Hydrostatic Pressure Regime Research Articles

3D numerical modelling of fluid flow in the Val-d’Or orogenic gold district: major crustal shear zones drain fluids from overpressured vein fields

Fluid flow patterns have been determined using oxygen isotope isopleths in the Val-d’Or orogenic gold district. 3D numerical modelling of fluid flow and oxygen isotope exchange in the vein field shows that the fluid flow patterns can be reproduced if the lower boundary of the model is permeable, which represents middle or lower crustal rocks that are infiltrated by a metamorphic fluid generated at deeper levels. This boundary condition implies that the major crustal faults so conspicuous in vein fields do not act as the only major channel for upward fluid flow. The upper model boundary is impermeable except along the trace of major crustal faults where fluids are allowed to drain out of the vein field. This upper impermeable boundary condition represents a low-permeability layer in the crust that separates the overpressured fluid from the overlying hydrostatic fluid pressure regime. We propose that the role of major crustal faults in overpressured vein fields, independent of tectonic setting, is to drain hydrothermal fluids out of the vein field along a breach across an impermeable layer higher in the crust and above the vein field. This breach is crucial to allow flow out of the vein field and accumulation of metals in the fractures, and this breach has major implications for exploration for mineral resources. We propose that tectonic events that cause episodic metamorphic dehydration create a short-lived pulse of metamorphic fluid to rise along zones of transient permeability. This results in a fluid wave that propagates upward carrying metals to the mineralized area. Earthquakes along crustal shear zones cause dilation near jogs that draw fluids and deposit metals in an interconnected network of subsidiary shear zones. Fluid flow is arrested by an impermeable barrier separating the hydrostatic and lithostatic fluid pressure regimes. Fluids flow through the evolving and interconnected network of shear zones and by advection through the rock matrix. Episodic breaches in the impermeable barrier along the crustal shear zones allow fluid flow out of the vein field.

Superconductivity in the charge-density-wave state of the organic metalα−(BEDT−TTF)2KHg(SCN)4

The superconducting transition in the layered organic compound \alpha-(BEDT-TTF)_2KHg(SCN)_4 has been studied in the two hydrostatic pressure regimes where a charge-density wave is either present or completely suppressed. Within the charge-density-wave state the experimental results reveal a network of weakly coupled superconducting regions. This is especially seen in a strong enhancement of the measured critical field and the corresponding positive curvature of its temperature dependence. Further, it is shown that on lowering the pressure into the density-wave state traces of a superconducting phase already start to appear at a much higher temperature.

Evolution of the magmatic-hydrothermal acid-sulfate system at Summitville, Colorado: integration of geological, stable-isotope, and fluid-inclusion evidence

