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1.
The Tiámaro deposit in Michoacán state has been dated as Lower Cretaceous (Valanginian), though most of the porphyry deposits in central Mexico were dated or have an attributed Eocene–Oligocene age. The host rocks belong to a volcanoplutonic complex overlain by red conglomerates. These rocks were intruded by pre-Valanginian plutonic and hypabissal rocks. Propylitic, phyllic, and argillic alteration assemblages developed, and their superimposition draws the evolution of the deposit. Stage I is represented by propylitic assemblages, stage II contains the main ore forming stockworks and both phyllic and argillic assemblages, and stage III contains late carbonatization assemblages. The obtained temperatures and salinities from inclusion fluids are low for a porphyry-type deposit, but we interpret that the known part of the deposit represents the shallow portion of a bigger deposit. The evolution of mineralizing fluids draws a dilution trend of brines from “porphyry-like” to “epithermal-like” stages. The richest ore zone is roughly located between the 300 and 350 °C isotherms, though unnoticed resources may occur at depth.  相似文献   

2.
Fe isotopes are a potential tool for tracing the biogeochemical redox cycle of Fe in the ocean. Specifically, it is hypothesized that Fe isotopes could enable estimation of the contributions from multiple Fe sources to the dissolved Fe budget, an issue that has received much attention in recent years. The first priority however, is to understand any Fe isotope fractionation processes that may occur as Fe enters the ocean, resulting in modification of original source compositions. In this study, we have investigated the Fe inputs from a basalt-hosted, deep-sea hydrothermal system and the fractionation processes that occur as the hot, chemically reduced and acidic vent fluids mix with cold, oxygen-rich seawater.The samples collected were both end-member vent fluids taken from hydrothermal chimneys, and rising buoyant plume samples collected directly above the same vents at 5°S, Mid-Atlantic Ridge. Our analyzes of these samples reveal that, for the particulate Fe species within the buoyant plume, 25% of the Fe is precipitated as Fe-sulfides. The isotope fractionation caused by the formation of these Fe-sulfides is δFe(II)–FeS = +0.60 ± 0.12‰.The source isotope composition for the buoyant plume samples collected above the Red Lion vents is calculated to be −0.29 ± 0.05‰. This is identical to the value measured in end-member vent fluids collected from the underlying “Tannenbaum” chimney. The resulting isotope compositions of the Fe-sulfide and Fe-oxyhydroxide species in this buoyant plume are −0.89 ± 0.11‰ and −0.19 ± 0.09‰, respectively. From mass balance calculations, we have been able to calculate the isotope composition of the dissolved Fe fraction, and hypothesize that the isotope composition of any stabilised dissolved Fe species exported to the surrounding ocean may be heavier than the original vent fluid. Such species would be expected to travel some distance from areas of hydrothermal venting and, hence, contribute to not only the dissolved Fe budget of the deep-ocean but also it’s dissolved Fe isotope signature.  相似文献   

3.
The Bujinhei Pb–Zn deposit is located in the southern Great Xing'an Range metallogenic belt. It is a representative medium‐ to high‐temperature hydrothermal vein type deposit controlled by fractures, and orebodies hosted in the Permian Shoushangou Formation. The hydrothermal mineralization is classified into three stages: pyrite ± arsenopyrite–quartz (Stage 1), polymetallic sulfide–quartz (Stage 2), and polymetallic sulfide–calcite (Stage 3). Fluid inclusion petrography, laser Raman analyses and microthermometry indicate that the liquid‐rich aqueous inclusions (L) and vapor‐rich CO2 ± CH4–H2O inclusions (C) occur in the Stage 1 and as medium‐ to high‐ temperature and low‐ to medium‐salinity NaCl–H2O–CO2–CH4 hydrothermal fluids. The liquid‐rich (L) and rare vapor‐rich CO2 ± CH4–H2O inclusions (C) occur in the Stage 2 with medium‐temperature and low‐salinity NaCl–H2O ± CO2 ± CH4 hydrothermal fluids. The exclusively liquid‐rich (L) fluid inclusions are observed in the Stage 3, and the hydrothermal fluid belongs to medium‐temperature and low‐salinity NaCl–H2O hydrothermal fluids. The results of hydrogen and oxygen isotope analyses indicate that ore‐forming fluids were initially derived from the magmatic water and mixed with local meteoric water in the late stage (δ18OH2O‐SMOW = 6.0 to 2.2‰, δDSMOW = ?103 to ?134‰). The carbon isotope compositions (?18.4‰ to ?26.5‰) indicate that the carbon in the fluid was derived from the surrounding strata. The sulfur isotope compositions (5.7 to 15.2‰) indicate that the ore sulfur was also primarily derived from the strata. The ore vein No. 1 occurs in fractures and approximately parallel to the rhyolite porphyry; orebodies have a close spatial and temporal relationship with the rhyolite porphyry. The rhyolite porphyry yielded a crystallization age of 122.9  ± 2.4 Ma, indicating that the Bujinhei deposit may be related to the Early Cretaceous magmatic event. Geochemical analyses reveal that the Bujinhei rhyolite porphyry is high in K2O and peraluminous, and derived from an acidic liquid as a result of strong interaction with hydrothermal fluid during the late magmatic stage; it is similar to A2‐type granites, and formed in a backarc extensional environment. These results indicate that the Bujinhei Pb–Zn deposit was a vein type system that formed in Early Cretaceous and influenced by the Paleo‐Pacific tectonic system. Bujinhei deposit is a representative hydrothermal vein type deposit on the genetic types, and occurs on the western slope of the southern Great Xing'an Range. The ore‐forming fluids were medium‐ to high‐temperature and low‐to medium‐salinity NaCl–H2O–CO2–CH4 hydrothermal fluids, which became medium‐temperature and low‐salinity NaCl–H2O hydrothermal fluids in later stages, and came from magmatic water and mixed with meteoric water, whereas the ore‐forming materials were mainly derived from the surrounding strata. The LA–ICP–MS zircon U–Pb dating indicates that the Bujinhei deposit formed at the period of late Early Cretaceous, potentially in a backarc extensional environment influenced by the Paleo‐Pacific tectonic system.  相似文献   

