Mississippi Valley-type Mineralization Research Articles

Barite Replacement as a Key Factor in the Genesis of Sediment-Hosted Zn-Pb±Ba and Barite-Sulfide Deposits: Ore Fluids and Isotope (S and Sr) Signatures from Sediment-Hosted Zn-Pb±Ba Deposits of Iran

Iran hosts more than 350 Precambrian to Cenozoic sediment-hosted Zn-Pb±Ba and barite-sulfide deposits, including shale-hosted massive sulfide (SHMS, also called SEDEX) and Irish-type and Mississippi Valley-type (MVT) mineralization, and barite is a common mineral in these deposits. In the SHMS deposits, barite is typically found as fine-grained disseminations in thin laminae. In these deposits, the sulfide laminae often occur as diagenetic replacements and as bands containing authigenic and diagenetic barite and pyrite framboids. In the Irish-type Zn-Pb-Ba and stratabound barite-sulfide deposits, barite exhibits various textures, including fine-grained disseminated barite, banded zebra textures, veins, and massive barite lenses. In some of the giant Irish-type deposits, as well as in the stratabound barite-sulfide mineralization, the main stratabound sulfide ore is developed within a barite envelope and is characterized by the replacement of barite and pyrite by chalcopyrite, galena, and sphalerite. In the MVT deposits, the formation of barite is often related to dolomitization, and sulfide mineralization involves the replacement of the dolomitized carbonate rocks, as well as associated barite. Fluid inclusion studies on the Irish-type deposits indicate that the temperatures and salinities of the sulfide-forming fluids are higher compared to those of the barite-forming fluids. Fluid inclusion analyses of coarse-grained barites from Irish and MVT deposits reveal their hydrothermal origin. The δ3⁴S values of sulfide minerals (pyrite, sphalerite, and galena) in Irish-type deposits exhibit a broad range of low values (mostly −28 to +5‰), primarily revealing a process of bacterial sulfate reduction (BSR). However, the textures (replacement, colloform, and banded) and more positive sulfur isotope values (+1 to +36‰) in the SHMS Zn-Pb deposits suggest that bacterial sulfate reduction (BSR) plays a less significant role. We suggest that thermochemical sulfate reduction (TSR) connected to the direct replacement of barite plays a more relevant role in providing sulfur for the sulfide mineralization in the SHMS, barite-sulfide, and MVT deposits. Based on the textual evidence, sulfur isotopic data, and fluid inclusion studies, barite has been identified as a key controller for the subsequent Zn-Pb mineralization by providing a suitable host and significant sulfur contribution in the sediment-hosted Zn-Pb and stratabound barite-sulfide deposits. This implies that diagenetic barite might be a precursor to all types of sediment-hosted Zn-Pb mineralization.

Vanished evaporites, halokinetic structure, and Zn-Pb mineralization in the world-class Angouran deposit, northwestern Iran

The Angouran deposit (19.3 Mt at 23.4% Zn and 4% Pb) is the second-largest Zn-Pb deposit in Iran. The deposit is hosted in a Neoproterozoic−Cambrian marble-schist sequence within a breccia pipe in a domal structure, with sulfide mineralization under low-temperature hydrothermal conditions (<200 ºC). The features of the ore-hosting breccias are similar to known halokinetic diapir breccias in the world but evaporite minerals are subtle. The common types of breccia clasts in the Angouran breccia pipe include a matrix-supported angular clast (float breccia) with highly variable sizes and orientations and exotic volcanic clasts. The volcanic clasts were derived from the underlying Miocene volcanic rocks, evidenced by the consistent petrography and zircon U-Pb ages dated at 20−19 Ma. Abundant smithsonite pseudomorphs after anhydrite and anhydrite inclusions within sphalerite and pre-ore marcasite in the breccia matrix indicate that the breccia pipe contains abundant anhydrite prior to the Zn-Pb mineralization. The enrichment of evaporite is also supported by the occurrence of considerable double-terminated quartz crystals that contain spherical and tabular carbonate inclusions and anomalously high Li, Na, and K concentrations, relatively high B concentration, and high δ18O values (up to 28.3‰). These observations suggest the Angouran deposit formed in a former halokinetic diapir breccia pipe. The halokinetic diapirism was possibly triggered by thrust loading of the marble-schist sequence over the Miocene evaporite beds during the Arabia-Eurasia continental collision. Halokinetic structures elsewhere in the Angouran region warrant this consideration. Most of the evaporite minerals in the breccia pipe were dissolved and replaced before and/or during subsequent Zn-Pb sulfide and smithsonite mineralization events. This study provides a good example for the identification of vanished evaporites, halokinetic structure, and associated Mississippi Valley-type mineralization.

