Quartz vein fabrics coupled to elevated fluid pressures in the Stawell gold deposit,south-eastern Australia     

C. J. L. Wilson  J. A. Robinson  A. L. Dugdale 《Mineralium Deposita》2009,44(3):245-263

Shear and extensional veins formed during the reactivation of the Magdala shear system at Stawell in western Victoria, Australia, contribute to the formation of the auriferous Central and Basalt Contact lodes. Within this shear system is a range of fault rocks accompanied by steep-dipping (>65°) quartz-rich laminated shear veins and relatively flat-lying extensional veins. Both vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant deviatoric stresses. The macro- and microstructures suggest that the dilatancy, that produced mineralized veins, formed under conditions of overpressure generated by fluid infiltration late in a tectonic regime. A new microfabric analysis technique is used to investigate the quartz-rich veins, which allows rapid integration of the microstructure with the crystallographic preferred orientations (CPOs). Both the shear and extensional quartz veins have a random CPO with ∼120° dihedral angles between the quartz–quartz grains, which is typical of a metamorphic equilibrium microfabric. The microstructures indicate that the quartz has undergone extensive grain adjustment in the solid-state, with grain shape and size affected by interfacial solution (pressure solution) effects. These features are consistent with inferences from experimental rock deformation studies, where grain boundary migration is enhanced in a water-rich environment. The onset of solution-transfer processes (pressure solution) developed as the quartz microfabric stabilized and continued to modify the CPO and microstructure significantly. It is concluded that grain growth and pressure solution are coupled diffusive mass transfer processes, related to fluctuations in pore fluid pressures in a region undergoing deformation at near lithostatic pressures.  相似文献   

A geochemical study of the Sweet Home Mine,Colorado Mineral Belt,USA: hydrothermal fluid evolution above a hypothesized granite cupola   总被引：1，自引：0，他引：1  

Volker Lüders  Rolf L. Romer  H. Albert Gilg  Robert J. Bodnar  Thomas Pettke  Dean Misantoni 《Mineralium Deposita》2009,44(4):415-434

Deposition of quartz–molybdenite–pyrite–topaz–muscovite–fluorite and subsequent hübnerite and sulfide–fluorite–rhodochrosite mineralization at the Sweet Home Mine occurred coeval with the final stage of magmatic activity and ore formation at the nearby world-class Climax molybdenum deposit about 26 to 25 m.y. ago. The mineralization occurred at depths of about 3,000 m and is related to at least two major fluid systems: (1) one dominated by magmatic fluids, and (2) another dominated by meteoric water. The sulfur isotopic composition of pyrite, strontium isotopes and REY distribution in fluorite suggest that the early-stage quartz–molybdenite–pyrite–topaz–muscovite–fluorite mineral assemblage was deposited from magmatic fluids under a fluctuating pressure regime at temperatures of about 400°C as indicated by CO₂-bearing, moderately saline (7.5–12.5 wt.% NaCl equiv.) fluid inclusions. LA-ICPMS analyses of fluid inclusions in quartz demonstrate that fluids from the Sweet Home Mine are enriched in incompatible elements but have considerably lower metal contents than those reported from porphyry–Cu–Au–Mo or Climax-type deposits. The ore-forming fluid exsolved from a highly differentiated magma possibly related to the deep-seated Alma Batholith or distal porphyry stock(s). Sulfide mineralization, marking the periphery of Climax-type porphyry systems, with fluorite and rhodochrosite as gangue minerals was deposited under a hydrostatic pressure regime from low-salinity ± CO₂-bearing fluids with low metal content at temperatures below 400°C. The sulfide mineralization is characterized by mostly negative δ³⁴S values for sphalerite, galena, chalcopyrite, and tetrahedrite, highly variable δ¹⁸O values for rhodochrosite, and low REE contents in fluorite. The Pb isotopic composition of galena as well as the highly variable ⁸⁷Sr/⁸⁶Sr ratios of fluorite, rhodochrosite, and apatite indicates that at least part of the Pb and Sr originated from a much more radiogenic source than Climax-type granites. It is suggested that the sulfide mineralization at the Sweet Home Mine formed from magmatic fluids that mixed with variable amounts of externally derived fluids. The migration of the latter fluids, that were major components during late-stage mineralization at the Sweet Home Mine, was probably driven by a buried magmatic intrusion.  相似文献   

