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1.
The Ixtahuacan Sb-W deposits are hosted by upper Pennsylvanian to Permian metasedimentary rocks of the central Cordillera of Guatemala. The deposits consist of gold-bearing arsenopyrite, stibnite and scheelite. Arsenopyrite and scheelite are early in the paragenesis, occurring as disseminations in pyritiferous black shale/sandstone and in argillaceous limestone, respectively. Some stibnite is disseminated, but the bulk of the stibnite occurs as massive stratabound lenses in black shales and in quartz-ankerite veins and breccias, locally containing scheelite.Microthermometric measurements on fluid inclusions in quartz and scheelite point to a low temperature (160–190°C) and low to moderate salinity (5–15 wt% NaCl eq.) aqueous ore fluid. Abundant vapour-rich inclusions suggest that the fluid boiled. Carbon dioxide was produced locally as a result of interaction of the aqueous fluid with the argillaceous limestone. Bulk leaching experiments and SEM-EDS analyses of decrepitated fluid inclusion residues indicate that the ore-bearing solution was NaCl-dominated. The 18O values of quartz, ankerite and scheelite from mineralized veins range from 19.7 to 20.5, 18.1 to 20.0 and 7.0 to 8.4 respectively. The average temperature calculated from quartz-scheelite oxygen isotopic fractionation is 170°C. The oxygen isotopic composition of the fluid, interpreted to have been in equilibrium with these minerals, ranged from 5.7 to 7.6, and is considered to represent an evolved meteoric water. Diagenetic or syngenetic pyrite has a sulphur isotopic composition of 0.5±0.3 which is consistent with bacterial reduction of sulphate. The 34S values of arsenopyrite and stibnite range from –2.8 to 2.0 and –2.7 to –2.3 respectively, and are though to reflect sulphur derived from pyrite.The Ixtahuacan deposits are interpreted to have formed at low temperature (<200°C) and a depth of a few hundred metres from a low fO2 (10–49–10–57), high pH (7–8) fluid. Arsenic was probably transported as arsenious acid, antimony and gold as thio-complexes and tungsten as the complex HWO 4 – .A model is proposed in which a meteoric fluid, heated by a felsic intrusion at depth, was focused to shallow levels along faults. The interaction of the fluid with pyritiferous beds caused the deposition of arsenopyrite as a result of sulphidation and/or decreasing fO2; gold probably co-precipitated with As or was adsorbed onto the arsenopyrite. The precipitation of stibnite was caused by boiling. Scheelite deposited in response to the increase in Ca2+ activity which accompanied interaction of the ore fluid with the argillaceous limestones.  相似文献   

2.
余盼  郑义  王岳军  虞鹏鹏 《地学前缘》2018,25(5):266-276
湖南新宁县星子岩锑矿位于钦杭结合带中段湘中盆地内,脉状矿体受NE向硅化破碎带控制,直接容矿围岩为下寒武统香楠组碳质板岩和硅质板岩。文中对星子岩锑矿进行系统的矿物生成序列研究,将成矿过程分为两期3个阶段,即沉积期以碳质板岩内球状黄铁矿为代表,热液期早阶段脉状辉锑矿石英脉(局部夹围岩角砾)和晚阶段辉锑矿方解石脉。为进一步限定其成因,选取热液期与辉锑矿共生的早阶段石英和晚阶段的方解石进行流体包裹体研究。包裹体岩相学研究表明,热液期石英和方解石内广泛发育流体包裹体,主要类型为纯H2O型和CO2-H2O型流体包裹体。激光拉曼测试表明,CO2-H2O型包裹体气相主要成分为CO2,液相主要成分为H2O。显微测温过程中,当降温时可见CO2-H2O型包裹体由气液两相变为三相,其初熔温度为-60.0~-56.6 ℃,CO2部分均一温度为3.7~28.5 ℃,完全均一温度介于113~266 ℃,CO2笼合物熔化温度为3.0~9.6 ℃,对应的流体盐度w(NaCleqv.)为0.8%~12.0%。星子岩锑矿流体包裹体的中低温、低盐度和含CO2的特征与造山型金矿的变质流体成矿特征相吻合。综合星子岩锑矿的成矿大地构造背景、矿床地质和成矿流体特征,星子岩锑矿的精细成矿过程为：钦杭结合带在印支-燕山期发生强烈的陆壳叠置作用,富碳质含黄铁矿的寒武系地层发生变质变形脱挥发分作用,形成中低温、低盐度、富CO2和富集锑元素的变质热液;变质热液向上运移过程中,温度压力降低发生相分离,导致CO2逸失和流体沸腾作用,大量辉锑矿沉淀,最终形成有经济价值的矿体。湖南星子岩锑矿的矿床地质特征和成矿过程与造山型金矿一致,因此其成因类型为造山型锑矿。  相似文献   