4.
The metaturbidites of the Palaeoproterozoic Jormua–Outokumpu thrust belt in eastern Finland enclose m- to km-scale ultramafic massifs that are distributed over an area of more than 5000 km2. These bodies, which almost entirely consist of highly depleted mantle peridotites (now metaserpentinites and metaperidotites), are intimately associated with massive to semimassive, polymetallic Cu–Co–Zn–Ni–Ag–Au sulphide deposits that sustained mining in the region between 1913 and 1988. Currently, one deposit (Kylylahti) is proceeding into a definitive feasibility study emphasising the renewed economic interest for Outokumpu-type deposits.The origin of these Outokumpu-type Cu–Co–Zn–Ni–Ag–Au deposits is now re-interpreted to be polygenetic. First, their formation requires deposition of a Cu-rich proto-ore within peridotitic sea floor at  1950 Ma. Close modern analogues to the proto-ore setting include, for example, the Logatchev and Rainbow fields at the Mid-Atlantic Ridge, where venting of high-T–low-pH hydrothermal fluid resulted in accumulations of Cu–Zn–Co–Ag–Au sulphides on serpentinised ultramafic seafloor. Second, the Ni-rich composition of Outokumpu sulphide ores calls for a separate source for nickel: Some 40 Ma after the deposition of the Cu-rich proto-ore – concomitant with the obduction of the ultramafic massifs – disseminated Ni sulphides formed through chemical interaction between obducting peridotite massifs and adjacent black schists. This process was related to listwaenite–birbirite type carbonate–silica alteration at margins of the ultramafic massifs. Due to this alteration, silicate nickel was released from the primary Fe–Mg silicates and redeposited as Ni sulphides in the alteration fringes of the massifs.We propose that syntectonic mixing of these two “end-member” sulphides, i.e., the primary Cu-rich proto-ore and the secondary Ni-sulphide disseminations, resulted in the uncommon metal combination of the Outokumpu-type sulphides. Late tectonic solid-state re-mobilisation, related to the duplexing of the ore by isoclinal folding, upgraded the sulphides into economic deposits.  相似文献   

5.
The French Massif Central constitutes an exceptional study area due to the diversity of its metallic deposits, its internal position in the Variscan belt, and the abundance of available geological, geophysical and metallogenic data obtained within the GeoFrance 3D programme. The deposits, formed towards the end of the orogenic evolution, represent the economic products of two distinct mineralizing systems, a Au ± Sb hydrothermal system and a W ± Sn and rare-metals magmatic–hydrothermal system, which were simultaneously active during a short time span between ca. 310 and 300 Ma.Two types of gold deposit can be distinguished on the basis of their depth of emplacement: “deep-seated” gold deposits developed under lithostatic to hydrostatic pressure during rapid exhumation, and “shallow” gold deposits emplaced under hydrostatic pressure with no significant uplift.Deposits of W ± Sn and rare-metals were emplaced in the upper crust during final crystallization of specialized magmas after their rapid ascent, perhaps enhanced by simultaneous regional uplift. The gold-bearing systems are associated with a complex network of re-activated crustal-scale faults initially active during the period between 335 and 315 Ma. Normal motion along the faults, coeval with 335 to 315 Ma granite–migmatite domes, played a major role in the 3D distribution of the hydrothermal plumbing system. Gold and related metals were carried within huge hydrothermal cells, which reached ca. 100 km by 10 km in area, and 30 km in depth. In contrast, granites rich in magmatophile elements (W, Sn, rare-metals) generated smaller hydrothermal cells (10 km by 10 km in area, and < 6 km deep). Extraction of metals, by both deep-seated fluids and specialized magmas, occurred during granulitization of the lower crust at 300 ± 15 Ma. In the French Massif Central, the genesis of the two late Carboniferous mineralizing systems coincided with the end of syn-collisional extension and ended just before post-collisional extension.  相似文献   