Genesis of the Assif El Mal Zn–Pb (Cu,Ag) vein deposit. An extension-related Mesozoic vein system in the High Atlas of Morocco. Structural,mineralogical, and geochemical evidence     

Mohammed Bouabdellah  Georges Beaudoin  David L. Leach  Fidel Grandia  Esteve Cardellach 《Mineralium Deposita》2009,44(6):689-704

The Assif El Mal Zn–Pb (Cu–Ag) vein system, located in the northern flank of the High Atlas of Marrakech (Morocco), is hosted in a Cambro-Ordovician volcaniclastic and metasedimentary sequence composed of graywacke, siltstone, pelite, and shale interlayered with minor tuff and mudstone. Intrusion of synorogenic to postorogenic Late Hercynian peraluminous granitoids has contact metamorphosed the host rocks giving rise to a metamorphic assemblage of quartz, plagioclase, biotite, muscovite, chlorite, amphibole, chloritoid, and garnet. The Assif El Mal Zn–Pb (Cu–Ag) mineralization forms subvertical veins with ribbon, fault breccia, cockade, comb, and crack and seal textures. Two-phase liquid–vapor fluid inclusions that were trapped during several stages occur in quartz and sphalerite. Primary inclusion fluids exhibit T _h mean values ranging from 104°C to 198°C. Final ice-melting temperatures range from −8.1°C to −12.8°C, corresponding to salinities of ∼15 wt.% NaCl equiv. Halogen data suggest that the salinity of the ore fluids was largely due to evaporation of seawater. Late secondary fluid inclusions have either Ca-rich, saline (26 wt.% NaCl equiv.), or very dilute (3.5 wt.% NaCl equiv.) compositions and homogenization temperatures ranging from 75°C to 150°C. The δ¹⁸O and δD fluid values suggest an isotopically heterogeneous fluid source involving mixing between connate seawater and black-shale-derived organic waters. Low δ¹³C_VPDB values ranging from −7.5‰ to −7.7‰ indicate a homogeneous carbon source, possibly organic matter disseminated in black shale hosting the Zn–Pb (Cu–Ag) veins. The calculated δ³⁴S_H2S values for reduced sulfur (22.5‰ to 24.3‰) are most likely from reduction of SO₄ ²⁻ in trapped seawater sulfate or evaporite in the host rocks. Reduction of sulfate probably occurred through thermochemical sulfate reduction in which organic matter was oxidized to produce CO₂ which ultimately led to precipitation of saddle dolomite with isotopically light carbon. Lead isotope compositions are consistent with fluid–rock interaction that leached metals from the immediate Cambro-Ordovician volcaniclastic and metasedimentary sequence or from the underlying Paleo-Neoproterozoic crustal basement. Geological constraints suggest that the vein system of Assif El Mal formed during the Jurassic opening of the central Atlantic Ocean.  相似文献   

Fluid–rock interaction at the roots of an Eopaleozoic epithermal system, Sierra Norte de Córdoba, Argentina: Paragenesis, thermometry and fluid sources     

R. Lira  M.F. Poklepovic  M.J. Dorais  H.A. Millone  G.M. Gomez 《Journal of South American Earth Sciences》2009,28(3):263-276

A deep epithermal vein system hosted in Late Proterozoic to Cambrian granodiorite has been identified in the Sierra Norte de Córdoba, the easternmost range of the Sierras Pampeanas Orientales of Argentina. The vein swarm extends over an area of 3 km² parallel to a mylonitic belt and formed in fractured granodiorite. Thicknesses of veins are less than 0.5 m and their visible strike length is less than 100 m. Veins are either barren or weakly mineralized in base-metal sulfides. Most veins have mineral associations dominated by calcite and quartz with lesser amounts of chlorite, sericite, pyrite, and minor illite. In other less exposed albite-rich, adularia-bearing veins, chalcopyrite, bornite, galena, sphalerite, chalcocite and covellite may occur. The widespread occurrence of bladed calcite without any petrographic or microthermometric evidence of boiling implies that this particular habit of calcite may also develop under sub-near boiling fluid conditions. Thermometric calculations based on fluid inclusion data, chlorite composition and oxygen isotopes in the quartz–calcite pair, constrain the formation of the system between 300 and 350 °C, at pressures between 42 and 64 MPa (1.5–2.3 km). Stable isotope data suggest that W/R interaction might have been the most probable mechanism of alteration, involving the participation of meteoric fluids; nevertheless, the metallic signature of some weakly mineralized veins as well as intermediate fluid inclusion salinities favor a magmatic input and a mixed origin for the fluids. Textures and mineral associations, as well as the absence of evidence of boiling in fluid inclusions, all suggest that the silica–carbonate vein system formed deeper than typically shallow Au and Ag-bearing boiling solutions. A 485 (±25) Ma lamprophyre dike crosscuts some of these veins locally producing metasomatic reactions and skarn formation, which constrains the age of the hydrothermal system to the Cambrian-Early Ordovician time span.  相似文献   