3.
Coexisting, liquid-rich and vapor-rich primary fluid inclusions in quartz provide direct evidence for fluid phase separation in high-grade quartz–roscoelite–gold veins and breccias from the Porgera alkalic-type gold deposit. Vapor-rich fluid inclusions are CO2-rich, and sometimes contain liquid CO2 at room temperature. The close spatial and paragenetic relationship between these “boiling assemblage” fluid inclusions and gold suggests that gold was precipitated by phase separation, at least locally. Additionally, the occurrence of carbonate and sulfate minerals in high-grade veins (reflecting pH increase and oxidation of the boiled fluid) and the appearance of hydrothermal breccias, are consistent with the process of fluid phase separation. Liquid CO2-bearing fluid inclusions are rare in near-surface epithermal deposits, and indicate that the Porgera vein system was formed at greater depths and pressures (our estimates suggest pressures between 250 and 340 bars). It is suggested that alkalic-type gold deposits may be distinguished from other epithermal deposit types by the more gaseous nature of the ore-forming fluids, in addition to their association with alkalic magmas. Received: 24 February 2000 / Accepted: 6 April 2000  相似文献   

4.
Here we report the occurrence of some uncommon mineral assemblages including pääkönenite, aurostibite, native arsenic, native antimony, and native bismuth found in the Baogutu gold deposit in the western Junggar, Xinjiang, NW China. The mineralization could be generally subdivided into two types: the gold-bearing quartz-vein type mineralization and disseminated mineralization in the wall rocks. The sulfide minerals in gold lodes commonly include pyrite, arsenopyrite, marcasite, and stibnite. However, the L7 lode in No. 4 orebody and the L1 lode in No. 11 orebody of the Baogutu gold deposit are quite different in terms of their mineral assemblages. The L7 lode contains native arsenic–quartz veins in shallow levels and stibnite–quartz veins at depth. Gold-bearing minerals (electrum, native gold, and rarely aurostibite) mainly coexist with pääkönenite, stibnite, native arsenic, and native antimony. The crystallization of As- and Sb-bearing minerals was likely to have consumed H2S from the hydrothermal fluid, which probably triggered the precipitation of native gold. The L1 lode consists of several discontinuous sulfide-dominated lensoid orebodies. The massive sulfide ores that produced most of the gold resource are characterized by an intimate association between native bismuth and native gold mineralization.  相似文献   

5.
Epithermal gold deposits are typical precious metal deposits related to volcanic and subvolcanic magmatism.Due to the lack of direct geological and petrographic evidences,the origin of the ore-forming fluid is deduced from the spatial diagenesis-mineralization relationship,chronological data,physicochemical characteristics of mineral fluid inclusions,mineral or rock elements and isotopic geochemical characteristics.By objectively examining this scientific problem via a geological field survey and petrographic analysis of the Gaosongshan epithermal gold deposit,we recently discovered and verified the following points:(1)Pyrite-bearing spherical quartz aggregates(PSQA)occur in the rhyolitic porphyry;(2)the mineralization is structurally dominated by WNW-and ENE-trending systems and occurs mostly in hydrothermal breccias and pyrite-quartz veins,and the ore types are mainly hematite-crusted quartz,hydrothermal breccia,massive pyrite-quartz,etc.;(3)the alteration types consist of prevalent silicification,sericitization,propylitization and carbonation,with local adularization and illitization.The ore minerals are mainly pyrite,primary hematite,native gold,and electrum,with lesser amounts of chalcopyrite,magnetite,sphalerite,and galena,indicating a characteristic epithermal low-sulfidation deposit.The ore-forming fluid may have been primarily derived from magmatic fluid exsolved from a crystallizing rhyolitic porphyry magma.Further zircon U-Pb geochronology,fluid inclusion,physicochemical and isotopic geochemical analyses revealed that(1)rhyolitic porphyry magmatism occurred at 104.6 ± 1.0 Ma,whereas the crystallization of the PSQA occurred at 100.8 ± 2.1 Ma;(2)the hydrothermal fluid of the pre-ore stage was an exsolved CO_2-bearing H20-NaCl magmatic fluid that produced inclusions mainly composed of pure vapor(PV),vapor-rich(WV)and liquid-rich(WL)inclusions with a small number of melt-(M)and solid-bearing(S)inclusions;mineralization-stage quartz contains WL and rare PV,WV and pure liquid(PL)inclusions characterized by the H_2 O-NaCl system with low formation temperatures and low salinities;(3)the characteristics of hydrogen,oxygen,sulfur,and lead isotopes and those of rare earth elements(REEs)provide insight into the affinity between PSQA and orebodies resulting from juvenile crust or enriched mantle.Combined with previous research on the mineralogenetic epoch(99.32± 0.01 Ma),we further confirm that the mineralization of the deposit occurred in the late Early Cretaceous,which coincides with the extension of the continental margin induced by subduction of the Pacific Plate beneath the Eurasian Plate.The formation of the ore deposit was proceeded by a series of magmatic and hydrothermal events,including melting of enriched juvenile crust,upwelling,the eruption and emplacement of the rhyolitic magma,the exsolution and accumulation of magmatic hydrothermal fluid,decompression,the cooling and immiscibility/boiling of the fluid,and mixing of the magmatic fluid with meteoric water,in association with water-rock interaction.  相似文献   

