Fluid Inclusion Characteristics Research Articles

Mineralogic, alteration and fluid-inclusion studies of epithermal gold-bearing veins at the Mt. Muro Prospect, Central Kalimantan (Borneo), Indonesia

The Mt. Muro prospect contains numerous steeply dipping, gold-bearing quartz veins that formed within an epithermal environment during mid-Tertiary calc-alkaline volcanism of Central Kalimantan. The two centers of mineralization studied, Gunung (Mt.) Baruh and Luit, are approximately 10 km apart, and represent the upflow zones of former geothermal systems. Gold mineralization throughout the region is structurally controlled, and grades are unevenly distributed within veins. The Kerikil veins at Gunung Baruh are characteristic of the mineralization style and are one of the most extensively drilled targets. These vein structures trend north-northwest and extend discontinuously over a 1000-m strike along the west flank of Gunung Baruh. Host rock consist of porphyritic andesite flows, a hypabyssal basaltic andesite intrusion, and volcanic breccias. Primary sulfide mineralization within the Kerikil veins occurred in four district stages related to hydrothermal brecciation (stage II), and local occurrences of laminated quartz-sulfide veins (stage IV). Quartz is the predominant gangue mineral throughout, with subordinate adularia, calcite, and rhodochrosite. Electrum and acanthite are fine grained (≤30 μm) and associated with minor disseminated pyrite, sphalerite, galena, chalcopyrite, and covellite. Bleached zones of quartz-illite-pyrite envelop the mineralized veins, but where host rock was less permeable, a weak alteration assemblage of epidote-chlorite-albite ± pyrite ± illite ± calcite formed instead. Fluid inclusion studies indicate that mineralizing solutions were dilute, ≤4.1 eq. wt.% NaCl, < 4.0 wt.% CO 2, and ranged between 207 and 253°C. These alteration and fluid inclusion features are characteristic of deeply convecting, near-neutral pH, alkali-chloride fluids found in active geothermal systems. Fluid-inclusion and mineralogic evidence suggest that fluids were boiling under conditions which favored gold deposition; however, most gold mineralization appears restricted spatially and temporally to the timing of hydrothermal brecciation. Mineralization at Luit is similar, although exposed veins here apparently formed at greater depths and higher temperatures (238–262°C); gold grades diminish sharply with depth. During the late collapse of the geothermal system, shallow acid fluids (produced from steam condensate) descended preferentially down vein structures and reacted with adjacent host rocks to produce a late kuolin overprint. These same fluids also partially digested vein carbonate, adularia and rock fragments, forming enhanced secondary permeability within vein structures. This secondary permeability was important in the development of supergene oxidation; the latter locally extends to 100 m below the present surface. Erratic supergene gold enrichment is mostly restricted to depths less than 25 m below the present surface and is closely associated with iron and manganese oxides; however, comparison of Au, Ag, Mn, Fe, and combined Cu+Pb+Zn contents between oxide and sulfide zone samples does not show significant trends. Microprobe analyses of supergene electrum indicate an average composition of 71 wt.% Au and 29 wt.% Ag, which is very similar to the average composition of primary electrum associated with sulfides. These compositions suggest that thiosulfate complexes in groundwaters acted as transporting agents for remobilization of gold during oxidation. Gold and silver adsorption onto amorphous oxides was ultimately important for their reprecipitation; however, where steeply dipping veins transect steep terrain, some gold may have been lost through dispersion into nearby soils and subsequent erosion.

