Precipitation Of Galena Research Articles

Genesis of Pb–Zn Mineralization in the Pulang Cu Polymetallic Deposit in Yunnan Province, China: Insights from Analyses of Geology, Fluid Inclusions and C–H–O–S Isotopes

The Pulang super-large porphyry Cu polymetallic deposit, located in the Sanjiang area of Yunnan Province, is one of the largest Cu deposits in China. This deposit hosts Cu resources of ~5 × 106 t and other ore-forming elements, such as Mo, Au, Ag, Pb, Zn, Pt and Pd. Recently, obvious hydrothermal vein-type Pb–Zn mineralization, with a Pb + Zn resource of ~0.4 × 106 t, has been detected in the North Ore Section of the deposit. However, the genesis of these Pb–Zn ore bodies, especially their relationship to the major Cu ore bodies in the South Ore Section, remains controversial. We conducted geologic description, fluid inclusion petrography and microthermometry, and C, H, O and S isotope studies to uncover the genesis of Pb–Zn vein-type mineralization in North Pulang. As a result, three types of Pb–Zn veins were identified: a quartz–pyrrhotite–chalcopyrite–sphalerite–galena vein, a quartz–pyrrhotite–sphalerite–galena vein, and a calcite–quartz–pyrrhotite–galena vein. All fluid inclusions in the quartz from different veins are liquid-rich inclusions, with homogenization temperatures in the range of 184 °C–235 °C and salinities between 10.4 wt.% and 17.8 wt.% NaCl eq., indicating that the Pb–Zn ore-forming fluid was a single-phase fluid with a low temperature and low-to-medium salinity. Hydrothermal quartz in different stages displays δDwater values ranging from −46.9‰ to −120.0‰ (V-SMOW), and the calculated δ18Owater values range from 2.4‰ to 4.3‰ (V-SMOW), implying that the mineralization fluids likely originated from magma, with a minor involvement of meteoric water. The δ13CCal values (−2.3‰ to −7.9‰ V-PDB) of calcite indicate that C likely originated from a deep-seated source. The δ34S values of chalcopyrite, pyrite, pyrrhotite and sphalerite reveal that S was possibly derived from magmatic rocks. Based on the above data, it is suggested that the Pb–Zn mineralization in North Pulang was the result of the northward migration of ore-forming fluids that originated from South Pulang along the NE-trending structural fractures. A strong water–rock interaction occurred during the migration process. However, the involvement of meteoric water and accompanied cooling of fluids were most likely responsible for the precipitation of galena and sphalerite.

Re-examination of the Aïn Allegua-Aguiba celestite-Pb-Zn salt diapir-related Mississippi Valley-type deposits (Nappes zone, Northwestern Tunisia): New conceptual model

The celestite-rich and Pb-Zn-poor Aïn Allegua deposits are located at the northeastern extremity of the Dougas diapir in the Nappes zone of northern Tunisia. The Sr-Ba-Zn-Pb mineral system studied in this paper is enclosed in a Triassic breccia, labeled cap rock, located between the Triassic diapir and the surrounding Cretaceous to Lutetian marine sediments. It consists of evaporite dissolution float breccia where Triassic dolostone clasts are floating in a Triassic matrix of anhydrite and argillites. Five mineralizing stages are recognized, which are from early to late: celestite (stage 1); hydrothermal dolomite (stage 2); crosscutting galena-barite or sphalerite-galena-barite-calcite (stage 3); (4) fracture and karst filled by barite, celestite, and galena (stage 4); and late supergene mineralization (strontianite, smithsonite, and cerussite) (stage 5). The present study shows that the celestite and Pb-Zn mineralization formed from the circulation in the cap rock of at least three fluids: (1) fluid A (100–150 °C; 10 to 25 wt% NaCl eq.) deposited the celestite; (2) fluid B (125 to 185 °C; 29 to 34 wt% NaCl eq.) deposited the hydrothermal dolomite, and (3) fluid C (temperatures around 90 °C; 14 to 22 wt% NaCl eq.) deposited the calcite that accompanied sphalerite and galena precipitation. The δ34S values for sphalerite and galena fall within a narrow range from + 10.4‰ to + 18.9‰ (avr. 13‰) indicating a homogeneous source of sulfur. The δ34S values of celestite and barite are also relatively homogeneous from + 14.7 to + 23‰ (ave. + 16.5‰) and match those of Tunisian Triassic gypsum and Triassic marine sulfates (δ34S = +15‰). The temperature of deposition together with sulfur isotope data indicate that the reduced sulfur in sulfides was derived through thermochemical sulfate reduction of Triassic sulfate. The lead isotopes in galena show that Pb, and probably Zn and Ba were extracted from the upper crustal rocks. Part of the strontium is derived from the Triassic anhydrite and the other part is probably transported to the cap rock in formation waters. The tectonic setting, the carbonated nature of the host rocks, the epigenetic style of the mineralization, and the mineral associations, together with fluid inclusions data and sulfur and lead isotopes show that the Aïn Allegua deposit belongs to the diapir-related Mississippi Valley-type (MVT) deposits of the Eastern Maghreb salt diapirs province, that formed, as the most of the MVT deposits and districts in the world, from basinal brines with no mixing with metamorphic and/or magmatic fluid or involvement of a heat source from a concealed sill or magmatic intrusion.