6.
The magma–ore deposit relationship of most low-sulfidation epithermal ore deposits is still unclear, partly because many stable isotopic studies of such deposits have indicated the predominance of meteoric waters within hydrothermal fluids. However, it is certainly true that hydrothermal systems are ultimately driven by magmatic intrusions, and epithermal gold deposits might therefore be produced by magmatic activity even in deposits having has no obvious links to a magma. We re-examine the genesis of two typical low-sulfidation epithermal gold deposits, the Kushikino and Hishikari deposits, using structural simulations and isotope data.Many epithermal gold deposits including the Kushikino and Hishikari deposits have been discovered in Kyushu, southwestern Japan. The Kushikino deposit comprises fissure-filling veins within Neogene andesitic volcanics that overlie unconformably Cretaceous sedimentary basement. The veins consist of gold- and silver-bearing quartz and calcite with minor amounts of adularia, sericite and sulfides. Although carbon and oxygen isotopic data for the veins indicate a meteoric origin of the ore fluid, finite element simulations suggest that the vein system might have formed in direct response to magma intrusion. In particular, geophysical data suggest that intruding magma has uplifted the basement rocks, thereby producing fractures and veins and a positive Bouguer anomaly, and providing the heat necessary to drive an ore-forming hydrothermal system.The second component of this study has been to investigate the nature and evolution of the Kushikino and Hishikari epithermal systems. Isotope data document the geochemical evolution of the hydrothermal fluids. We conclude that the existence of sedimentary basement rocks at depth might have affected the strontium and carbon isotopic ratios of the Kushikino and Hishikari ore fluids. The 87Sr/86Sr ratios and δ13C–δ18O trend reveal that major ore veins in the Hishikari deposit can be distinguished from shallow barren veins. It was suggested isotopically that fluids responsible for the barren veins in nearby shallow and barren circulation systems were only controlled by the shallow host rocks. Such multi-isotope systematics provide a powerful tool with which to determine the center of hydrothermal activity and thereby document the evolution of hydrothermal fluids.  相似文献   

7.
Major and trace elements, noble gases, and stable (δD, δ18O) and cosmogenic (3H, 14C) isotopes were measured from geothermal fluids in two adjacent geothermal areas in NW-Mexico, Las Tres Vírgenes (LTV) and Cerro Prieto (CP). The goal is to trace the origin of reservoir fluids and to place paleoclimate and structural-volcanic constraints in the region. Measured 3He/4He (R) ratios normalized to the atmospheric value (Ra = 1.386 × 10−6) vary between 2.73 and 4.77 and are compatible with mixing between a mantle component varying between 42 and 77% of mantle helium and a crustal, radiogenic He component with contributions varying between 23% and 58%. Apparent U–Th/4He ages for CP fluids (0.7–7 Ma) suggest the presence of a sustained 4He flux from a granitic basement or from mixing with connate brines, deposited during the Colorado River delta formation (1.5–3 Ma). Radiogenic in situ 4He production age modeling at LTV, combined with the presence of radiogenic carbon (1.89 ± 0.11 pmC – 35.61 ± 0.28 pmC) and the absence of tritium strongly suggest the Quaternary infiltration of meteoric water into the LTV geothermal reservoir, ranging between 4 and 31 ka BP. The present geochemical heterogeneity of LTV fluids can be reconstructed by mixing Late Pleistocene – Early Holocene meteoric water (58–75%) with a fossil seawater component (25–42%), as evidenced by Br/Cl and stable isotope trends. CP geothermal water is composed of infiltrated Colorado River water with a minor impact by halite dissolution, whereas a vapor-dominated sample is composed of Colorado River water and vapor from deeper levels. δD values for the LTV meteoric end-member, which are 20‰–44‰ depleted with respect to present-day precipitation, as well as calculated annual paleotemperatures 6.9–13.6 °C lower than present average temperatures in Baja California point to the presence of humid and cooler climatic conditions in the Baja California peninsula during the final stage of the Last Glacial Pluvial period. Quaternary recharge of the LTV geothermal reservoir is related to elevated precipitation rates during cooler-humid climate intervals in the Late Pleistocene and Early Holocene. The probable replacement of connate water or pore fluids by infiltrating surface water might have been triggered by enhanced fracture and fault permeability through contemporaneous tectonic–volcanic activity in the Las Tres Vírgenes region. Fast hydrothermal alteration processes caused a secondary, positive δ18O-shift from 4‰ to 6‰ for LTV and from 2‰ to 4‰ for CP geothermal fluids since the Late Glacial infiltration.  相似文献   

