Mississippi Valley-type Ore Deposits Research Articles

Multiple Paleokarst Events in the Cambrian Potosi Dolomite, Illinois Basin

The Cambrian (Furongian) Potosi Dolomite (100-183 m) in Illinois is part of the Cambro-Ordovician Knox Group. It is a uniformly dolomitized unit with very low intercrystalline porosity but contains very permeable vug, fracture/cavern porosity intervals. Here, we interpret the characteristics of the widespread porous zones in the Potosi as paleokarst features formed by rising hypogenic basinal/hydrothermal fluids. The conformity bounded Potosi Dolomite is characterized by massive dolomitization, overdolomitization and occlusion of previously generated intercrystalline porosity, void filling mineralization, and extensive dissolution and formation of cavity-conduit systems. The pore spaces are typically lined with drusy quartz or are characterized by partial to complete infilling with chalcedonic silica and/or dolomite cements. Clay minerals may partially fill pore spaces; physical properties and thorium-potassium crossplot suggest chlorite as the main clay mineral present. Dolomite crystals typically are planar-s or nonplanar with open-space filling, inclusion rich saddle dolomite displaying curved and zigzag crystal faces. Void filling cement does not exhibit sign of pressure solution and in places vug porosity is developed along bedding parallel stylolite indicating post burial origin of these features. Cavern reservoirs in the Potosi are laterally extensive and often stacked with intervening very low porosity dolomite; very low bulk density, excursion of caliper log signature from the baseline, and loss of fluid circulation during drilling in these intervals signify anomalously high porosity and permeability interpreted as being the result of cavern forming multiple paleokarst events. Post burial origin of cavities and void filling cements, association of saddle dolomite and chlorite, and occurrence of Mississippi Valley-type (MVT) ore deposits in Missouri suggest karstification by hypogenic warm basinal/hydrothermal fluids. Dissolution and mineralization likely occurred by flow of deep basinal formation waters and hydrothermal fluids (sourced from the crystalline basement underlying the Reelfoot Rift and the Illinois Basin) along numerous basement-rooted normal, reverse, and strike-slip faults, and the associated fold and fractures. Expansion and contraction because of fault-related seismicity likely developed fracture porosity in brittle host dolomite and possibly ruptured any underling impermeable units to enable large-scale upward and outward fluid movement. The Potosi fracture/cavern porosity intervals are confined by thick very low porosity dolomite intervals that could serve as effective seal. There is no report of any show of oil in the Potosi Dolomite, but the unit has an excellent potential to serve as a combined reservoir and seal for storing anthropogenic CO2 and waste material.

Pb-Zn-Ba deposits in the Nigerian Benue Trough: A synthesis on deposits classification and genetic model

The Benue Trough stretching about 800–1000 km with a width ranging between 100 and 150 km, is a megarift structure whose origin was related to the opening of the south Atlantic during the early Cretaceous. This mega rift structure contains economically significant Pb-Zn-Ba-F deposits that form well-defined metallogenic belt about 600 km long in Nigeria. In the Southern Benue Trough (SBT), the Pb-Zn-Ba-F deposits are localized in four districts namely Abakaliki, Ishiagu, Wanikande and Gabu-Oshina and are hosted by carbonaceous shale; in the Central Benue Trough (CBT) the deposit is hosted in the silicified limestone and arkosic sandstone localized in Arufu, Akwana and Azara districts; while in the Northern Benue Trough (NBT) ores deposits occur in the arenaceous sediments of Yolo, Gombe and Gulani districts. Previously published sulfur (S) isotopic data of sulfides suggest that the ore-forming S was sourced from marine sulfates and the reduction of sulfate mainly by thermochemical sulfate reduction process in the CBT and NBT. Whereas in the SBT, bacterial sulfate reduction process is dominant. The compiled C-O isotopic compositions suggest that limestone and slightly metamorphosed sedimentary rocks supplied ore-forming materials to the hydrothermal systems through water/rock interaction. The available bulk Pb isotopic compositions for the ore minerals, the host sedimentary and crystalline rocks comprising the basement rocks as well as the basaltic rocks from the Trough suggest that the arkosic sandstone which is composed of detrital feldspars that was eroded from the Precambrian basement rocks could be the likely source of ore-forming metals. Our model proposed that the ore-forming fluids were sourced from the hot connate-brine that was expelled through compaction during the development of the deep sedimentary basins. Mesozoic magmatic events distributed throughout the basin likely provided the initial thermal flow and elevated the geothermal gradient. The hot brines could have leached the metals from the syn-rift continental formations. The tensional fractures serve as the conduit through which the metals migrated, leached abundant sulphate and precipitated their contents in the transcurrent fault system as sulfides, barite and associated polymetallic ores. The geological, mineralogical, and geochemical characteristics of the Nigerian Benue Trough Pb-Zn deposits are consistent with Mississippi Valley-type (MVT) Pb-Zn deposits in North Africa and the typical MVT ore deposits worldwide.