Basin scale evolution of zebra textures in fault‐controlled, hydrothermal dolomite bodies: Insights from the Western Canadian Sedimentary Basin

AbstractStructurally controlled dolomitization typically involves the interaction of high‐pressure (P), high‐temperature (T) fluids with the surrounding host rock. Such reactions are often accompanied by cementation and recrystallization, with the resulting hydrothermal dolomite (HTD) bodies including several ‘diagnostic’ rock textures. Zebra textures, associated with boxwork textures and dolomite breccias, are widely considered to reflect these elevated P/T conditions. Although a range of conceptual models have been proposed to explain the genesis of these rock textures, the processes that control their spatial and temporal evolution are still poorly understood. Through the detailed petrographical and geochemical analysis of HTD bodies, hosted in the Middle Cambrian strata in the Western Canadian Sedimentary Basin, this study demonstrates that a single genetic model cannot be applied to all the characteristics of these rock textures. Instead, a wide array of sedimentological, tectonic and metasomatic processes contribute to their formation; each of which is spatially and temporally variable at the basin scale. Distal to the fluid source, dolomitization is largely stratabound, comprising replacement dolomite, bedding‐parallel zebra textures and rare dolomite breccias (non‐stratabound, located only proximal to faults). Dolomitization is increasingly non‐stratabound with proximity to the fluid source, comprising bedding‐inclined zebra textures, boxwork textures and dolomite breccias that have been affected by recrystallization. Petrographical and geochemical evidence suggests that these rock textures were initiated due to dilatational fracturing, brecciation and precipitation of saddle dolomite as a cement, but significant recrystallization occurred during the later stages of dolomitization. These rock textures are closely associated with faults and carbonate‐hosted ore deposits (e.g. magnesite, rare earth element and Mississippi Valley–type mineralization), thus providing invaluable information regarding fluid flux and carbonate metasomatism under elevated P/T conditions.

A tool for Zn-Pb MVT exploration by combining C and O isotopes and REE geochemistry of dolomite

An exploration guide for Mississippi Valley-type (MVT) deposits is proposed and illustrated in the Riópar area (SE Spain), where extensive dolostone geobodies hosting Zn-(Fe-Pb) MVT mineralization occur. These base metal deposits are found within stratabound and patchy dolostones replacing Upper Jurassic to Lower Cretaceous limestones. However widespread stratabound dolostones, with no Zn-(Fe-Pb) mineralization associated, are also found in the same area replacing Lower Jurassic to Upper Cretaceous carbonates. A detailed sampling of both, Zn-(Fe-Pb) mineralized dolostones (fertile), as well as unmineralized ones (barren), has been performed in an area of ∼106 km2 to characterize their rare earth element (REE) and isotopic (C, O, Sr) compositions. Barren Lower and Middle Jurassic and Upper Cretaceous dolostones show C, O and Sr isotopic characteristics (δ13C = +2.1 to +3.8‰ VPDB, δ18O = +27.6 to +29.8 ‰ VSMOW, 87Sr/86Sr = 0.70736 to 0.70764) like the Jurassic-Cretaceous undolomitized limestones and marine carbonates, and consistent with dolostones produced by low temperature seawater. On the contrary, barren and fertile Upper Jurassic to Lower Cretaceous stratabound and patchy dolomitized limestones are depleted in δ13C and δ18O and relatively enriched in 87Sr/86Sr compared to the host limestones. This δ13C and δ18O composition can be explained by an interaction of warm fluids with regional carbonates (2–3% of fluid/rock interaction at 190–230 °C). The hydrothermal character for the dolomitizing fluid is supported by REE geochemical data (e.g., (Pr/Pr*)PAASN and (Ce/Ce*)PAASN anomalies). Thus, C and O isotopes, as well as (Ce/Ce*)PAASN and (Pr/Pr*)PAASN ratios, can be used to discriminate between low temperature dolostones, formed from cold seawater that do not contain Zn-(Fe-Pb) mineralization (barren), from hydrothermal dolomites (HTDs), which may host Zn-(Fe-Pb) mineralization. Furthermore, most barren HTDs show more restricted and higher δ13C (+0.4 to +0.9 ‰ VPDB) and δ18O values (+26.4 to +27.1 ‰ VSMOW), as well as lower ∑REE contents (5.41 to 7.38 mg kg−1), compared to Zn-(Fe-Pb) mineralized HTDs (δ13C: −2.3 to +0.6 ‰ VPDB; δ18O: +25.1 to +27.1 ‰ VSMOW; ∑REE 14.10 to 54.79 mg kg−1). Therefore, the obtained REE contents, and partially the δ13C values, can be used to discriminate between HTDs associated with Zn-(Fe-Pb) mineralization (fertile dolostones) and barren HTDs, revealing their potential as a new geochemical tool for MVT exploration in dolomitized terrains.