The Summitville Au–Ag–Cu deposit is a classic volcanic dome-hosted high-sulfidation deposit. It occurs in the Quartz Latite of South Mountain, a composite volcanic dome that was emplaced along the coincident margins of the Platoro and Summitville calderas at 22.5±0.5 Ma, penecontemporaneous with alteration and mineralization. A penecontemporaneous quartz monzonite porphyry intrusion underlies the district and is cut and overlain by pyrite–quartz stockwork veins with traces of chalcopyrite and molybdenite. Alteration and mineralization proceeded through three hypogene stages and a supergene stage, punctuated by at least three periods of hydrothermal brecciation. Intense acid leaching along fractures in the quartz latite produced irregular pipes and lenticular pods of vuggy silica enclosed sequentially by alteration zones of quartz–alunite, quartz–kaolinite, and clay. The acid-sulfate-altered rocks host subsequent covellite+enargite/luzonite+chalcopyrite mineralization accompanied by kaolinite, and later barite–base-metal veins, some containing high Au values and kaolinite. The presence of both liquid- and vapor-rich fluid inclusions indicates the episodic presence of a low-density fluid at all levels of the system. In the mineralized zone, liquid-rich fluid inclusions in healed fractures in quartz phenocrysts and in quartz associated with mineralization homogenize to temperatures between 160 and 390 °C (90% between 190 and 310 °C), consistent with the range (200–250 °C) estimated from the fractionation of sulfur isotopes between coexisting alunite and pyrite. A deep alunite–pyrite pair yielded a sulfur-isotope temperature of 390 °C, marking a transition from hydrostatic to lithostatic pressure at a depth of about 1.5 km. Two salinity populations dominate the liquid-rich fluid inclusions. One has salinities between 0 and 5 wt.% NaCl equivalent; the other has salinities of up to 43 wt.% NaCl equivalent. The occurrence of high-salinity fluid inclusions in vein quartz associated with mineralization, as well as in the deep stockwork veins, suggests that brines originating deep in the system transported the metals. The δ 34S values of sulfides in magnetite (−2.3‰) and of sulfate in apatite (5.4‰) in unaltered quartz latite indicate that δ 34S ΣS was near 0‰. The δ 34S values of coexisting alteration alunite and pyrite are 18.2‰ to 24.5‰ and −8.1‰ to −2.2‰, respectively. Deep in the system, most of the change in δ 34S values occurs in the sulfates, indicating that the fluids were initially H 2S-dominant, their redox state buffered at depth by equilibration with igneous rocks. However, in the main alteration zone, most of the change in δ 34S values occurs in pyrite, indicating that the fluids moved off the rock buffer and became SO 4 2−-dominant as pyrite precipitated and SO 2 disproportionation produced the sulfuric acid requisite for acid leaching. The δ 34S values of the late-stage barite and sulfides indicate that the system returned to high H 2S/SO 4 2− ratios typical of the original rock-buffered fluid. The δD H2O of alunite parent fluids was near −45‰ and their δ 18O ranged from 7‰ to −1‰, depending on the degree of exchange in the alteration zone at low water–rock ratio, or mixing with unexchanged meteoric water. The low δD values of some alunite samples are interpreted to result from postdepositional exchange with later ore fluids. Fluid exsolved from the magma at depth had δD H2O and δ 18O H2O values near −70‰ and 10‰, respectively. During and following migration to the top of the magma chamber, the fluid underwent isotopic exchange with the partially crystallized magma and its solid and cooler, but still plastic, carapace just below the transition from a lithostatic to hydrostatic pressure regime. These evolved magmatic fluids had δD H2O and δ 18O H2O values close to −40‰ and 5‰, respectively, prior to release into the superjacent hydrostatically pressured fracture zone, wherein the fluids separated into a low-density phase and hypersaline brine. The low-density phase rose to the level of acid-sulfate alteration, where disproportionation of SO 2 commenced upon condensation of the vapor plume causing the acid-sulfate alteration. Hydrogen and oxygen isotopic compositions estimated from deep kaolinitic wallrock alteration and fluid inclusions in quartz stockwork veins indicate the brine mixed with highly exchanged meteoric water at depth and rose to the ore zone, producing the Cu–Au–Ag mineralization. The composition of inclusion fluids in a single sample of enargite also indicates mixing between ore fluids and unexchanged meteoric water at shallow depths. The sulfate in fluids responsible for precipitation of most barite samples was of magmatic origin, but the δ 34S of a single sample suggests mixing with sulfate derived from the atmospheric oxidation of H 2S in an overlying steam-heated zone.

Effects of pore pressure on compressional wave attenuation in a young oceanic basalt

Laboratory data are reported for ultrasonic compressional wave attenuation (αp) as a function of pore pressure in a Juan de Fuca Ridge dredge basalt. Four experiments have been made to determine the relationships between attenuation and quality factor (Qp) and confining and pore pressures in the shallow ocean crust. Attenuation was measured at 1) a constant differential pressure of 40 MPa; 2) confining pressures to 120 MPa and atmospheric pore pressures; 3) a constant confining pressure of 50 MPa while varying pore pressures; 4) a constant confining pressure of 100 MPa while varying pore pressures. For atmospheric pore pressures, αp ranges from 1.64 dB/cm to 7.08 dB/cm (Qp = 24 to 10). In addition, attenuation increases systematically with increasing pore pressure and decreasing differential pressure (confining pressure ‐ pore pressure). Results from DSDP and ODP downhole packer experiments suggest that the hydrostatic pore pressure regime may best approximate in situ conditions for young oceanic crust. Hydrostatic pore pressures (@ 5000 meters depth) reduce Qp as much as 35% from normal atmospheric pressure conditions; therefore, pore pressures generated in the upper oceanic crust may be responsible in part for the observed low seismic Qp within layer 2A. Qp measurements at elevated pore pressures agree well with seismic Qp data.