Hydrologic windows into the crystalline basement and their controls on groundwater flow patterns across the Paradox Basin, western USA

Conceptual models of sedimentary basin groundwater flow systems typically assume that the crystalline basement acts as an impermeable boundary and can be neglected. In this study, we use hydrologic models constrained by isotopic and geochemical datasets to argue that the La Sal Mountains, Utah, USA, act as a hydrologic window into the Paradox Basin’s lower aquifer system and underlying crystalline basement. We conducted a sensitivity study in which we varied crystalline basement/laccolith permeability as well as fault zone connectivity along a cross-sectional transect from the La Sal Mountains to Lisbon Valley. When the crystalline basement/laccolith units are set at relatively permeable levels (10−14 m2), simulated tracers that include total dissolved solids, oxygen isotopic composition of pore fluids (δ18O), and groundwater residence times are in closest agreement with field measurements. Model results indicate that pore fluids in the basal aquifer system underlying the Paradox Formation confining unit are a mixture of relatively young meteoric fluids and older Paradox Formation brines. The presence of faults did not significantly modify fluid exchange between the upper and lower aquifer systems. This was due, in part, to underpressuring within the Paradox Formation. Our study concludes that the Paradox Basin represents a regional recharge area for the Colorado Plateau, with groundwater discharge occurring along the Colorado River within the Grand Canyon some 375 km away to the southwest. This is only possible with a permeable crystalline basement. Our findings help explain the genesis of Mississippi Valley-type ore deposits of the US Midcontinent, where the presence of a permeable basement may be useful in addressing issues related to solute mass and energy balance.

Sulfur and lead isotopic compositions of ore sulfides and mining economic potential of the High Atlas Mississippi Valley-type ore province, Morocco

The Moroccan High Atlas metallogenic province hosts numerous PbZn (Ba-Sr-Fe) ore deposits hosted in the Jurassic carbonates. The high Atlas basin was formed during the Triassic-Jurassic rifting and underwent a Cenozoic tectonic inversion during the Alpine orogeny. This work presents a study of Pb and S isotopes of ore sulfides (galena, sphalerite, pyrite) from the Central and Oriental High Atlas of Morocco (CHA, OHA). The investigated ore deposits are Mississippi Valley-type (MVT) ore deposits with a simple paragenesis consisting of galena and sphalerite with subordinate barite, celestine, and pyrite. These ore deposits belong to three geographic zones; the northern border zone (NZ), the southern border (SZ), the central zone also called the diapiric zone (CZ), and the western zone (WZ) in the High Atlas. Based on the sulfur isotopic data, we suggest the Triassic-Jurassic sulfates and/or coeval pore water sulfates as the source of sulfur. Sulfur isotope compositions (δ34S = −18.6 to 22.2‰) are regionally zoned with isotopically heavy δ34S (14.3‰) in the center of the basin, negative δ34S values at the northern border (−16.9‰) and western border (−9.1‰), and intermediate δ34S values (4.4‰) at the southern border. This δ34S zonation suggests different sulfate reduction mechanisms; a low-temperature bacterial sulfate reduction (BSR) that induced the negative δ34S values and a high-temperature thermochemical sulfate reduction (TSR) that yielded positive δ34S values. The intermediate sulfur isotopic composition reflects the mixing of the TSR- derived sulfur and the bacteriogenic sulfur. The western part of the High Atlas near the Precambrian-Paleozoic paleohigh is characteristically more radiogenic (206Pb/204Pb = 18.709 207Pb/204Pb = 15.658 and 208Pb/204Pb = 38.808) than the rest of the basin (206Pb/204Pb = 18.151 to 18.369, 207Pb/204Pb = 15.631 to 15.668 and 208Pb/204Pb = 38.433 to 38.555). The Paleozoic rocks are the primary source of lead and by inference other metals. The metallogenic zone CZ with heavy sulfur isotopic composition and abundant Triassic evaporites (diapiric zone) constitutes a potential zone for future ore exploration. The SZ is also a promising target for ore exploration since it hosts ore sulfides that contain the thermally reduced sulfur (TSR).

Effects of Mississippi valley-type minerogenetic metal sulfates on thermochemical sulfate reduction, studied by hydrous pyrolysis

Mississippi valley-type (MVT) ore deposits are epigenetic carbonate-hosted Pb-Zn deposits, that are formed by fluid expulsion from sedimentary sulfide successions. The sulfides were generated by thermochemical sulfate reduction (TSR) of the evaporitic sulfates dissolved in fluids. The initiation, processes and principles for TSR occurring in MVT ore deposition and the respective influence of major minerogenetic metal ions such as Pb2+ and Zn2+, as well as Fe2+, Sr2+, Ba2+ has not been clearly resolved. To evaluate the TSR activity of metal cations in MVT minerogenetic systems, a series of 300 °C to 450 °C gold-tube hydrous-pyrolysis experiments were separately conducted with FeSO4, PbSO4, ZnSO4, BaSO4 or SrSO4 and with n-octadecane (n-C18) as the hydrocarbon substrate. Based on the yields (from gas chromatography (GC) analysis) and carbon isotopic compositions (determined by GC-irMS) of the gases produced in the hydrous-pyrolysis gold-tube experiments, the TSR reactivity of the minerogenetic metal sulfates was ranked FeSO4 > ZnSO4 > Sr/BaSO4 > PbSO4. TSR occurred easily in the FeSO4 experiments at 300 °C, but hardly at all in PbSO4 experiments at 450 °C. Hence, S2− for the formation of the gelenite (PbS), sphalerite (ZnS) and pyrite (FeS2) in the MVT ore deposits appears to be related to Fe2+, which could initiate the TSR easily to produce reduced sulfur.The following two potential routes would provide good support for the TSR of FeSO4: (1) Hydrolysis of Fe2+, or the formation of a ferrous hydroxide-sulfate-hydrate complex, that increases the H+ concentration, resulting in the formation of HSO4− that initiates TSR; and (2) The oxidation of Fe2+ to Fe3+, which on subsequent hydrolysis (or the formation of iron-hydroxide-sulfate-hydrate complex), would greatly increase the concentrations of H+ and HSO4−, reduce the pH of the brine fluids and maintain acidic conditions favorable to TSR. However, the precipitation of pyrite greatly consumes S2−, limiting the concentration of H2S and thereby affecting the rate of TSR.In short, TSR is difficult to simulate using Zn2+ and Pb2+ sulfates, but easy with sulfates containing Fe2+ and Mg2+. This suggests that the occurrence of the FeSO4 and MgSO4 is critical in the formation of the large-scale MVT ore deposits, where they react as acid buffering agents, decreasing and maintaining a lower pH in the brine fluids and accelerating and sustaining TSR with higher concentrations of HSO4−. H2S/S2− would then be produced continually and participate as FeS2, ZnS and PbS in the large-scale MVT ore deposits.