Fluid inclusion studies on the gold deposits of the upper Anzasca Valley, northwestern Alps, Italy

The gold deposits of the upper Anzasca Valley are part of the Monte Rosa gold district, northwestern Alps, Italy. The mineralization occurs within polymetamorphic Paleozoic metasedimentary and metagranitic rocks of the Pennidic Monte Rosa unit. The orebodies consist of more or less concordant veins where they are hosted by metasedimentary rocks, whereas in the metagranites they occupy fractures and shear zones discordant with the foliation. Field evidence suggests, however, a common late Alpine age for the emplacement of both types of orebodies.The mineralogy of the orebodies is dominated by quartz. Gold is invariably associated with sulfides (pyrite, arsenopyrite, galena, sphalerite, chalcopyrite, and sulfosalts).The characteristics of fluid inclusions in quartz from gold-bearing veins are remarkably consistent: the inclusions are moderately saline brines containing, at room temperature, about 20 to 60 percent vol of a low-density, rather pure CO 2 phase; they were probably homogeneous at the time of trapping. Small amounts of CH 4 occur in some inclusions. Total homogenization occurs either in the liquid or in the gas phase, or by near-critical phenomena, at temperatures mostly on the order of 300 degrees to 350 degrees C. By combining these temperatures with geologic pressure estimates and with temperatures inferred from arsenopyrite compositions, trapping conditions of about 400 degrees to 450 degrees C at pressures of 1 to 1.5 kbars are derived. A less common, presumably later, type of CO 2 -poor inclusion is interpreted as a late influx of cooler and less saline fluids.Quartz from barren veins also contains CO 2 -bearing inclusions; however, these show higher T (sub m clathrate ) , lower T (sub h (sub CO 2 ) ) , and lower total homogenization temperatures than mineralized veins.Results of reconnaissance studies on fluid inclusions from other gold deposits (Val Sesia, Val Toppa) in the Monte Rosa district are somewhat different from those of the upper Anzasca Valley: the CO 2 phase is of higher density, and total homogenization temperatures are lower.Combined with field evidence and available Pb and O isotope data, the fluid inclusion characteristics are consistent with a model of ore genesis in which the gold deposits of the upper Anzasca Valley were formed in the late stages of the Alpine orogeny by presumably metamorphic fluids, probably produced by devolatilization of deep unexposed portions of Monte Rosa crustal rocks. The strict association of gold with sulfides suggests that gold possibly was transported as thio complexes. Possible precipitation mechanisms include cooling, dilution, minor wall-rock reaction, and decrease of sulfur activity in the fluid.Similar processes probably were active in the formation of other neighboring gold deposits, but there is evidence that gold deposition, on a regional scale, was related to at least two distinct hydrothermal systems separate in space and also in time.

P-T evolution and fluid inclusion characteristics of retrograded eclogites, M�nchberg Gneiss Complex, Germany

Kyanite eclogites occur as part of the Munchberger nappe pile in NE-Bavaria, West Germany. Eclogites are overprinted by subsequent amphibolite facies metamorphism. The preservation of primary eclogitic textures as well as symplectitic textures are indicative of rapid decompression. Eclogite formation is estimated to have occurred under conditions of high H2O-activities at pressures between 20 and 26 kbar and temperatures ranging between 590 and 660° C, as is shown by the coexistence of omphacite (Jd 50), kyanite, zoisite and quartz. Minimum pressure estimates, independent of the water activity, range between 9 and 16 kbar at the relevant temperatures. Detailed studies of fluid inclusion reveal two predominant groups of aqueous-brine inclusions: high salinity (14–17 wt% NaCl equiv.) and low salinity (0–8 wt% NaCl equiv.) inclusions. Fluid compositions of both groups of inclusions yield isochores passing close to the estimated amphibolite facies PT-field. The compositions of these fluids are in good agreement with fluid compositions considered from mineral equilibria. None of the fluid inclusions has densities appropriate for eclogite facies metamorphism, but probably reflect later amphibolite facies metamorphism.