8.
大兴安岭南段发育大井、双尖子山、布金黑、拜仁达坝、维拉斯托等多个具有典型后生特征的热液脉状铅锌银锡多金属矿床。为了查明上述矿床在成矿流体、成矿物质等方面的特征与差异,进而总结大兴安岭南段热液脉状矿床成矿作用特点,本次研究在野外地质调研的基础上,对上述矿床进行了流体包裹体、激光拉曼和氢氧硫同位素的研究,并取得了如下主要的认识:(1)双尖子山银多金属矿床成矿流体属简单盐水体系热液,维拉斯托和布金黑矿床成矿流体属富碳质盐水体系热液,而大井铜矿成矿流体则为含子矿物的不均匀盐水体系热液;(2)大井、布金黑、拜仁达坝和维拉斯托等矿床早期成矿流体均来自于岩浆热液,但布金黑、拜仁达坝和维拉斯托矿床成矿流体在运移过程中明显地受到了大气降水和地层中有机质的影响;(3)大兴安岭南段多数热液脉状多金属矿床矿石硫同位素δ34S值具有岩浆来源特点,个别矿床硫同位素δ34S值偏高可能是由复杂的岩浆源区性质及地层硫混入所引起。总的来说,大兴安岭南段不同热液脉状矿床在物质来源和流体演化方面的差异明显,而这也体现了该区中生代热液成矿作用的多样性和复杂性特点。  相似文献   

9.
A geochemical survey of thermal waters collected from submarine vents at Panarea Island (Aeolian Islands, southern Italy) was carried out from December 2002 to March 2007, in order to investigate (i) the geochemical processes controlling the chemical composition of the hydrothermal fluids and (ii) the possible relations between the chemical features of the hydrothermal reservoir and the activity of the magmatic system. Compositional data of the thermal water samples were integrated in a hydrological conceptual model, which describes the formation of the vent fluid by mixing of seawater, seawater concentrated by boiling, and a deep, highly-saline end-member, whose composition is regulated by water-rock interactions at relatively high temperature and shows clear clues of magmatic-related inputs. The chemical composition of concentrated seawater was assumed to be represented by that of the water sample having the highest Mg content. The composition of the deep end-member was instead calculated by extrapolation assuming a zero-Mg end-member. The Na–K–Ca geothermometer, when applied to the thermal end-member composition, indicated an equilibrium temperature of approximately 300 °C, a temperature in agreement with the results obtained by gas-geothermometry.  相似文献   

10.
新疆东天山康古尔金矿控矿构造特征   总被引：6，自引：1，他引：5  
作者通过构造控矿分析发现,康古尔金矿是受一韧性挤压带控制的中低温岩浆-变质热液型金矿床。成矿作用与控矿韧性挤压带从韧性向脆、韧性变形演化密切相关。本文具体分析了控矿构造类型和构造演化对矿化的控制程序以及矿体定位机制,建立了构造成矿模式指出了找矿方向。   相似文献   