Origins and mixing contributions of deep warm groundwater in a carbonate-hosted ore deposit, Sichuan-Yunnan-Guizhou Pb-Zn triangle, southwestern China

Sichuan-Yunnan-Guizhou (SYG) carbonate-hosted Pb-Zn triangle is a world-class metallogenic belt and classified as Mississippi Valley-Type (MVT), which provides the greatest ore production in China. However, little is known about the effects of water-rock interactions involving the ore deposits and the modern hydrologic system. Measured chemical and isotopic compositions of the deep groundwater are used to evaluate water origins, potential circulation depth, reaction temperatures, and mixing process with shallow groundwater to assess possible linkages between regional hydrogeology and ore deposits. Within the Maoping sub-deposit, deep groundwater is warmer (21.2–32.0 °C), has lower pH, higher HCO3− and SO42− concentrations, higher average 87Sr/86Sr ratio (0.7148) and Sr concentration (4.1 mg/L), more positive δ13CDIC values (−4.5 to −4.2‰), and lighter average isotopic value (δD = −98 and δ18O = −13.7‰) relative to shallow groundwater, suggesting an origin distinct from modern surface recharge. The light isotopic composition of the deep warm water indicates recharge at a cooler temperature in the past, perhaps during the Last Glacial Maximum. The δ13CDIC values reflect isotopic equilibrium between deep groundwater and surrounding rocks along with a possible deep source of CO2. The oxidation process of sulfides, including H2S and pyrite linked to the ore-forming processes, could produce sulfuric acid that enhances carbonates dissolution and generates elevated HCO3− and SO42− concentrations. Reaction temperatures for the deep groundwater appear to have been around 50–65 °C based on quartz geothermometers and given a regional geothermal gradient of 24.1 °C/km suggest reactions occurred at depths ~1900 m below the land surface. Elevated Sr concentrations with radiogenic Sr isotope ratios reflect reaction with basement rocks and carbonates during the deep circulation. Sr concentration and isotope mixing models demonstrate different recharge sources and suggest the contribution of deep warm groundwater captured in mine drainage. These results reflect the importance of deep groundwater circulation and reactions in a large ore deposit that may improve understanding of potential effects of these important ore bodies on regional water compositions in the SYG triangle and other MVT ore deposits.

Geochemical characteristics of trace occurrences of Mississippi Valley-type mineralization in the U.S. mid-continent: Implications for deposit growth

The Paleozoic strata of the U.S. mid-continent host several major Mississippi Valley-type (MVT) ore deposits and many trace occurrences of MVT mineralization. The present study sought to test the hypothesis that systematic differences in fluid composition led to the differences in scale of mineralization. To conduct this test, fluid inclusions in trace MVT occurrences were characterized and compared to previously characterized fluid inclusions in mid-continent MVT ore deposits.Primary sphalerite-hosted fluid inclusions in trace MVT occurrences from across the U.S. mid-continent were analyzed using microthermometry, Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry. Interquartile ranges (25th to 75th percentile) of trace MVT fluid inclusion properties were found to be as follows: homogenization temperature = 78 to 107 °C, salinity = 13.8 to 23.9 equivalent weight percent NaCl, atomic ratios of K/Na = 0.017 to 0.038, Ca/Na = 0.18 to 0.31, Mg/Na = 0.0081 to 0.013, Sr/Na = 0.0093 to 0.015, Ba/Na = 0.00013 to 0.00041. Compared to MVT ore deposits, fluid inclusions in trace MVT occurrences have lower homogenization temperatures and K/Na and Ba/Na ratios, similar salinities and Ca/Na and Sr/Na ratios, and slightly higher Mg/Na ratios (excluding Mg-rich fluid inclusions from the Upper Mississippi Valley district). In addition, fluid inclusions in trace MVT occurrences have low to undetectable methane contents whereas MVT ore deposits have high methane concentrations on the order of 100's to 1000's of ppm. Sphalerite matrix trace metal concentrations of trace MVT occurrences generally overlap those of MVT ore deposits.Differences in elemental concentrations and temperature between trace MVT and MVT ore-forming fluids are insufficient to account for the differences in deposit size. This suggests that differences in the local availability of sulfide, which could not be measured in this study but may correspond with the large observed differences in methane content, may have governed MVT deposit size.

