Geology Reports⌕ Search

Geology topics

D. L. Leach

Publications and source records attributed to D. L. Leach.

At least 19 recordsLinked to original sources
USGS2014

Mississippi Valley-type lead-zinc deposits through geological time: Implications from recent age-dating research

Remarkable advances in age dating Mississippi Valley-type (MVT) lead-zinc deposits provide a new opportunity to understand how and where these deposits form in the Earth's crust. These dates are summarized and examined in a framework of global tectonics, paleogeography, fluid migration, and paleoclimate. Nineteen districts have been dated by paleomagnetic and/or radiometric methods. Of the districts that have both paleomagnetic and radiometric dates, only the Pine Point and East Tennessee districts have significant disagreements. This broad agreement between paleomagnetic and radiometric dates provides added confidence in the dating techniques used. The new dates confirm the direct connection between the genesis of MVT lead-zinc ores with global-scale tectonic events. The dates show that MVT deposits formed mainly during large contractional tectonic events at restricted times in the history of the Earth. Only the deposits in the Lennard Shelf of Australia and Nanisivik in Canada have dates that correspond to extensional tectonic events. The most important period for MVT genesis was the Devonian to Permian time, which corresponds to a series of intense tectonic events during the assimilation of Pangea. The second most important period for MVT genesis was Cretaceous to Tertiary time when microplate assimilation affected the western margin of North America and Africa-Eurasia. There is a notable paucity of MVT lead-zinc ore formation following the breakup of Rodinia and Pangea. Of the five MVT deposits hosted in Proterozoic rocks, only the Nanisivik deposit has been dated as Proterozoic. The contrast in abundance between SEDEX and MVT lead-zinc deposits in the Proterozoic questions the frequently suggested notion that the two types of ores share similar genetic paths. The ages of MVT deposits, when viewed with respect to the orogenic cycle in the adjacent orogen suggest that no single hydrologic model can be universally applied to the migration of the ore fluids. However, topographically driven models best explain most MVT districts. The migration of MVT ore fluids is not a natural consequence of basin evolution; rather, MVT districts formed mainly where platform carbonates had some hydrological connection to orogenic belts. There may be a connection between paleoclimate and the formation of some MVT deposits. This possible relationship is suggested by the dominance of evaporated seawater in fluid inclusions in MVT ores, by hydrological considerations that include the need for multiple-basin volumes of ore fluid to form most MVT districts, and the need for adequate precipitation to provide sufficient topographic head for topographically-driven fluid migration. Paleoclimatic conditions that lead to formation of evaporite conditions but yet have adequate precipitation to form large hydrological systems are most commonly present in low latitudes. For the MVT deposits and districts that have been dated, more than 75% of the combined metal produced are from deposits that have dates that correspond to assembly of Pangea in Devonian through Permian time. The exceptional endowment of Pangea and especially, North America with MVT lead-zinc deposits may be explained by the following: (1) Laurentia, which formed the core of North America, stayed in low latitudes during the Paleozoic, which allowed the development of vast carbonate platforms; (2) intense orogenic activity during the assembly of Pangea created ground preparation for many MVT districts through far-field deformation of the craton; (3) uplifted orogenic belts along Pangean suture zones established large-scale migration of basin fluids; and (4) the location of Pangea in low latitudes with paleoclimates with high evaporation rates led to the formation of brines by the evaporation of seawater and infiltration of these brines into deep basin aquifers during Pangean orogenic events.

Mineralium Deposita↗
USGS2009

Genesis of the Assif El Mal Zn-Pb (Cu, Ag) vein deposit. An extension-related Mesozoic vein system in the High Atlas of Morocco. Structural, mineralogical, and geochemical evidence