11.
The Qingchengzi orefield in northeastern China, is a concentration of several Pb–Zn, Ag, and Au ore deposits. A combination of geochronological and Pb, Sr isotopic investigations was conducted. Zircon SHRIMP U–Pb ages of 225.3 ± 1.8 Ma and 184.5 ± 1.6 Ma were obtained for the Xinling and Yaojiagou granites, respectively. By step-dissolution Rb–Sr dating, ages of 221 ± 12 Ma and 138.7 ± 4.1 Ma were obtained for the sphalerite of the Zhenzigou Zn–Pb deposit and pyrargyrite of the Ag ore in the Gaojiabaozi Ag deposit, respectively. Pb isotopic ratios of the Ag ore at Gaojiabaozi (206Pb/204Pb = 18.38 to 18.53) are higher than those of the Pb–Zn ores (206Pb/204Pb = 17.66 to 17.96; Chen et al. [Chen, J.F., Yu, G., Xue, C.J., Qian, H., He, J.F., Xing, Z., Zhang, X., 2005. Pb isotope geochemistry of lead, zinc, gold and silver deposit clustered region, Liaodong rift zone, northeastern China. Science in China Series D 48, 467–476.]). Triassic granites show low Pb isotopic ratios (206Pb/204Pb = 17.12 to 17.41, 207Pb/204Pb = 15.47 to 15.54, 208Pb/204Pb = 37.51 to 37.89) and metamorphic rocks of the Liaohe Group have high ratios (206Pb/204Pb = 18.20 to 24.28 and 18.32 to 20.06, 207Pb/204Pb = 15.69 to 16.44 and 15.66 to 15.98, 208Pb/204Pb = 37.29 to 38.61 and 38.69 to 40.00 for the marble of the Dashiqiao Formation and schist of the Gaixian Formation, respectively).Magmatic activities at Qingchengzi and in adjacent regions took place in three stages, and each contained several magmatic pulses: ca. 220 to 225 Ma and 211 to 216 Ma in the Triassic; 179 to 185 Ma, 163 to 168 Ma, 155 Ma and 149 Ma in the Jurassic, as well as ca. 140 to 130 Ma in the Early Cretaceous. The Triassic magmatism was part of the Triassic magmatic belt along the northern margin of the North China Craton produced in a post-collisional extensional setting, and granites in it formed by crustal melting induced by mantle magma. The Jurassic and Early Cretaceous magmatism was related to the lithospheric delamination in eastern China. The Triassic is the most important metallogenic stage at Qingchengzi. The Pb–Zn deposits, the Pb–Zn–Ag ore at Gaojiabaozi, and the gold deposits were all formed in this stage. They are temporally and spatially associated with the Triassic magmatic activity. Mineralization is very weak in the Jurassic. Ag ore at Gaojiabaozi was formed in the Early Cretaceous, which is suggested by the young Rb–Sr isochron age, field relations, and significantly different Pb isotopic ratios between the Pb–Zn–Ag and Ag ores. Pb isotopic compositions of the Pb–Zn ores suggest binary mixing for the source of the deposits. The magmatic end-member is the Triassic granites and the other metamorphic rocks of the Liaohe Group. Slightly different proportions of the two end-members, or an involvement of materials from hidden Cretaceous granites with slightly different Pb isotopic ratios, is postulated to interpret the difference of Pb isotopic compositions between the Pb–Zn–(Ag) and Ag ores. Sr isotopic ratios support this conclusion. At the western part of the Qingchengzi orefield, hydrothermal fluid driven by the heat provided by the now exposed Triassic granites deposited ore-forming materials in the low and middle horizons of the marbles of the Dashiqiao Formation near the intrusions to form mesothermal Zn–Pb deposits. In the eastern part, hydrothermal fluids associated with deep, hidden Triassic intrusions moved upward along a regional fault over a long distance and then deposited the ore-forming materials to form epithermal Au and Pb–Zn–Ag ores. Young magmatic activities are all represented by dykes across the entire orefield, suggesting that the corresponding main intrusion bodies are situated in the deep part of the crust. Among these, only intrusions with age of ca. 140 Ma might have released sufficient amounts of fluid to be responsible for the formation of the Ag ore at Gaojiabaozi.Our age results support previous conclusions that sphalerite can provide a reliable Rb–Sr age as long as the fluid inclusion phase is effectively separated from the “sulfide” phase. Our work suggests that the separation can be achieved by a step-resolution technique. Moreover, we suggest that pyrargyrite is a promising mineral for Rb–Sr isochron dating.  相似文献   

12.
Studies of hot springs have focused mainly on the properties of fluids and solids. Fewer studies focus on the relationship between the hot springs and groundwater/surface-water environments. The differences in temperature and dissolved solids between hot-spring water and typical surface water and groundwater allow interactions to be traced. Electromagnetic terrain (EMT) conductivity is a nonintrusive technique capable of mapping mixing zones between distinct subsurface waters. These interactions include zones of groundwater/surface-water exchange and groundwater mixing. Herein, hydrogeological techniques are compared with EMT conductivity to trace hot-spring discharge interactions with shallow groundwater and surface water. Potentiometric-surface and water-quality data determined the hydrogeochemistry of two thermally influenced areas in Yellowstone National Park, Wyoming (USA). Data from the sites revealed EMT conductivity contrasts that reflected the infiltration of conductive hot-spring discharge to local groundwater systems. The anomalies reflect higher temperatures and conductivity for Na+–Cl?-rich hydrothermal fluids compared to the receiving groundwater. EMT conductivity results suggested hot springs are fed by conduits largely isolated from shallow groundwater; mixing of waters occurs after hot-spring discharge infiltrates groundwater from the surface and, generally, not by leakage in the subsurface. A model was proposed to explain the growth of sinter mounds.  相似文献   

13.
In the spring of 1979, 350°C springs precipitating hydrothermal sulphides and sulphates directly on to the sea-floor were discovered on the crest of the East Pacific Rise (EPR) at 21°N by the astonished scientific party of the RISE submersible expedition. These hot springs are within a linear field of active and inactive hydrothermal vents extending 6 km along the rise axis. Typically the mineral deposits at EPR, 21°N consist of basal sulphide mounds surmounted by mineralized sulphide-sulphate edifices, or “chimneys”, reaching heights up to 13 m above the sea floor. The mounds rest directly on fresh basalt and cover areas up to 450 m2. Chimneys atop mounds may be active or dead. The hottest active chimneys (350°C) spew forth fluids blackened by fine-grained sulphide precipitates, dominantly hexagonal pyrrhotite and iron-rich sphalerite. These “black smokers” are distinguished from cooler “white smoker” chimneys which are encrusted by worm tubes and emit milky fluids bearing amorphous silica, barite, and pyrite.  相似文献   

