Sulfide mineralization and rubidium-strontium geochronology at Ore Knob, North Carolina, and Ducktown, Tennessee (original) (raw)
Related papers
Geological Society of America Bulletin, 1988
Strontium isotopic ratios of wall-rock, ore, and gangue minerals from the Mascot-Jefferson City, Copper Ridge, and Sweetwater Mississippi Valley-type (MVT) districts and from the Lost Creek barite deposit in East Tennessee were measured in an effort to determine the age of this mineralization and its relation to the tectonic evolution of the Appalachian orogen. The Lower Ordovician Knox Group, which hosts the MVT mineralization, contains limestones and dolomite karst breccias with 87 Sr/ 86 Sr ratios of 0.70900 to 0.70916 that are similar to those of Early Ordovician sea water. Primary (diagenetic) Knox dolomite, as well as "recrystalline" dolomite, which formed by later alteration of limestone, exhibits ^Sr/^Sr ratios of 0.70930 to 0.71024, reflecting varying degrees of contamination by solutions with more radiogenic strontium. ^Sr/^Sr isotopic ratios of mineralization in the districts increase in the order Lost Creek, Sweetwater, Copper Ridge, Mascot-Jefferson City and extend to values significantly higher than those of the enclosing rocks. Of the three possible source basins for the MVT brines, the Late Proterozoic Ocoee, Cambrian Luttrell, and Ordovician Sevier basins, the Sevier basin appears to be the only one that could have supplied brines of the appropriate composition at geologically reasonable times. Estimates of the change with time in the isotopic composition of Sevier basin brines indicate that the maximum ages of the Lost Creek, Sweetwater, Copper Ridge, and Mascot-Jefferson City mineralization are 520, 460, 405, and 395 m.y., respectively. Comparison of fluid-inclusion temperatures for the Sweetwater and Mascot-Jefferson City mineralization with the thermal evolution curve for the Sevier basin supports these estimates. These ages suggest that the Lost Creek barite mineralization formed as an exhalative deposit in the Sevier basin and that the Sweetwater fluorite-barite mineralization formed during the later part of the Taconic orogeny. The Copper Ridge and Mascot-Jefferson City mineralization could have formed either prior to or during the Alleghanian orogeny (but prior to deformation of the enclosing rocks), depending on the rate of isotopic evolution of the Sevier basin brines.
Economic geology and the bulletin of the Society of Economic Geologists, 1980
SCIENTIFIC COMMUNICATIONS POSSIBLE EXCESS 4ø,qr IN HORNBLENDE ,4NI) BIOTITE FROM THE ,qPPAL,qCHI,qN M,qSSII 'E SULFIDE DEPOStTS AT ORE KNOB, NORTH CAROLIN,q, ,qND DUCKTO•FN, TENNESSEE Introduction Major massive sulfide deposits occur in the Blue Ridge province of the southern Appalachians at Ore Knob, North Carolina, and Ducktown, Tennessee (Fig. 1). The geology and origin of these deposits have been discussed by many investigators (e.g.,
Rb-Sr dating of sphalerites from Mississippi Valley-type (MVT) ore deposits
… et Cosmochimica Acta, 1993
Valley-type (MVT) ore deposits arc epigenetic carbonate-hosted Pb-Zn deposits that contain galena, sphalerite, fluorite, bar&e, dolomite, calcite, and quartz. Although they are thought to form from basinal brines, their exact origins are still unclear, partly because of the scarcity of reliable geochronological data. Rb-Sr dating of sphalerites has recently been shown to be a promising technique for the direct dating of ore minerals in MVT deposits. This paper reports the results of a reconnaissance study of sphalerites, their fluid inclusions, and associated minerals from MVT deposits of North America. Sphalerites from Immel mine, Mascot-Jefferson City district, east Tennessee, define a Rb-Sr age of 347 f 20 Ma consistent with a Rb-Sr age of 377 +-29 Ma for sphalerites from Coy mine in the same district, but inconsistent with models that ascribe their genesis to the effects of the late Paleozoic Alleghenian orogeny. Rb-Sr isotopic analyses of K-feldspar from Immel mine preclude the possibility that the Rb-Sr data reflect feldspar inclusions. Sphalerites from the main ore zone of Daniel's Harbour mine, Newfoundland, do not form a linear isochron and open behavior of the Rb-Sr system is suspected. Sphalerites from the Pine Point district, Northwest Territories, Canada, define a Rb-Sr age of 36 1 + 13 Ma, indicating that the mineralization took place shortly after the deposition of the middle Devonian host carbonate rocks. These results are not compatible with mineralization models based on regional fluid migration related to early Tertiary Cordilleran deformation. Sphalerites from northern Arkansas have very low Rb and Sr concentrations (less than 0.1 ppm). The Rb-Sr data do not form isochrons and the sphalerites have higher '%r/%r ratios than expected, given their Rb/Sr ratios and reasonable constraints on their ages. The sphalerites are suspected to contain clay inclusions; and it is likely that the Sr isotopic compositions of these sphalerites, which have very low Sr concentrations, were affected by small amounts of inherited inclusions. Except for sphalerite from northern Arkansas, SEM studies and isotope dilution trace element measurements have so far failed to identify any suitable phases other than sphalerite that might be a host for the Sr. A Permian Rb-Sr age for sphalerite from the Upper Mississippi zinc lead district, Wisconsin. Nafure 356,509-5 I 1. CHAUDHURI S. ( 1978) Strontium isotopic composition of several oilfield brines from Kansas and Colorado. Geochim. Cosmochim. Acta. 42, 329-33 I. CHESLEY J. T., HALLIDAY A. N., and SCRIVENER R. C. ( 199 1) Samarium-neodymium direct dating of fluorite mineralization, Science 252,949-95 1.