The Assif El Mal Zn-Pb (Cu-Ag) vein system, located in the northern flank of the High Atlas of Marrakech (Morocco), is hosted in a Cambro-Ordovician volcaniclastic and metasedimentary sequence composed of graywacke, siltstone, pelite, and shale interlayered with minor tuff and mudstone. Intrusion of synorogenic to postorogenic Late Hercynian peraluminous granitoids has contact metamorphosed the host rocks giving rise to a metamorphic assemblage of quartz, plagioclase, biotite, muscovite, chlorite, amphibole, chloritoid, and garnet. The Assif El Mal Zn-Pb (Cu-Ag) mineralization forms subvertical veins with ribbon, fault breccia, cockade, comb, and crack and seal textures. Two-phase liquid-vapor fluid inclusions that were trapped during several stages occur in quartz and sphalerite. Primary inclusion fluids exhibit T h mean values ranging from 104??C to 198??C. Final ice-melting temperatures range from -8.1??C to -12.8??C, corresponding to salinities of ???15 wt.% NaCl equiv. Halogen data suggest that the salinity of the ore fluids was largely due to evaporation of seawater. Late secondary fluid inclusions have either Ca-rich, saline (26 wt.% NaCl equiv.), or very dilute (3.5 wt.% NaCl equiv.) compositions and homogenization temperatures ranging from 75??C to 150??C. The ?? 18 O and ??D fluid values suggest an isotopically heterogeneous fluid source involving mixing between connate seawater and black-shale-derived organic waters. Low ?? 13 C VPDB values ranging from -7.5??? to -7.7??? indicate a homogeneous carbon source, possibly organic matter disseminated in black shale hosting the Zn-Pb (Cu-Ag) veins. The calculated ?? 34 S H2S values for reduced sulfur (22.5??? to 24.3???) are most likely from reduction of SO 4 2- in trapped seawater sulfate or evaporite in the host rocks. Reduction of sulfate probably occurred through thermochemical sulfate reduction in which organic matter was oxidized to produce CO 2 which ultimately led to precipitation of saddle dolomite with isotopically light carbon. Lead isotope compositions are consistent with fluid-rock interaction that leached metals from the immediate Cambro-Ordovician volcaniclastic and metasedimentary sequence or from the underlying Paleo-Neoproterozoic crustal basement. Geological constraints suggest that the vein system of Assif El Mal formed during the Jurassic opening of the central Atlantic Ocean. ?? Springer-Verlag 2009.

Mineralium Deposita↗
USGS2008

Reactive flow models of the Anarraaq Zn-Pb-Ag deposit, Red Dog district, Alaska

The Red Dog ore deposit district in the Brooks Range of northern Alaska is host to several high-grade, shale-hosted Zn + Pb deposits. Due to the complex history and deformation of these ore deposits, the geological and hydrological conditions at the time of formation are poorly understood. Using geological observations and fluid inclusion data as constraints, numerical heat and fluid flow simulations of the Anarraaq ore deposit environment and coupled reactive flow simulations of a section of the ore body were conducted to gain more insight into the conditions of ore body formation. Results suggest that the ore body and associated base metal zonation may have formed by the mixing of oxidized, saline, metal-bearing hydrothermal fluids (<200??C) with reducing, HS-rich pore fluids within radiolarite-rich host rocks. Sphalerite and galena concentrations and base metal sulfide distribution are primarily controlled by the nature of the pore fluids, i.e., the extent and duration of the HS- source. Forward modeling results also predict the distribution of pyrite and quartz in agreement with field observations and indicate a reaction front moving from the initial mixing interface into the radiolarite rocks. Heuristic mass calculations suggest that ore grades and base metal accumulation comparable to those found in the field (18% Zn, 5% Pb) are predicted to be reached after about 0.3 My for initial conditions (30 ppm Zn, 3 ppm Pb; 20% deposition efficiency). ?? Springer-Verlag 2008.

Mineralium Deposita↗
USGS2007

Paleomagnetic and mineral magnetic constraints on Zn-Pb ore genesis in the Pend Oreille Mine, Metaline district, Washington, USA

