The Ozark region of the U.S. midcontinent is host to a number of Mississippi Valley-type districts, including the world-class Viburnum Trend, Old Lead Belt, and Tri-State districts and the smaller Southeast Missouri barite, Northern Arkansas, and Central Missouri districts. There is increasing evidence that the Ozark Mississippi Valley-type districts formed locally within a large, interconnected hydrothermal system that also produced broad fringing areas of trace mineralization, extensive subtle hydrothermal alteration, broad thermal anomalies, and regional deposition of hydrothermal dolomite cement. The fluid drive was provided by gravity flow accompanying uplift of foreland thrust belts during the Late Pennsylvanian to Early Permian Ouaehita orogeny.In this study, we use chemical speciation and reaction path calculations, based on quantitative chemical analyses of fluid inclusions, to constrain likely hydrothermal brine compositions and to determine which precipitation mechanisms are consistent with the hydrothermal mineral assemblages observed regionally and locally within each Mississippi Valley-type district in the Ozark region. Deposition of the regional hydrothermal dolomite cement with trace sulfides likely occurred in response to near-isothermal effervescence of CO 2 from basinal brines as they migrated to shallower crustal levels and lower confining pressures. In contrast, our calculations indicate that no one depositional process can reproduce the mineral assemblages and proportions of minerals observed in each Ozark ore district; rather, individual districts require specific depositional mechanisms that reflect the local host-rock composition, structural setting, and hydrology.Both the Northern Arkansas and Tri-State districts are localized by normal faults that likely allowed brines to rise from deeper Cambrian-Ordovician dolostone aquifers into shallower carbonate sequences dominated by limestones. In the Northern Arkansas district, jasperoid preferentially replaced limestones in the mixed dolostone-limestone sedimentary packages. Modeling results indicate that the ore and alteration assemblages in the Tri-State and Northern Arkansas districts resulted from the flow of initially dolomite-saturated brines into cooler limestones. Adjacent to fluid conduits where water/rock ratios were the highest, the limestone was replaced by dolomite. As the fluids moved outward into cooler limestone, jasperoid and sulfide replaced limestone. Isothermal boiling of the ore fluids may have produced open-space filling of hydrothermal dolomite with minor sulfides in breccia and fault zones. Local mixing of the regional brine with locally derived sulfur undoubtedly played a role in the development of sulfide-rich ore runs.Sulfide ores of the Central Missouri district are largely open-space filling of sphalerite plus minor galena in dolostone karst features localized along a broad anticline. Hydrothermal solution collapse during ore deposition was a minor process, indicating dolomite was slightly undersaturated during ore deposition. No silicification and only minor hydrothermal dolomite is present in the ore deposits. The reaction path that best explains the features of the Central Missouri sulfide deposits is the near-isothermal mixing of two dolomite-saturated fluids with different H 2 S and metal contents. Paleokarst features may have allowed the regional brine to rise stratigraphically and mix with locally derived, H 2 S-rich fluids. The Viburnum Trend and Old Lead Belt ores are galena rich with lesser amounts of sphalerite; they replace the most permeable dolostone facies in the Bonneterre Dolomite. Hydrothermal dissolution of host dolostone was concurrent with sulfide deposition, but dolomite deposition occurred episodically between periods of sulfide deposition. The important ore controls in these districts are a variety of sedimentary and geologic features that allowed cross-stratigraphic fluid flow and provided opportunities for fluid mixing. The reaction path which best reproduces the broad features of the Viburnum Trend and Old Lead Belt ores is one in which a dolomite-saturated, lead-rich, zinc- and H 2 S-poor brine mixes with a less saline, H 2 S-rich fluid. The brine became enriched in K, Mg, and Pb and depleted in H 2 S as it flowed through sandstone and redbed aquifers prior to entering the district. This mixing model is consistent with the abundant fluid inclusion and stable isotope evidence for fluid mixing in the districts. Small amounts of cooling associated with the mixing may have contributed to sulfide deposition.