Tectonic hydrothermal gold mineralisation in the outboard zone of the Southern Alps, New Zealand

Abstract Gold‐bearing veins cut a belt of low‐grade (pumpellyite‐actinolite/greenschist facies) schist in the Ben Ohau Range to the east of the Main Divide, in the outboard zone of the Southern Alps continental collisional zone, New Zealand. The schist has been exposed along the currently active Ostler Fault system, which has had c. 5 km of reverse motion since the Pliocene. The veins consist of quartz, ankerite, calcite, chlorite, and pyrite, with minor chalcopyrite and galena. Hydrothermal chlorite contains about 490 ppm Zn, and the gold contains 3–5 wt% Ag. Hydrothermal alteration of host rock is minor apart from Sr enrichment (up to four times background). Fluid inclusions in quartz are aqueous with minor dissolved CO2 and salts (<4 wt% NaCl equivalent), and homogenise at 236–270°C. The veins formed at 300 ± 20°C and 1000 ± 800 bars fluid pressure, probably under a hydrostatic fluid pressure regime. Oxygen and carbon isotopic data (δ18O = +10 to +14; δ13C = ‐6 to ‐10‰) are similar to data from economic metamorphogenic Au deposits of the nearby Otago Schist, but minor meteoric incursion may have occurred. Isotopic data are also similar to veins formed in the inboard zone of the Southern Alps orogen. The Ben Ohau veins demonstrate that gold can be concentrated in low‐grade schists distant from the most active part of the hydrothermal system driven by continental collision.

Diagenesis as Related to Burial History, Pressure Regimes, and Low-Permeability Zones in the Upper Cretaceous Lower Tuscaloosa Formation, Mississippi and Louisiana: ABSTRACT

Sandstone reservoirs in the lower part of the Upper Cretaceous Tuscaloosa Formation in the Gulf Coast contain large petroleum and natural gas reserves and many are presently overpressured at depths greater than 3,350 to 4,900 m. A petrologic study was initiated to determine the types of diagenetic seals present and the timing of cementation. Hydrocarbons stain most authigenic phases including early iron-rich chlorite and quartz overgrowths and later pore-filling kaolinite and albite, and they are present in dissolution vugs in both ferroan and nonferroan carbonate cements from depths of 2,800 to 6,300 m. In sandstones from 4,575 to 6,300 m depths, two-phase hydrocarbon and hydrous inclusions are abundant between detrital quartz grains and their overgrowths and are common within the overgrowths. Uncorrected (because of included petroleum and (or) methane ) homogenization temperatures of these inclusions average 125[degrees]C, indicating that they formed in the late oil or early gas window. Present bottom hole temperatures of wells in the lower Tuscaloosa at depths of about 4,600 m average 145[degrees]C and temperatures at depths of about 5,600 m average 170[degrees]C. These values indicate that the quartz overgrowths formed before maximum burial of the Tuscaloosa. According to published hydrologic models, the Tuscaloosa was undermore » a hydrostatic pressure regime through the early Tertiary. Burial history reconstructions indicate that shales in the Tuscaloosa in central Louisiana reached the gas generation window (about 125[degrees]C) at a depth of 4 km in the Miocene. At this time, much of the lower Tuscaloosa in the downdip gas trend (greater than approximately 4,300 m) became overpressured and silica and hydrocarbon (pyrobitumen ) cements created low-permeability zones in sandstones, preserves overpressured compartments and trapping hydrocarbons.« less

Relationships between a porphyry Cu-Mo deposit, base and precious metal veins and Laramide intrusions, Mineral Park, Arizona

In the Wallapai mining district, porphyry-style copper-molybdenum mineralization occurs within and near Laramide granitoid stocks at the center of an elongate zone of polymetallic quartz veins. The principal mineralized veins form a well-defined paragenetic sequence, with a simplified mineralogy progressing through: (1) anhydrite-molybdenite (AM) veins of quartz + K-feldspar + anhydrite + pyrite + biotite + molybdenite, (2) quartz-molybdenite (QM) veins with quartz + molybdenite + pyrite, (3) anhydrite-chalcopyrite (AC) veins containing quartz + K-feldspar + anhydrite + pyrite + magnetite + chalcopyrite + chlorite + rutile, (4) quartz-pyrite (QP) veins of quartz + sericite + pyrite, and quartz + pyrite + chalcopyrite + epidote + chlorite, and (5) polymetallic quartz (PMQ) veins, with a gangue assemblage of quartz + sericite + calcite + kaolinite(?) + epidote + chlorite, and a variety of base and precious metal minerals.The coexistence of vapor- and liquid-rich fluid inclusions in vein quartz shows that the molybdenite-bearing anhydrite and quartz veins formed at 360 degrees to 410 degrees C from boiling, low-salinity brines. Chalcopyrite deposition in the anhydrite-chalcopyrite veins was at 380 degrees to 420 degrees C from nonboiling brines averaging 20 equiv wt percent NaCl and 25 equiv wt percent KCl. The quartz-pyrite veins formed at 320 degrees to 350 degrees C from brines of 1 to 13 equiv wt percent NaCl. The polymetallic quartz veins formed from dilute brines of 1 to 7 equiv wt percent NaCl. Temperatures in the first stage of mineralization of the polymetallic-quartz veins abruptly increased to 400 degrees to 450 degrees C in and near Mineral Park, but declined outward to less than 300 degrees C on the district periphery. In subsequent stages, fluid temperatures decreased across the district to less than 200 degrees C. Pressures were 250 to 300 bars, probably in a hydrostatic pressure regime, during boiling related to molybdenite mineralization, but they may have increased to about 1,100 bars lithostatic pressure during chalcopyrite deposition in the anhydrite-chalcopyrite veins. A depth during mineralization of 3 to 4 km is suggested.Copper and molybdenum mineralization occurred in a lithocap environment above a progenitor intrusion but was not directly related to the exposed Ithaca Peak stocks. The evolution from hypersaline fluids in the anhydrite-chalcopyrite veins to low-salinity, lower temperature fluids in the quartz-pyrite veins suggests an influx of meteoric water into the waning porphyry-style hydrothermal system. Brines up to 100 degrees C hotter appeared during the formation of the paragenetically later polymetallic quartz veins and constituted a new, and much larger, hydrothermal system. The heat source, presumably an unexposed intrusion, was much larger than the porphyry-style system. The zonation in base metal ratios of polymetallic quartz veins is symmetrical with respect to the proposed heat source.