6.
CO2 inclusions with density up to 1,197 kg m−3 occur in quartz–stibnite veins hosted in the low-grade Palaeozoic basement of the Gemericum tectonic unit in the Western Carpathians. Raman microanalysis corroborated CO2 as dominant gas species accompanied by small amounts of nitrogen (<7.3 mol%) and methane (<2.5 mol%). The superdense CO2 phase exsolved from an aqueous bulk fluid at temperatures of 183–237°C and pressures between 1.6 and 3.5 kbar, possibly up to 4.5 kbar. Low thermal gradients (∼12–13°C km−1) and the CO2–CH4–N2 fluid composition rule out a genetic link with the subjacent Permian granites and indicate an external, either metamorphogenic (oxidation of siderite, dedolomitization) or lower crustal/mantle, source of the ore-forming fluids.According to microprobe U–Pb–Th dating of monazite, the stibnite-bearing veins formed during early Cretaceous thrusting of the Gemeric basement over the adjacent Veporic unit. The 15- to 18-km depth of burial estimated from the fluid inclusion trapping PT parameters indicates a 8- to 11-km-thick Upper Palaeozoic–Jurassic accretionary complex overlying the Gemeric basement and its Permo-Triassic autochthonous cover.  相似文献   

7.
Gold-bearing quartz veins fill late-Alpine brittle structures in Pennine nappes of Austria (in the Tauern window) and in northern Italy. The veins formed in the latter stages of uplift of the Alps. Fluid inclusions in veins sampled from Böckstein, Austria, and Valle Anzasca, Italy have a wide variety of compositions, ranging from aqueous brine (about 5 wt% NaCl equiv.) to about 50 mol% CO2. At room temperature, the inclusions range with increasing CO2 content from two-phase aqueous, through three-phase in which the CO2 homogenizes to vapour, to three-phase with CO2 homogenizing to liquid. This wide range of inclusion compositions is interpreted as evidence for fluid immiscibility, with most inclusions being accidental mixtures of the two end-member immiscible fluids. The homogenization temperatures of the aqueous inclusions, 200–280°C, gives the best estimate of temperature of formation of the veins. Vein formation fluid pressure at Böckstein and Valle Anzasca was about 1 kbar, and Böckstein veins formed at lower pressure than Valle Anzasca veins. Fluid immiscibility may have contributed to deposition of gold at both Valle Anzasca and Böckstein, and possibly many other uplift-related Alpine gold localities.  相似文献   

8.
Gold mineralisation in classic Australian slate belt gold deposits at Ballarat, Bendigo, St. Arnaud and Inglewood occurred very late in the orogenic history of these rocks rather than during formation of the main slaty cleavage. This has been revealed through the examination of microstructural relationships in gold-bearing quartz veins and their host rocks from these deposits, which has established a D1 to D4 deformation-stage history and consistent timing for gold mineralisation over a wide area. The gold was deposited synorogenically but during the fourth deformation stage (D4) of the orogeny, a relatively weak event occurring two deformations after the main slaty cleavage producing event, D2. Previously, D2 had been regarded as both the source and control of gold mineralisation as most of the quartz veins that occur in these deposits formed before or during this deformation event. However, most gold is hosted in breccia veins that formed during D4. The wallrock clasts within these breccia veins contain a young rotated foliation and the breccia veins are spatially associated with a paragenetically consistent alteration of the host rocks in the deposits. This alteration both crosscuts and preferentially mineralises wallrock S4 allowing the timing of the breccia veins, alteration and gold deposition to be defined as syn-D4 in age.  相似文献   

