaining cavities and represents the waning stage of the hydrothermal system. Fluid inclusions studies distinguished magmatic (>300–350d?C, 9–15 wt% NaCl equiv.) and meteoric (<150–200d?C, 1–2 wt% NaCl equiv.) fluids (Holzberger et al., 1996). Temperatures and salinities of fluid inclusions from barite associated with Cu sulfides show a general decrease from NW (330d?C, 9–15 wt% NaCl equiv.) to SE (172d?C, 10 wt% NaCl equiv.) parts of the deposit, indicating gradual entrainment of ground water (Hitchman and Espi, 1997). Interaction of magmatic fluids with meteoric water accompanied by changes in temperature, salinity, acidity and oxidation state of the resultant fluids is interpreted to have been the main cause of metal precipitation. Finally, supergene processes generated Au zone with an underlying chalcocite‐covellite‐digenite blanket over the primary sulfides at depth. Gold occurs as lattice constituent in scorodite, limonite‐goethite and jarosite. Chalcocite is more abundant and widespread than other Cu sulfides. Acidic fluids deposited powdery alunite and kaolinite, vein alunite and amorphous silica. Weakly secondary biotite‐quartz altered porphyry located below the known HS Cu‐Au deposit contains chalcopyrite‐bornite and is overprinted by quartz‐alunite‐pyro‐phyllite‐pyrite assemblage. This feature indicates close temporal, spatial and genetic relation between the two deposit types.  相似文献   

17.
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   

18.
The Bilimoia deposit (2.23 Mt, 24 g/t Au), located in the eastern Central Mobile Belt of mainland Papua New Guinea, is composed of fault‐hosted, NW–NNW‐trending Irumafimpa–Kora and Judd–Upper Kora Au‐quartz veins hosted by Middle–Late Triassic basement that was metamorphosed to medium‐grade greenschist facies between Middle–Late Triassic and Early–Middle Jurassic. Mineralizing fluids were introduced during crustal thickening, rapid uplift, change of plate motions from oblique to orthogonal compression, active faulting and S3 and S4 events in an S1–S4 deformation sequence. The Bilimoia deposit is spatially and temporally related to I‐type, early intermediate to felsic and late mafic intrusions emplaced in Late Miocene (9–7 Ma). Hydrothermal alteration and associated mineralization is divided into 10 main paragenetic stages: (1) chlorite–epidote‐selvaged quartz–calcite–specularite vein; (2) local quartz–illite–pyrite alteration; (3) quartz–sericite–mariposite–fuchsite–pyrite wall‐rock alteration that delimits the bounding shears; (4) finely banded, colloform‐, crustiform‐ and cockade‐textured and drusy quartz ± early wolframite ± late adularia; (5) hematite; (6) pyrite; (7) quartz ± amethyst‐base metal sulfides; (8) quartz–chalcopyrite–bornite–Sn and Cu sulfides–Au tellurides and Te ± Bi ± Ag ± Cu ± Pb phases; (9) Fe ± Mn carbonates; and (10) supergene overprint. Fluid inclusions in stage 4 are characterized by low salinity (0.9–5.4 wt% NaCl equivalent), aqueous–carbonic fluids with total homogenization temperatures ranging from 210 to 330°C. Some of the inclusions that homogenized between 285 and 330°C host coexisting liquid‐ and vapor‐rich (including carbonic) phases, suggesting phase separation. Fluid inclusions in quartz intergrown with wolframite have low salinity (0.9–1.2 wt% NaCl equivalent), aqueous–carbonic fluids at 240–260°C, defining the latter’s depositional conditions. The ore fluids were derived from oxidized magmatic source initially contaminated by reduced basement rocks. Wall‐rock alteration and involvement of circulating meteoric waters were dominant during the first three stages and early part of stage 4. Stage 5 hematite was deposited as a result of stage 4 phase separation or entrainment of oxygenated groundwater. Gold is associated with Te‐ and Bi‐bearing minerals and mostly precipitated as gold‐tellurides during stage 8. Gold deposition occurred below 350°C due to a change in the sulfidation and oxidation state of the fluids, depressurization and decreasing temperature and activities of sulfur and tellurium. Bisulfides are considered to be the main Au‐transporting complexes. The Bilimoia deposit has affinities that are similar to many gold systems termed epizonal orogenic and intrusion‐related. The current data allow us to classify the Bilimoia deposit as a fault‐controlled, metamorphic‐hosted, intrusion‐related mesothermal to low sulfidation epithermal quartz–Au–Te–Bi vein system.  相似文献   

