陕西蓝田灞源一带金矿地质特征及成因浅析

本文通过分析灞源地区的赋矿地层、岩浆岩及断裂构造与金矿形成的关系，查明了控制金矿形成与分布的地质条件，并重点论述了金矿床的产出、矿化类型、围岩蚀变和含金矿物等主要地质特征。最后根据矿床地球化学特征综合分析阐明本区金矿床的岩浆热液成因。     

Borosilicate- and phengite-bearing veins from the Grenville Province of Labrador: evidence for rapid uplift

Quartzo‐feldspathic veins emplaced within a migmatite terrane near Wilson Lake in the Grenville Province of central Labrador record a metamorphic event not evident in the host rocks. The discordant veins are undeformed and have undisturbed primary igneous/hydrothermal textures. Most of the veins contain euhedral kyanite, as well as aggregates of kyanite, K‐feldspar, phlogopite and minor dumortierite which are likely pseudomorphs after primary phengite. The reconstructed phengite compositions range from 3.1 to 3.2 Si per 11 oxygen formula unit. The pseudomorph assemblage is interpreted as the product of phengite + quartz melting under H₂O‐undersaturated conditions, which brackets P–T conditions of formation to about 9–16 kbar and 775–875 °C. A parallel vein that is likely of the same generation contains the borosilicate phases, dumortierite, prismatine and grandidierite, but no kyanite. The borosilicate assemblages constrain the P–T conditions of vein crystallization to ≥10 kbar and c. 750–850 °C. Vein emplacement is constrained to T ≤ 875 °C at the same pressures, which is well within the kyanite zone. Because the host rocks and veins must have experienced the same P–T history following vein emplacement, the presence of unreacted sillimanite in the host migmatites implies insufficient time for host rock equilibration. Slow reaction rates because of anhydrous conditions are not a likely explanation given the abundance of biotite and hornblende in the host rocks. The ductility implied by the breakdown of a hydrous phase (phengite) and the production of an H₂O‐undersaturated melt in the veins contrasts with the apparently brittle behaviour of the host rocks. The absence of deformation since the time of vein emplacement, even at temperatures above 750 °C, suggests that the deep crust in this part of Labrador had a very short residence time under conditions of the kyanite zone. Rapid decompression from those conditions is consistent with quartz + phengite melting and accounts for the relatively brittle behaviour of the terrane as it was uplifted.     

华北地台中元古代串岭沟组页岩中的砂脉构造:17亿年前甲烷气逃逸的沉积标识?

华北地台中元古界串岭沟组暗色页岩中发育一种特殊的砂质脉状构造;层面上表现为不规则密集分布的细砂脊,垂向上由不连续薄砂层和近于直立的"肠状"砂脉体相交互组成.浅色砂脉由较纯的石英粉砂-细砂组成,含微量自生白云石、菱铁矿及微晶碳酸盐岩斑块,与黑色泥质围岩边界截然.研究表明,砂脉构造可能是在早期成岩阶段前由来自薄砂层的细-粉砂灌入甲烷缓慢逃逸通道而成,并由于压实缩短而褶皱成"肠状".薄砂层与黑色页岩形成的能量条件完全不同,可能系由风暴将海岸带或砂坝细砂带入低能环境而形成;甲烷源自沉积中埋藏的微生物席腐烂分解.围岩层面有微生物席成因微皱痕和气体逃逸形成的气泡构造,围岩中发现有细菌化石和草霉状黄铁矿.串岭沟组中密集发育的砂脉构造是目前地层中识别的最古老的甲烷排放证据,并有可能作为指示地质时期陆源碎屑沉积环境甲烷逃逸的沉积标识.大量高效温室气体进入大气圈可能是导致元古宙地球表层高温室气候和无冰川发育的重要原因.     