9.
The Campbell-Red Lake gold deposit in the Red Lake greenstone belt, with a total of approximately 840 t of gold (past production + reserves) and an average grade of 21 g/t Au, is one of the largest and richest Archean gold deposits in Canada. Gold mineralization is mainly associated with silicification and arsenopyrite that replace carbonate veins, breccias and wallrock selvages. The carbonate veins and breccias, which are composed of ankerite ± quartz and characterized by crustiform–cockade textures, were formed before and/or in the early stage of penetrative ductile deformation, whereas silicification, arsenopyrite replacement and gold mineralization were coeval with deformation. Microthermometry and laser Raman spectroscopy indicate that fluid inclusions in ankerite and associated quartz (Q1) and main ore-stage quartz (Q2) are predominantly carbonic, composed mainly of CO2, with minor CH4 and N2. Aqueous and aqueous–carbonic inclusions are extremely rare in both ankerite and quartz. H2O was not detected by laser Raman spectroscopic analyses of individual carbonic inclusions and by gas chromatographic analyses of bulk samples of ankerite and main ore-stage quartz (Q2). Fluid inclusions in post-mineralization quartz (Q3) are also mainly carbonic, but proportions of aqueous and aqueous–carbonic inclusions are present. Trace amounts of H2S were detected by laser Raman spectroscopy in some carbonic inclusions in Q2 and Q3, and by gas chromatographic analyses of bulk samples of ankerite and Q2. 3He/4He ratios of bulk fluid inclusions range from 0.008 to 0.016 Ra in samples of arsenopyrite and gold. Homogenization temperatures (T h–CO2) of carbonic inclusions are highly variable (from −4.1 to +30.4°C; mostly to liquid, some to vapor), but the spreads within individual fluid inclusion assemblages (FIAs) are relatively small (within 0.5 to 10.3°C). Carbonic inclusions occur both in FIAs with narrow T h–CO2 ranges and in those with relatively large T h–CO2 variations. The predominance of carbonic fluid inclusions has been previously reported in a few other gold deposits, and its significance for gold metallogeny has been debated. Some authors have proposed that formation of the carbonic fluid inclusions and their predominance is due to post-trapping leakage of water from aqueous–carbonic inclusions (H2O leakage model), whereas others have proposed that they reflect preferential trapping of the CO2-dominated vapor in an immiscible aqueous–carbonic mixture (fluid unmixing model), or represent an unusually H2O-poor, CO2-dominated fluid (single carbonic fluid model). Based on the FIA analysis reported in this study, we argue that although post-trapping modifications and host mineral deformation may have altered the fluid inclusions in varying degrees, these processes were not solely responsible for the formation of the carbonic inclusions. The single carbonic fluid model best explains the extreme rarity of aqueous inclusions but lacks the support of experimental data that might indicate the viability of significant transport of silica and gold in a carbonic fluid. In contrast, the weakness of the unmixing model is that it lacks unequivocal petrographic evidence of phase separation. If the unmixing model were to be applied, the fluid prior to unmixing would have to be much more enriched in carbonic species and poorer in water than in most orogenic gold deposits in order to explain the predominance of carbonic inclusions. The H2O-poor, CO2-dominated fluid may have been the product of high-grade metamorphism or early degassing of magmatic intrusions, or could have resulted from the accumulation of vapor produced by phase separation external to the site of mineralization.Geological Survey of Canada contribution 2004383.  相似文献   