19.
The Asachinskoe epithermal Au‐Ag deposit is a representative low‐sulfidation type of deposit in Kamchatka, Russia. In the Asachinskoe deposit there are approximately 40 mineralized veins mainly hosted by dacite–andesite stock intrusions of Miocene–Pliocene age. The veins are emplaced in tensional cracks with a north orientation. Wall‐rock alteration at the bonanza level (170–200 m a.s.l.) consists of the mineral assemblage of quartz, pyrite, albite, illite and trace amounts of smectite. Mineralized veins are well banded with quartz, adularia and minor illite. Mineralization stages in the main zone are divided into stages I–IV. Stage I is relatively barren quartz–adularia association formed at 4.7 ± 0.2 Ma (K‐Ar age). Stage II consists of abundant illite, Cu‐bearing cryptomelane and other manganese oxides and hydroxides, electrum, argentite, quartz, adularia and minor rhodochrosite and calcite. Stage III, the main stage of gold mineralization (4.5–4.4 ± 0.1–3.1 ± 0.1 Ma, K‐Ar age), consists of a large amount of electrum, naumannite and Se‐bearing polybasite with quartz–adularia association. Stage IV is characterized by hydrothermal breccia, where electrum, tetrahedrite and secondary covellite occur with quartz, adularia and illite. The concentration of Au+Ag in ores has a positive correlation with the content of K2O + Al2O3, which is controlled by the presence of adularia and minor illite, and both Hg and Au also have positive correlations with the light rare‐earth elements. Fluid inclusion studies indicate a salinity of 1.0–2.6 wt% NaCl equivalent for the whole deposit, and ore‐forming temperatures are estimated as approximately 160–190°C in stage III of the present 218 m a.s.l. and 170–180°C in stage IV of 200 m a.s.l. The depth of ore formation is estimated to be 90–400 m from the paleo‐water table for stage IV of 200 m a.s.l., if a hydrostatic condition is assumed. An increase of salinity (>CNaCl≈ 0.2 wt%) and decrease of temperature (>T ≈ 30°C) within a 115‐m vertical interval for the ascending hydrothermal solution is calculated, which is interpreted as due to steam loss during fluid boiling. Ranges of selenium and sulfur fugacities are estimated to be logfSe2 = ?17 to ?14.5 and logfS2 = ?15 to ?12 for the ore‐forming solution that was responsible for Au‐Ag‐Se precipitation in stage III of 200 m a.s.l. Separation of Se from S‐Se complex in the solution and its partition into selenides could be due to a relatively oxidizing condition. The precipitation of Au‐Ag‐Se was caused by boiling in stage III, and the precipitation of Au‐Ag‐Cu was caused by sudden decompression and boiling in stage IV.  相似文献   

20.
The Wolyu mine is one of the largest vein-type gold-silver-bearing epithermal systems in the Youngdong district and is the first gold-silver deposit in Korea found to contain significant germanium, in the form of argyrodite (Ag8GeS6). Mineralized veins (78.9 ± 1.2 Ma) crosscutting Late Cretaceous hostrock tuff and quartz porphyry (81.5 ± 1.8 Ma) consist of three stages of quartz and carbonates, the first of which contains pyrite, basemetal sulfides and Au-Ag-minerals. Stage I Au-Ag-Ge-mineralized veins show a systematic variation of mineral assemblage with time: (1) quartz + pyrite; (2) quartz + pyrite + sphalerite + electrum + argentite; (3) carbonate + quartz + sphalerite + electrum + argentite; (4) carbonate + native silver + argentite + Ag-sulfosalts + argyrodite + sphalerite. Calculated values of temperature and sulfur activity are: assemblage (1), 360-280°C and 10–7-10–10; (2), 280-210°C and 10–10-10–14; (3), 210-180°C and 10–14-10–16; (4), 180-155°C and 10–17-10–18. These data, the frequent association of gold with sulfides, and the abundance of pyrite in alteration zones indicate that decreasing sulfur activity and cooling were important in triggering gold deposition. Hydrogen and oxygen isotope compositions of ore fluids display a systematic variation with increasing time. Within the main Ag-Au-Ge mineralization, D and 18O values decrease with the transition from quartz to carbonate deposition (from -78 and –2.8% to –90 and –8.7%., respectively), indicating increasing involvement (mixing) of less evolved meteoric water which resulted in progressive cooling and dilution of ore fluids in the shallow ( 370–600 m) Wolyu epithermal system.  相似文献   

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