Dolomite cement microstratigraphy: A record of brine evolution and ore precipitation mechanisms, upper Knox Group, Tennessee and Kentucky, USA

AbstractTrace element changes in fluids associated with ore-forming events in sedimentary basins may be recorded by contemporaneous cements, especially zoned carbonate minerals (microstratigraphy). Cement analysis using advanced mapping and analytical techniques including scanning electron microscopy cathodoluminescence (SEM-CL), charge contrast imaging, high-resolution X-ray computed tomography (XCT), and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) documents geochemical changes associated with Mississippi Valley–type mineralization in solution-collapse breccias of the Cambrian–Ordovician Knox Group (Tennessee and Kentucky, USA). Dolomite cement zonation coincident with changes in Fe and Mn can be observed with optical microscope CL in bands as narrow as 5 µm, whereas panchromatic SEM-CL reveals microfractures and cement subzones coincident with changes in La and Ce concentrations in bands as narrow as 0.1 µm. XCT scans image a high-density (Fe-rich) dolomite zone at the onset of late sulfide precipitation. The transition from pre-ore to ore-stage cementation is marked by increased Fe, Mn, Zn, Cd, Ga, Pb, and Sr and decreased La and Ce concentrations. Fine-scale metal depletion cycles during this transition may record metal precipitation from brine in response to the availability of reduced sulfur. Except for Fe and Mn, post-ore dolomite zones generally have low metal concentrations. Thus, dolomite microstratigraphy tracks systematic changes in brine metal concentrations modified by episodes of localized sulfide mineral precipitation.

Mineralization of MVT Pb-Zn Deposits in the Process of Hydrocarbon Accumulation and Destruction in the Strong Structural Deformation Area of Eastern Sichuan, South China

The Mississippi Valley-type (MVT) Pb-Zn deposits and hydrocarbon reservoirs coexist around the margin of the eastern Sichuan basin. This study examined the fluid inclusions, Sr isotope and systematic Rb-Sr, Sm-Nd geochronology for the distinct ore and gangue minerals of four orebodies from two MVT Pb-Zn deposits in the margin of the eastern Sichuan basin, combined with the existing research foundation of oil and gas accumulation and evolution, which was designed to understand the internal relationship between oil and gas accumulation and the involvement of organic matter in metal mineralization. High-density methane, moderate temperature, and salinity inclusions were discovered in the studied MVT Pb-Zn deposits, combined with relatively higher 87Sr/86Sr ratios (0.71088~0.714749), indicating that the ore-forming fluids were derived largely from the hydrocarbon associated basinal brines. Rb-Sr isochron of paragenetic sphalerites and pyrites and Sm-Nd isochron of paragenetic fluorites and calcite from the studied MVT Pb-Zn deposits define isochron ages of 144.5 Ma~138.5 Ma, and 147.6 Ma, respectively, indicating that the Late Jurassic to Early Cretaceous Yanshanian orogeny was an important metallogenic event in the margin of the eastern Sichuan basin. The close temporal and spatial relationship between the MVT mineralization and hydrocarbon accumulation and destruction in the strong structural deformation area of the margin of the eastern Sichuan basin allows us to propose a possible model in which the Yanshanian compressional tectonics drove a large-scale flow of metal- and sulfate- bearing basinal fluids passed under or through methane-bearing carbonate reservoirs, resulting in the MVT mineralization and hydrocarbon accumulation and destruction.

Controls on Timing of Hydrothermal Fluid Flow in South-Central Kansas, North-Central Oklahoma, and the Tri-State Mineral District

Paleozoic sedimentary rocks in the southern midcontinent of the United States have been affected by multiple events of deformation and fluid flow, resulting in petroleum migration, thermal alteration, Mississippi Valley-type mineralization, and a complex diagenetic history. This record is a hidden history of how cratonal settings respond to tectonic and non-tectonic drivers. The aim of this contribution is to better understand the controls on fluid migration in Paleozoic strata to evaluate whether hydrothermal activity is forced by tectonic or non-tectonic processes. This paper summarizes and vets the distribution of published dates related to thermal events in the southern midcontinent. In addition, we present new U-Pb dates obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) on calcite cements that were formed from hydrothermal fluids. These are from three samples from the Berexco Wellington KGS 1-32 core in Sumner County, Kansas; an ore sample from the Tri-State Mineral District, Neck City, Missouri; and a core sample from the Blackbird 4-33 well in Osage County, Oklahoma. Previous studies of these calcite samples provided evidence for hydrothermal fluid flow, with one of the Wellington samples possibly recording vertical hydrothermal fluid flow out of the basement. The sample from the Tri-State Mineral District (Missouri) yields a mid-Cretaceous age of 115.6±3.1 Ma. This age falls into the timing of the Sevier Orogeny along the west coast and the development of its foreland basin in the midcontinent. Calcites from the Mississippian interval in the Wellington KGS 1-32 core yield dates of 305±10.5 Ma and 305.1±9.1 Ma. Calcite in Mississippian strata from the Blackbird 4-33 core yields a date of 308.6±2.5 Ma. These dates from Mississippian calcite cements indicate hydrothermal fluid flow in the Late Pennsylvanian that coincides with the timing of the Marathon-Ouachita Orogeny or the Ancestral Rocky Mountains Orogeny. A calcite sample from the Ordovician Arbuckle Group from the Berexco Wellington KGS 1-32 core yielded an age of 5.6±1.6 Ma, coinciding with a time after high elevation uplift of the Rocky Mountains was already far advanced. We propose that this hydrothermal fluid flow may have been associated with increased meteoric recharge and increased regional fluid pressure in a basement aquifer that activated local seismic events far into the continental interior. The distribution of ages of hydrothermal fluid flow confirms a syntectonic driver during the Ouachita Orogeny and Ancestral Rocky Mountains Orogeny deformation. Continuation of hydrothermal fluid flow well into the Permian and tailing off early in the Triassic indicates a post-tectonic driver, where uplifted areas continued to provide the recharge from gravity-driven fluid flow, until the mountains were mostly beveled by the early part of the Triassic. A dearth of Triassic and Jurassic hydrothermal events suggests Gulf of Mexico rifting and extension were less important. Rejuvenation of hydrothermal fluid flow in the Cretaceous and continuing into the Paleogene indicates that elevation and regional flexure from both the Sevier and Laramide events continued to drive hydrothermal fluid flow far from the main sites of mountainous uplift and deformation. Finally, hydrothermal fluid flow associated with more recent uplift of the Rocky Mountains may have been activated by recharge events that pressurized a regional basement aquifer and triggered seismic activity.