Geologic, mineralogic, and fluid inclusion characteristics of polymetallic veins, Real de Guadalupe mining district, Guerrero

Geologic, mineralogic, and fluid inclusion characteristics of polymetallic veins, Real de Guadalupe mining district, Guerrero

Fluid inclusion characteristics and U-Pb rutile age of late hydrothermal alteration and veining at the Musoshi stratiform copper deposit, Central African copper belt, Zaire

Hydrothermal veining at the Musoshi stratiform copper deposit in Zaire has caused extensive alteration in the arkosic footwall sediments and in the mineralized ore shale (argillaceous siltstone) horizon. Pervasive growth of secondary rutile, biotite, and muscovite in the wall rocks accompanied this activity, whereas hematite, biotite, calcite, K-feldspar, barite, and accessory rutile occur with quartz in footwall veins. In contrast, veins cutting the ore shale carry Cu-Fe sulfides, uraninite, and molybdenite, with quartz, calcite, biotite, K-feldspar, and minor rutile. Linear zones of particularly intense alteration and veining probably represent structurally controlled fluid conduits. Fluid inclusions in vein quartz from such zones identify an apparently halite-saturated liquid at 397 degrees + or - 5 degrees C, with approximately 39 wt percent NaCl, 15 wt percent KCl, and minor amounts of CO 2 and other components.Rutile is found pervasively, both as a wall-rock alteration phase and as large euhedra (up to 3 cm) in the quartz veins. The hydrothermal growth of this normally very insoluble mineral is explained by the high temperature and salinity of the aqueous phase.The widespread occurrence of rutile in the deposit makes it suitable for determining the age of hydrothermal alteration. Six samples of rutile picked from footwall and ore-shale veining and alteration give a U-Pb concordia upper intercept of 514 + or - 2 m.y. (2 Sigma ). This value is confirmed on a 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb plot (four of these samples contained common Pb concentrations in excess of blank), which yields a slope age of 513 + or - 5 m.y. (2 Sigma ). A seventh sample from an unusual footwall hematite vein gave a slightly younger 207 Pb/ 206 Pb age (496 + or - 1 m.y.).The 514 + or - 2 m.y. alteration event is considerably younger than the minimum age of the host sediments (602 + or - 20 m.y.), and probably also postdates ore deposition, believed to be older than 600 m.y. No unambiguous evidence was seen at Musoshi to support a relationship between hydrothermal alteration and stratiform Cu mineralization, although sulfide textures were modified at this time (as shown by intergrowths with rutile, dated here at 514 m.y.). The hydrothermal activity is concluded, therefore, to be a late overprint, unrelated to Cu ore deposition, but perhaps related to compressional deformation and metamorphism associated with the Lufilian orogeny.

Mineralogy and geochemistry of a stratabound manganese deposit in the Tangganshan region of South China

The Tangganshan manganese ore deposit is a typical sedimentary-magmatic hydatopneumatogenic superimposed ore deposit. In this paper this deposit is discussed in more detail from the following aspects: geology, ore mineralogy, and geochemistry. On the basis of its occurrence, mineral assemblage, element geochemistry, isotopic geochemistry, and characteristics of organic matter and fluid inclusions, the Tangganshan manganese ore deposit has proved to be a new-type manganese ore deposit that has been enriched by magmatic-pneumatolic solutions. Most of the ore-forming elements were derived from the ore deposit itself, and the rest from the magmatic-pneumatolitic solutions.

Fluid inclusion study of the Gordonsville zinc deposit, central Tennessee

A complementary paragenetic and fluid inclusion investigation of the carbonate-hosted Gordonsville zinc deposit was undertaken to trace thermal and chemical evolution of the mineralizing fluid. The following paragenetic sequence was established from field and petrographic study: white (early) calcite-sphalerite-galena-fiuorite-white to lavender (main stage) calcite-barite-clear to amber (late stage) calcite. Sphalerite occurs in three textural varieties: disseminated (yellowish brown), massive (reddish brown), and vug fill (dark brown). The disseminated sphalerite, with somewhat lower Fe and Cd contents, may represent an earlier stage of mineralization, but its fluid inclusion characteristics are similar to the other two.Homogenization and freezing temperatures of two-phase (liquid + vapor) inclusions show a pronounced change in the nature of mineralizing fluids between early-stage and late-stage minerals. The early-stage minerals (early calcite through main-stage calcite) precipitated from moderately hot (97 degrees -133 degrees C) and highly saline Na-Ca-Mg-Cl fluids (salinity 21-23 equiv wt % NaCl). Variations in homogenization temperatures of the early-stage minerals probably reflect periodic influx of basinal fluids of similar salinites. However, the lack of correlation between homogenization and freezing temperatures suggests, but does not prove, that fluid mixing was not the cause of precipitation of these minerals. The fluid inclusion data for sphalerite are consistent with its emplacement from a single solution carrying both zinc and sulfur, although the model remains untested without a knowledge of the pH of the Gordonsville ore fluid.The late-stage minerals (barite and late-stage calcite) precipitated from a fluid of some-what lower temperature (96 degrees -122 degrees C) but markedly lower salinity (7-9 equiv wt % NaCl), suggesting an influx of a more dilute fluid, probably heated meteoric water, at the end of main-stage calcite deposition.