14.
This paper deals with the tectonic control on the hydrothermal system that gave rise to Sb–Hg ore deposits in the Monte Amiata area that was one of the most relevant mining district for the exploitation of mercury in Italy. The study area (Selvena mining district) is located in southern Tuscany (inner Northern Apennines) one of the most important mineralized area in the western Mediterranean region. Southern Tuscany was severely affected by Middle–Late Miocene low-angle normal faults, later dissected by Pliocene–Pleistocene faults, coeval magmatism (Late Miocene–Pleistocene) and hydrothermal activity (Pliocene–Present). The Selvena mining district is located south of Middle Pleistocene Monte Amiata volcanic complex. Our structural and kinematic study is based on the integration among fieldwork, borehole and mine data. The results highlight two Pleistocene–Holocene left-lateral transtensional shear zones linked by normal faults, defining a coeval pull-apart structure. Here, the Sb–Hg mineralization, transported by meteoric hydrothermal fluids mainly, is particularly diffuse and concentrated in the cataclasites and in damage zones of the normal faults. Furthermore, a widespread mineralization also occurs in the cataclasites of Miocene low-angle normal faults. Mine evidence suggests that ore-bearing fluids percolated through structural conduits located along the fault planes and resulting parallel to the intermediate stress axis. Geological structures and ore deposit distribution are related to a single hydrothermal circuit, with meteoric water channelled to depth through conduits parallel to the intermediate stress axis of the transcurrent shear zones; then, hydrothermal fluids mainly ascended through the almost vertical deformation zones located at the intersection between normal and strike-slip faults. Thus, hydrothermal fluids permeated also the Middle–Late Miocene cataclasites. This study shed light on the relationships between geological structures and mineralization in southern Tuscany and underlines the importance to investigate mine areas to understand hydrothermal fluids path.  相似文献   

15.
Molecular and isotopic compositions of crude oils in the Beaufort–Mackenzie Basin confirm three genetic end-member oil groups and suggest extensive cross-formational hydrocarbon fluid flows in the Tertiary deltaic system. Inter- and intra-fractional variations in the geochemistry of the Tertiary-reservoired oils indicate that the oil source/maturity signatures were substantially masked by biomarkers that were picked up along migration pathways. Thus, many of the previously recognized “immature non-marine oils” are in fact thermally mature, probably derived from unpenetrated deeper marine source rocks. Although the effective source rock volumes have not been evaluated and their exact stratigraphic levels remain unknown, the relative timing of oil generation versus trap formation, rather than poor source quality, may be the cause of under-filled traps in the offshore area.  相似文献   

16.
CSA mine exploits a ‘Cobar-type’ Cu–Pb–Zn±Au±Ag deposit within a cleaved and metamorphosed portion of the Cobar Supergroup, central New South Wales. The deposit comprises systems of ‘lenses’ that encompass veins, disseminations and semi-massive to massive Cu–Pb–Zn ores. The systems and contained lenses truncate bedding, are approximately coplanar with regional cleavage and similarly oriented shear zones and plunge parallel to the elongation lineation. Systems have extreme vertical continuity (>1000 m), short strike length (400 m) and narrow width (100 m), exhibit vertical and lateral ore-type variation and have alteration haloes. Models of ore formation include classical hydrothermalism, structurally controlled remobilisation and polymodal concepts; syntectonic emplacement now holds sway.Fluid inclusions were examined from quartz±sulphide veins adjacent to now-extracted ore, from coexisting quartz–sulphide within ore, and from vughs in barren quartz veins. Lack of early primary inclusions precluded direct determination of fluids associated with D2–D3 ore and vein emplacement. Similarly, decrepitation (by near-isobaric heating) of the two oldest secondary populations precluded direct determination of fluid phases immediately following D2–D3 ore and vein emplacement. Post-decrepitation outflow (late D3 to early post-D3) is recorded by monophase CH4 inclusions. Entrained outflow of deeply circulated meteoric fluid modified the CH4 system; modification is recorded by H2O+CH4 and H2O+(trace CH4) secondary populations and by an H2O+(trace CH4) primary population. The contractional tectonics (D2–D3) of ore emplacement was superseded by relaxational tectonics (D4P) that facilitated meteoric water penetration and return flow.Under D2 prograde metamorphism, entrapment temperatures (Tt) and pressures (Pt) for pre-decrepitation secondary inclusions are estimated as Tt300–330 °C and Pt1.5–2 kbar≈Plith (the lithostatic pressure). Decrepitation accompanied peak metamorphism (T350–380 °C) in mid- to late-D3, while in late-D3 to early post-D3, essentially monophase CH4 secondary inclusions were entrapped at Tt350 °C and Pt=1.5–2 kbar≈Plith. Subsequently, abundant CH4 and entrained meteoric water were entrapped as H2O+CH4 secondaries under slowly decreasing temperature (Tt330–350 °C) and constant pressure (Pt1.5–2 kbar). Finally, with increasingly dominant meteoric outflow, H2O+(trace CH4) populations record decreasing temperatures (Tt>300 to <350 down to 275–300 °C) at pressures of Phydrostatic<Pt (1 kbar) <Plith (1.5 kbar).The populations of inclusions provide insight into fluid types, flow regimes and P–T conditions during parts of the deposit's evolution. They indirectly support the role of basin-derived CH4 fluids in ore formation, but provide no insight into a basement-sourced ore-forming fluid. They fully support post-ore involvement of meteoric water. The poorly constrained entrapment history is believed to span 10 Ma from 395 to 385 Ma.  相似文献   