Zinc-lead mineralization in the Metaline mining district of northeastern Washington, USA, is hosted by the Cambrian Metaline Formation and is classified into Yellowhead-type (YO) and Josephine-type (JO) ore based on texture and mineralogy. Paleomagnetic results are reported for four Cambrian Metaline Formation sites, one Ordovician Ledbetter slate site, 12 YO and 13 JO (including two breccia sites) mineralization sites in the Pend Oreille Mine, and eight sites from the nearby Cretaceous Kaniksu granite batholith. Thermal and alternating field step demagnetization, saturation isothermal remanence analysis, and synthetic specimen tests show that the remanence in the host carbonates and Zn-Pb mineralization is carried mostly by pseudosingle (PSD) to single domain (SD) pyrrhotite and mostly by PSD to SD magnetite in the Kaniksu granite. Based on thermomagnetic measurements, sphalerite and galena concentrates and tailings from the mine's mill contain hexagonal and monoclimc pyrrhotite. The postfolding characteristic remanent magnetization (ChRM), known thermal data, and paleoarc method of dating suggest that the Zn-Pb mineralization carries a primary chemical remanent magnetization (CRM), and Metaline Formation carbonates a secondary CRM that were acquired during the Middle Jurassic (166 ??6 Ma) during the waning stages of the Nevadan orogeny. A paleomagnetic breccia test favours a solution-collapse origin for the Josephine breccia. Finally, the Kaniksu paleopole is concordant with the North American Cretaceous reference paleopole, suggesting the Kootenay terrane has not been rotated since emplacement of the batholith at ???94 Ma. ?? 2007 NRC Canada.

Canadian Journal of Earth Sciences↗
USGS2004

Sulfur and oxygen isotopes in barite deposits of the western Brooks Range, Alaska, and implications for the origin of the Red Dog massive sulfide deposits

Sulfur and oxygen isotope analyses have been obtained for barite samples from the giant stratiform sulfide barite deposits at Red Dog in the western Brooks Range of Alaska, from stratiform barite deposits elsewhere in the Red Dog district, and from stratiform and vein and breccia barite occurrences in the central Brooks Range. Twelve of the 15 deposits studied lie within middle to Upper Mississippian black shale and chert units. The data reveal two different patterns on ?? 34S versus ??18O plots. The first, which is best illustrated by the barite deposit at Anarraaq, shows linear trends with slopes that vary with barite texture. For most samples, ??34S and ??18O values are both higher than the values characteristic of Mississippian marine sulfate. The second pattern, which is evident at the Red Dog deposits, shows no correlation between ??34S and ??18. In most samples, ??18O is below the value for Mississippian marine sulfate. Comparisons with sulfate in modern marine environments suggest a possible model for the mineralizing process. Anarraaq-type barite formed at sea-floor vents where ascending fluids carrying barium and methane encountered sulfate-bearing pore waters or bottom waters. Barite deposition was accompanied by the reduction of sulfate to H2S by means of microbially mediated anaerobic methane oxidation. Red Dog-type barite was formed in a manner similar to Anarraaq-type barite but was over-printed by a massive sulfide-forming event. Red Dog sulfides precipitated where metal-bearing hydrothermal fluids encountered pore waters that had been charged with H2S by anaerobic methane oxidation. Textural and isotopic evidence indicates that the sulfide bodies grew by consuming the available H2S and then by reductively dissolving barite. Dissolution of barite caused barium to be released to higher stratigraphic levels where it was reprecipitated on encountering sulfate. Isotopic evidence is pre sented for a link between methane venting and barite formation and raises the possibility that the coexistence of barite and sulfide at Red Dog, and the occurrence elsewhere in the district of barite-only and sulfide-only deposits, can be explained by a spectrum of vent types in the Mississippian basin analogous to the spectrum that is observed today along the modern continental margins. Authigenic barite formed at some but not all methane seeps, perhaps owing to differences in the barium content of vent fluids, differences in the relative proportion of aqueous fluid and gas emanating from vents, or differences in sulfate availability in local bottom waters. Some barite-forming seeps were later replaced by sulfides (Red Dog deposits) whereas others were not (e.g., Anarraaq barite horizon, Gull Creek, Moil). At sulfide occurrences where there is little evidence of preexisting barite (e.g., Anarraaq, Wulik, Suds), methane venting is indicated by fossils suggestive of chemosynthetic fauna. Mammiform sedimentary structures that are widespread in black chert at the top of the Kuna Formation may represent seeps that supported neither authigenic mineral formation nor chemosynthetic megafauna. ?? 2004 by Economic Geology.