Sulphur and metal sources of polymetallic vein-type, sedimentary exhalative-type and Mississippi Valley-type Zn–Pb deposits along the southeast margin of the Yangtze Block

Three types of zinc-lead deposits, including polymetallic vein-type, sedimentary-exhalative (SEDEX), and Mississippi valley-type (MVT) Zn–Pb deposits, have been discovered along the southeast margin of the Yangtze Block. A systematically comparative study on the sulphur and metal sources for these deposits, is absent, which significantly limits the understanding of the metallogenic model and geological exploration. Thus, in order to reveal the S and Pb sources of Pb–Zn deposits, an investigation of trace elements of sphalerite and of S–Pb isotopic compositions of sulphides in these three types of Zn–Pb deposits was conducted in this study. We found that, in the vein-type, SEDEX, and MVT Zn–Pb deposits, (1) the sulphur was derived respectively from granitic magmatism related to the Guangxi Orogeny, early Cambrian stratified seawater and connate water; (2) the lead isotope variation indicates that the ore-forming metals were mainly derived from the granitic hydrothermal fluids for vein-type deposits, from basement rocks for SEDEX deposits, and mainly from early Cambrian sedimentary rocks for MVT mineralization. The Nanhua rifting lead to the syngenetic Zn-Pb mineralization, while the Guangxi Orogeny resulted in magmatic-hydrothermal vein mineralization at depth and in MVT mineralization at the shallow crustal levels.

Identifying the source of groundwater contaminants in West-Central Wisconsin, U.S.A.: Geochemical and mineralogical characterization of the Cambrian sandstone aquifer

Naturally-sourced groundwater quality issues are common, but there is rarely a geochemical and mineralogical dataset of aquifer properties for comparison. This study utilizes geochemical and mineralogical data to characterize naturally-occurring minerals that are potential groundwater contaminants in the Cambrian Eau Claire – Mount Simon aquitard-aquifer system of west-central Wisconsin, U.S.A. A high-resolution portable x-ray fluorescence elemental analysis was used for initial characterization of geochemistry and chemostratigraphy of well cutting and drill core samples. Then, a subset of sample materials was analyzed mineralogically and geochemically using XRD and ICP-MS, respectively. Elevated concentrations of arsenic, phosphorous, strontium, and various metals within the aquitard-aquifer sandstone were identified, mostly associated with sulfide minerals and iron (hydr)oxides and suggestive of Mississippi Valley-type mineralization. Similar elemental contaminants in surface and groundwater in the study area indicate that the observed trace element-bearing minerals are a natural source of groundwater contamination, most likely through release into groundwater during fluctuating redox and pH conditions near the water table. Co-occurrence of iron (hydr)oxide-coated sulfide minerals near the water table, and observations of sulfide oxidation post-drilling, suggests sulfides in these units are actively oxidizing. Well construction recommendations based on these results should mitigate current or future pumping of trace element-contaminated groundwater and in the vast majority of cases eliminate naturally-occurring contaminants as a potential source. This study provides an extensive baseline dataset of aquifer mineralogy and trace element composition (and an efficient approach for data collection) that is necessary for interpreting and attributing possible future groundwater quality issues.

Origin of sediment-hosted Pb-Zn mineralization in the Paleoproterozoic Mârmorilik and Qaarsukassak formations, Karrat Group, West Greenland