Chemical and Isotopic Composition of Sulfide Minerals from the Noho Hydrothermal Field in the Okinawa Trough

Studies of the element contents and isotopic characteristics of sulfide minerals from seafloor hydrothermal sulfide deposits are a significant method of investigating seawater-fluid mixing and fluid-rock and/or sediment interactions in hydrothermal systems. The seafloor hydrothermal sulfide ores from the Noho hydrothermal field (NHF) in the Okinawa Trough (OT) consist of pyrrhotite, isocubanite, sphalerite, galena, and amorphous silica. The Rh, Ag, Sb, and Tl contents mostly increase in galena as the fluid temperature decreases in the late ore-forming stage. In the sulfide minerals, the rare earth elements are mainly derived from the hydrothermal fluids, while the volcanic rocks and/or sediments are the sources of the sulfur and lead in the sulfide minerals. After the precipitation of galena, the redox state becomes oxidizing, and the pH value of the fluid increases, which is accompanied by the formation of amorphous silica. Finally, neither pyrite nor marcasite has been observed in association with pyrrhotite in the NHF sulfides, likely indicating that the amount of sulfur was limited in this hydrothermal system, and most of the residual Fe was incorporated into the sphalerite. This suggests that the later pyrite and/or marcasite precipitation in the seafloor hydrothermal sulfide deposit is controlled by the sulfur content of the fluid. Furthermore, it is possible to use hydrothermal sulfides and their inclusions to trace subseafloor fluid circulation processes.

Sphalerite‐galena replacement in sodium chloride solution: A thermodynamic approach

AbstractConsidering the material balances of the constituents including solid phases, replacement reaction of the sphalerite‐galena pair in chloride solution is examined quantitatively under equilibrium conditions of 250°C, water saturation vapor pressure, and initial Cl concentration of 1 mol/L. NaCl+PbCl2 solution with solid sphalerite, dissolves and releases both total Zn and total S of 1.26 × 10−5 mol/L into the solution under without or lower PbCl2 concentration. If the PbCl2 concentration is higher than 1.32 × 10−6 mol/L, precipitation of galena as replacement occurs, suggesting that sphalerite has an ability to trap a lower concentration of Pb. If PbCl2 concentration of the solution is higher than 1.32 × 10−6 mol/L, the majority of Pb deposited as galena with using sulfur originated from solid sphalerite dissolved, and the amount of Zn from sphalerite equivalent to the amount of galena deposited releases into the solution. On the other hand, NaCl+ZnCl2 solution with solid galena under the same environmental conditions, dissolves and releases both total Pb and total S of 6.43 × 10−6 mol/L into the solution under without or lower ZnCl2 concentration. Over the ZnCl2 concentration of 6.40 × 10−5 mol/L in the solution, precipitation of sphalerite occurs, indicating that galena cannot trap a low concentration of Zn. Zinc would drain away from the hydrothermal depositional environment under the presence of only galena. These relationships are controlled mainly by the reaction of predominant metal chloride or metal hydroxide species in the solution. Sphalerite is a good scavenger for Pb, but galena is not for Zn.