An experimental study on metal precipitation driven by fluid mixing: implications for genesis of carbonate-hosted lead–zinc ore deposits

A type of carbonate-hosted lead–zinc (Pb–Zn) ore deposits, known as Mississippi Valley Type (MVT) deposits, constitutes an important category of lead–zinc ore deposits. Previous studies proposed a fluid-mixing model to account for metal precipitation mechanism of the MVT ore deposits, in which fluids with metal-chloride complexes happen to mix with fluids with reduced sulfur, producing metal sulfide deposition. In this hypothesis, however, the detailed chemical kinetic process of mixing reactions, and especially the controlling factors on the metal precipitation are not yet clearly stated. In this paper, a series of mixing experiments under ambient temperature and pressure conditions were conducted to simulate the fluid mixing process, by titrating the metal-chloride solutions, doping with or without dolomite, and using NaHS solution. Experimental results, combined with the thermodynamic calculations, suggest that H2S, rather than HS− or S2−, dominated the reactions of Pb and/or Zn precipitation during the fluid mixing process, in which metal precipitation was influenced by the stability of metal complexes and the pH. Given the constant concentrations of metal and total S in fluids, the pH was a primary factor controlling the Pb and/or Zn metal precipitation. This is because neutralizing or neutralized processes for the ore-forming fluids can cause instabilities of Pb and/or Zn chloride complexes and re-distribution of sulfur species, and thus can facilitate the hydrolysis of Pb and Zn ions and precipitation of sulfides. Therefore, a weakly acidic to neutral fluid environment is most favorable for the precipitation of Pb and Zn sulfides associated with the carbonate-hosted Pb–Zn deposits.

Pyrite Re-Os age constraints on the Irankuh Zn-Pb deposit, Iran, and regional implications

The investigation of epigenetic carbonate-hosted Pb-Zn deposits from China and Europe in the Eastern and Western orogens of the Tethyan Domain, have provided a new perspective on the range of processes operating in this deposit class globally. However, improved metallogenic models require new constraints on the timing of mineralization throughout the Domain. To address this need, we applied Re-Os pyrite dating to the world class Irankuh Zn-Pb deposit, Iran. The Irankah deposit is located in the thrust belt of the Malayer-Esfahan Pb-Zn Metallogenic Belt (MEMB) in the Zagros Orogen, Iran. Mineralization is hosted by the Goushfil Main Fault and adjacent dolomitized limestones. The hydrothermal mineral assemblage comprises sphalerite, galena, pyrite, minor chalcopyrite and sulfosalt minerals, together with dolomite, barite and abundant quartz. Pyrite associated with main-stage sphalerite mineralization has low concentrations of 3 to 37 ng/g Re and 6 to 49 pg/g Os that are typical of low-level highly radiogenic (LLHR) sulfides. Nonetheless, vein and replacement pyrites from the Goushfil orebody yielded a 5-point isochron age of 66.5 ± 1.6 Ma that is interpreted as the main Zn-Pb mineralization age. The new mineralization age confirms an epigenetic origin for the Irankuh Zn-Pb deposit and supports an early onset for compressional deformation in the Zagros Orogen. The geochronological data support textural evidence that mineralization in veins occurred during regional compression in an active tectonic environment. Therefore the possibility that magmatic fluids and/or heat contributed to the mineralizing system should be incorporated into regional exploration models. Further work is required to test the extent to which epigenetic carbonate-hosted Pb-Zn deposits in the thrust and fold belt settings of the Central and Eastern Tethys differ from those in the Western Tethys and classic Mississippi Valley Type ore deposits.

Lead isotope trends and metal sources in the Mississippi Valley-type districts from the mid-continent United States

This study identifies metal-contributing source regions for Mississippi Valley-type (MVT) ore deposits in the Northern Arkansas district, the Tri-State district, and the Burkesville deposit in the Central Kentucky district. The Pb isotope ratios of sphalerite ores from the Tri-State district (208Pb/204Pb between 40.7443 and 41.2626; 207Pb/204Pb between 15.8633 and 15.9571; 206Pb/204Pb between 21.8373 and 22.1956) are congruent with the Pb isotope ratios defined by samples from the Central Missouri district, suggesting similar Pb sources. The ores from the Tri-State district plot in a narrow field and have higher 208Pb/204Pb values than samples from the Southeast Missouri district (Viburnum Trend and Old Lead Belt). The sphalerite samples from the Northern Arkansas district yield lower Pb isotope values (208Pb/204Pb between 39.4633 and 40.8863; 207Pb/204Pb between 15.8216 and 15.9176; 206Pb/204Pb between 20.2396 and 21.6438) than the Tri-State district ores and overlap the Southeast Missouri ores. The Northern Arkansas samples show a wide range of isotopic ratios with a linear trend, suggesting mixing of two end-member components. Age-corrected (250 Ma) Pb isotope ratios of Devonian-Mississippian Chattanooga shales from the Arkoma basin analyzed in this study plot close to the non-radiogenic end of the Northern Arkansas ores, implying that the shales may represent the non-radiogenic end-member that contributed Pb to the ores. The slope of the linear Pb isotope trend for the Northern Arkansas ores corresponds to an age of about 1.23 Ga, suggesting that the radiogenic Precambrian igneous basement may have supplied some of the ore Pb. Therefore, current data may indicate that the linear trend defined by Northern Arkansas ores may be the result of mixing of two end-members: the radiogenic basement rocks and the non-radiogenic shales.Pb isotope values of sphalerite from the Burkesville deposit are lower (208Pb/204Pb between 39.294 and 39.583; 207Pb/204Pb between 15.709 and 15.806; 206Pb/204Pb between 19.639 and 19.803) than those from the Northern Arkansas and Tri-State districts. The Pb isotope ratios of the Burkesville ores are similar to the Pb isotope values of the Central Tennessee (Elmwood deposit) and East Tennessee (Young deposit) ores and show evidence, the first of its kind for the region, of fluid mixing between the Illinois and Appalachian basins along the Cincinnati Arch that divides them.On the 208Pb/204Pb vs. 206Pb/204Pb diagram, the Pb isotope values of MVT ore samples from districts within the mid-continent US show two collinear, parallel trends, suggesting a different Th/U trend: (1) Tri-State and Central Missouri ore samples are collinear with Southeast Missouri and Northern Arkansas district samples; (2) Upper Mississippi Valley and Illinois-Kentucky ores are collinear with the Central Appalachian, Southern Appalachian (including East Tennessee), Central Tennessee, and Burkesville (Central Kentucky) ores.