• We compare Pb-Zn mineralization at Black Angel and Qaarsukassak, West Greenland. • S-isotopes and Pb-isotopes address S and Pb sources to mineralizing systems. • A Re-Os isotopes demonstrate Paleoproterozoic-aged sulfide mineralization. • Black Angel, Qaarsukassak represent MVT and SEDEX mineralization, respectively. Sediment-hosted Pb-Zn mineralization occurs in the Mârmorilik and Qaarsukassak formations of the Paleoproterozoic Karrat Group in arctic West Greenland. The timing and model of mineralization of the historical Pb-Zn Black Angel Mine in the Mârmorilik Formation and how it might be related to Pb-Zn mineralization in the nearby Qaarsukassak Formation is enigmatic. This study aims to understand the origin of the Pb-Zn mineralization in both formations by petrographic analysis, pyrite- and ore-sulfide sulfur isotope analysis, Pb-Pb isotope analysis, and Re-Os isotope analysis of pyrite associated with the mineralization. Sulfur isotope results from secondary ion mass spectrometry show a range of δ 34 S values between 0‰ and +6.9‰ on sulfide minerals (pyrite, sphalerite, galena, pyrrhotite); isotopic fractionations and geothermometry suggest contributions from both bacterial sulfate reduction and abiotic thermochemical sulfate reduction processes. Isotopic analysis (Pb-Pb) of galena from ore zones hosted by the Mârmorilik Formation shows a relatively homogenous Pb isotope signature that is consistent with local basement as a likely source for the metals. Results from Re-Os analysis of pyrite indicate that the Mârmorilik and Qaarsukassak mineralizing events are Paleoproterozoic. Ore textures and mineralogy, as well as S and Pb isotope data, associated with the carbonate-hosted Mârmorilik Formation are most consistent with a Mississippi Valley-type (MVT) deposit model, whereas characteristics of the dominantly siliciclastic-hosted Qaarsukassak mineralization textures are most consistent with a sedimentary exhalative (SEDEX) mineralization model. The Mârmorilik (Black Angel) mineralization represents the oldest documented MVT deposit outside of the Neoarchean–Paleoproterozoic Transvaal basin in South Africa.

Multi-dataset analysis to assess mineral potential of MVT-type zinc-lead deposits in Malayer-Isfahan metallogenic belt, Iran

The Malayer-Isfahan metallogenic belt (MIMB) is located in the Sanandaj-Sirjan zone. The belt hosts the important Zn-Pb deposits of Iran which are predominantly hosted by Cretaceous limestone, limestone-dolomite, limestone-shale, and carbonate sandstones. These deposits, which are associated with dolomitization, silicification, and calcitization, are believed to be mainly epigenetic stratabound Mississippi Valley-type (MVT) mineralization. This research aims at presenting an efficient geological-based exploration model through the integration of structural, lithological, alteration, and geochemical datasets from Malayer-Isfahan metallogenic belt. The main genetic factors of epigenetic stratabound Zn-Pb MVT-type mineralization and those of the well-known MVT deposits of the study area were used to extract the recognition criteria for potential mapping of these deposits. The predictive maps of geological, geochemical, structural, and alteration data were generated by fuzzy and simple weighted overlay (SWO) methods, while final favorability maps were created by SWO and technique for order preference by similarity ideal solution (TOPSIS) methods. The results were finally validated by using the known mineralized locations. The results showed that Lower Cretaceous dolomitized carbonate rocks and their contact with Jurassic shales, deep thrust faults, and joint predictive maps of Pb and Zn are the most important factors for exploration of these deposits in regional scale. The validation results showed that the moderate and high favorable areas identified by SWO and TOPSIS methods occupy 12.6% and 12.9% of the study area, which include 83.3% and 75% of the known MVT-type deposits, respectively. The well-known deposits such as Irankuh, Emarat, and Tiran occur in high potential areas of favorability maps. The results also delineated new potential targets for MVT-type mineralization in Malayer-Isfahan metallogenic belt for follow-up next exploration programs.

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.

Hydrothermal influence on barite precipitates in the basal Ediacaran Sete Lagoas cap dolostone, São Francisco Craton, central Brazil

Most Precambrian sediment-hosted barites have been interpreted to be diagenetic, hydrothermal exhalation, or methane-seepage in origin. Seafloor barite precipitates and void-filling barite cements in basal Ediacaran cap dolostones have been interpreted as sedimentary and early diagenetic in origin, and they have been used to infer ocean geochemistry in the aftermath of the terminal Cryogenian (or Marinoan) Snowball Earth glaciation. Barite crystals precipitated syndepositionally from seawater or authigenically from marine porewaters can potentially offer insights into Neoproterozoic ocean geochemistry. In this study, we analyzed void-filling barite cements from the basal Ediacaran Sete Lagoas cap dolostone to determine whether the barite records seawater geochemical signatures and whether it experienced post-depositional hydrothermal alteration. Sete Lagoas barite occurs as veins and major void-filling cement in multiple horizons, sometimes interbedded with carbonate fans. Barite crystals, commonly forming rosettes, grew upwards and downwards as radiating bladed crystals, isolated crystals in the matrix, and inclusions within carbonate fan crystals. Sulfur isotope compositions of coexisting barite and carbonate-associated sulfate (CAS) show similar values, suggesting a similar sulfate source, potentially seawater sulfate from a shallow marine environment. Assuming that CAS was sourced from seawater sulfate, we infer that the Sete Lagoas barite was precipitated from marine porewaters with seawater as a source of the sulfate. Carbonaceous material (CM) and fluid inclusions in barite crystals share a similar thermal history with a peak temperature in the range of 206–257 °C. CM was not detected in the host rock, possibly due to pervasive dolomitization, thermodegradation of CM or the fine-grained nature of the host rock. Estimated temperatures and salinity based on fluid inclusions are both higher than seawater, suggesting that hydrothermal activity, perhaps those related to Mississippi-Valley type mineralization in the São Francisco Craton, may have influenced the Sete Lagoas barite. Thus, although the Sete Lagoas barite may have originally precipitated from seawater or marine porewaters in communication with seawater, it may have been subsequently influenced by hydrothermal activity and therefore a more thorough diagenetic assessment must be done before the Sete Lagoas barites can be used to infer seawater geochemistry and environment in the aftermath of Marinoan Snowball Earth.