Economic Geology↗
USGS2004

Textural, compositional, and sulfur isotope variations of sulfide minerals in the Red Dog Zn-Pb-Ag deposits, Brooks Range, Alaska: Implications for Ore Formation

The Red Dog Zn-Pb deposits are hosted in organic-rich mudstone and shale of the Mississippian Kuna Formation. A complex mineralization history is defined by four sphalerite types or stages: (1) early brown sphalerite, (2) yellow-brown sphalerite, (3) red-brown sphalerite, and (4) late tan sphalerite. Stages 2 and 3 constitute the main ore-forming event and are volumetrically the most important. Sulfides in stages 1 and 2 were deposited with barite, whereas stage 3 largely replaces barite. Distinct chemical differences exist among the different stages of sphalerite. From early brown sphalerite to later yellow-brown sphalerite and red-brown sphalerite, Fe and Co content generally increase and Mn and Tl content generally decrease. Early brown sphalerite contains no more than 1.9 wt percent Fe and 63 ppm Co, with high Mn (up to 37 ppm) and Tl (126 ppm), whereas yellow-brown sphalerite and red-brown sphalerite contain high Fe (up to 7.3 wt %) and Co (up to 382 ppm), and low Mn (<27 ppm) and Tl (<37 ppm). Late tan sphalerite has distinctly lower Fe (< 0.9 wt %) and higher Tl (up to 355 ppm), Mn (up to 177 ppm), and Ge (426 ppm), relative to earlier sphalerite. Wide ranges in concentrations of Ag, Cu, Pb, and Sb characterize all sphalerite types, particularly yellow-brown sphalerite and red-brown sphalerite, and most likely reflect submicroscopic inclusions of galena, chalcopyrite and/or tetrahedrite in the sphalerite. In situ ion microprobe sulfur isotope analyses show a progression from extremely low ??34S values for stage 1 (as low as -37.20???) to much higher values for yellow-brown sphalerite (mean of 3.3???; n = 30) and red-brown sphalerite (mean of 3.4; n = 20). Late tan sphalerite is isotopically light (-16.4 to -27.2???). The textural, chem ical, and isotopic data indicate the following paragenesis: (1) deposition of early brown sphalerite with abundant barite, minor pyrite, and trace galena immediately beneath the sea floor in unconsolidated mud; (2) deposition of yellow-brown sphalerite during subsea-floor hydrothermal recrystallization and coarsening of preexisting barite; (3) open-space deposition of barite, red-brown sphalerite and other sulfides in veins and coeval replacement of barite; and (4) postore sulfide deposition, including the formation of late tan sphalerite breccias. Stage 1 mineralization took place in a low-temperature environment where fluids rich in Ba mixed with pore water or water-column sulfate to form barite, and metals combined with H2S derived from bacterial sulfate reduction to form sulfides. Higher temperatures and salinities and relatively oxidized ore-stage fluids (stages 2 and 3) compared with stage 1 were probably important controls on the abundances and relative amounts of metals in the fluids and the resulting sulfide chemistry. Textural observations and isotopic data show that preexisting barite was reductively dissolved, providing a source of H2S for sulfide mineral formation. In stage 3, the continued flow of hydrothermal fluids caused thermal alteration of organic-rich mudstones and a build-up of methane that led to fluid overpressuring, hydrofracturing, and vein formation. Barite, red-brown sphalerite, and other sulfides were deposited in the veins, and preexisting barite was pervasively replaced by red-brown sphalerite. Hydrothermal activity ceased until Jurassic time when thrusting and large-scale fluid flow related to the Brookian orogeny remobilized and formed late tan sphalerite in tectonic breccias. ?? 2004 by Economic Geology.

Economic Geology↗
USGS2004

Origin of the Red Dog Zn-Pb-Ag deposits, Brooks Range, Alaska: Evidence from regional Pb and Sr isotope sources