Skarn zonation and fluid evolution in the Groundhog Mine, Central mining district, New Mexico

The Groundhog mine contains one of the largest and best exposed Zn-Pb-Cu-Ag skarn deposits in the United States. Skarn is formed in Carboniferous limestones spatially associated with a nearly vertical swarm of Early Tertiary granodiorite porphyry dikes. Metal ratios, mineralogy, composition, and fluid inclusion characteristics of the skarn are systematically zoned from northeast to southwest along the >3-km strike length of the system and relative to the original dike-limestone contact. Zn/Cu, Zn/Ag, Pb/Cu, Pb/Zn, and Pb/Ag ratios all increase toward the distal southwest end of the mine where highest Zn, Pb, and total metal grades also occur. Endoskarn is zoned in terms of the relative proportions and manganese content of epidote and chlorite. In the Northeast zone, skarn is zoned away from dikes in the sequence: garnet to pyroxene to pyroxenoid to marble. In the Central skarn zone, garnet is absent and pyroxene begins the zonation sequence at the dike contact. In the Southwest zone, pyroxene is less abundant and pyroxenoid locally begins the zonation sequence. Sphalerite-galena-pyrite-quartzcarbonate are present in all skarn zones and in carbonate replacement mantos and chimneys beyond skarn. Distal mineralogical zones (e.g., pyroxenoid and manto) are more extensive to the southwest. In general, the skarn zonation sequence is wider and more fully developed in pure, coarse-grained limestone than in silty, carbonaceous, or fine-grained limestone. High permeability and water/rock ratio are considered the dominant controls on skarn size. Most skarn minerals, including garnet, pyroxene, pyroxenoid, ilvaite, amphibole, chlorite, carbonate, and sphalerite, are enriched in manganese. Pyroxene becomes more manganese rich (up to 85 mole % johannsenite):(1) marble, (2) along strike to the southwest, (3) at higher elevations, and (4) in coarser grained, cleaner limestones. The more manganese-rich pyroxenes formed from fluids which are distal in the overall zonation sequence. In contrast, pyroxene formed in proximal locations (dike contact, Northeast skarn zone, low elevation, fine-grained, impure limestone) is enriched in magnesium. Pyroxene-forming fluids are depleted first in Mg, then in Fe, and finally in Mn, as proposed by Burt (1977). Fluid inclusion homogenization temperatures are lower than for most zinc skarns. In the Northeast skarn zone the average homogenization temperature is 337oC and salt daughter minerals are present in garnet and pyroxene (inclusions without daughter minerals average 8.5 wt % NaCI), whereas salt daughter minerals are absent in the Central and Southwest skarn zones and average homogenization temperatures and salinities are 320oC, 7.3 wt percent NaC1, and 293oC, 6.2 wt percent NaC1, respectively. Pressure estimates from rare clusters of fluid inclusions with evidence for boiling are between 155 and 175 bars. Skarn thermal gradients calculated from fluid inclusion homogenization temperatures are 35oC from dike to marble front and 50oC from dike to the limit of alteration (carbonate replacement mantos and chimneys). The calculated average thermal gradient along strike ranges from 8oC/km along the dike contact to 23oC/km along the marble front. The calculated thermal gradients and measured pyroxene compositions are in general agreement with published experimental studies of clinopyroxene stability relations. The variations in skarn metal ratios, mineralogy, composition, and fluid inclusion characteristics have been incorporated into a general model for zinc skarn formation which can be applied to the hierarchy of zinc skarns summarized by Einaudi et al. (1981) and as a guide to exploration. The distal end of this zinc skarn hierarchy is transitional to manto and chimney carbonate replacement deposits which contain few, if any, calc-silicate minerals.