Multimethod (K-Ar, Rb-Sr, Sm-Nd) dating of bentonite minerals from the eastern United States
Basin Research, 1998
Isotopic determinations (K-Ar, Rb-Sr and Sm-Nd), and trace and rare-earth elemental analyses were made on a few biotite and clay fractions of Palaeozoic bentonite units from the eastern United States. The clay fractions were gently leached with dilute hydrochloric acid to study separately the acid-soluble minerals intimately associated with the extracted clay particles. The data highlight interesting potentials for this integrated approach to decipher complex tectonothermal evolutions of sedimentary basins. Biotite K-Ar ages are consistent with a Middle Ordovician stratigraphic age for the bentonite units with a mean age of 459±10 Ma. The clay residues give a Sm-Nd isochron age of 397±44 Ma, indicative of their crystallization during Acadian tectonothermal activity at about 200°C. The clay leachates, which are considered to represent mineral phases different from clay material, yield a distinct Sm-Nd isochron age of 285±18 Ma which is indistinguishable from K-Ar ages obtained previously on the clays, suggesting a thermally induced diffusion of radiogenic 40 Ar from clay particles during Alleghenian-Ouachita orogenic activity. The Rb-Sr system of the clay material seems to have been variably disturbed, except for the sample taken near the Allegheny Front for which an age of 179±4 Ma suggests a further localized activity of the thrust system at about 130-150°C.
Economic Geology, 1995
Mississippi Valley-type mineralization from two major host formations in the central Appalachians was studied in order to gain a better understanding of depositional conditions, sources of ore constituents, and the distribution of mineralizing fluids in the region. Sulfur isotope data obtained in this study and from the literature (Ault and Wilbur et al., 1990) show that mineralization in each formation acquired sulfur from sources of similar isotopic composition. Barite deposits in the Beckmantown Group have (534S values ranging from 25 to 36 per mil; the values probably closely reflect the isotopic composition of the sulfur-bearing fluid involved in the barite formation. Sulfide deposits in this formation have more variable compositions and more complex isotopic systematics. In both the Timberville district and Nittany Arch area, main-stage sulfide minerals have 634S values ranging from about 15 to 30 per mil. Each district also has a smaller, isotopically lighter substage of sphalerite mineralization that has (5•4S values between i and 19 per mil. Sulfate minerals in the Nittany Arch area are isotopically heavier than the sulfides, with 634S values between 30 and 40 per mil. The lightest sulfur in the central Appalachian Mississippi Valley-type districts is found in the Friedensville district, where (•34S values of sulfide minerals range between -9 and +1 per mil. Sulfide minerals from the two Silurian Tuscarora-Shawangunk-hosted deposits have similar (534S values of 21 to 35 per mil. Minor amounts of sulfate minerals in this formation are isotopically heavy with •34S values between 34 and 43 per mil. New fluid inclusion data were also obtained from the Timberville district and reveal the presence of at least two distinct fluids involved in mineralization-a more saline or calcium-rich fluid with final melting temperatures between -42 ø and -29øC, and a less saline or more calcium-poor fluid with final melting temperatures of -24 ø to -12øC. Homogenization temperatures of both inclusion types ranged for the most part between 100 ø and 170øC. Geologic evidence suggests that the Beckmantown Group and Tuscarora-Shawangunk Formations were part of distinct palcoaquifers, and together with the sulfur isotope data, also suggests that the source of sulfur for Mississippi Valley-type deposits in each palcoaquifer was cormate (formational) fluids derived from seawater, or alternatively in the case of the Beckmantown Group, intraformational evaporites. Furthermore, evidence of mixing and a temporal increase in the oxidation state of mineralizing conditions in at least two of the districts suggests that much of the Mississippi Valley-type mineralization in the central Appalachian region may be the result of mixing of oxidizing tectonically driven Zn-Pb or Ba-rich fluids with sulfur-rich fluids of varying oxidation state present in the Beckmantown Group and Tuscarora-Shawangunk Formations.