10.
The Jiaodong peninsula contains the most important concentration of gold deposits in China, which can be divided into Jiaojia-type and Linglong-type deposits based on mineralization style. The former is characterized by disseminated- and stockwork-style mineralization hosted in first-order regional faults, with relatively larger tonnages and lower gold grades. The latter is characterized by massive auriferous quartz veins commonly hosted in subsidiary second- or third-order faults, with smaller tonnage but higher grade orebodies. Despite these differences, both groups of deposits have the same alteration assemblages, mineral paragenesis, element concentrations, and ore-forming ages.The mainly Jiaojia-type Luoshan gold deposit and the mainly Linglong-type Fushan gold deposit are characterized by H-O-S-Pb isotope data that indicate the ore-forming fluids have a dominantly metamorphic source. The fluids were derived during the Yanshanian orogenic event, and were most likely associated with dehydration and decarbonization processes near the top of the subducting paleo-Pacific plate. The Linglong-type ores have relatively lighter calculated δ18O compositions (−3.9 to −2.3‰) than the Jiaojia-type ores (0.3–8.0‰), possibly because of a greater degree of mixing with meteoric water. Petrographic, cathodoluminescence, microthermometric, and laser Raman spectroscopic analyses of fluid-inclusion assemblages in quartz from the two types of ores indicate fluids were similar, in both cases characterized by medium–high homogenization temperatures (211–393 °C), significant CO2 (∼15% mol), minor CH4 (⩽18% in the carbonic phase), and low salinity (⩽11.2 wt% NaCl eq.). The Linglong-type ores, however, have a wider range of CO2 and CH4 concentration and salinity than the Jiaojia-type ores. Fluid immiscibility, occurred in main ore stage of both ore types, with the trapping conditions of 77–185 MPa and 284–328 °C, although the unmixing is more intense and widespread in the Linglong-type ores. Both fluid-wallrock interaction and fluid immiscibility are important gold-deposition processes in the two types, but immiscibility is more important in the Linglong-type ores and that has led to the typical higher gold grade.In general, there is little geochemical differences between the ore-forming fluids for Jiaojia- and Linglong-type gold deposits. Both Jiaojia- and Linglong-type ores can exist in a single deposit and form in the same metallogenic event. The Linglong-type ores developed as more massive veins, because of their location in zones of more extensive extension and they lack significant post-ore cataclastic deformation.  相似文献   

11.
蒋少涌  徐耀明  周巍  朱志勇  孔凡斌  孙岩 《岩石学报》2012,28(10):3076-3086
在九瑞矿集区研究叠合断裂和叠加成矿作用的基础上,我们进一步详细研究了出露在洋鸡山-丁家山-望夫山一线的硅质角砾岩,指出它们不是原先认为的石炭系沉积硅质岩,而应属于一种断裂磨砾岩,并深入探究其形成过程及与成矿之关系.断裂磨砾岩是断裂分带结构成熟的标志之一,多在剪切作用和热液作用下,断裂岩石经硅化-破裂-碎裂-粉碎-研磨,形成具有一定圆度和球度,大小差异较大的磨砾或磨粒,且又会反复的集结-破碎,不断拓宽断裂构造形成磨砾-角砾岩带.本文研究的断裂磨砾岩,呈北东向展布,延长达十几千米.成分上以硅化角砾岩为主,SiO2含量一般大于90％,石英颗粒由隐晶到显晶.一些角砾岩中含Fe2O3较高,有可能是原先的硫化物经氧化形成的褐铁矿.本区洋鸡山-丁家山-望夫山一线产出的断裂磨砾-角砾岩带,很可能是燕山期构造-岩浆-成矿事件的产物.在城门山和武山铜矿,我们之前的工作己发现存在产于泥盆系五通组和石炭系黄龙组层滑构造体系中的黄铁矿角砾岩,则有可能属于海西期同生断裂活动的产物.因此,这些不同的角砾岩具有多阶段活动和叠加成矿的特征.本文还进一步指出,九瑞地区其他层位(如泥盆系与志留系之间、志留系与奥陶系之间)发育的层滑构造体系和断裂角砾岩及热液蚀变岩,也很可能是成矿有利部位,值得今后找矿工作的关注.  相似文献   

12.
The Salu Bulo prospect is one of the gold prospects in the Awak Mas project in the central part of the western province, Sulawesi, Indonesia. The gold mineralization is hosted by the meta‐sedimentary rocks intercalated with the meta‐volcanic and volcaniclastic rocks of the Latimojong Metamorphic Complex. The ores are approximately three meters thick, consisting of veins, stockwork, and breccias. The veins can be classified into three stages, namely, early, main, and late stages, and gold mineralization is related to the main stage. The mineral assemblage of the matrix of breccia and the veins are both composed of quartz, carbonate (mainly ankerite), and albite. High‐grade gold ores in the Salu Bulo prospect are accompanied by intense alteration, such as carbonatization, albitization, silicification, and sulfidation along the main stage veins and breccia. Alteration mineral assemblage includes ankerite ± calcite, quartz, albite, and pyrite along with minor sericite. Pyrite is the most abundant sulfide mineral that is spatially related to native gold and electrum (<2–42 μm in size). It is more abundant as dissemination in the altered host rocks than those in veins. This suggests that water–rock interaction played a role to precipitate pyrite and Au in the Salu Bulo prospect. The Au contents of intensely altered host rocks and ores have positive correlations with Ag, Ni, Mo, and Na. Fluid inclusions in the veins of the main stage and the matrix of breccia are mainly two‐phase liquid‐rich inclusions with minor two‐phase, vapor‐rich, and single‐phase liquid or vapor inclusions. CO2 and N2 gases are detected in the fluid inclusions by Laser Raman microspectrometry. Fluid boiling probably occurred when the fluid was trapped at approximately 120–190 m below the paleo water table. δ18OSMOW values of fluid, +5.8 and +7.6‰, calculated from δ18OSMOW of quartz from the main stage vein indicate oxygen isotopic exchange with wall rocks during deep circulation. δ34SCDT of pyrite narrowly ranges from ?2.0 to +3.4‰, suggesting a single source of sulfur. Gold mineralization in the Salu Bulo prospect occurred in an epithermal condition, after the metamorphism of the host rocks. It formed at a relatively shallow depth from fluids with low to moderate salinity (3.0–8.5 wt% NaCl equiv.). The temperature and pressure of ore formation range from 190 to 210°C and 1.2 to 1.9 MPa, respectively.  相似文献   