Chemical and isotopic evolution of hydrothermal solutions at Bingham, Utah

Systematic changes in mineral assemblages, mineral compositions, fluid inclusion characteristics, and hydrogen and oxygen isotope compositions in altered igneous rocks in the Bingham, Utah, porphyry copper deposit record the chemical and isotopic evolution of the hydrothermal solutions. Hydrothermal alteration in and surrounding the Bingham porphyry copper deposit consists of an outer propylitic zone of chlorite, epidote, and actinolite; a transition zone of hydrothermal biotite, actinolite, epidote, and chlorite; and an inner potassic zone of hydrothermal biotite and K-feldspar. Later sericitic and argillic alteration are superimposed on early potassic and propylitic alteration.Compositions of alteration minerals determined by electron microprobe methods are: biotite, mole fraction phlogopite (Mg/Mg + Fe) 0.58 to 0.83; actinolite, mole fraction tremolite (Mg/Mg + Fe) 0.58 to 0.79; epidote, mole fraction pistacite (Fe/Fe + Al) 0.27 to 0.30; and chlorite, mole fraction clinochlore (Mg/Mg + Fe) 0.66 to 0.76. Mineral compositions, especially MnO and MgO in chlorite, MnO in epidote, and TiO 2 in biotite, vary systematically with distance from disseminated ore. MgO in chlorite increases from 20.6 to 24.6 wt percent. MnO in chlorite and vein epidote increases from less than 0.1 to 0.59 wt percent and from 0.08 to 0.73 wt percent, respectively. The average mole proportion Ti in biotite increases from 0.1 to 0.24.Temperatures and salinities from fluid inclusions range from greater than 600 degrees C and 50 equiv wt percent NaCl for the potassic zone to approximately 370 degrees C and 33 wt percent for the propylitic zone. Coexisting liquid-rich and vapor-rich inclusions which homogenize at temperatures similar to the liquid and vapor phase, respectively, suggest boiling from the potassic core out to the potassic-propylitic transition zone. Compositions and homogenization temperatures for fluid inclusions suggest average P (sub (f)) -T conditions which vary smoothly from the potassic core to the propylitic fringe: 600 degrees C and 800 bars for the potassic zone, 450 degrees C and 500 bars for the transition zone, and 350 degrees C and 200 bars for the outer propylitic zone.Thermochemical calculations show that log f (sub H 2 O) decreases from 2.74 in the potassic core to 2.17 in the propylitic zone fringe, log f (sub S 2 ) decreases from -1 to -3.3 in the core to -8.7 in the propylitic zone, and log f (sub O 2 ) decreases from -16 to -17.5 in the core to -29 in the same interval. The log activity ratios of Ca, Mg, and K to hydrogen ion increase from the potassic to the propylitic zone. K (super +) /H (super +) increases from 3.1 to 3.4, Mg (super +2) /(H (super +) ) 2 ) increases from -2.25 to 4.25, and Ca (super +2) /(H (super +) ) 2 increases from less than 1 to 7.75.The extent of oxygen isotope exchange between the hydrothermal vein fluids and the igneous rock matrix in the Bingham stock increases with increasing temperature, abundance of alteration and quartz veins, thickness of veins, and fracture intensity. All these parameters generally increase inward from the propylitic zone into the core of the potassic zone. Within the potassic zone, there is pervasive exchange equilibrium between vein fluids and the selvage and rock matrix domains of the highly altered, veined, and fractured igneous rock. In contrast, vein fluids closely approach exchange equilibrium only with vein selvage in the propylitic zone; the igneous rock matrix has not equilibrated extensively, as might be expected in this lower temperature, less altered, and veined portion of the hydrothermal system.Calculated delta 18 O and delta D values of early fluids range from 6.8, -67 per mil in the potassic core to 5.1, -41 per mil in the outer propylitic zone, showing a systematic trend of 18 O depletion and deuterium enrichment with decreasing temperature outward from the innermost potassic zone. This trend is inconsistent with the progressive influx of local meteoric waters (delta D 18 O and delta D values for these fluids could have resulted from isotopic exchange between meteoric water and igneous rock at very low water (W)/rock (R) ratios over a range of temperature (> or =600 degrees -350 degrees C). Such water/rock exchange likely would be confined to deeper parts of the system with the exchanged fluids subsequently focused up into the present exposure of the deposit.Late-stage fluids responsible for sericitic and argillic alteration are progressively depleted in 18 O and deuterium, consistent with an increasing, and finally dominant, meteoric water component. If fluid-rock isotopic exchange has been the dominant mechanism in defining the isotopic compositions of the hydrothermal fluids at Bingham, then this later hydrothermal system was characterized by shallower circulation and higher water/rock ratios than was the system responsible for the development of the early hydrothermal fluids.