Tracing mineralization history from the compositional textures of sulfide association: A case study of the Zhenzigou stratiform Zn-Pb deposit, NE China

The Qingchengzi Orefield located in northeastern China hosts more than ten Pb-Zn deposits and several Au-Ag deposits. Among those, the Zhenzigou Zn-Pb deposit displays a typical stratiform morphology, comprising ore bodies in layers and lenses. The ore minerals are simply composed of sphalerite, galena, pyrite, and pyrrhotite, together with minor arsenopyrite, marcasite, and argentite. A sulfide association of sphalerite-galena-pyrite-arsenopyrite was selected for this study; the interior compositional texture of the sulfide association was investigated by means of LA-ICPMS spot and mapping analysis, with the aim of constraining the ore genesis, fluid evolution history and element partitioning behavior. The results show that three distinct fluids were involved in the mineralization, namely an episode of sedimentary fluids at ∼2052 Ma which is responsible for the precipitation of pyrite core (Py0), an episode of metamorphic fluids at ∼1800 Ma which formed an overgrowth of pyrite (Py 1) and new crystallization of sphalerite (Sph1) and arsenopyrite (Aspy1), and a final episode of hydrothermal fluids related to the Triassic magmatism, forming another overgrowth of pyrite (Py2), sphalerite (Sph2) and arsenopyrite (Aspy2), together with minor precipitation of galena (Ga). The sedimentary fluids are relatively low in temperature (<150 °C), enriched in Cu, Pb, Zn, Ag, and depleted in Co, Ni, and As. Pyrite is the main precipitated sulfide mineral, scavenging most of the available trace elements. The late metamorphic and magmatic fluids are high in temperature (>300 °C) and have remobilized As, Co and Ni but only at mineral grain scale. The late metamorphic and magmatic fluids are enriched in In, Cd, Au, As, Ag, Cu, Pb, Sb, Sn, Co and Ni, and depleted in Bi, W, Mo, Te, and Se. The partitioning of most of the metals from the fluids into the sulfide minerals was controlled by the metals’ availability in the fluids and by the compatibility with crystal structure through direct or coupled substitutions. Pyrite acts as the main host of Co, Ni, Pb, and As; arsenopyrite hosts a major amount of Sb and Pb, and a minor amount of Au; sphalerite is the primary host of Cd, In, Cu, and the secondary host of Ag and Sn; galena is the primary host of Ag, Sb, Sn, and subordinately hosts Cd, In and As. In particular, a remarkable amount of In and Cd was carried by the hydrothermal fluids associated with the Triassic magmatism, indicating that the contribution of magmatic hydrothermal fluids to the mineralization is significant. In conclusion, our in-situ geochemical evidence of sulfide minerals reinforces a sedimentation-metamorphism-magmatic-hydrothermal reworking model for the Zhenzigou stratiform Zn-Pb mineralization. This case study underlines the significance of compositional textures of coexisting sulfide minerals when assessing the ore genesis. Overall, our study demonstrates that, by integrating trace element mapping and spot analysis on coexisting sulfide minerals, it is possible to re-construct the fluid evolution history, resolve the metal behaviors associated with fluids evolution, and review the contribution of individual fluids to mineralization.

Order of sphalerite and galena precipitation: A case study from lead-zinc deposits in southwest China

Most of the lead and zinc deposits in Southwest China, are characterized by mineral zoning, which is especially true for the Huize and Zhaotong deposits. The mineral assemblage zoning is consistent for both horizontal and vertical zoning, from the base (center) of the ore body to the top (outermost), the mineral zones are as follows. coarse-grained pyrite and a little puce sphalerite; I-2: brown sphalerite, galena, and ferro-dolomite; I-3: galena, sandy beige and pale yellow sphalerite, and calcite; and I-4: fine-grained pyrite, dolomite, and calcite. Among them, sphalerite is the landmark mineral of different zoning. From I-1 to I-3, the color of sphalerite changes from dark to light, its crystalline size changes from coarse to fine, and its structure changes from disseminated to veinlet. This mineral zoning is seen not only on a microscopic scale, but is also clear on a mesoscopic and macroscopic scale. It is caused by the order of the sphalerite and galena precipitation. We studied the metallic minerals and fluid inclusions using a thermodynamic phase diagram method, such as $$\lg {f_{{{\rm{O}}_2}}} - \lg {f_{{{\rm{S}}_2}}}$$, $${\rm{pH}} - \lg {f_{{{\rm{O}}_2}}}$$, pH−lg[Pb2+] and pH−lg[HS−], discussed the constraints on the order of the sphalerite and galena precipitation in the migration and precipitation process of lead and zinc under different pH values, oxygen fugacity, sulfur fugacity, and ionic activity. We also explain the formation mechanism and propose that the main controlling factor of the order of the sphalerite and galena precipitation is sulfur fugacity.