Fluid-pressure controlled soft-bed deformation sequence beneath the surging Breiðamerkurjökull (Iceland, Little Ice Age)     

Michaël Denis  Jean-Franois Buoncristiani  Michel Guiraud 《Sedimentary Geology》2009,221(1-4):71-86

The general subject of this paper is subglacial deformation beneath Breiðamerkurjökull, a surging Icelandic glacier. More specifically it discusses the evolution and the role of fluid pressure on the behaviour of subglacial sediments during deformation. During Little Ice Age maximum, the two outcrops studied, North Jökulsarlon (N-Jk) and Brennhola-Alda (BA), were located at 2550 m and 550 m respectively from the front of the Breiðamerkurjökull. Sedimentological analysis at the forefield of the glacier shows thick, coarse glaciofluvial deposits interbedded with thin, fine-grained shallow lacustrine/swamp deposits, overlain by a deformed till unit at N-Jk. BA outcrop shows fine-grained shallow lacustrine/swamp deposits overlain by a deformed till unit. The sequence of deformation events from one outcrop to the other is similar. First, major thrust planes, which were rooted in shallow lacustrine/swamp deposits developed by glacially induced simple shear. Next, the thrusts were folded, indicating the deformation of hydroplastic sediment assisted by moderate fluid pressure. Then clastic dyke swarms crosscut the sedimentary succession, proving that fluid overpressure caused hydrofracturing associated with fluidisation. Finally, as water escaped from the glacier bed, fluid pressure dropped, and normal faulting occurred in brittle-state subglacial sediments. Fluid-pressure variations are related to glacier dynamics. They control the deformation sequence by modifying subglacial rheological behaviour and the nature of the subglacial tectonism.  相似文献   

An experimental investigation of gravity-driven shale tectonics in progradational delta   总被引：1，自引：0，他引：1  

R. Mourgues  E. Lecomte  B. Vendeville  S. Raillard 《Tectonophysics》2009,474(3-4):643-656

Many deltas exhibit gravitational deformation of their sedimentary cover. In these systems, the décollement layers do not always consist of rock salt but sometimes of overpressured shale. Unlike salt, the efficiency of detachment in shale depends on the magnitude of fluid overpressures and it varies through time and space, as rapid sedimentary burial progrades into deeper water. As a result, the gravity deformational domains are progressively translated seaward. Sandbox models involving high air pore pressures were used to simulate such gravity-driven shale tectonics in prograding deltas. Models were built with sand of various permeabilities and air was injected to simulate the mechanical effects of fluid overpressure. Our apparatus for the injection of air allowed us to control subsurface pressures in space and time during the experiments, and it was used to simulate the advance of the front of the overpressured domain during the sedimentary progradation. In our models, sand kept obeying a frictional behavior, for medium to high pore pressures, and the detachment appeared as very thin shear bands. Compressional belts that formed during the experiment were dominated by asymmetric basinward-verging fore-thrusts, as is often observed in deep-water, shale-detached foldbelts. Where the value of fluid pressures approached that of the lithostatic stress, sand was fluidized, resulting in ductile strains analogous to what occurs in highly overpressured mobile shale. During progradation, ancient buried thrustbelts were reactivated, thereby controlling later extension. During the experiments, sand volcanoes, analogous to mud volcanoes, formed in relation with tectonic structures. Some of them developed near normal faults but many of them formed directly above old buried thrusts.  相似文献   

Genesis and evolution of the structurally controlled vein mineralization (Sb–Hg) in the Escarlati deposit (León,Spain): Evidence from fault population analysis methods,fluid-inclusion research and stable isotope data     

A. Martín-Izard  P. Gumiel  M. Arias  A. Cepedal  M. Fuertes-Fuente  R. Reguilón 《Journal of Geochemical Exploration》2009