13.
Fluid inclusions in quartz globules and quartz veins of a 3.8-3.7 Ga old, well-preserved pillow lava breccia in the northeastern Isua Greenstone Belt (IGB) were studied using microthermometry, Raman spectrometry and SEM Cathodoluminescence Imaging. Petrographic study of the different quartz segregations showed that they were affected by variable recrystallization which controlled their fluid inclusion content. The oldest unaltered fluid inclusions found are present in vein crystals that survived dynamic and static recrystallization. These crystals contain a cogenetic, immiscible assemblage of CO2-rich (+H2O, +graphite) and brine-rich (+CO2, +halite, +carbonate) inclusions. The gas-rich inclusions have molar volumes between 44.8 and 47.5 cm3/mol, while the brine inclusions have a salinity of ∼33 eq. wt% NaCl. Modeling equilibrium immiscibility using volumetric and compositional properties of the endmember fluids indicates that fluid unmixing occurred at or near peak-metamorphic conditions of ∼460 °C and ∼4 kbar. Carbonate and graphite were precipitated cogenetically from the physically separated endmember fluids and were trapped in fluid inclusions.In most quartz crystals, however, recrystallization obliterated such early fluid inclusion assemblages and left graphite and carbonate as solid inclusions in recrystallized grains. Intragranular fluid inclusion trails in the recrystallized grains of breccia cementing and crosscutting quartz veins have CO2-rich assemblages, with distinctly different molar volumes (either between 43.7 and 47.5 cm3/mol or between 53.5 and 74.1 cm3/mol), and immiscible, halite-saturated H2O-CO2-NaCl(-other salt) inclusions. Later intergranular trails have CH4-H2 (XH2 up to ∼0.3) inclusions of variable density (ranging from 48.0 to >105.3 cm3/mol) and metastable H2O-NaCl(-other salt?) brines (∼28 eq. wt% NaCl). Finally, the youngest fluid inclusion assemblages are found in non-luminescent secondary quartz and contain low-density CH4 (molar volume > 105.33 cm3/mol) and low-salinity H2O-NaCl (0.2-3.7 eq. wt% NaCl). These successive fluid inclusion assemblages record a retrograde P-T evolution close to a geothermal gradient of ∼30 °C/km, but also indicate fluid pressure variations and the introduction of highly reducing fluids at ∼200-300 °C and 0.5-2 kbar. The quartz globules in the pillow fragments only contain sporadic CH4(+H2) and brine inclusions, corresponding with the late generations present in the cementing and crosscutting veins. We argue that due to the large extent of static recrystallization in quartz globules in the pillow breccia fragments, only these relatively late fluid inclusions have been preserved, and that they do not represent remnants of an early, seafloor-hydrothermal system as was previously proposed.Modeling the oxidation state of the fluids indicates a rock buffered system at peak-metamorphic conditions, but suggests a change towards fluid-graphite disequilibrium and a logfH2/fH2O above the Quartz-Fayalite-Magnetite buffer during retrograde evolution. Most likely, this indicates a control on redox conditions and on fluid speciation by ultramafic rocks in the IGB.Finally, this study shows that microscopic solid graphite in recrystallized metamorphic rocks from Isua can be deposited inorganically from a fluid phase, adding to the complexity of processes that formed reduced carbon in the oldest, well-preserved supracrustal rocks on Earth.  相似文献   