Research Article| February 01, 1988 Metamorphic origin of the Coeur d'Alene base- and precious-metal veins in the Belt basin, Idaho and Montana D. L. Leach; D. L. Leach 1U.S. Geological Survey, M.S. 973, Box 25046, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar G. P. Landis; G. P. Landis 1U.S. Geological Survey, M.S. 973, Box 25046, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar A. H. Hofstra A. H. Hofstra 1U.S. Geological Survey, M.S. 973, Box 25046, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Geology (1988) 16 (2): 122–125. https://doi.org/10.1130/0091-7613(1988)016<0122:MOOTCD>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation D. L. Leach, G. P. Landis, A. H. Hofstra; Metamorphic origin of the Coeur d'Alene base- and precious-metal veins in the Belt basin, Idaho and Montana. Geology 1988;; 16 (2): 122–125. doi: https://doi.org/10.1130/0091-7613(1988)016<0122:MOOTCD>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Fluid inclusions, geochronology, and field studies show that the world-class base- and precious-metal veins of the Coeur d'Alene district, Idaho and Montana, are the product of deformation coupled with regional metamorphism of the Belt basin around 850 Ma. Fluid-inclusion data show that the mineralizing solutions were complex CO2-CH4-CnHm-N2-H2O-NaCl fluids that changed in composition from CH4-CnHm-rich fluids in the older Zn-rich veins to CO2-rich fluids in the younger Ag-rich veins. Ore deposition occurred at about 250 to 350 °C and at pressures in excess of 1 kbar. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Porphyry and epithermal deposits are important sources of base and precious metals. Most actively mined deposits have been exhumed such that ore bodies are relatively close to the surface and are therefore locatable and economic to extract. Identifying and characterizing concealed deposits, particularly more deeply buried porphyry deposits, represents a far greater challenge for mineral exploration, and will become progressively more important as near-surface resources are gradually exhausted over time. We report high-precision 40Ar/39Ar dates for coarsely crystalline alunite that precipitated from magmatic steam in open fractures in Oligocene dacitic volcanic rocks, and a SHRIMP 206Pb/238U zircon date for one of several rhyolite dikes present at Alunite Ridge and Deer Trail Mountain, Utah. Both the magmatic-steam alunite and rhyolite dikes are related to concealed intrusions. The rhyolite dike yielded an age of 30.72 ± 0.36 Ma, which is older than a commonly cited 27.1 Ma age estimate for the Three Creeks Tuff Member of the Bullion Canyon Volcanics that is cut by the dike. 40Ar/39Ar data for samples of magmatic-steam alunite and sericite from six mines and prospects provide evidence for at least two periods of episodic hydrothermal activity at ca. 15.7–15.1 Ma and ca. 14.7–13.8 Ma, with the older and younger pulses of activity recorded at the more eastern and western sites, respectively. These two periods of hydrothermal activity are consistent with previous interpretations that Alunite Ridge and Deer Trail Mountain are underlain by two concealed porphyry stocks. 40Ar/39Ar analyses of individual bands in a sample of massive, centimeter-scale banded vein alunite yield indistinguishable ages with a weighted mean of 13.98 ± 0.12 Ma, consistent with a short-lived (≲250 ka) magmatic event with episodic vapor discharge recurring on short timescales (≲36 ka). 40Ar/39Ar geochronology of magmatic-steam alunite is a valuable tool to constrain the timing and duration of magmatic hydrothermal activity associated with unexposed intrusions and potentially porphyry deposits, and therefore may be useful in exploration.
Tabular uranium-vanadium mineralization characteristic of the Colorado Plateau occurs in fluvial sandstones of the Salt Wash Member of the Morrison Formation (Jurassic) within the Henry structural basin, south-central Utah. The ore consists of a mineralized interval (MI) of two closely spaced uranium and vanadium-rich zones separated by one barren of uranium but enriched in vanadium. No known stratigraphic feature controls the position of this MI which occurs at successively higher stratigraphic levels toward the interior of the basin. The dominant clay mineral throughout the MI is an unusual vanadium-rich di,trioctahedral chlorite. Laterally continuous with and below the MI, mixed-layer chlorite/smectite and illite/smectite (greater than 75% expandable layers) predominat . Above the MI, kaolinite in sandstone beds and illite/smectite plus kaolinite in bentonitic beds are the dominant authigenic clay minerals. The MI and its unmineralized lateral extensions are bounded, both above and below, by zones rich in authigenic dolomite cement. Petrographic evidence places the dolomite as pre-ore to contemporaneous with ore, and the chlorite contemporaneous with ore. Geochemical and mineralogical data, ^dgr18O to ^dgrD values of clay minerals and ^dgr18O to ^dgr13C values of dolomite indicate the presence of an interface between two isotopically and chemically distinct fluids. The lower fluid was typical of closed-basin evaporated brines with a high Mg/Ca ratio and high SO42- content. The upper fluid was meteo ic water. Elemental zoning patterns and isotopic data suggest that the upper (meteoric) fluid carried the uranium and vanadium to the solution interface, but that ore grade mineralization occurred only where the brine-meteoric water interface intersected horizons with anomalous concentrations of organic matter (dominantly detrital plant debris). End_of_Article - Last_Page 613------------
Analysis of shallow firn pore gas from the Wrangell-St.Elias range, Alaska, the GISP2 site in Greenland, and Taylor Dome site, Antarctica, indicates anomalous carbon dioxide contents 2x to 3x that of modern atmosphere.Average CO2 enrichment of shallow firn gases shows a strong correlation with mean annual temperature of the field site, with low values of 710 ppmV and -45°C at Taylor Dome in Antarctica, and high values of 1,922 ppmV and -17°C on the Klutlan Glacier below Mt.Churchill and Mt.Bona in Alaska.Important selective wet deposition and in-cloud scavenging processes are indicated.This enriched carbon dioxide content is observed predictably in the relative solubility of carbon dioxide compared to nitrogen, oxygen, and argon in atmospheric water droplets.Very shallow firn pore spaces are dominated by air gases but in non-atmospheric concentrations.Recrystallization, sublimation, and densification of firn cause loss of this desorbed gas to voids and interstitial spaces.Below a depth of 15-20 meters, desorbing anomalous gas no longer dominates the composition of larger pore volumes but continues below this crossover point to exist in isolated micropores, grain boundaries, junctions, and crystal defect sites as "matrix" gas.The major permeability paths control exchange with the atmosphere and firn gas is like modern atmosphere in large interconnected pore spaces.Atmospheric gas transfer processes through the firn column to closure at the firn-ice transition include complex mixing and isolation of this anomalous gas with atmospheric gas.The likely persistence of this anomalous gas as matrix gas at depth complicates wet and dry extraction methods and interpretation of all ice gas data and clathrate behavior.