Geological setting and fluid inclusion characteristics of a lead-copper-barium occurrence hosted in a Neoproterozoic mafic sill at Kiatak, Northumberland Island, Northwestern Greenland

We describe a Pb-Cu-Ba occurrence in Northumberland Island, Northwestern Greenland. Mineralization occupies the brecciated upper contact domain of a Neoproterozoic diabase sill belonging to the Franklin-Thule dike swarm (720–716Ma), with pyrite-bearing black shales of the Dundas Group, upper Thule Supergroup. The host tholeiitic diabase sill is of different composition (low TiO2 and P2O5) than the locally crosscutting dikes (high TiO2 and P2O5). Chloritization, carbonatization and silicification are intense in proximity to sulfides. Coarse grained, open space-filling galena and minor chalcopyrite are accompanied by 2 generations of calcite and 2 of barite. Galena contains significant amounts of Ag (av. 400ppm), Sb (av. 700ppm), Se (av. 20ppm), traces of Bi, Cd, and Sn. Fluid inclusions in the gangue of the Kiatak occurrence indicate two fluid types. Prior to galena precipitation, a CaCl2-NaCl-rich aqueous brine (~20wt.%eq NaCl) cooled from temperatures >300°C and was trapped first in early calcite, and with further cooling, in barite together with solid bitumen inclusions. Following galena crystallization, secondary inclusions containing a similar brine, but of lower salinity, higher Ca:Na ratio, and lower temperature, were trapped in calcite. Corrosion of galena was followed by precipitation of lower temperature (~100°C) barite from a second fluid, comprising immiscible water and CH4. Despite its location in the contact between shale and large mafic sill, the low-temperature mineralization postdates the cooling of the sill, and may be related to basinal fluid circulation controlled by regional extensional faults parallel to diabase dikes. Although uneconomic, the Kiatak occurrence may be witness to a larger metallogenic process that could have formed significant SEDEX type metal concentrations in strata within the Thule Supergroup.

The carbonate rock-hosted epithermal gold deposit of Agdarreh, Takab geothermal field, NW Iran—hydrothermal alteration and mineralisation

The disseminated gold deposit of Agdarreh (24.5 t at 3.7 g/t Au) is hosted in hydrothermally leached Miocene reefal limestone in the Takab geothermal field, which is part of the Cenozoic Urumieh–Dokhtar volcanic arc of NW Iran. Alteration and mineralisation are largely bedding controlled blanket-like and include: (1) pre-ore decalcification; (2) first-stage silicification associated with pyrite (early pyrite with 3–4 wt% As, late pyrite with <1–3 wt% As) and sphalerite; (3) second-stage silicification with precipitation of galena, Pb–Sb–As sulphides, sulphosalts, tellurides and native bismuth; (4) late-stage cinnabar and barite in vugs; (5) oxide ore stage and carbonate alteration (complex Mn–Fe-rich oxyhydroxides, arsenates, sulphates, APS minerals and rutile in residual leached rock and infill of karstic cavities). Gold occurs invisibly in the jasperoids and is enriched in the Mn–Fe oxyhydroxide surface cap of the jasperoids. Gold mineralisation is associated with the hydrothermal metal suite of As, Sb, Hg, Te, Se, Tl, Ba, Zn, Ag, Cd, Bi and Pb, and is characterised by very low Cu contents. Arsenian pyrite probably carried most of the primary (invisible) gold. Native gold occurs in association with the late-stage cinnabar and the oxide ore. The Agdarreh deposit shows many similarities with Carlin-type ore and is interpreted to have resulted from near-surface hydrothermal activity related to the Cenozoic arc volcanism that developed within the extensional Takab graben. The extensive oxidation at Agdarreh may be partly due to the waning stages of hydrothermal activity. Active H2S-bearing thermal springs are locally depositing extremely high contents of Au and Ag, and travertine is present over large areas, suggesting that ore-forming hydrothermal activity occurred periodically from the Miocene to Recent in the Takab geothermal field.

