David L. Leach

Genetic link between Ouachita foldbelt tectonism and the Mississippi Valley–type lead-zinc deposits of the Ozarks

Fluids expelled from the Arkoma basin in response to Late Pennsylvanian–Early Permian orogenesis in the Ouachita foldbelt may have been ultimately responsible for formation of the Mississippi Valley–type lead-zinc deposits in the Ozark region of Missouri, Arkansas, Kansas, and Oklahoma. Fluid inclusions in both mineralized and barren rock record the passage of hot, saline fluids regionally throughout most of the Paleozoic section in the Ozarks. Dating studies in addition to structural and geologic observations provide further evidence for fluid migration and mineralization approximately coincident with the Ouachita orogeny. Formation of foreland basins, such as the Arkoma, during convergent tectonism creates conditions exceptionally favorable to the migration of fluids from deep sedimentary basins. Proximity to a basin whose margin has undergone some form of tectonic deformation or uplift may be the unifying factor in the genesis of Mississippi Valley–type deposits in geologically diverse settings.

Origin of lode-gold deposits of the Juneau gold belt, southeastern Alaska

Mesothermal gold-bearing quartz veins along the Juneau gold belt formed in the early Tertiary from H 2 O-CO 2 (±N 2 ,H 2 S,CH 4 )-rich, low-salinity fluids at temperatures of 200 to 325 °C and pressures in excess of 1 - 1.5 kbar. Isotopically heavy ore-forming fluids, with δ 18 O values between +8‰ and +12‰ and δD values of -20‰ to -30‰ , indicate a deep-seated fluid source. The data are compatible with a model of ore-fluid generation via metamorphic devolatilization within material subducted below the southern Alaska continental margin. Prograde metamorphic fluids infiltrated up zones of mechanically enhanced permeability associated with the Coast Range megalineament and formed the gold-bearing veins within brittle units at higher, retrograding crustal levels. Deposition of gold was accomplished by several mechanisms, including boiling, fluid-wall rock reactions, and decreases in pressure and temperature of the hydrothermal fluids.

Strontium isotopic constraints on the origin of ore-forming fluids of the Viburnum Trend, southeast Missouri

Abstract We have measured 87 Sr 86 Sr and Rb and Sr concentrations in several minerals, primarily sulfides, spanning the paragenesis of hydrothermal mineralization in the Viburnum Trend in southeast Missouri. Separate measurements were made for fluid inclusions opened by crushing or thermal decrepitation and for the solids. For comparison, measurements were also made on samples of probable local aquifers, the Bonneterre Formation and the Lamotte Sandstone. For some of the samples, concentrations of K, Ca, Cl, Na, and Mg are also reported. In several cases 87 Sr 86 Sr ratios are different (higher) in the solids than in the fluid inclusions. We have investigated the possibility that either type of sample gives spurious results, e.g., that the fluid inclusions are secondary or contaminated by host dolomite, or that Sr in the solids reflects a detrital rather than an authigenic source. Consideration of mass balance, overall solute chemistry, and examination of non-sulfide dissolution residue, however, suggests that both types of sample reflect primary fluid Sr composition. We thus adopt the working hypothesis that Sr isotopic composition in fluids at the time of hydrothermal mineralization was highly variable. The observed results for the Viburnum Trend do not conform well to expected trends for Mississippi Valley-type (MVT) mineralization based primarily on analyses of gangue carbonates, barite, and fluorite. Fluid inclusion Sr in some of the Viburnum Trend samples is more radiogenic than in the host dolomite but only moderately so; in other samples, notably main-stage octahedral galena, fluid inclusion Sr composition is within the range observed for the host dolomite. In contrast, Sr in some of the sulfides is very radiogenic, much more so than previously reported for MVT minerals, and is very radiogenic early in the paragenesis (pyrite, chalcopyrite, sphalerite), less radiogenic during main-stage ore deposition, and again more radiogenic in later paragenetic stages.