Characteristics of fluid inclusions in the porphyry copper deposit at La Granja, Peru

At La Granja, Northern Peru, a granodiorite porphyry of probably Mid-Tertiary age intruded into an Upper Triassic to Lower Tertiary volcano-sedimentary sequence. It was followed by intensive hydrothermal alteration and cogenetic mineralization processes which influenced the intrusive body as well as its sedimentary cover and host rocks, producting hydrothermal alteration assemblages typical of porphyry Cu deposits (potassic, propylitic, phyllic, advanced argillic, silicic). The investigation of fluid inclusions showed that: (1) generally three types of fluid inclusions can be distinguished. Apparently coexisting vapour- and liquid-rich inclusions are taken as evidence for boiling in the hydrothermal system; and (2) the overall abundance of different inclusion types correlates well with different alteration types. The microthermometric data obtained fit well into the published range for other Cu porphyry deposits. Together with the petrographic data the following model of fluid and alteration development is proposed: Highly saline fluids of probably magmatic origin developed - possibly by dilution - towards a boiling system of moderate salinity, producing potassic alteration centers surrounded by propylitic alteration and/or skarnification. A younger low-salinity fluid regime affected the centers by overprinting phyllic and/or advanced argillic alteration. Silicification partly occurred during the latest stage of hydrothermal activity. Possible practical aspects of the fluid inclusion investigations are discussed.

Fluid inclusion characteristics of the Variscan and post-Variscan mineralizing fluids in the Federal Republic of Germany

A systematic mapping of crustal fluids using fluid inclusion analysis has been carried out for the Variscan basement and the overlying Permo-Mesozoic sedimentary cover in the Federal Republic of Germany. Several distinctive fluid systems can be differentiated. These systems are regionally distributed and can be assigned to certain tectonic, metamorphic or magmatic processes: 1. (1) rock fluids associated with medium-pressure-high-temperature metamorphism; 2. (2) rock fluids associated with low-pressure-high-temperature metamorphism; 3. (3) low-salinity high-temperature vein mineralizations associated with vein and polymetallic sulfide deposits of Variscan age; 4. (4) late- and postmagmatic hydrothermal fluids enriched in K associated with Variscan granites; 5. (5) highly saline (CaCl 2NaCl) low-temperature fluids derived from basinal brines of the sedimentary Permian molasse stockwork associated with widely distributed post-Variscan PbZn and fluorite-barite deposits. The regional mapping of fluid inclusions provides new information on the genetic/temporal classification of hydrothermal deposits in the Central European Variscan for the area studied.