Lead and sulfur isotope composition of trace occurrences of Mississippi Valley-type mineralization in the U.S. midcontinent

Mississippi Valley-type (MVT) Zn-Pb mineralization in the U.S. mid-continent has formed in numerous widely dispersed trace occurrences and in a few large ore deposits that have been mined. Based on similarly high fluid inclusion salinities and high homogenization temperatures relative to inferred burial depth, the trace MVT occurrences and MVT ore deposits may be genetically related by having precipitated from the same regional groundwater flow system mobilized by Pennsylvanian-Permian tectonism. The purpose of the present study was to test the robustness of this inferred genetic relationship by characterizing the Pb and S isotope composition of trace MVT occurrences in light of the much more thoroughly studied MVT ore deposits.Lead and sulfur isotope compositions have been obtained for trace MVT occurrences from three broad regions of the U.S. mid-continent: the Forest City basin (FCB), the Illinois basin (ILB), and the Greater Upper Mississippi Valley (GUMV) region. Lead isotope compositions of trace MVT occurrences from each of these three areas define discrete linear trends that are similarly oriented but not strictly collinear with one another or with the Pb isotope trends of MVT ore deposits, suggesting that trace MVT occurrences in each of these three regions derived their Pb from separate pairs of end-member Pb sources. An exception is the GUMV trace MVT occurrences and the Upper Mississippi (UMV) district ores, which appear to have collinear Pb isotope trends and thus may be the most closely related genetically with respect to sources of Pb. All of the trace MVT occurrences have radiogenic Pb suggesting derivation from Precambrian igneous basement rocks and not from the formations that host trace MVT mineralization. Trace MVT occurrences tend to have less radiogenic Pb than the MVT ore deposits, although trace MVT occurrences from the GUMV region are the most heterogeneous and can have Pb that is as radiogenic as the MVT ore deposits. Regionally, Pb isotope compositions of trace MVT occurrences tend to become less radiogenic southwestward and southward, but do not correlate with host rock age, and by proxy, distance from the Precambrian basement.The sulfur isotope compositions of sphalerite and galena from the trace MVT occurrences are light compared to the MVT ore deposits and likely reflect incomplete reduction of marine sulfate reservoirs or derivation from organic matter. The Pb and S isotope compositions of the trace MVT occurrences do not correlate well, suggesting that Pb and S were not transported together in the same fluid.

Hydrothermal Fluid Origins of Carbonate-Hosted Pb-Zn Deposits of the Sanjiang Thrust Belt, Tibet：Indications from Noble Gases and Halogens

The Sanjiang metallogenic belt includes a variety of economically important carbonate-hosted Pb-Zn deposits that share some similarities with classic Mississippi Valley-type (MVT) ore deposits but are hosted within a thrust belt rather than an orogenic foreland. This study aims to clarify the origin of mineralizing fluids responsible for this style of mineralization. Fluid inclusions trapped in ore-stage carbonate and fluorite from these deposits have salinities of ~6 to 28 wt % NaCl equiv and homogenization temperatures of 70° to 370°C that extend to much higher values than are typical of MVT deposits. The majority of ore-stage samples have fluid inclusion molar Br/Cl ratios of between seawater (1.5 × 10 −3 ) and (2.86 ± 0.04) × 10 −3 , but low-salinity fluid inclusions in late calcite have lower Br/Cl of less than (0.55 ± 0.01) × 10 −3 . In contrast, fluid inclusion molar I/Cl ratios are uniformly greater than the seawater value of ~0.8 × 10 −6 and extend from (2.1 ± 1.1) × 10 −6 to (506 ± 12) × 10 −6 . This range of Br/Cl and I/Cl values is similar to what has been reported for fluid inclusions in other MVT districts and together with the fluid salinity implies the ore-forming fluids had a dominant origin from basinal brines (e.g., sedimentary formation waters) formed by the subaerial evaporation of seawater; all the fluids were influenced by addition of organic Br and I derived from the sedimentary host rocks and some fluids were locally modified by interaction with evaporites producing low Br/Cl ratios. The fluid inclusions have 40 Ar/ 36 Ar ratios of up to 441 that are higher than the atmospheric value of 296 and typical of carbonate sedimentary rocks. The fluid inclusions have high concentrations of atmospheric 36 Ar and variable 129 Xe/ 36 Ar and 84 Kr/ 36 Ar ratios that are outside the range expected from mixing air and air-saturated water. These data are likely to reflect a complex fluid history involving acquisition of atmospheric ( 36 Ar, 84 Kr, 129 Xe) and radiogenic (e.g., 40 Ar ✼ ) noble gases trapped in sedimentary rocks and fractionation of these gases between water and hydrocarbons. The 3 He/ 4 He ratios of fluorite fluid inclusions range from a typical crustal value of 0.061 ± 0.004 to values of >0.7 Ra, indicating a minor component of mantle-derived 3 He. The fluids with the highest 3 He/ 4 He also have 4 He/ 40 Ar ✼ close to the mantle value, suggesting the 3 He could have been introduced by a volumetrically minor fluid of either magmatic or deep metamorphic origin ( 40 Ar ✼ = radiogenic 40 Ar). The new halogen and noble gas data are consistent with a model in which regional Pb-Zn mineralization formed by mixing two modified basinal brines that were transported through independent aquifers and fluid pathways to the sites of mineralization. A low-temperature brine contained organic Br, I, and H 2 S, and a high-temperature metal-rich brine (>370°C) that included a volumetrically minor magmato-metamorphic component was channeled up deeply penetrating thrust structures.