柿竹园钨多金属矽卡岩-云英岩与铅锌银矿脉C、H、O、Pb同位素地球化学研究

柿竹园W_Sn_Mo_Bi_Pb_Zn_Ag矿田是南岭地区典型多金属矿田,其具有显著的蚀变_成矿分带特征:近接触带矽卡岩-云英岩型W_Sn_Mo_Bi矿床(如柿竹园、金船塘、柴山地区)和远接触带脉状Pb_Zn_Ag矿床(如蛇形坪、横山岭、百步窿、枞树板地区)。通过对远—近接触带中主要蚀变岩石和矿石样品的C、H、O、Pb同位素研究,发现在近接触带块状矽卡岩中方解石的δ~(13)C和δ~(18)O值分别在-6.5‰~-3.6‰和4.8‰~8.4‰之间,远接触带Pb_Zn_Ag矿脉中团块状和脉状方解石的δ~(13)C和δ~(18)O值分别在-3.5‰~-1.5‰和8.1‰~15.8‰之间,表明矽卡岩形成于岩浆热液流体的渗滤交代作用,而远接触带碳酸盐脉的形成与热液流体沿灰岩裂隙充填过程中的水岩反应相关。近接触带块状云英岩中石英的δD_(H_2O)和δ_(18)O_(H_2O)值分别为-77‰和5.1‰,矽卡岩中石榴子石的δD_(H_2O)值为-97‰,δ_(18)O_(H_2O)值范围在9.2‰~9.4‰,网脉状云英岩中石英的δD_(H_2O)和δ_(18)O_(H_2O)值分别为-83‰和-8.2‰,退化蚀变矽卡岩中石英的δD_(H_2O)和δ_(18)O_(H_2O)值分别为-65‰和-4.2‰,远接触带石英脉中石英样品的δD_(H_2O)和δ_(18)O_(H_2O)值分别为-83‰和2.8‰,表明近接触带的块状矽卡岩和云英岩与远接触带石英脉成矿流体具有岩浆水的特征,而网脉状云英岩和退化蚀变矽卡岩成矿流体具有岩浆流体与大气水混合的特征。远—近接触带中方铅矿的206Pb/204Pb值范围18.565~18.622,207Pb/204Pb值范围15.694~15.738,208Pb/204Pb值范围38.819~38.986,表明Pb等金属元素来源于上地壳。     

Genesis of the Taishanmiao Au Deposit in the Ningshan–Zhenan Ore Field, South Qinling Orogen, China: Evidence from Geochemical, Geochronological and S-Pb-H-O Isotopical Study

The Taishanmiao Au deposit is in the western part of the Ningshan–Zhenan ore field, in the South Qinling orogen. Based on geological and geochemical features, we propose that the Taishanmiao Au deposit is a magmatic-hydrothermal type of deposit. All samples have high SiO2, K2O + Na2O contents and differentiation index values, low CaO, MgO, P2O5, and TiO2 contents, are enriched in high field-strength elements, and depleted in large ion lithophile element. The stable isotope δ34S values of pyrite vary from 6.8%–7.8%, and the H-O isotopic compositions of quartz from quartz-pyrite veins indicate the ore-forming fluid is a mixture of a small amount of magmatic-hydrothermal solution and groundwater. Lead isotope ratios of pyrite and silicalite can show that the ore-forming materials were derived from a mixed source containing mantle and crustal materials. At the same time, the LA-ICP-MS U-Pb dating of monzogranite is 198.4 ± 4.2 Ma. Combined with the regional geological background, the intracontinental extension in the late collisional orogeny and large-scale lithospheric thinning associated with mantle uplift may lead to large-scale mineralisation in the region.     

大庆油田古龙凹陷青山口组页岩油储层方解石脉中发现藻生物层

<正>顺层方解石脉是页岩油储层中比较常见的一种构造,虽然它的体量不大,但是携带的地质信息很多,近年来颇受重视,取得了很多成果(何文渊等,2022,2023;杨勇等,2023)。近期大庆油田在古龙凹陷青山口组获得了页岩油的资源量可达15.1 Gt,已引起广泛关注,但还有一些地质问题没有解决,严重地影响了页岩油的勘探开发。因此,对古龙凹陷青山口组页岩的基础地质进行深入研究具有重要意义。     

Genesis of Fibrous Calcite in the Chang 7 Member of the Yanchang Formation, Ordos Basin, China

To explore the genesis of the laminated calcite veins developed in the black shale of Chang 73 submember of the Ordos Basin, the petrology, microstructure and geochemistry of calcite veins are studied using thin sections, fluid inclusions, trace elements and isotopic geochemistry. The source, the time of formation, the mechanism of formation, and the dynamic background of the veins are discussed. The veins are mostly made up of calcite, mixed with lenticular or spindle-shape solid wall rock inclusions. Three structures are identified in the calcite minerals: fibrous, rhombic cleavage, and wedge-like structure. Trace elements and isotopes of carbon and oxygen confirm that the calcite veins were formed from a high density hydrothermal fluid. It is assumed that calcite veins formed prior to wall rock consolidation during the formation of the Qinling Orogenic Belt in the Middle and Late Triassic. The results show that the sedimentary sources of Chang 73 submember were influenced by hydrothermal materials besides terrigenous detritus. The fibrous calcite is different from the fibrous calcite reported in the morphology and mechanism of formation. In this way, the research complements and improves the morphology and the mechanism of formation of fibrous calcite.     