The Idaho cobalt belt is a 60-km-long alignment of deposits composed of cobaltite, Co pyrite, chalcopyrite, and gold with anomalous Nb, Y, Be, and rare-earth elements (REEs) in a quartz-biotite-tourmaline gangue hosted in Mesoproterozoic metasedimentary rocks of the Lemhi Group. It is the largest cobalt resource in the United States with historic production from the Blackbird Mine. All of the deposits were deformed and metamorphosed to upper greenschist-lower amphibolite grade in the Cretaceous. They occur near a 1377 Ma anorogenic bimodal plutonic complex. The enhanced solubility of Fe, Co, Cu, and Au as chloride complexes together with gangue biotite rich in Fe and Cl and gangue quartz containing hypersaline inclusions allows that hot saline fluids were involved. The isotopes of B in gangue tourmaline are suggestive of a marine source, whereas those of Pb in ore suggest a U ± Th-enriched source. The ore and gangue minerals in this belt may have trapped components in fluid inclusions that are distinct from those in post-ore minerals and metamorphic minerals. Such components can potentially be identified and distinguished by their relative abundances in contrasting samples. Therefore, we obtained samples of Co and Cu sulfides, gangue quartz, biotite, and tourmaline and post-ore quartz veins as well as Cretaceous metamorphic garnet and determined the gas, noble gas isotope, and ion ratios of fluid inclusion extracts by mass spectrometry and ion chromatography. The most abundant gases present in extracts from each sample type are biased toward the gas-rich population of inclusions trapped during maximum burial and metamorphism. All have CO 2 /CH 4 and N 2 /Ar ratios of evolved crustal fluids, and many yield a range of H 2 -CH 4 -CO 2 -H 2 S equilibration temperatures consistent with the metamorphic grade. Cretaceous garnet and post-ore minerals have high R H and R S values suggestive of reduced sulfidic conditions. Most extracts have anomalous 4 He produced by decay of U and Th and 38 Ar produced by nucleogenic production from 41 K. In contrast, some ore and gangue minerals yield significant SO 2 and have low R H and R S values of a more oxidized fluid. Three extracts from gangue quartz have high helium R/R A values indicative of a mantle source and neon isotope compositions that require nucleogenic production of 22 Ne in fluorite from U ± Th decay. Two extracts from gangue quartz have estimated 40 K/ 40 Ar that permit a Precambrian age. Extracts from gangue quartz in three different ore zones are biased toward the hypersaline population of inclusions and have a tight range of ion ratios (Na, K, NH 4 , Cl, Br, F) suggestive of a single fluid. Their Na, Cl, Br ratios suggest this fluid was a mixture of magmatic and basinal brine. Na-K-Ca temperatures (279°–347°C) are similar to homogenization temperatures of hypersaline inclusions. The high K/Na of the brine may be due to albitization of K silicate minerals in country rocks. Influx of K-rich brines is consistent with the K metasomatism necessary to form gangue biotite with high Cl. An extract from a post-ore quartz vein is distinct and has Na, Cl, Br ratios that resemble metamorphic fluids in Cretaceous silver veins of the Coeur d’Alene district in the Belt Basin. The results show that in some samples, for certain components, it is possible to “see through” the Cretaceous metamorphic overprint. Of great import for genetic models, the volatiles trapped in gangue quartz have 3 He derived from a mantle source and 22 Ne derived from fluorite, both of which may be attributed to nearby ~1377 Ma basalt-rhyolite magmatism. The brine trapped in gangue quartz is a mixture of magmatic fluid and evaporated seawater. The former requires a granitic intrusion that is present in the bimodal intrusive complex, and the latter equatorial paleolatitudes that existed in the Mesoproterozoic. The results permit genetic models involving heat and fluids from the neighboring bimodal plutonic complex and convection of basinal brine in the Lemhi Group. While the inferred fluid sources in the Idaho cobalt belt are similar in many respects to those in iron oxide copper-gold deposits, the fluids were more reduced such that iron was fixed in biotite and tourmaline instead of iron oxides.