Fluid inclusion evidence for minimum 11 km vertical offset on the Wasatch fault, Utah

The footwall of the Wasatch fault in the Corner Creek area near Salt Lake City is hydrothermally altered and deformed quartz monzonite of the Oligocene Little Cottonwood stock. Secondary fluid inclusions are associated with hydrothermal alteration minerals and structural deformation. Thermometric measurements of fluid inclusion characteristics indicate entrapment of a CO2-H2O-NaCl fluid at minimum temperatures of 223–353 °C and minimum fluid pressures of 900–2800 bar. The 2800-bar fluid pressure is near lithostatic pressure at a depth of 11 km, the minimum displacement of the fault that is required for exhumation of the observed alteration and fluid inclusions and more than three times greater than most previous estimates. The large displacement estimate is supported by the occurrence of pyrophyllite in shales in the footwall, by the petrology of metamorphosed shale surrounding an unroofed pluton in the footwall, and by geologic reconstruction of the eroded footwall.

Pore fluid and seismogenic characteristics of fault rock at depth on the Wasatch Fault, Utah

Fluid inclusions and structural fabric in fault rock in the exhumed footwall of the Wasatch fault allow measurement of fluid pressure, temperature, and composition and estimation of rheological characteristics of fault behavior. Hydrothermally altered and deformed Oligocene quartz monzonite of the Little Cottonwood stock forms a partially preserved footwall carapace at the southern end of the Salt Lake segment of the Wasatch normal fault zone. Cataclasite and phyllonite contain two syndeformational alteration mineral assemblages that formed during progressive faulting and displacement of the footwall. An early epidote‐chlorite‐sericite‐magnetite alteration assemblage developed in phyllonite and cataclasite, and a later laumontite‐prehnite‐hematite alteration assemblage developed during cataclasis. Fluid inclusions associated with the formation of alteration minerals are preserved in healed fractures in quartz grains. Fluid inclusions that record the transition temperature from quasi‐plastic flow to frictional deformation are associated with hydrothermal alteration to chlorite, epidote, and sericite and were trapped at minimum temperatures of 223°C–353°C and minimum fluid pressures of 1150–2800 bars. The fluids average 13 mol% CO2 and 8.2 wt % NaCl. A radiometric age of hydrothermal sericite suggests that the age of the alteration is 17.6±0.7 m.y. If maximum fluid pressure is near lithostatic, the minimum depth of formation of the alteration was 11 km at 337°C consistent with a thermal gradient of 30°C/km. The ratio of pore fluid pressure to lithostatic pressure varied from 0.52 to nearly 1.0 and indicates that effective normal stress on the fault at these depths may have approached zero. Structural fabrics and fluid inclusion characteristics are used to estimate seismogenic characteristics of the fault. Cleavage lineation and S‐C bands in the phyllonite indicate that deformation at temperatures above 350°C and depths greater than 11 km was dominated by aseismic creep. Unstable fracture propagation in cataclasite developed during seismic slip at shallower levels. The downdip, seismogenic length of the Salt Lake segment is estimated as 16 km using a fault zone dip of 45° and a minimum depth of 11 km for the quasi‐plastic/frictional transition. The segment strike length is 35 km, and the seismogenic area is about 600 km2, compatible with generation of M = 7.0 earthquakes.

A Color Video Tape Introduction to Fluid Inclusions For Economic Geology Classes

A 40-minute color videotape which introduces the novice student to fluid inclusions and some of the operations that can be performed on them has been produced at Michigan Technological University. Because fluid inclusions are studied microscopically, they are particularly well-suited for the videotape teaching medium. A microscope equipped with a TV camera allowed filming of fluid inclusion features and their reactions to heating and cooling. The tape focuses on microscopic aspects of fluid inclusions that are difficult to explain or diagram on the blackboard, and combines lecture materials and color diagrams for a complete presentation on the basics of fluid inclusions.