Fluid inclusion characteristics of hematite-bearing quartz vein from the Daenam mine, Republic of Korea

The Daenam mine, which produced over 9250 tons of iron oxide ore from 1958 to 1962, is situated in the Early Cretaceous Yeongyang subbasin of the Gyeongsang basin. It consists of two lens-shaped, hematite-bearing quartz veins that occur along faults in Cretaceous leucocratic granite. The hematite-bearing quartz veins are mainly composed of massive and euhedral quartz and hematite with minor amounts of pyrite, pyrrhotite, mica, feldspar and chlorite.Fluid inclusions in quartz can be divided into three main types: CO₂-rich, CO₂–H₂O, and H₂O-rich. Hydrothermal fluids related to the formation of hematite are composed of either H₂O–CO₂–NaCl ± CH₄ (homogenization temperature: 262–455 °C, salinity <7 eq. wt.% NaCl) or H₂O–NaCl (homogenization temperature: 182–266 °C, and salinity <5.1 eq. wt.% NaCl), both of which evolved by mixing with deeply circulating meteoric water. Hematite from the quartz veins in the Daenam mine was mainly deposited by unmixing of H₂O–CO₂–NaCl ± CH₄ fluids with loss of the CO₂ + CH₄ vapor phase and mixing with downward percolating meteoric water providing oxidizing conditions.     

Evidence for volatile-influenced differentiation in a layered alkali basalt flow, Penghu Islands, Taiwan

An approximately 20-m-thick alkali basalt flow on the Penghu Islands contains ∼20 cm thick, horizontally continuous (>50 m), vesicular layers separated by ∼1.5 m of massive basalt in its upper 8.5 m. The three layers contain ocelli-like "vesicles" filled with nepheline and igneous carbonate. They are coarse grained and enriched in incompatible elements relative to the massive basalt with which they form sharp contacts. These vesicular layers (segregation veins) formed when residual liquid in the underlying crystal mush was forced (gas filter pressing) or siphoned into three thermally induced horizontal cracks that opened successively in the advancing crystal mush of the flow's upper crust. Most vesicular layer trace elements can be modelled by residual melt extraction after 25–40% fractional crystallization of massive basalt underlying each layer. Sulphur, Cl, As, Zn, Pb, K, Na, Rb, and Sr show large concentration changes between the top, middle, and bottom layers, with each vesicular and underlying massive basalt forming a chemically distinct "pair." The large changes between layers are difficult to account for by crystal fractionation alone, because other incompatible elements (e.g., La, Sm, Yb, Zr, Nb) and the major elements change little. The association of these elements (S, Cl, etc.) with "fluids" in various geologic environments suggests that volatiles influenced differentiation, perhaps by moving alkali, alkaline earth, and chalcophile elements as magma-dissolved volatile complexes. Volatiles may have also led to large grain sizes in the segregation veins by lowering melt viscosities and raising diffusion rates. The chemical variability between layers indicates that a convection and concentration mechanism acted within the flow. The specific process cannot be determined, but different rates of vesicle plume rise (through the flow) and/or accumulation in the upper crust's crystal mush might account for the chemical pairing and extreme variations in Cl, S, As, and C. This study emphasizes the importance of sampling vesicular rocks in flows. It also suggests that volatiles play important physical and chemical roles in rapidly differentiating mafic magmas in processes decoupled from crystal fractionation. Received: 11 November 1996 / Accepted: 20 September 1998     

P-T conditions of quartz-calcite boudins and vein formation within a low-angle detachment fault in Tinos Island (Aegean Sea): a fluid-inclusion study

Fluid inclusions were investigated in successive generations of quartz-calcite boudins and veins in the vicinity of, and within, the low-angle detachment fault (of Early Miocene age) exposed on Tinos Island (Cyclades, Greece). Abundant boudins, veins, tension gashes and mineral segregations were formed during various stages of ductile and brittle shear along the detachment indicating fluid flow at various crustal levels, assisted motion and slip along the detachment. Three generations of fluid inclusions were identified: (1) syn-deformation aqueous inclusions; (2) local- late-deformation H₂O-CO₂ inclusions; (3) post-slip deformation, partly decrepitated aqueous and H₂O-CO₂ inclusions. The conditions of inclusion trapping correspond to the greenschist facies, at temperatures between 300 and 450°C and pressures in the range of 1.5–4.0 kb. A gradient of pressure of about 0.5 kb from the fault zone to the rocks of upper and lower plates is suggested. The results indicate that the syn-deformation fluid was NaCl-KCl-sulfate-dominated with a salinity of about 5 wt% NaCl equiv. Downward infiltration of meteoric water is a suitable source for this fluid. During the late stage of fault activity, in the brittle field, fluid was of H₂O-CO₂ composition. Very rapid exhumation of the entire section (unrelated to motion on the investigated fault) is marked by numerous decrepitation clusters of the fluid inclusions.