Metal Transport and Deposition in Hydrothermal Veins Revealed by 213nm UV Laser Ablation Microanalysis of Single Fluid Inclusions

213nm UV laser ablation provides a powerful and cost-effective new method for the chemical microanalysis of individual fluid inclusions. The instrument offers significantly improved energy absorption in many geological materials compared to 266nm UV lasers resulting in improved ablation characteristics. The laser is not strongly absorbed by either air or commonly used optical material, permitting relatively easy delivery and control of the beam. Ablation of multielement standard solutions in pure quartz glass capillaries was found to be an effective method for external calibration of instrument sensitivity. Internal calibration for calculation of absolute element concentrations in inclusion fluids was carried out using chloride concentration estimated from microthermometric data. Using this approach, analyses of groups of cogenetic primary and secondary inclusions using a 30 element analyte menu gave typical relative standard deviations for absolute element concentrations of around 20 percent for alkali and alkali earth elements and 20 to 50 percent for metals, comparable with those obtained by 193nm Excimer laser ablation. Results for Ca are consistent with independent estimates made from microthermometric measurements and a range of other elements are in reasonable agreement with previously published bulk chemical analyses. Overall, the data give good charge balance, within 5 percent. The technique was used to test the hypothesis that the metal content of hydrothermal ore deposits is primarily controlled by the metal content of the mineralizing solutions rather than selective precipitation mechanisms. We studied Pb(-Zn) mineralized ‘crosscourse’ veins from Southwest England that are known to have formed from low temperature brines expelled from Permo-Triassic red-bed basins adjacent to the Cornubian peninsula, probably during the Triassic. These fluids are comparable with the solutions that formed Mississippi Valley-type Pb-Zn deposits. Analyses of primary fluid inclusions across growth-zoned quartz crystals from one of these veins indicate the orefluids at an early stage of vein growth are relatively Pb-rich (up to 295 ppm Pb, up to 86 ppm Zn; Pb/Zn = 1.3-16.1) and display marked temporal variations in chemistry. Gradual increases in K, Ca, Sr, Ba and Zn are observed across a significant (∼10 mm) interval of quartz growth. These increases are interpreted to reflect mineral dissolution-precipitation reactions occurring within the fracture system exploited by the fluids after being expelled from the basin source area. Superimposed on this trend are comparatively short time-scale (∼1 mm of quartz growth) chemical excursions characterized by decreases in the concentrations of Cl, K, Ca, Ba, Sr, Pb and Zn of up to two orders of magnitude. These excursions are believed to be due to local, volumetrically significant, mineral precipitation events and indicate the operation of processes that result in the simultaneous supersaturation of a number of phases. It is suggested that initial galena precipitation (>98% Pb loss from solution) was due to reduction as oxidized fluids encountered the first strongly reducing lithology on their flow path out of the basin source region. Several later dilution events were caused by fluid mixing and are inferred to have caused the co-precipitation of sulfides (galena, sphalerite; up to 98% loss of metals from solution) with sulfates (anhydrite, barite; up to 91% Ca and 97% Ba loss from solution) and/or silicates (K-mica; up to 96% loss of K from solution). However, at only one stage is galena (±anhydrite, K-mica) observed in the sample studied suggesting that fluid inclusions may commonly trap depleted fluids after mineral precipitation has already occurred some distance upstream, rather than reflecting precipitation events in their immediate vicinity. The primary ore-forming fluid in this case study contained elevated Pb and Zn and, at least initially, displayed a higher Pb/Zn ratio than predicted for fluids saturated with respect to both sphalerite and galena. Therefore, we suggest that the Pb-rich tenor of the vein deposit is primarily controlled by the metal content of the ore fluids, which is ultimately a function of the geochemistry of the basin source. In this example, the source area is dominated by oxidized terrestrial clastic rocks. Reactions within the aquifer and selective precipitation mechanisms at the site of ore formation also appear to have modified fluid geochemistry but played a second order role in controlling the metal budget of the mineral deposit formed.