Mineralogical and fluid inclusion features of rock alterations in the Seigoshi gold-silver mining district, western part of the Izu Peninsula, Japan

Propylitic and advanced argillic alterations occur in the Seigoshi gold-silver mining district, in the western part of the Izu Peninsula, Japan. The propylitic alteration has the following zonal arrangement from the deeper portion to the shallower portion: an epidote-prehnite-K-feldspar-chlorite zone; a wairakite-laumontite zone; and a stilbite-heulandite-montmorillonite zone. Lateral and vertical zoning is conspicuous in the advanced argillic alteration. The inner zone is silica-rich and this grades laterally and vertically through alunite- to clay-rich zones. Fluid inclusions from the epidote-prehnite-K-feldspar-chlorite zone are liquid-dominated and filling temperatures of this zone are in the range of 225–285°C. Filling temperatures for the zeolite zone are variable, being in the range of 240–380°C. This wide range suggests that boiling of the fluids was responsible for the zeolite zone. Filling temperatures of the advanced argillic alteration range widely from 210° to 430°C. This wide range and the coexistence of liquid- and vapor-dominated fluid inclusions in a given sample suggest that liquid-vapor separation simultaneously occurred during the advanced argillic alteration process. It is deduced that the ranges of gaseous fugacity are quite different for each type of alteration. For instance, f S 2 and f O 2 of the advanced argillic alteration are estimated to be higher than those of the propylitic alteration. Based on the alteration mineral assemblage, chemical composition and mode of occurrence of alteration minerals, fluid inclusions, estimated ranges of gaseous fugacities, and comparison of these features with those of active geothermal systems, it is concluded that the propylitic alteration minerals were probably precipitated due to the loss of gases such as CO 2, and that the formation of advanced argillic alteration was caused by subsurface mixing of volcanic gas and/or condensed hot water with groundwater. These coexisting propylitic and advanced argillic alterations are commonly found in the other AuAg mining districts in Japan.

Origin of lead-zinc mineralization in the southern Benue Trough, Nigeria ? Fluid inclusion and trace element studies

The epigenetic Pb-Zn deposits of the southern Benue Valley (Nigeria) are localized within Cretaceous sediments of an intracontinental rift basin. Fluid inclusion studies of vein minerals from the Abakaliki and Ishiagu orebodies show that sphalerite and quartz were deposited at relatively low temperatures (102–175 °C), with ore-fluid salinity mostly in the range of 17–25 equiv. wt% NaCl. Trace-element contents of sphalerite and galena are also consistent with the low temperature of formation and epigenetic origin. On the basis of the geotectonic setting, the mode of occurrence and fluid-inclusion characteristics, mineralization is attributed to connate brines set into motion by a high geothermal gradient accompanying continental rifting. Mineral deposition was caused principally by rapid cooling due either to reaction with wall rocks or mixing with meteoric or descending water of low salinity.

Geological, geochemical, stable isotope, and fluid inclusion characteristics of epithermal gold mineralization, Velvet District, Nevada

Abstract Gold mineralization in the Velvet District occurs in an eastward dipping sequence of late Tertiary rhyolitic ash-flow tuffs, flows, and tuffaceous sediments in northwestern Nevada. Minor gold and silver concentrations are associated with irregular zones of brecciation, argillic alteration, and quartz veining along north-northeast trending normal faults. Reaction of mineralizing fluids with wallrock produced an argillic alteration assemblage of illite, mixed-layer clays, smectite, and kaolinite. Illite alteration and highest gold concentrations appear to be associated with zones of high water/rock ratios. Kaolinite, smectite, alunite, and opal are postulated to have formed during a steam-dominated episode of alteration. Fluid inclusion studies indicate that the quartz veins were deposited in the temperature range 230 to 280°C from fluids which had salinities equivalent to 0.2–0.8 weight percent NaCl. δ 18O of quartz veins varies from −2.5 to +6.7 ‰ and indicates that the ore fluid must have been Tertiary meteroric water. Stable isotope data appear to define a zone of concentrated fluid flow and potential subsurface mineralization in the southeastern part of the district. Fluid inclusion and isotope studies can be used in combination with more standard geochemical, geophysical, and geological information to provide site-specific targets for epithermal metal concentrations.