17.
New data are discussed on the distribution of mercury in the host volcanosedimentary and igneous rocks, hydrothermal—metasomatic rocks, and all types of modern newly formed materials (hydrothermal clays, argillized soil—pyroclastic beds, siliceous and limonite—hematite covers, bottom sediments, salt ‘sweat-outs’ of various compositions, etc.) typical of the supergene zone of geothermal deposits. By the example of the Nizhne-Koshelevskii (vapor-dominated) and Pauzhetka (water-type) geothermal deposits and thermal fields of the Koshelevskii volcanic massif and Kambal’nyi volcanic range (southern Kamchatka), the role of mercury was illustrated as an indicator element of the temperature, phase state, and dynamics of hydrothermal systems; intensity of rock argillization; and relative age (maturity) of geothermal deposits and thermal anomalies.  相似文献   

18.
The Vani manganese deposit is located in the rugged NW sector of Milos Island. It occurs within the Vani volcano-sedimentary basin, which is underlain by dacitic domes and flows of Upper Pliocene age (3.5–2.0 Ma). The end of the emplacement of the dacites was marked by the collapse of the magma chamber, which resulted in a huge pyroclastic episode and the deposition of a thick layer of pyroclastic material within a shallow submarine basin. This pyroclastic material subsequently compacted to form the volcaniclastic sandstone, which became the host for the manganese ore beds which were about 4 m thick in the two sections studied. Hydrothermal fluids penetrated these sandstone horizons via fractures and fissures to produce the manganese deposit. The permeable nature of the sandstone facilitated the retention of the hydrothermal fluids within these layers. This permitted the fluids to cool slowly and deposit the manganese oxides almost quantitatively. Formation of the hydrothermal manganese deposit took place fairly rapidly over a period of several tens of thousands of years at most. Strong tectonic activity resulted in rapid uplift of the area which elevated the deposit above sea level.Two generations of manganese oxides have been identified within this deposit; the first generation consists of pyrolusite and ramsdellite; the second generation of oxides of the isostructural series cryptomelane–hollandite–coronadite plus hydrohetaerolite characterized by high contents of K, Ba, Pb and Zn, respectively. This sequence is the result of a two-stage process of formation of the manganese-oxide minerals in which a second high-salinity hydrothermal fluid enriched in Ba, Pb and Zn as a result of the dissolution of sulphide minerals remineralized the original manganese-oxide assemblage. It is this two-stage process of formation, which was mainly responsible for the unique characteristics of this deposit. Although formed in a submarine setting, the deposit shows marked differences in mineralogy and composition from known submarine hydrothermal manganese deposits and is most analogous to the epithermal vein deposits of the southwestern United States.  相似文献   