Growth of sulfate-reducing bacteria with solid-phase electron acceptors.

Hannebachite (CaSO3 x 0.5H2O), gypsum (CaSO4 x 2H2O), anglesite (PbSO4), and barite (BaSO4) were tested as electron acceptors for sulfate-reducing bacteria with lactate as the electron donor. Hannebachite and gypsum are commonly associated with flue gas desulfurization products, and anglesite is a weathering product found in lead mines. Barite was included as the most insoluble sulfate. Growth of sulfate-reducing bacteria was monitored by protein and sulfide (dissolved H2S and HS-) measurements. Biogenic sulfide formation occurred with all four solid phases, and protein data confirmed that bacteria grew under these electron acceptor conditions. Sulfide formation from gypsum was almost comparable in rate and quantity to that produced from soluble sulfate salt (Na2SO4); hannebachite reduction to sulfide was not as fast. Anglesite as the electron acceptor was also reduced to sulfide in the solution phase and galena (PbS) was detected in solids retrieved from spent cultures. Barite as the electron acceptor supported the least amount of growth and H2S formation. The results demonstrate that low-solubility crystalline phases can be biologically reactive under reducing conditions. Furthermore, the results demonstrate that galena precipitation through sulfide production by sulfate-reducing bacteria serves as a lead enrichment mechanism, thereby also alleviating the potential toxicity of lead. In view of the role of acidophilic thiobacilli in the oxidation of sulfides, the present work accentuates the role of anaerobic and aerobic microbes in the biogeochemical cycling of solid-phase sulfates and sulfides.

Lead and barium sources in Cambrian siliciclastics and sediment provenance of a sector of the Taconic Orogen, Quebec: a mixing scenario based on Pb-isotopic evidence

To test the hypothesis that siliciclastic rocks constituted the major source of Pb and Ba in barite-galena deposits of the Taconic Orogen, we determined Pb-isotope ratios in galena, barren rocks and contained minerals, as well as concentrations of Pb, U, Th and Ba in the latter (detrital feldspars, sandstones, mudstones, rock clasts and carbonate cements and clasts). Ranges in 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb of 28 galena samples are 17.96–18.05, 15.56–15.59 and 37.75–37.93, respectively; ranges for 41 barren rocks and minerals are 16.17–23.31, 15.26–15.86 and 35.98–42.51, respectively. The lowest ratios are in feldspar, and the highest in carbonate and mudstone. Values of the mudstones samples overlap those of galena when corrected for in situ decay of U and Th since galena precipitation (∼450Ma). We thus propose that mudstones constituted a source of lead. Corrected ratios for anomalously Pb-rich mudstones are virtually identical to galena-Pb ratios and may be due to contamination by lead-bearing brines. Assuming that burial diagenesis did not disturb the Pb-isotope values of sandstones, these rocks contributed only a minor fraction of lead to the galena, estimated at∼20% for one deposit. The source of barite-Ba was probably perthite. Low Ba and Pb concentrations of sandstone adjacent to this deposit, compared to high concentrations remote from it, support leaching of barium (and minor lead) from feldspar penecontemporaneous with feldspar dissolution. Geological data indicate that the provenance of the siliciclastic rocks was mainly from Grenville terrane. A comparison of our Pb-isotopic data for Taconic perthite with those of Grenville K-feldspar, as well as ratios of trace elements, support this provenance for both sandstones and mudstones. The presence of carbonate platforms peripheral to the orogen, and the Middle Ordovician-Middle Devonian depositional range of the studied and Mississippi Valley type deposits north (Newfoundland) and south (U.S.A. Appalachians) of our study area suggest targets for exploration in these platforms.