A Predictive Geospatial Exploration Model for Mississippi Valley Type Pb–Zn Mineralization in the Southeast Missouri Lead District

The Pb–Zn sulfide concentrations hosted by dolomitized Cambrian carbonates in Southeast Missouri are world-class Mississippi Valley Type (MVT) deposits. These deposits commonly are in sites where local Precambrian basement highs resulted in depositional pinchouts of the basal Cambrian sandstones that served as a regional aquifer for basinal fluid migration driven by late Paleozoic Ouachita deformation. Mineralization also appears to be spatially related to regional faults that probably served as local fluid conduits. Understanding spatial associations between sites of known mineralization and regional geology, geochemistry, and geophysics in Southeast Missouri will be a useful guide in future exploration efforts in this region and for similar geologic settings globally. The weights-of-evidence method is used to evaluate regional geology, geochemistry, and geophysical datasets and produce favorability maps for MVT deposits in Southeast Missouri. Host rock characteristics, regional structural controls, stream sediment geochemistry, and proximity to basement highs appear to be the most useful data for predicting the location of the major deposits. This work illustrates the potential utility of mineral potential modeling to prioritize areas for exploration and identify permissive areas for undiscovered MVT mineralization.

Woodcutters 55 Years Later: A New Look at an Old Discovery

Abstract The Woodcutters Zn-Pb-Ag deposit in the Rum Jungle district of the Pine Creek orogen in northern Australia was discovered in 1964 and produced 4.6 Mt of ore grading 12.3% Zn, 5.6% Pb, and 83 g/t Ag between 1985 and 1999. Woodcutters, together with several other polymetallic, uranium, and phosphate deposits, is within a Paleoproterozoic sequence of fluviatile and shallow marine sediments deposited in a deepening basin between ~2100 and 2025 Ma around the margins of an Archean granitic and gneissic dome. These sediments were overlain by turbidites and volcaniclastic rocks until the basin was inverted and the sediments and mineral deposits were deformed and metamorphosed at 1860 Ma. Whereas the polymetallic and uranium bodies at Rum Jungle are considered to be syngenetic or syndiagenetic, sulfides in the Woodcutters orebody replace dolomitic horizons in an otherwise carbonaceous unit. This suggests that Woodcutters is similar to Mississippi Valley-type mineralization and rules out affinities with younger sedimentary exhalative-style deposits elsewhere in the Pine Creek orogen. A model is proposed whereby metals were eroded from Archean basement rocks into Paleoproterozoic sandstone aquifers following the Great Oxidation Event, which also liberated sulfur by oxidation of pyrite. Evaporative conditions, as suggested by the widespread occurrence of dolomite and magnesite, may have increased the chloride content of seawater and enhanced its capacity to transport metals. Subsequently, deeply circulating seawater leached metals from the aquifers and ascended up a deep, basin-penetrating fault until it intersected carbonaceous sediments. In this environment, Zn and Pb sulfides were deposited under reducing conditions, while sulfur may have been provided by H2S from organic material. The Woodcutters and other deposits at Rum Jungle show how metals formerly locked up in Archean cratons were delivered by erosion under an oxygenated atmosphere to Paleoproterozoic shorelines, where they were further mobilized and concentrated by a variety of processes.

Rare Earth Elements and Yttrium (REY) in coal mine drainage from the Illinois Basin, USA