Formation waters from Cambrian-age strata, Illinois Basin, USA: Constraints on their origin and evolution

Recently, brine samples from the Cambrian-age Mount Simon Formation (the deepest, most inaccessible sedimentary rock formation of the Illinois Basin) and the overlying Ironton–Galesville Formation were collected as part of a major research effort evaluating the feasibility of sequestration of carbon dioxide in deep geologic formations. Halide and halide/cation ratios (especially Cl/Br and Na/Br ratios) from groundwater samples collected during this investigation suggest that the brines of the Cambrian-age strata formed by the evaporation of seawater well beyond the point of halite precipitation. The Cl/Br and Na/Br ratios, the presence of Mississippi-Valley-Type (MVT) ore mineralization in close proximity to the Illinois Basin, and the tectonic history of the region and the Illinois Basin suggest that components of ore-forming brines and perhaps crystalline basement brine are likely still present within the Mount Simon Formation. Halide and cation/halide ratio plots show that these brines have mixed with and have been diluted by subaerially evaporated seawater, seawater and dilute groundwater. Movement of brines out of the Mount Simon Formation and/or exchange with brines of other formations is constrained by the overlying, siltstone- and shale-rich Eau Claire Formation, a low-permeability layer.The most plausible interpretation of the halide and halide/cation ratio data is that the brines of the Cambrian-age strata were introduced to the Illinois Basin from outside of the basin, perhaps when the Illinois Basin was connected to the Arkoma (Oklahoma and Arkansas) and Black Warrior Basins (Alabama and Mississippi) via the Reelfoot Rift during Cambrian and early Ordovician time. In addition, the presence of some percentage of high NaCl, low Cl/Br brines from the crystalline basement is suggested given the geochemical relationships of the halide and cation/halide ratios and the tectonic history of the Illinois Basin. Finally, halide and cation/halide ratios determined by this investigation, and regional geochemical evidence and hydrogeologic modeling (by others) suggest that the brines of these strata probably were affected by regional hydrothermal activity during Permian time that was responsible for the MVT ore deposits of the Midwestern U.S. Thus, the brines of the deepest strata of the Illinois Basin constitute a different, more complex type of fluid than those found elsewhere in the basin. Halide and halide-cation ratios suggest that these deep brines are dominated by residual evaporitic brine (possibly originating as ore-forming brines) with dilution by seawater and dilute groundwater. Other components may include subaerially evaporated seawater and crystalline basement brines.

Polaris as a guide to northern exploration: Ore textures, paragenesis and the origin of the carbonate-hosted Polaris Zn–Pb Mine, Nunavut, Canada

The Polaris Zn–Pb Mine in Nunavut, Canada was one of the largest single Mississippi Valley-type ore deposits in the world. Over 20Mt of sphalerite (ZnS) and galena (PbS) was hosted in brecciated carbonate rocks of the Upper Ordovician Thumb Mountain Formation. Three paragenetic stages are recognized: 1) early dolomite and marcasite; 2) main stage sulphide and dolomite; and 3) late calcite, marcasite and barite. Ore mineral textures range from discrete crystals to massive crystal aggregates and formed as replacements of the dolomite host rock or as fracture- and open space-filling mineralization. Zinc concentration is highest in the core of the deposit where botryoidal aggregates predominate, whereas iron is concentrated in the upper part. Observations of temperature and in situ sulphur isotope fractionation support a genetic model for the Polaris deposit in which thermochemical sulphate reduction occurred within the deposit, with locally generated hydrocarbons acting as a reducing agent. Information from the Polaris Mine indicates that hydrothermal alteration including dolomite, marcasite and barite; complex paragenesis with numerous ore textures; Th values >100°C associated with organic-rich strata; and a geochemical signature that includes in situ sulphur fractionation are effective predictors for determining which showings are prospective in the vast central Arctic Pb–Zn district.

Coupling meteorology, metal concentrations, and Pb isotopes for source attribution in archived precipitation samples

A technique that couples lead (Pb) isotopes and multi-element concentrations with meteorological analysis was used to assess source contributions to precipitation samples at the Bondville, Illinois USA National Trends Network (NTN) site. Precipitation samples collected over a 16month period (July 1994–October 1995) at Bondville were parsed into six unique meteorological flow regimes using a minimum variance clustering technique on back trajectory endpoints. Pb isotope ratios and multi-element concentrations were measured using high resolution inductively coupled plasma‐sector field mass spectrometry (ICP-SFMS) on the archived precipitation samples. Bondville is located in central Illinois, ~250km downwind from smelters in southeast Missouri. The Mississippi Valley Type ore deposits in Missouri provided a unique multi-element and Pb isotope fingerprint for smelter emissions which could be contrasted to industrial emissions from the Chicago and Indianapolis urban areas (~125km north and east, of Bondville respectively) and regional emissions from electric utility facilities. Differences in Pb isotopes and element concentrations in precipitation corresponded to flow regime. Industrial sources from urban areas, and thorogenic Pb from coal use, could be differentiated from smelter emissions from Missouri by coupling Pb isotopes with variations in element ratios and relative mass factors. Using a three endmember mixing model based on Pb isotope ratio differences, industrial processes in urban airsheds contributed 56±19%, smelters in southeast Missouri 26±13%, and coal combustion 18±7%, of the Pb in precipitation collected in Bondville in the mid-1990s.