Origin of late dolomite cement by CO2-saturated deep basin brines: Evidence from the Ozark region, central United States

Studies of fluid inclusions in regionally extensive late dolomite cement (LDC) throughout the Ozark region show that CO2 effervescence was widespread during dolomite precipitation. On the basis of quantitative analyses of inclusion fluids, reaction-path modeling shows that LDC with trace amounts of sulfides can be deposited by effervescence of a CO2-saturated basin brine as it migrates to shallower levels and lower confining pressures. This precipitation mechanism best explains occurrences of LDC in the Ozark region and may account for LDC found in sedimentary basins worldwide.

Banded iron formation to iron ore: A record of the evolution of Earth environments?

Banded iron formations (BIF) are the protolith to most of the world’s largest iron ore deposits. Previous hypogene genetic models for Paleoproterozoic “Lake Superior” BIF-hosted deposits invoke upwards, down-temperature fl ow of basinal brines via complex silica and carbonate precipitation/dissolution processes. Such models are challenged by the necessary SiO 2 removal. Thermodynamic and mass balance constraints are used to refi ne conceptual models of the formation of BIF-hosted iron ore. These constraints, plus existing isotope and halogen ratio evidence, are consistent with removal of silica by down- or up-directed infi ltration of high-pH hypersaline brines, with or without a contribution from basinal brines. The proposed link to surface environments suggest that Paleoproterozoic BIF-ore upgrade may provide a record of a critical time in the evolution of the Earth’s biosphere and hydrosphere.

Coupled heat and fluid flow modeling of the CarboniferousKuna Basin, Alaska: implications for the genesis of the Red Dog PbZnAgBa ore district

The Red Dog deposit is a giant 175 Mton (16% Zn, 5% Pb), shale-hosted PbZnAgBa ore district situated in the Carboniferous Kuna Basin, Western Brooks Range, Alaska. These SEDEX-type ores are thought to have formed in calcareous turbidites and black mudstone at elevated sub-seafloor temperatures (120–150 °C) within a hydrogeologic framework of submarine convection that was structurally organized by large normal faults. The theory for modeling brine migration and heat transport in the Kuna Basin is discussed with application to evaluating flow patterns and heat transport in faulted rift basins and the effects of buoyancy-driven free convection on reactive flow and ore genesis. Finite element simulations show that hydrothermal fluid was discharged into the Red Dog subbasin during a period of basin-wide crustal heat flow of 150–160 mW/m2. Basinal brines circulated to depths as great as 1–3 km along multiple normal faults flowed laterally through thick clastic aquifers acquiring metals and heat, and then rapidly ascended a single discharge fault zone at rates ∼ 5 m/year to mix with seafloor sulfur and precipitate massive sulfide ores.

Reaimantation regionale eocene, migration de fluides et mineralisations sur la bordure cevenole (France)

Widespread remagnetization has been identified in fold belts and forelands inboard of mountain ranges and has usually been interpreted as resulting from fluid migration related to orogenesis in these mountain ranges. The geochemical properties of these fluids should be compatible with the formation or the transformation of ferrimagnetic minerals, thus allowing acquisition of remanent magnetization during fluid migration. Carbonate hosted lead-zinc (+ or -barite and fluorite) mineralization of the Mississippi Valley-type (MVT) are also generally considered to have formed during the migration of enormous volumes of fluids and are commonly located in foreland fold belts or their forelands. This suggests a similar origin for widespread carbonate remagnetization and MVT mineralization. The paleomagnetic dating of MVT deposits has been successfully applied in MVT districts, mainly of North America. Thus, it was used for the MVT deposits hosted by Mesozoic carbonate rocks of the Cevennes region of southern France. In view of the structurally complexity in the region, and because the results presented here are intended to provide an initial reference direction on which further results can be based, only sites belonging to the most stable parts of this border are considered here. Most samples were collected from three surface sites in the area of the Largentiee mine and from independently oriented cores of the two deep boreholes (Balazuc and Morte-Merie) from the Geologie Profonde de la France program. Several samples were also collected from three sites around the area of the Saint Felix-de-Pallieres mine. All these samples come from different stratigraphic levels: Carboniferous, Permian, Triassic, Liassic and Middle Jurassic.