The ever-growing demand in the USA and in the modern economy for rare earth elements, defined here as the lanthanide elements plus yttrium (REY), motivates the development of economically feasible and environmentally friendly approaches for domestic REY resources. Non-conventional sources, including coal mine drainage (CMD) hosting elevated concentrations of REY, have been explored as attractive secondary sources for metals recovery. Consequently, in this study we investigate CMD from abandoned coal mines in the Illinois Basin as a potential REY resource.We collected CMD from 35 abandoned coal mine sites in the Illinois Basin and investigated trends in REY enrichment and their association with CMD geochemical characteristics, including pH, the presence of other chemical elements of economic significance (i.e., Zn, Ni, Cr, Cu, V) as well as spatial patterns in relation to Permian magmatic intrusions of the Hicks Dome, a potential source of REY in the Illinois Basin. Additionally, the REY contents in possible source rocks for Illinois CMD were measured including coal from an underground mine and coal preparation plant in southern Illinois, mafic igneous rocks associated with Hicks Dome, and hydrothermal fluorite from Illinois fluorspar district.The REY concentrations in samples of CMD (filtered fraction) and associated coal and non-coal solids were determined by ICP-MS. The total concentration of REY in CMD, ΣREYCMD varied between 1 and 9879 μg/L with an average of 1059 μg/L and critical-ΣREYCMD (Nd, Eu, Tb, Dy, and Y) varied between 0.3 and 7212.5 μg/L with an average of 611 μg/L. Basin-wide, we found no statistical correlation between ΣREYCMD and (1) pH of the CMD, or (2) proximity to the Permian magmatic intrusion at Hicks Dome in southern Illinois. The REYCMD values normalized to North American Shale Composite (NASC) REYCMD/REYNASC exhibit enrichments in middle-REY for all CMD, but also enrichments in light-REY in some CMD. Statistically, we observed that ΣREYCMD displays significant direct correlations with Al, Si, SO4, Zn, Ni, Co, and Cu and no-correlation with Fe, Ca, PO4, Ba, and V.Synthesis of the geochemical data suggests that coal mining sites with the highest ΣREYCMD and critical-ΣREY contents include non-coal bedrock dominated by silicates (high clay content) and coal mine wastes that were impacted by metal-rich hydrothermal solutions, which are predominantly located along the western and northern margins of the Illinois Basin. Particularly, sites with the highest hydrothermal input as approximated by Mississippi Valley-type mineralization (i.e., Zn contents) are prime candidates for REY, as well as other elements of potential economic value such as Al, Ni, Cr, V, and Co. Thus, CMD in the Illinois Basin could become a potential alternative domestic source of extractable REY, especially critical-ΣREY, as well economically valuable metals.

A geochemical evaluation of the genetic relationship between Ouachita Mountains Paleozoic rocks and the Mississippi Valley-type mineralization in the southern Ozark Region, USA

The Pb and/or Zn deposits of the Tri-State and the Northern Arkansas Mississippi Valley-type (MVT) districts in the southern Ozark Region, located north of the Arkoma Basin and the Ouachita fold-thrust belt in North America, are genetically connected to the Pennsylvanian-Permian Ouachita orogeny. This orogenic event triggered a south-north topographic gradient flow of basinal brines, leaching metal-rich sediments en route. To establish whether the organic-rich shales of the Ouachita Mountains and the Ozark Region provided metals during the mineralization event, the Pb isotope compositions of the ores (sphalerite) have been compared to those of the sedimentary rocks (whole-rocks and leachates) of the Ouachita Mountains (Cambrian Collier Shale, Early Ordovician Mazarn Shale, Middle Ordovician Womble Shale, Late Ordovician Polk Creek Shale, Mississippian Stanley Shale, and Pennsylvanian Jackfork Sandstone) and the Ozark Plateau (Devonian/Mississippian Chattanooga Shale and Mississippian Fayetteville Shale). The Pb isotope compositions of ores in both districts show a broad and a linear range of isotopic values. This trend suggests mixing of Pb from two distinct end-member components. One end-member must be highly radiogenic, with Pb isotope ratios equal to or higher than the highest noticed value for ores. The other end-member must be less radiogenic, with Pb isotope ratios equal to or lower than the lowest value recorded for ores. The Pb isotope compositions of the analyzed whole rocks and leachates indicate that the best candidates for contributing the less radiogenic Pb to the ores are the sandstone members of the upper and middle Jackfork Sandstone and the lowermost section of the Chattanooga Shale.The depositional environment of the potential source rocks and their ability to sequester metals has been appraised using redox sensitive trace elements (Cd, Co, Cr, Cu, Mo, Ni, Pb, Th, U, V, Zn). Paleoredox proxies (e.g., enrichment factors of redox sensitive trace elements and V/V + Ni ratios) indicate deposition of the analyzed rocks mainly under anoxic and euxinic conditions, which are favorable for metal enrichment. However, oxic-suboxic conditions are also indicated by other geochemical proxies, suggesting a possible first order control of primary production that has resulted in an increased flux of organic matter. A disagreement in the redox conditions yielded by the paleoredox proxies has been noticed in several cases. This inconsistent behavior may be associated with several factors, including redox potential, pH, sulfur abundance, chemical availability, TOC values, and sediment provenance. The authigenic flux is the only fraction needed to unravel past redox conditions; however, the analyzed bulk sediments consist of authigenic, biogenic, and siliciclastic components, which may also have led to the observed disagreement in the redox conditions. Additional factors such as the basin geometry, the rates of sediment accumulation, the post depositional alteration processes associated with diagenesis and low-grade metamorphism during the Ouachita orogeny might have overprinted the depositional signatures.