Pb and Sr isotope data were obtained on the shale-hosted Zn-Pb-Ag Red Dog deposits (Qanaiyaq, Main, Aqqaluk, and Paalaaq), other shale-hosted deposits near Red Dog, and Zn-Pb-Ag sulfide and barite deposits in the western and central Brooks Range. The Red Dog deposits and other shale-hosted Zn-Pb-Ag deposits near Red Dog are hosted in the Mississippian Kuna Formation, which is underlain by a sequence of marine-deltaic clastic rocks of the Upper Devonian to Lower Mississippian Endicott Group. Ag-Pb-Zn vein-breccias are found in the Endicott Group. Galena formed during the main mineralization stages in the Red Dog deposits and from the Anarraaq and Wulik deposits have overlapping Pb isotope compositions in the range 206Pb/204Pb = 18.364 to 18.428, 207Pb/204Pb = 15.553 to 15.621, and 208Pb/204Pb = 38.083 to 38.323. Galena and sphalerite formed during the main ore-forming stages in the Red Dog deposits define a narrow field on standard uranogenic and thorogenic Pb isotope diagrams. Lead in sulfides of the Red Dog district is less radiogenic (238U/204Pb: ?? = 9.51-9.77) than is indicated by the average crustal lead evolution model (?? = 9.74), a difference consistent with a long history of evolution at low ratios of ?? before the Carboniferous. The homogeneous regional isotopic reservoir of Pb may indicate large-scale transport and leaching of minerals with various ?? ratios and Th/Pb ratios. Younger and genetically unrelated fluids did not significantly disturb the isotopic compositions of galena and sphalerite after the main mineralization event in the Red Dog district. Some pyrite shows evidence of minor Pb remobilization. The overall lead isotope homogeneity in the shale-hosted massive sulfide deposits is consistent with three types of control: a homogeneous regional source, mixing of lead during leaching of a thick sedimentary section and fluid transport, or mixing at the site of deposition. Isotopic variability of the hydrothermal fluids, as represented by galena in the Red Dog district, appears to be consistent with a simple mixing system. Evidence indicates that galena was deposited from largely similar hydrothermal solutions throughout the Red Dog district. A shared regional isotopic reservoir is also supported by the correspondence of Pb isotope compositions of galena in deposits of the Red Dog district and galena in clastic rocks (vein-breccias). Leaching of metals and progressive extraction of radiogenic lead from the clastic rocks in the Endicott Group may account for the trend of increasing 206Pb/204Pb in galena of the Red Dog district. Galena in the Red Dog deposits is unlikely to have been derived entirely from the same isotopic reservoir as that represented by the lead in the Kuna Formation or from the igneous rocks in the Red Dog district. Sr isotope data for barite, calcite, and witherite from the Red Dog deposits are compared with data from regional barite that is associated with sulfides and from barite in sulfide-poor occurrences. Fluids with heterogeneous Sr isotope signatures are indicated. Barite in the Main deposit extends to higher ratios of 87Sr/86Sr (0.709034-0.709899) than barite in the Anarraaq deposit (0.708615-0.709256). All barite is more radiogenic than Carboniferous seawater. Other Mississippian(?) shale-hosted deposits and mineral occurrences containing barite in the Red Dog district and barite in regional occurrences east of Red Dog in the western and central Brooks Range also have heterogeneous 87Sr/86Sr ratios. Carbonate (87Sr/86Sr = 0.710319-0.713637) and witherite (87 Sr/86 Sr = 0.710513) in the Main deposit are more radiogenic than barite. In contrast, carbonate (87Sr/86Sr = 0.708196-0.709740) intergrown with massive sulfides at Anarraaq has isotopic compositions similar to that of barite. Paragenetic and isotop ic studies suggest that early barite is similar to barite typically formed in cold seeps along continental margins. This early fine-grained barite formed before the main mineralizat