14.
A prominent set of veins was formed during post-metamorphic deformation of the Caledonian Dalradian metamorphic belt. These veins are concentrated in dilational zones in fold hinges, but apophyses follow schistosity and fold axial surface fractures. The veins are most common in the cores of regional structures, especially the Dalradian Downbend and consist of quartz, calcite, chlorite and metallic sulphides and oxides. Metals, including gold, have been concentrated in the veins. The fluid which formed the veins was low salinity (1–5 wt% NaCl and KCl) CO2-bearing (3–16 wt% CO2) water of metamorphic origin. The fluid varies slightly in composition within and between samples, but is essentially uniform in composition over several hundred km2. Vein formation occurred at about 350±50 °C and 200–300 MPa pressure. Further quartz mineralization occurred in some dilational zones at lower temperatures (160–180 °C). This later mineralization was accompanied by CO2 immiscibility. Dilution and oxidation of the metamorphic fluid occurred due to mixing with meteoric water as the rocks passed through the brittle-ductile transition. A similar metamorphic fluid is thought to have been responsible for gold mineralization in the nearby Tyndrum Fault at a later stage in the Dalradian uplift.  相似文献   

15.
Quartz-pebbles of the early Proterozoic Au-bearing Tarkwaian conglomerates in Ghana reveal several original (inherited) pre-sedimentary fluid inclusions. These inclusions are CO2-N2 rich and display a distinct high density (up to 1.15 g/cm3). The unusual high density and composition compare well with CO2-N2-rich inclusions in quartz-vein type gold deposits of the Birimian Supergroup in Ghana and Burkina Faso. This type of fluid inclusions has not been reported from any other lode-gold deposit of greenstone affiliation and is thus a specific characteristic for Birimian-hosted gold deposits. Therefore, it can be used as an unequivocal pathfinder for epigenetic as well as for syn-sedimentary gold mineralization of the early Proterozoic of West Africa. The inherited fluid inclusions with the unique physicochemical characteristics suggest that the Tarkwaian quartz-pebbles and possibly some gold were derived from Au-quartz vein deposits comparable in mineralogy, petrography and genesis to those along the NW-margin of the Ashanti belt (e.g. Ashanti Mine, Prestea Mine).  相似文献   

16.
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17.
The Archaean lode-gold deposits at Norseman, Western Australia, consist of auriferous quartz veins in dextral-reverse ductile-brittle shear zones within tholeiitic metabasalts of upper-greenschist to amphibolite facies metamorphic grade. Three types of deposits (Northern, Central, Southern) are delineated on the basis of their spatial distribution, veining style, alteration mineraloty and metamorphic grade of host rocks. Northern deposits, hosted in upper-greenschist to lower-amphibolite facies rocks, comprise massive to laminated quartz veins with selvedges of quartz-chlorite-calcite-biotite-plagioclase assemblages. Central deposits, hosted in lower-amphibolite facies rocks, consist of laminated to massive quartz veins with selvedges of quartz-actinolite-biotite-plagioclasecalcite assemblages. Southern deposits, hosted in middleamphibolite facies metabasalts, consist of banded quartz-diopside-calcite-microcline-zoisite veins. All deposits exhibit variable ductile deformation of veins and contiguous alteration haloes, consistent with a syn-deformational genesis at high temperatures. From Northern to Southern deposits, the alteration assemblages are indicative of higher temperatures of formation, and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals. These observations imply that a thermal variation of gold-related hydrothermal alteration exists within the Norseman Terrane over a distance of 40 km, with TNorthern<TCentral<TSouthern This thermal zonation is corroborated by T−XCO 2 phase relations between vein selvedge assemblages, which signify formation temperatures of approximately 420°–475°C, 470°–495°C and >500°C for Northern, Central and Southern deposits, respectively. The sum of structural, petrographic and mineral chemistry data indicates that the alteration assemblages formed in high-temperature, open hydrothermal systems and have not been subsequently metamorphosed. The thermal differences between the deposit groups may reflect (1) a temperature gradient, at relatively constant P, corresponding to the proximity of the deposits to regional granitoid complexes, or (2) formation of the deposits at progressively deeper crustal levels from north to south. In either case the deposits represent a continuum of gold deposition from upper-greenschist to amphibolite facies, now exposed in an oblique section through the Archaean crust at Norseman.  相似文献   