19.
Formation of volcanogenic massive sulfide deposits: The Kuroko perspective   总被引：3，自引：0，他引：3  
The main objective of this paper is to identify the geochemical, hydrological, igneous and tectonic processes that led to the variations in the physical (size, geometry) and chemical (mineralogy, metal ratios and zoning) characteristics of volcanogenic massive sulfide deposits with respect to space (from a scale of mining district size area to a global scale) and time (from a < 10 000 year time scale to a geologic time scale).All volcanogenic massive sulfide deposits (VMSDs) appear to have formed in extensional tectonic settings, such as at mid ocean spreading centers, backarc spreading centers, and intracontinental rifts (and failed rifts). All VMSDs appear to have formed in submarine depressions by seawater that became ore-forming fluids through interactions with the heated upper crustal rocks. Submarine depressions, especially those created by submarine caldera formation and/or by large-scale tectonic activities (e.g., rifting), become most favorable sites for the formation of large VMSDs because of hydrological, physical and chemical reasons.The fundamental processes leading to the formation of VMSDs include the following six processes:
1. (1) Intrusion of a heat source (typically a 103 km size pluton) into an oceanic crust or a submarine continental crust causes deep convective circulation of seawater around the pluton. The radius of a circulation cell is typically 5 km. The temperature of fluids that discharge on the seafloor increases with time from the ambient temperature to a typical maximum of 350°C, and then decreases gradually to the ambient temperatures in a time scale of 100 to 10 000 years. The majority of sulfide and sulfate mineralization occurs during the waxing stage of hydrothermal activity.
2. (2) Reactions between low temperature (T < 150°C) country rocks with downward percolating seawater cause to precipitate seawater SO2−4 as disseminated gypsum and anhydrite in the country rocks.
3. (3) Reactions of the “modified” seawater with higher-temperature rocks at depths during the waxing stage cause the transformation of the “seawater” to metal- and H2S-rich ore-forming fluids. The metals and sulfide sulfur are leached from the county rocks; the previously formed gypsum and anhydrite are reduced by Fe2+-bearing minerals and organic matter, providing additional H2S. The mass of high temperature rocks that provide the metals and reduced sulfur is typically 1011 tons ( 40 km3 in volume). The roles of magmatic fluids or gases are minor in most massive sulfide systems, except for SO2 to produce acid-type alteration in some systems.
4. (4) Reactions between the ore-forming fluids and cooler rocks in the discharge zone cause alteration of rocks and precipitation of some ore minerals in the stockwork ores.
5. (5) Mixing of the ore-forming fluids with local seawater within unconsolidated sediments and/or on the seafloor causes precipitation of “primitive ores” with the black ore mineralogy (sphalerite + galena + pyrite + barite + anhydrite).
6. (6) Reactions between the “primitive ores” with later and hotter hydrothermal fluids cause transformation of “primitive ores” to “matured ores” that are enriched in chalcopyrite and pyrite.
Variations in the mineralogical and elemental characteristics, the geometry, and the size of submarine hydrothermal deposits are controlled by the following four parameters:
1. (A) The chemical and physical characteristics of seawater (composition, temperature, density), which depend largely on the geographical settings (e.g., equatorial evaporating basins),
2. (B) The chemical and physical characteristics of the plumbing system (lithology, fractures),
3. (C) The thermal structure of the plumbing system, which is determined largely by the ambient geothermal gradient, and the size and temperature of the intrusive, and
4. (D) The physical characteristics of the seafloor (depth, basin topography).
For example, the submarine hydrothermal deposits developed in basaltic plumbing systems are generally poor in Pb and Ba compared to those developed in felsic plumbing systems. The lower temperature systems are generally poorer in sulfides, but richer in iron oxides and sulfates. The higher temperature and larger hydrothermal systems tend to produce chalcopyrite and pyrite rich ores. Contrasts in the metal ratios between the Noranda-type Archean VMSDs and the younger VMSDs reflect the differences in the geothermal gradient of the plumbing systems. The submarine hydrothermal deposits developed in the near equatorial regions tend to form large continuous bedded type ores because of the likeliness of creating large stratified basins.The basic processes of submarine hydrothermal mineralization have remained essentially the same throughout the geologic history, from at least 3.5 billion year ago to the present.  相似文献   

20.
The Ernest Henry Fe oxide Cu–Au (IOCG) deposit (>ca. 1.51 Ga) is hosted by breccia produced during the waning stages of an evolving hydrothermal system that formed a number of tens of metres to a kilometre scale, pre- and syn-ore alteration halos, although no demonstrable patterns have been attributed to fluids expelled through the outflow zones. However, the recognition of a population of hypersaline fluid inclusions representing the ‘spent’ fluids after Cu–Au deposition at Ernest Henry provides the basis to model the geochemical characteristics of the deposit's outflow zones. Geochemical modeling at 300 °C was undertaken at both high and low fluid/rock ratios via FLUSH models involving three host rock types: (1) granite, (2) calc–silicate rock, and (3) graphitic schist. In models run at high fluid/rock ratios, all rock types are essentially fluid-buffered, and produce an albite–quartz–hematite–barite-rich assemblage, although in low fluid–rock environments, the pH, redox, and geochemical character of the host rock exerts a greater influence on the mineralogy of the alteration assemblages (e.g., andradite, Fe–chlorite, and magnetite). Significant sulphide mineralization was predicted in graphitic schist where sphalerite occurred in both low- and high-porosity models, which indicates the possibility of an association between high-temperature IOCG mineralization and lower temperature base metal mineralization.Cooling experiments (from 300 to 100 °C) using the ‘spent fluids’ predict early high-T (300–200 °C) Na-, Ca-, Fe-, and Mn-rich, magnetite-bearing hydrothermal associations, whereas with cooling to below 200 °C, and with progressive fluid–rock interaction, the system produces rhodochrosite-bearing, hematite–quartz–muscovite–barite-rich assemblages. These results show that the radical geochemical and mineralogical changes associated with cooling and progressive fluid influx are likely to be accompanied by major transformations in the geophysical expression (e.g., spectral and magnetic character) of the alteration in the outflow zone, and highlight the potential link between magnetite- and hematite-bearing IOCG hydrothermal systems.  相似文献   

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