Economic Geology↗
USGS2004

Paleomagnetism of the Red Dog Zn-Pb massive sulfide deposit in northern Alaska

Paleomagnetic methods have isolated two ancient magnetizations in and around the Paleozoic shale-hosted Red Dog ore deposit in northern Alaska. A high-latitude, westerly magnetization carried by magnetite, termed characteristic remanent magnetization A, was found in rocks that have barite and/or substantial quartz replacement of barite. An intermediate- to low-latitude, southerly magnetization (characteristic remanent magnetization B) is carried by pyrrhotite and was found in rocks dominated by galena and sphalerite. The ages the two components are constrained by their relationship with geochemistry, radiometric age dating, and hypotheses for the Mesozoic tectonic history of the Brooks Range. Characteristic remanent magnetization A fails the fold test so it must postdate the end of Brookian orogenesis (??? 150 Ma). It is always found with replacement quartz that has a radiometric date (white mica from a vug, 39Ar/40Ar) of 126 Ma. The paleolatitude for characteristic remanent magnetization B is too shallow to be Mesozoic or younger, regardless of the model for the tectonic origin of northern Alaska, and must predate Brookian orogenesis. Geologic mapping suggests that most of the ore is syngenetic, formed at 330 to 340 Ma, and a radiometric date (Re-Os on pyrite) yields an age of 338 Ma. Since characteristic remanent magnetization B predates deformation, is found in mineralized rocks and is carried by pyrrhotite, it was probably acquired during the mineralizing process as well. The combined radiometric ages and paleomagnetic data sets can be best interpreted by assuming that northern Alaska was part of an accreted terrane that was translated northward by about 30?? into its current location relative to the rest of North America and then rotated counterclockwise by 50?? to 70??. This tectonic interpretation yields plausible magnetization ages for both characteristic remanent magnetization A and B. Geologic evidence, isotopic ages, and paleomagnetic data indicate formation of the deposit at a paleolatitude that is much lower than today. ?? 2004 by Economic Geology.

Economic Geology↗
USGS2004

Re-Os sulfide geochronology of the Red Dog sediment-hosted Zn-Pb-Ag deposit, Brooks Range, Alaska

The Red Dog sediment-hosted deposit in the De Long Mountains of northern Alaska is the largest Zn producer in the world. Main stage mineralization is characterized by massive sulfide ore and crosscutting subvertical veins. Although the vein mineralization is clearly younger than the massive ore, the exact temporal relationship between the two is unclear. Re-Os geochronology of pyrite is used to determine the absolute age of main stage ore at Red Dog. A 10-point isochron on both massive and vein pyrite yields an age of 338.3 ?? 5.8 Ma and is interpreted to represent the age of main stage ore. The Re-Os data indicate that both massive and vein ore types are coeval within the resolution of the technique. Formation of the Red Dog deposit was associated with extension along a passive continental margin, and therefore the Re-Os age of main stage ore constrains the timing of rifting as well as the age of the host sedimentary rocks. Sphalerite from both massive and vein ore yields imprecise ages and shows a high degree of scatter compared to pyrite. We suggest that the Re-Os systematics of sphalerite can be disturbed and that this mineral is not reliable for Re-Os geochronology. ?? 2004 by Economic Geology.

Economic Geology↗
USGS2003

Tectonic controls of Mississippi Valley-type lead-zinc mineralization in orogenic forelands

Most of the world's Mississippi Valley-type (MVT) zinc-lead deposits occur in orogenic forelands. We examine tectonic aspects of foreland evolution as part of a broader study of why some forelands are rich in MVT deposits, whereas others are barren. The type of orogenic foreland (collisional versus Andean-type versus inversion-type) is not a first-order control, because each has MVT deposits (e.g., Northern Arkansas, Pine Point, and Cevennes, respectively). In some MVT districts (e.g., Tri-State and Central Tennessee), mineralization took place atop an orogenic forebulge, a low-amplitude (a few hundred meters), long-wavelength (100-200 km) swell formed by vertical loading of the foreland plate. In the foreland of the active Banda Arc collision zone, a discontinuous forebulge reveals some of the physiographic and geologic complexities of the forebulge environment, and the importance of sea level in determining whether or not a forebulge will emerge and thus be subject to erosion. In addition to those on extant forebulges, some MVT deposits occur immediately below unconformities that originated at a forebulge, only to be subsequently carried toward the orogen by the plate-tectonic conveyor (e.g., Daniel's Harbour and East Tennessee). Likewise, some deposits are located along syn-collisional, flexure-induced normal and strike-slip faults in collisional forelands (e.g., Northern Arkansas, Daniel's Harbour, and Tri-State districts). These findings reveal the importance of lithospheric flexure, and suggest a conceptual tectonic model that accounts for an important subset of MVT deposits-those in the forelands of collisional orogens. The MVT deposits occur both in flat-lying and in thrust-faulted strata; in the latter group, mineralization postdated thrusting in some instances (e.g., Picos de Europa) but may have predated thrusting in other cases (e.g., East Tennessee).