Paleomagnetic mapping of the regional fluid flow event that mineralized the Upper Mississippi Valley Zn–Pb ore district, Wisconsin, U.S.A.

The Upper Mississippi Valley (UMV) zinc–lead district contains archetypal examples of Mississippi Valley-type (MVT) ore deposits. Specimens (332) from 25 sites in the Lower Ordovician Oneota dolomite were collected from around the northern and eastern sides of the UMV district on the Wisconsin Arch in southwestern Wisconsin. Most specimens yield one of two characteristic remanence (ChRM) components, A and B, in varying proportion at each site. Component B resides in magnetite mostly and gives a primary Early–Middle Ordovician paleopole at 15.1°N, 150.5°E (δp = 2.4°, δm = 4.7°), and component A resides mainly in pyrrhotite and gives an Early Permian remagnetization paleopole at 47.4°N, 127.3°E (δp = 1.1°, δm = 2.2°) that matches the UMV zinc–lead ore's paleopole. The percentage of A component specimens at each site is used to map the boundary of the Early Permian fluid flow that formed the UMV ore over a distance of > 200 km. The map also delineates areas of intense pyrrhotite authigenesis created by sulphidic fluid flow within the UMV district, and locates potential conduits in the Oneota dolomite through which fluid flow was delivered from an adjacent basin.

Geochemical constraints on the origin of the Kicking Horse and Monarch Mississippi Valley-type lead-zinc ore deposits, southeast British Columbia, Canada

Two Mississippi Valley-type (MVT) ore deposits, Kicking Horse and Monarch, have been studied with the aim of comparing the ores at the two localities and to characterize the origin of the mineralizing fluids and the ore formation process(es). Both deposits are hosted by the Middle Cambrian Cathedral Formation carbonate host rocks, Kicking Horse on the north and Monarch on the south flank of the Kicking Horse valley near Field (SE British Columbia). The ore bodies are situated at the transition of (western) basinal to (eastern) shallow-water strata of the paleo-Pacific passive margin succession in the Cordilleran Foreland Province of the Western Canada Sedimentary Basin. Both deposits are related spatially to normal faults. In both localities, the ore minerals are dominated by pyrite, sphalerite, and galena. Dolomite, minor quartz, and calcite are also present in close association with the ores. The salinity (21–30 wt% NaCl eq.) and homogenization temperatures (63–182°C) measured in fluid inclusions in carbonate, quartz, and sphalerite lie within the typical range of MVT fluid conditions. The good stoichiometry (50–53 mol% CaCO3), low δ18O values (−21 to −14‰ Vienna Peedee belemnite) and relatively high homogenization temperatures (>95°C) of the dolomite suggest the dolomites were formed under burial diagenesis. The ore-forming fluids probably interacted with siliciclastic units, based on elevated Li contents and 87Sr/86Sr ratios, which are highest in the dolomite type after the main ore stage. We propose that the ores formed from the mixing of a downward-infiltrating, sulfur-bearing halite-dissolution fluid with an upward-migrating, metal-rich evaporated seawater fluid, which had already undergone minor mixing with a dilute fluid.

The fate of trace elements during several coal pretreatment processes : Yan, R. et al. Energy & Fuels, 2002, 16, (5), 1160-1160

Recently, brine samples from the Cambrian-age Mount Simon Formation (the deepest, most inaccessible sedimentary rock formation of the Illinois Basin) and the overlying Ironton–Galesville Formation were collected as part of a major research effort evaluating the feasibility of sequestration of carbon dioxide in deep geologic formations. Halide and halide/cation ratios (especially Cl/Br and Na/Br ratios) from groundwater samples collected during this investigation suggest that the brines of the Cambrian-age strata formed by the evaporation of seawater well beyond the point of halite precipitation. The Cl/Br and Na/Br ratios, the presence of Mississippi-Valley-Type (MVT) ore mineralization in close proximity to the Illinois Basin, and the tectonic history of the region and the Illinois Basin suggest that components of ore-forming brines and perhaps crystalline basement brine are likely still present within the Mount Simon Formation. Halide and cation/halide ratio plots show that these brines have mixed with and have been diluted by subaerially evaporated seawater, seawater and dilute groundwater. Movement of brines out of the Mount Simon Formation and/or exchange with brines of other formations is constrained by the overlying, siltstone- and shale-rich Eau Claire Formation, a low-permeability layer.The most plausible interpretation of the halide and halide/cation ratio data is that the brines of the Cambrian-age strata were introduced to the Illinois Basin from outside of the basin, perhaps when the Illinois Basin was connected to the Arkoma (Oklahoma and Arkansas) and Black Warrior Basins (Alabama and Mississippi) via the Reelfoot Rift during Cambrian and early Ordovician time. In addition, the presence of some percentage of high NaCl, low Cl/Br brines from the crystalline basement is suggested given the geochemical relationships of the halide and cation/halide ratios and the tectonic history of the Illinois Basin. Finally, halide and cation/halide ratios determined by this investigation, and regional geochemical evidence and hydrogeologic modeling (by others) suggest that the brines of these strata probably were affected by regional hydrothermal activity during Permian time that was responsible for the MVT ore deposits of the Midwestern U.S. Thus, the brines of the deepest strata of the Illinois Basin constitute a different, more complex type of fluid than those found elsewhere in the basin. Halide and halide-cation ratios suggest that these deep brines are dominated by residual evaporitic brine (possibly originating as ore-forming brines) with dilution by seawater and dilute groundwater. Other components may include subaerially evaporated seawater and crystalline basement brines.