The genesis of the Hehuashan Pb–Zn deposit and implications for the Pb–Zn prospectivity of the Tongling district, Middle–Lower Yangtze River Metallogenic Belt, Anhui Province, China

The Hehuashan lead–zinc deposit is located in the central Tongling Cu–Au–Mo mining district of eastern China and consists of stratabound ore bodies that are generally hosted by dolostone and limestone units of the Lower Triassic Lower Nanlinghu Formation. The deposit contains two types of sphalerite, namely tan to brown fine-grained (∼40 μm) type I sphalerite that is hosted by a dolomite, calcite, quartz and sericite gangue assemblage and colorless to brown coarse-grained (∼2 mm) type II sphalerite hosted within clasts in calcite-cemented breccias or within dolostone-hosted veins. Plotting these sphalerites in a Fe vs. Cd diagram indicates they have a similar composition as those within typical Mississippi Valley-type (MVT) deposits, suggesting that the Hehuashan deposit may have an MVT affinity. In addition, in-situ sulfur stable isotopic analysis of fine-grained (−7.8‰ to +3.0‰) and coarse-grained (+3.4‰ to +9.7‰) sphalerite indicates that the sulfur in the hydrothermal fluids that formed the deposit was derived from the country rocks within the Hehuashan area. Fluid inclusions within calcite and fluorite from the Hehuashan deposit have low homogenization temperatures (mean of 126 °C) and moderate salinities (mean of 7.1 wt% NaCl equivalent) that demonstrate the deposit formed from low temperature and moderate salinity hydrothermal fluids. This suggests that the Hehuashan deposit was almost certainly not formed from magmatic-hydrothermal fluids but instead was formed as a result of the mixing of basinal brines and meteoric water, again consistent with an MVT origin for this deposit. In summary, the Hehuashan lead–zinc deposit is an MVT deposit, suggesting that the Tongling district of Anhui Province, China is prospective for MVT mineralization and that the Triassic dolostones and limestones in this area represent high priority exploration targets for this style of mineralization.

In situ geochemical characterization of pyrite crystals in burial dolomites of St. George Group carbonates

Secondary ion mass spectrometry (SIMS) was used to measure the δ34S of pyrite disseminated in burial dolomite matrix of Boat Harbour formation at Main Brook and Daniel’s Harbour (about 130 km apart). At Main Brook, δ34S values for the pyrite grains show wide variation (–15 to +20‰ (n = 20)), but are mostly negative. Combined with a paucity of two-phase fluid inclusions in the host burial dolomite, the depleted δ34S values suggest that the pyrite is a direct product of bacterial sulfate reduction (BSR). Predominantly negative δ34S values were also obtained for sampled pyrite at Daniel’s Harbour and Port au Choix; however, relatively high homogenization temperatures (>100 °C) of two-phase fluid inclusions in the host dolomite is incompatible with a BSR process for pyrite formation. More so, Daniel’s Harbour is a site for main stage sulfide mineralization (Mississippi Valley-type (MVT) system), hosted in similarly burial dolomite and that has been previously constrained to be associated with thermochemical sulfate reduction (TSR). A relative proximity of the currently studied pyrite samples to this MVT system deposit is thus inconsistent with an in-situ BSR for these pyrites. The analyzed pyrite grains are commonly encased in bitumen, and they postdate their host burial dolomite and predate deep burial saddle dolomite. Taken together, the depleted δ34S signature in the pyrite was likely inherited from migrated hydrocarbons in the reservoir. Incursion of an initial pulse of hot sulfate-rich brine into the formation can cause thermal cracking of hydrocarbons, thereby releasing its low δ34S. Thus the inherited low δ34S signature was likely a product of an earlier BSR that occurred in the kerogen or source organic materials in source rock. Subsequently, the main stage sulfide mineralization (MVT deposit) occurred via in-situ TSR. These findings have an implication for the paragenetic history of sulfide minerals precipitated during MVT mineralization episode.

Controls on Mississippi Valley-Type Zn-Pb mineralization in Behabad district, Central Iran: Constraints from spatial and numerical analyses

The Behabad district, located in the central Iranian microcontinent, contains numerous epigenetic stratabound carbonate-hosted Zn-Pb ore bodies. The mineralizations formed as fault, fracture and karst fillings in the Permian-Triassic formations, especially in Middle Triassic dolostones, and comprise mainly non-sulfides zinc ores. These are all interpreted as Mississippi Valley-type (MVT) base metal deposits. From an economic geological point of view, it is imperative to recognize the processes that have plausibly controlled the emplacement of MVT Zn-Pb mineralization in the Behabad district. To address the foregoing issue, analyses of the spatial distribution of mineral deposits comprising fry and fractal techniques and analysis of the spatial association of mineral deposits with geological features using distance distribution analysis were applied to assess the regional-scale processes that could have operated in the distribution of MVT Zn-Pb deposits in the district. The obtained results based on these analytical techniques show the main trends of the occurrences are NW–SE and NE-SW, which are parallel or subparallel to the major northwest and northeast trending faults, supporting the idea that these particular faults could have acted as the main conduits for transport of mineral-bearing fluids. The results of these analyses also suggest that Permian-Triassic brittle carbonate sedimentary rocks have served as the lithological controls on MVT mineralization in the Behabad district as they are spatially and temporally associated with mineralization.