The Escarlati deposit is located in the Cantabrian Zone of the Variscan Massif and is one of the best examples in the Iberian Peninsula of Sb and Hg both coexisting in the same paragenesis. The Sb–Hg mineralization appears filling hydraulic and collapse breccias hosted in Late-Variscan fractures affecting Carboniferous black limestones.  相似文献   

新疆阿拉套山成矿作用：区域背景、矿源和模式          下载免费PDF全文

倪守斌  满发胜  陈江峰  孙盼盼 《地质科学》2009,44(1):128-136

本文研究了新疆阿拉套山地区成矿作用的区域地质背景、成矿物质来源、成矿的物理化学条件和矿质的迁移富集,据此建立了该区的成矿模式,提供了一个在较小范围内研究和对比两类花岗岩对应两套成矿系列的典型范例.  相似文献   

Coupled trace element mobilisation and strain softening in quartz during retrograde fluid infiltration in dry granulite protoliths   总被引：1，自引：0，他引：1  

Bjørn Eske Sørensen  Rune B. Larsen 《Contributions to Mineralogy and Petrology》2009,157(2):147-161

This study focuses on the retrograde rheological and chemical evolution of quartz and the behaviour of quartzites during retrograde metamorphism following dry high grade metamorphism at 750°C, 7 kbar. SEM-CL and LA-HR-ICP-MS are applied to document quartz texture and chemistry, respectively. Four generations of quartz were distinguished by SEM-CL; Qz1, Qz2, Qz3 and Qz4. Qz1, brecciated and partly dissolved old grains, is enriched in B, Al and Ti when compared with the other types. Qz2, formed during brecciation and partial dissolution of Qz1, has low Al contents (<50 ppm) but, due to rutile inclusions, variable Ti contents when occurring in amphibolite (210–10 ppm) but more consistent values when occurring in quartzites (peak value 32 ppm). Qz3, dark grey luminescent quartz forming fluid migration channels (fluid pathways), has Ti < 5 ppm and Al contents below 10 ppm and B < 1 ppm. Qz4, comprises are group of quartz later than Qz3 filling micron thick cracks and pods with very low luminescent quartz, i.e. darker than Qz3. The textural and chemical evolution of quartz in our study is explained by two major influxes of aqueous fluids during regional uplift and retrogression. They facilitated rehydration and recrystallisation in the otherwise dry high grade quartzites. The first introduction of aqueous fluids was associated with brecciation of the high grade quartz (Qz1) and dissolution/precipitation of quartz (Qz2). Ti in quartz geothermometry (Wark and Watson, Contrib Mineral Petrol 152(6):637–652) gives 626°C in agreement with the retrograde PT-path deduced from phase diagrams. Later fluid influx associated with scapolitisation of amphibolite caused localised recrystallisation (Qz3) and alteration of biotite to muscovite along mm-wide fluid migration channels. During subsequent deformation, Qz3 deformed plastically and recovered by subgrain rotation recrystallisation (SGR), resulting in a reduction of grain size, whereas Qz1 quartz formed micro faults. Qz2 was plastic but did not experience SGR to the same degree as Qz3 quartz. Increased plasticity and recovery rates most likely relate to an increased H₂O fugacity and the depletion in trace elements of the quartz lattice by promoting strain softening processes dislocation climb and recovery. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

Upscaling of Navier–Stokes equations in porous media: Theoretical, numerical and experimental approach     

Guillermo A. Narsilio  Olivier Buzzi  Stephen Fityus  Tae Sup Yun  David W. Smith   《Computers and Geotechnics》2009,36(7):1200-1206

The accurate estimation of hydraulic conductivity is important for many geotechnical engineering applications, as the presence of fluids affects all aspects of soil behaviour, including its strength. Darcy’s law is the key experimental (or phenomenological) equation employed to model ground water flow. Yet, this phenomenological equation can be linked to a more fundamental microscale model of flow through the pore spaces of the porous material. This paper provides an experimental verification of the relationships between Darcy’s law (macroscale) and the Navier–Stokes equations (microscale) for actual complex pore geometries of a granular material. The pore geometries are experimentally obtained through state-of-the-art X-ray computer assisted micro-tomography. From the numerical modelling of the microscale flow based on actual pore geometries, it is possible to quantify and visualize the development of pore-scale fluid preferential flow-paths through the porous material, and to assess the importance of pore connectivity in soil transport properties.  相似文献