A vein and disseminated Ba-Pb-Zn deposit in the Appalachian thrust belt, St.-Fabien, Quebec

The St.-Fabien deposit occurs in the Taconic thrust belt of the Quebec Appalachians. It consists of intimately associated veins and disseminations of barite, galena, and sphalerite hosted in interdigitated sandstones and limestone conglomerates comprising a proximal, deep submarine facies of the Upper Cambrian St.-Damase Formation.At a late stage in the tectonic evolution of the host rocks, hydraulic fracturing driven by the buildup of a geopressured zone took place in a dilatant stress field without mechanical relationships to folding. Episodic opening of the fractures enhanced corrosive fluid flow and led to the development of a secondary dissolution porosity followed by pervasive dolomitization of large volumes of host rocks and partial sealing of the fractures and pores by dolomite, followed by ferroan dolomite, in a reducing environment of low sulfur activity.We propose that an interconnected network of incompletely cemented fracture and dissolution porosities channeled oxidizing, sulfate-bearing fluids into this reducing environment. These oxidizing fluids mixed with deep orogenic metalliferous fluids, set in motion by the Taconic orogeny, and were partially reduced to aqueous sulfide. At this stage, barite, galena, and sphalerite precipitated in the remaining open spaces of varying redox potential and partially replaced precursor cements and framework constituents. Remaining open spaces were cemented by calcite.Rare earth element patterns for the various generations of gangue dolomite and late calcite are characterized by positive europium anomalies ascribed to the partial dissolution of detrital potassium feldspars before, during, and after mineralization. Detrital feldspars are suggested as a major source of lead and barium in the St.-Fabien deposit. Lead isotopes support this crustal source for the lead, and their model age is consistent with the precipitation of galena during the late stages of, or slightly after, the Taconic orogeny. Homogeneity of the lead isotope ratios of both vein and disseminated galenas throughout the St.-Fabien deposit suggests that the lead isotopes were efficiently averaged throughout this part of the Taconic belt by a large volume of fluid.

Present-day precipitation of lead and zinc from groundwaters

Small quantities of galena and sphalerite are currently precipitating 26m below the surface, at the Vinemount quarry, Ontario, Canada from sulfurous spring waters, issuing from carbonates of the Lockport Formation. At the spring orifices, these sulfides are present in both fracture coatings and an associated black sludge. These precipitates are overlain by a crust rich in native sulfur. The mineralogy of the precipitates, and the chemistry of waters taken from three consistent spring horizons, indicates that downward-moving meteoric groundwaters leach metals from the host rocks and precipitate galena and sphalerite on encountering H2S-rich waters at the water table. While our observation of the transport and deposition of lead and zinc by shallow groundwaters lends support to intra-karstic models for the direct precipitation of galena and sphalerite, it also enables us to propose that many carbonate-hosted Pb-Zn deposits, of differing theories of origin, may have been enriched during periods of uplift.

The origin of a mississippi valley-type deposit in the Viburnum Trend, Southeast Missouri