18.
The Archaean lode gold deposits in the Mt. York District, Pilbara Block, Western Australia are hosted in banded iron formation (Main Hill/Breccia Hill prospect) of the ca. 3.33 Ga Gorge Creek Group and in amphibolites (Zakanaka prospect) of the ca. 3.46 Ga Warrawoona Group. Gold mineralisation at the Main Hill/Breccia Hill prospect is associated with breccias comprising quartz clasts in a quartz-pyrrhotite matrix, and quartz-amphibole veins, with löllingite being the major host for gold. Minimum temperatures for gold mineralisation at the prospect are constrained as 455°C to 550°C by arsenopyrite thermometry. Gold mineralisation at the Zakanaka prospect is spatially associated with quartzclinopyroxene-calcite-microcline-calcic-amphibole veins and biotite altered wallrock adjacent to the veins. Temperatures for vein emplacement are estimated as 480°C to 570°C using both plagioclase-amphibole thermometry and mineral equilibria with respect to T and XCO2. The timing of gold mineralisation relative to the peak of metamorphism is constrained by mineral textures and the relative temperatures of hydrothermal alteration and metamorphism. Gold mineralisation at both deposits was broadly synchronous with the peak of regional amphibolite facies metamorphism, which reached temperatures of 520°C to 640°C based on amphibole-plagioclase and garnet-biotite thermometry. In this respect, the deposits are similar to other well documented syn-amphibolite facies lode gold deposits from the Archaean Southern Cross greenstone belt in the Yilgarn Block, and represent the deeper section of a crustal continuum of lode gold deposits that includes mesothermal deposits such as those at Kalgoorlie at higher crustal levels.  相似文献   

19.
A low-salinity, mixed aqueous-carbonic fluid is common to all Archæan lode-gold deposits throughout the range of mineralising conditions from sub-greenschist to lower-granulite facies temperatures. Alteration assemblages and fluid-inclusion data give constraints on the fluid composition. Fluid XCO 2 is 0.1–0.3 in typical greenschist-facies (mesothermal) deposits. At higher temperatures, the assemblages are consistent with formation from a fluid of similar composition, but slightly higher or lower XCO 2 cannot be ruled out, and fluid-inclusion data indicate that CH4 may be an important component in ore fluids at these temperatures. Fluid pH is neutral or weakly alkaline at all conditions. A range of relative oxidation states of four orders of magnitude fO 2 is indicated at any temperature, with deposits more oxidising relative to QFM at lower temperature. Sulphur contents of the fluids vary from ≈ 10 to 10?3.5m∑S, with a trend towards lower sulphur contents at lower temperatures. The relative concentrations of major cations in solution are similar at all conditions with Na ? K ≥ Ca, although Ca may be less abundant at low temperatures. The broad similarities in ore-fluid composition at all temperatures give support to ‘crustal-continuum’ models, in which Archæan lode-gold mineralisation involved either a single fluid moving through the middle and upper crust, or derivation of ore fluids by similar processes at different crustal levels. Many of the compositional differences between high- and lowtemperature ore-fluids may be attributed to evolution of deep-sourced hydrothermal solutions as they rise along structurally-controlled conduits. The constancy of major ore-fluid component concentration (e.g. CO2, Cl, ± K) suggests fluid-buffering and high fluid-rock ratios along fluid pathways. Fluid-buffered conditions can also explain the ore-fluid fO 2-temperature relations; with equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state. In contrast, the concentrations of components present in lesser abundance in Archæan gold ore-fluids (e.g. S, Ca, H+) were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature, or were rock-buffered through fluid-rock reactions. Extrapolation to high temperatures of the K, Na and Ca contents of the gold-bearing fluids indicates that their composition is consistent with derivation from, or final equilibration with, rocks of intermediate-granitic composition, thus giving support to isotopic and geological arguments for ore-fluid source regions external to the greenstone belts. The fluid oxidation states are characteristic of a wide range of potential source rocks, including mantle-derived igneous rocks, calc-alkaline granitoids and magmas, and seaflooraltered metabasalts. Strongly oxidised magmatic sources or unusually oxidising source processes (e.g. CO2-streaming during granulitisation of the lower crust) are therefore not required in the genesis of Arch?an lode-gold deposits.  相似文献   

20.
论界牌峪地区“陀状”角砾岩成因及找矿意义   总被引：6，自引：0，他引：6  
湖南石门界牌峪地区存在大量的所谓“陀状”角砾岩，它们互不相连，外观圆形，有的聚集成群或者呈完全孤立的山包。角砾岩地表多为椭圆状和等轴状，与围岩多呈刺穿状接触。通过地质学、岩石学和地球化学等方面研究，认为该区“陀状”角砾岩是一种由热泉气体及部分物质在封闭还原条件下产生急剧爆发所形成的特殊的角砾岩群。它们是一种极好的容矿控矿体，具有重要的找矿意义  相似文献   

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