A fluid inclusion study of the Panguna porphyry copper deposit, Bougainville, Papua New Guinea

Copper mineralization at Panguna took place at a pressure of 200 to 300 bars and at temperatures between 350 degrees and 700 degrees C or higher. Cu, Fe sulfides, quartz, anhydrite, and hematite were deposited by dense, boiling, salt-rich liquids of magmatic origin. The composition of the salt-rich liquids (those which nucleated both KCl and NaCl in fluid inclusions) in terms of the system NaCl-KCl-H 2 O varied between 76 percent salts (60% NaCl, 16% KCl) and 46 percent salts (30% NaCl, 16% KCl) by weight. Other liquids, apparently more dilute, nucleated only NaCl. The salt-rich liquids also contained Fe, Ca, and SO 4 and minor quantities of Mg, Cu, Mn, and Zn. A Cu concentration of 1,900 ppm has been estimated in one liquid. The atomic K/Na ratios of salt-rich liquids from three principal phases of vein mineralization and from quartz pheno-crysts conformed to a single trend, increasing from 0.17 to 0.46 as the NaCl content decreased. The first phase of mineralization (phase A) took place when the southern part of the Kaverong Quartz Diorite was at temperatures higher than 700 degrees C. The temperature and salinity distributions for this phase were asymmetric; cooling and dilution of the salt-rich liquids took place south and southwest of the orebody but have not been detected to the north or east. The zone of cooling and dilution corresponds closely with the 0.3 percent Cu ore-grade contour and the edge of major quartz deposition and is paralleled by a pyritic halo. The system cooled below 400 degrees C before undergoing renewed mineralization at temperatures over 400 degrees C in the two approximately concurrent phases B and C. These phases were accompanied by the intrusion of porphyritic stocks. Phase B formed a well-defined cell bounded by a pyritic halo and centered on the Leucocratic Quartz Diorite. Phase C was expressed as veining of the Biotite Granodiorite, the Biuro Granodiorite, and the area between them. Ground water, generally of less than 10 percent salinity, inundated the orebody between phase A and phases B and C, and again after phases B and C, at temperatures below 400 degrees C. Within the orebody there was no salinity continuum between ground water and the salt-rich liquids. Ground water deposited quartz-pyrite and probably pyrite-clay and sphalerite-pyrite veins at temperatures near 300 degrees C and caused local phyllic alteration. The distribution of fluids in space and time is consistent with the porphyry copper model in which a magmatic-hydrothermal system is surrounded by a cooler meteoric-hydrothermal system, the latter collapsing inward as the former wanes. Given a hydrostatic pressure regime in the meteoric-hydrothermal system, the depth of formation was 2 to 3 km.Extract from Melanesian pidgin abstractLiklik Tok I Autim Bikpela Tok I Kamap AnanitBipo tru, kopa i kam pinis long graun long Panguna taim graun i hat moa moa yet, na taim ston i karamapim main inap tripela kilomita. Wara i bin karkarim kopa i kam. na dispela lain wara i hevi tru bikos em i gat planti sol insait long en. ...TranslationAbstract (Literally, little talk (which) divulges big talk that follows underneath)A long time ago copper arrived in the ground at Panguna when the ground was very very hot and when the mine was covered by 3 kilometers of rock. Water carried the copper in, and this type of water was very dense because it contained much salt. . . .