Petrologic and isotopic studies of the base metal sulfide mineralization in the Buick mine have established the following paragenesis: an early stage of fine-grained pyrite and marcasite replaced the dolostone host rock; the iron disulfides and dolostone were replaced by sphalerite during the main stage of sphalerite precipitation (delta 34 S sp 16.5 to 19.0ppm). Following the sphalerite, two main stages of galena were precipitated--a cuboctahedral stage (delta 34 S gn 21.3 to 10.0ppm), during which galena replaced dolostone and sphalerite and grew in vugs in the dolostone and in the sphalerite, and a cubic stage (delta 34 S gn 10.5 to 0.9ppm). The two stages of galena precipitation were separated by a period during which the main stage of sphalerite and the cuboctahedral galena were partially dissolved and bladed marcasite (delta 34 S mc 2.3 to 4.1ppm) was precipitated, and followed by a period of mild etching of the cubic galena. Minor amounts of pale sphalerite (delta 34 S sp 14.0 to 16.5ppm) were then precipitated. White and colorless rhombs of dolomite were precipitated at the same time as some of the main stage of sphalerite, the cubic galena, and the pale sphalerite. After the pale sphalerite was precipitated, chalcopyrite, quartz, and siegenite were precipitated, followed by calcite and finally kaolinite. The paragenetic and textural data for both galena and sphalerite in the Buick mine strongly suggest that these minerals were precipitated and dissolved repetitively during ore formation. Similar textural evidence for galena reported by other workers for different mines in the Viburnum Trend implies that repetitive precipitation and dissolution of galena was probably an important process during the formation of the entire Viburnum Trend.Fluid inclusions in late quartz crystals coprecipitated with chalcopyrite have filling and freezing temperatures within the ranges reported for sphalerite from other parts of the Viburnum Trend, demonstrating that the ore-forming solutions in the Viburnum Trend were probably at temperatures in the range 105 degrees to 141 degrees C (assuming a maximum pressure correction of about 11 degrees C) and were extremely saline, containing about 4.0 to 5.0 molal chloride. Consideration of typical chemical analyses of fluid inclusions from other Mississippi Valley-type deposits suggests that the maximum likely concentration of dissolved CO 2 in the ore-forming solutions was about 0.05 molal. At these concentrations, the ore-forming solutions that travelled to the site of deposition could have had pH9s about two units less than neutral and still have been in equilibrium with dolomite or calcite.The ranges of sulfur and lead isotopic compositions of galena from the Buick mine are very similar to those established by earlier studies of the Old Lead Belt and adjacent subdistricts. However, the correlation of isotopically lighter sulfur with more radiogenic lead is much stronger for the Viburnum Trend, suggesting that galena was precipitated from many solutions, each of which carried lead and sulfur together and had different lead and overall sulfur isotopic compositions.The textural, paragenetic, and isotopic data for the Buick mine cannot be easily explained either by a sulfate reduction model in which sulfate was reduced by organic matter or methane, or mixing models in which base metals and reduced sulfur were transported separately to the site of deposition. Instead, the data strongly support a reduced sulfur model for ore formation according to which the base metals were transported together with reduced sulfur (as H 2 S), in the same solutions, to the sites of deposition. The reduced sulfur model is also supported by a theoretical calculation of the solubilities of galena and sphalerite, which demonstrate that low, but economically significant, concentrations of lead or zinc could have been transported as chloride complexes, together with equivalent concentrations of reduced sulfur, at temperatures of 100 degrees to 150 degrees C and pH9s of one to two units less than neutral, and could have been precipitated by pH increases, cooling, or dilution at the sites of deposition.

Cyclicity in the lead-zinc-silver-bearing sediments at Mount Isa mine, Queensland, Australia, and rates of sulfide accumulation

Markov probability matrices have been constructed showing the degree of upward and downward ordering in the lead-zinc-silver sulfide ores at Mount Isa mine, Queensland. Application of the difference matrix approach (Gingerich, 1969) and the concept of entropy (Hattari, 1976) revealed the sediments to be strongly ordered and cyclic. Vertical repetitions of the cyclic sequences dolomite --> sphalerite --> galena --> sphalerite --> dolomite; dolomite --> sphalerite --> dolomite; dolomite --> sphalerite with pyrite --> dolomite; and dolomite --> pyrite --> dolomite were found to be a function of lateral zonation of sulfide-rich layers. It is suggested that the metals and sulfur were supplied in pulses, possibly pumped up a fault by earth movements. Progressive mixing of this fluid with the sea water led to sequential precipitation of galena, sphalerite, and pyrite across the deposit. Transgression and regression of the system is responsible for the low-probability upward and downward transitions between the sulfide layers of lower probability. A generalized cross section has been drawn of a typical sulfide bed which, if repeated in the sequences, would give probability matrices approximating those found.A major conclusion of this model is that sulfides in submarine exhalative systems are probably rapidly deposited and a few centimeters of sediment may represent only a few weeks of deposition. Consideration of some general features of other submarine exhalative deposits also indicates high deposition rates.

L'association sulfures—Carbonates: Application au cas de la galène en milieu hydrothermal

The paragenetic association of sulfides and carbonates is studied experimentally for galena and calcite, by means of galena solubility measurements in a H 2OCO 2 system. Ionic Pb carbonate and bicarbonate complexes are shown, and K D's at 120, 150 and 180°C are calculated. The thermodynamic simulation of the behavior of a PbCa solution under CO 2 and H 2S pressure, submitted to a degasing procedure is done using those data; this simulation leads to the precipitation of galena and (or) calcite.