Geology and Geochemistry of the Reocin Zinc-Lead Deposit, Basque-Cantabrian Basin, Northern Spain

The Reocin Zn-Pb deposit, 30 km southwest of Santander, Spain, occurs within Lower Cretaceous dolomitized Urgonian limestones on the southern flank of the Santillana syncline. The Reocin deposit is one of the largest known strata-bound, carbonate-hosted, zinc-lead deposits in Europe. The total metal endowment of the deposit, including past production and remaining reserves, is 62 Mt of ore grading 8.7 percent Zn and 1.0 percent Pb. The epigenetic mineralization consists of sphalerite and galena, with lesser marcasite and trace pyrite with dolomite as gangue. Microprobe analyses of different generations of dolomite revealed nonstoichiometric compositions with various amounts of iron (up to 14 mol % of FeCO3). Replacement of host dolomite, open-space filling of fractures, and cementation of breccias derived from dissolution collapse are the principal types of ore occurrence. Detailed cross-section mapping indicates a stratigraphic and structural control on the deposit. A stratiform morphology is present in the western part of the orebody (Capa Sur), whereas mineralization in the eastern part is highly discordant but strata bound (Barrendera). Stratigraphic studies demonstrate that synsedimentary tectonic activity, related to the rifting of the North Atlantic (Bay of Biscay), was responsible for variation in sedimentation, presence of unconformities (including paleokarsts), local platform emergence and dolomitization along the N60 fault trend. In the Reocin area, two stages of dolomitization are recognized. The first stage is a pervasive dolomitization of the limestone country rocks that was controlled by faulting and locally affected the upper part of the Aptian and the complete Albian sequence. The second dolomitization event occurred after erosion and was controlled by karstic cavities. This later dolomitization was accompanied by ore deposition and, locally, filling of dolomite sands and clastic sediments in karstic cavities. The circulation of hydrothermal fluids responsible for sulfide deposition and the infilling of karst cavities were broadly contemporaneous, indicating a post-Albian age. Vitrinite reflectance data are consistent with previously measured fluid inclusion temperatures and indicate temperatures of ore deposition that were less than 100°C. Carbon and oxygen isotopic data from samples of regional limestone, host-rock dolostone and ore-stage dolomite suggest an early hydrothermal alteration of limestone to dolostone. This initial dolomitization was followed by a second period of dolomite formation produced by the mixing of basinal metal-rich fluids with local modified seawater. Both dolomitization events occurred under similar conditions from fluids exhibiting characteristics of basinal brines. The δ 34S values of sulfides are between –1.8 and +8.5 per mil, which is consistent with thermochemical sulfate reduction involving organic matter as the main source of reduced sulfur. Galena lead isotope compositions are among the most radiogenic values reported for Zn-Pb occurrences in Europe, and they are distinct from values reported for galena from other Basque-Cantabrian deposits. This suggests that a significant part of the lead was scavenged from the local underlying Asturian sediments. The stratigraphic and structural setting, timing of epigenetic mineralization, mineralogy, and isotopic geochemistry of sulfide and gangue minerals of the Reocin deposit are consistent with the features of most of Mississippi Valley-type ore deposits.

Laramide orogenic fluid flow into the Western Canada sedimentary basin; evidence from paleomagnetic dating of the Kicking Horse mississippi valley-type ore deposit

The Kicking Horse and Monarch Mississippi Valley-type zinc-lead ore deposits are in the southern Rocky Mountains of the foreland belt in the Canadian Cordillera of British Columbia. These epigenetic sphalerite-galena deposits are hosted by a massive sparry dolomite sheet in Middle Cambrian dolostones of the Cathedral Formation on the southwestern margin of the western Canada sedimentary basin. These strata were folded, thrust faulted, and uplifted during the Laramide orogeny. Paleomagnetic analysis was done on 316 specimens from 32 mineralized and host-rock sites, using alternating field and thermal step demagnetization and isothermal remanence methods. The results show that the characteristic remanent magnetization is carried by both pyrrhotite and magnetite. The increase in characteristic remanent magnetization intensity near Mississippi Valley-type mineralization ties the magnetization to the mineralization event. Fold tests on a small scale ( approximately 10 m) and a large scale ( approximately 10 km) show that the characteristic remanent magnetization postdates the main Laramide deformation event. Comparison of the characteristic remanent magnetization directions shows that the host rocks were remagnetized when the mineralization was emplaced with a primary characteristic remanent magnetization. The Kicking Horse mineralization and adjacent host rocks form the bulk of the collection and give a Late Cretaceous pole position of 64.0 degrees N, 152.8 degrees W (semi-axes of oval of 95% confidence: delta p = 5.4 degrees , delta m = 5.8 degrees ; number of sites (specimens) = 19 (180)). The age of this pole coincides with the Cretaceous normal superchron and conforms to the finding of all normal characteristic remanent magnetization directions except for one sample with an antipodal direction. No site retains a characteristic remanent magnetization that predates the Laramide orogeny. The evidence for and against a pre-Laramide Mississippi Valley-type mineralizing and dolomitizing event in the western Canada sedimentary basin is reviewed, and it is concluded that the main fluid flow event in the basin occurred during the Late Cretaceous-Paleocene Laramide orogeny.