The world-class Jinding Zn–Pb deposit: ore formation in an evaporite dome, Lanping Basin, Yunnan, China

The Jinding Zn–Pb sediment-hosted deposit in western Yunnan, China, is the fourth largest Zn deposit in Asia. Based on field observations of the ore textures, breccias, and the sandstone host rocks, the ores formed in a dome that was created by the diapiric migration of evaporites in the Lanping Basin during Paleogene deformation and thrust loading. Most of the ore occurs in sandstones that are interpreted to be a former evaporite glacier containing a mélange of extruded diapiric material, including breccias, fluidized sand, and evaporites that mixed with sediment from a fluvial sandstone system. A pre-ore hydrocarbon and reduced sulfur reservoir formed in the evaporite glacier that became the chemical sink for Zn and Pb in a crustal-derived metalliferous fluid. In stark contrast to previous models, the Jinding deposit does not define a unique class of ore deposits; rather, it should be classified as MVT sub-type hosted in a diapiric environment. Given that Jinding is a world-class ore body, this new interpretation elevates the exploration potential for Zn–Pb deposit in other diapir regions in the world.

Age and duration of the Mississippi Valley-type mineralizing fluid flow event in the Viburnum Trend, southeast Missouri, USA, determined from palaeomagnetism

Abstract The Viburnum Trend is a world-class Mississippi Valley-type (MVT) lead-zinc ore deposit in platform carbonates of the Upper Cambrian Bonneterre Dolomite in the midcontinent of the USA. Palaeomagnetic methods have been used to analyse 233 specimens from early octahedral (nine sites) and late-stage cubic (13 sites) galena ore from four mines along the c. 70 km north-south length of the Trend. The characteristic remanence is carried by single to pseudo-single domain pyrrhotite and magnetite. This is the first MVT deposit in which pyrrhotite is shown to be a remanence carrier and present in galena crystals. The remanence directions define an Early Permian mean age of 273 ± 10 Ma for the ore-stage mineralization, a maximum duration for the mineralization event of 12 Ma, and a time difference of 5 Ma between the early octahedral and late cubic galena ore stages. The Early Permian age for the ore is consistent with models of ore genesis that invoke fluid flow from the Ouachita orogen during Ouachitan orogenesis.

Geochemical exploration for uranium in playas.

Abstract Playas and closed-basin lakes represent unique geochemical environments for the entrapment and concentration of mobile chemical elements. For this reason, playas may be considered as potentially favorable areas for the accumulation of uranium. We investigated playa sediments to determine their value as possible sample media for determining the presence of accumulations of uranium in the basins and playas. Instrumental neutron activation analyses were made on near-surface [0–50 ft. (0–15 m)] sediments, pore fluids, and both surface and groundwater in closed basins at Winnemucca Dry Lake, Smoke Creek Desert, Roach Lake, Columbus Marsh, and Clayton Valley, Nevada, and Sevier Lake, Utah. Although playa sediments at spring discharge areas contain as much as 58 ppm uranium, evaporation and mobilization processes were found to limit the usefulness of playa sediments as geochemical sampling media for uranium. Other metals, however, such as zinc, tungsten, and gold may have a geochemical signature in the playas. Most playas have little potential for resources of uranium. However, there are certain conditions that would make some playas more favorable than others for the accumulation of uranium. These conditions include: (1) the presence of precursor carbonate-rich saline lakes; (2) nearby uranium-rich source rocks; (3) restricted groundwater discharge into the playa; (4) localized precipitation mechanisms or phases in the playa; (5) large drainage basin and adequate precipitation; and (6) stability of a hydrologically closed basin for a long period of time.