Mineralium Deposita↗
USGS2003

Coupled heat and fluid flow modeling of the Carboniferous Kuna Basin, Alaska: Implications for the genesis of the Red Dog Pb-Zn-Ag-Ba ore district

The Red Dog deposit is a giant 175 Mton (16% Zn, 5% Pb), shale-hosted Pb-Zn-Ag-Ba 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. ?? 2003 Elsevier Science B.V. All rights reserved.

Conference Paper↗
USGS1998

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

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.

Geological Society Special Publication↗
USGS1998

Evidence for Proterozoic and late Cretaceous-early Tertiary ore-forming events in the Coeur d'Alene district, Idaho and Montana

New 40 Ar/ 39 Ar age spectra on sericite and lead isotope data on tetrahedrite, siderite, galena, bournonite, and stibnite, together with previously published isotopic, geochemical, and geologic studies provide evidence for two major vein-forming events in the Coeur d'Alene district and surrounding areas of the Belt basin. The data suggest that the zinc- and lead-rich veins (e.g., Bunker Hill and Star-Morning mines) formed in the Proterozoic (1.0 Ga), whereas the silver-rich veins (e.g., Silver belt mines), antimony veins (e.g., U.S. Antimony mine), and gold-bearing quartz veins (Murry subdistrict) formed in Late Cretaceous to early Tetitary time.

Economic Geology↗
USGS1997

Evidence for hot Mississippi Valley-type brines in the Reelfoot Rift complex, south-central United States, in Late Pennsylvanian-Early Permian

Petrographic and fluid inclusion studies of sparry dolomite cement from Upper Cambrian to Lower Ordovician rocks and conodont thermal-alteration indices provide evidence that hot mississippi valley-type brines were once present in the Reelfoot Rift complex. The cathodoluminescent microstratigraphy of sparry dolomite cement in the Reelfoot Rift resembles that of sparry dolomite cement associated with widespread mississippi valley-type deposition in the Ozark region. If correlative cathodoluminescent zones in the sparry dolomite from the Ozark and Reelfoot Rift regions indicate broadly contemporaneous dolomite deposition, then the results show that the Ozark MVT-type hydrothermal system extended into the Reelfoot region and onto the western flank of the Nashville Dome. Independent evidence supports migration of MVT-type brines into the Ozark region from the Reelfoot Rift complex in late Paleozoic time.

Professional Paper↗
USGS1996

Fluid-inclusion gases in sphalerite, galena, and dolomite from the Silesian-Cracow Zn-Pb district, Poland

The gases in fluid inclusions from samples of sphalerite, galena, and sparry dolomite from ore deposits in the Silesian-Cracow zinc-lead district were determined by mass spectrometry. The results show that CO2 is the major gas in the fluid inclusions with lesser amounts of CH4. Samples of galena and sphalerite also commonly contain H2S and short-chain hydrocarbon gases. Gases in sphalerite and galena appear to contain two end-member compositions, CO2-CH4 and CO2-H2S bearing fluid inclusions. The apparent end member compositions could represent chemically distinct generations of fluid inclusions trapped at different times or at least two distinct fluids present during ore formation. The presence of distinct fluid inclusion gas compositions is consistent with previous fluid inclusion studies that show the presence of multiple fluids during ore deposition. The presence of multiple fluids in the ore-forming environment could lead to ore precipitation through processes of fluid mixing.

Prace - Panstwowego Instytutu Geologicznego↗
USGS1996

Paleomagnetism of the Mississippi Valley-type Zn-Pb deposits of the Silesian-Cracow area, Poland

Paleomagnetic analysis of zinc-lead ore and host rocks from 35 sites in three mines and several quarries was completed using alternating field and thermal step demagnetization and saturation isothermal remanent magnetization methods. Paleomagnetic conglomerate, breccia and fold tests were used to test for remanence stability. Most limestone and dolostone sites of the Middle Triassic Muschelkalk Formation carry a dual-polarity primary or diagenetic Middle to Upper Triassic remanence. Late dolomite and Mississippi Valley-type (MVT) mineralization from 14 sites carry a dual-polarity Tertiary magnetization that is shown to be post-brecciation and syndeformational by the paleomagnetic tests. This age supports gravity-driven fluid flow models for ore genesis that are associated with the Alpine orogeny.

Prace - Panstwowego Instytutu Geologicznego↗