Mineralium Deposita, 2001
The Barite Hill gold deposit, at the southwestern end of the Carolina slate belt in the southeastern United States, is one of four gold deposits in the region that have a combined yield of 110 metric tons of gold over the past 10 years. At Barite Hill, production has dominantly come from oxidized ores. Sulfur isotope data from hypogene portions of the Barite Hill gold deposit vary systematically with pyrite±barite associations and provide insights into both the pre-metamorphic Late Proterozoic hydrothermal and the Paleozoic regional metamorphic histories of the deposit. The d 34 S values of massive barite cluster tightly between 25.0 and 28.0&, which closely match the published values for Late Proterozoic seawater and thus support a sea¯oor hydrothermal origin. The d 34 S values of massive sul®de range from 1.0 to 5.3& and fall within the range of values observed for modern and ancient sea¯oor hydrothermal sul®de deposits. In contrast, d 34 S values for ®ner-grained, intergrown pyrite (5.1±6.8&) and barite (21.0±23.9&) are higher and lower than their massive counterparts, respectively. Calculated sulfur isotope temperatures for the latter barite±pyrite pairs (D 15.9± 17.1&) range from 332±355°C and probably re¯ect post-depositional equilibration at greenschist-facies regional metamorphic conditions. Thus, pyrite and barite occurring separately from one another provide premetamorphic information about the hydrothermal origin of the deposit, whereas pyrite and barite occurring together equilibrated to record the metamorphic conditions. Preliminary¯uid inclusion data from sphalerite are consistent with a modi®ed seawater source for the mineralizing¯uids, but data from quartz and barite may re¯ect later metamorphic and (or) more recent meteoric water input. Lead isotope values from pyrites range for 206 Pb/ 204 Pb from 18.005±18.294, for 207 Pb/ 204 Pb from 15.567±15.645, and for 208 Pb/ 204 Pb from 37.555± 38.015. The data indicate derivation of the ore leads from the country rocks, which themselves show evidence for contributions from relatively unradiogenic, mantlelike lead, and more evolved or crustal lead. Geological relationships, and stable and radiogenic isotopic data, suggest that the Barite Hill gold deposit formed on the Late Proterozoic sea¯oor through exhalative hydrothermal processes similar to those that were responsible for the massive sul®de deposits of the Kuroko district, Japan. On the basis of similarities with other gold-rich massive sul®de deposits and modern sea¯oor hydrothermal systems, the gold at Barite Hill was probably introduced as an integral part of the formation of the massive sul®de deposit.
Mineralium Deposita, 2001
The Barite Hill gold deposit, at the southwestern end of the Carolina slate belt in the southeastern United States, is one of four gold deposits in the region that have a combined yield of 110 metric tons of gold over the past 10 years. At Barite Hill, production has dominantly come from oxidized ores. Sulfur isotope data from hypogene portions of the Barite Hill gold deposit vary systematically with pyrite±barite associations and provide insights into both the pre-metamorphic Late Proterozoic hydrothermal and the Paleozoic regional metamorphic histories of the deposit. The d 34 S values of massive barite cluster tightly between 25.0 and 28.0&, which closely match the published values for Late Proterozoic seawater and thus support a sea¯oor hydrothermal origin. The d 34 S values of massive sul®de range from 1.0 to 5.3& and fall within the range of values observed for modern and ancient sea¯oor hydrothermal sul®de deposits. In contrast, d 34 S values for ®ner-grained, intergrown pyrite (5.1±6.8&) and barite (21.0±23.9&) are higher and lower than their massive counterparts, respectively. Calculated sulfur isotope temperatures for the latter barite±pyrite pairs (D 15.9± 17.1&) range from 332±355°C and probably re¯ect post-depositional equilibration at greenschist-facies regional metamorphic conditions. Thus, pyrite and barite occurring separately from one another provide premetamorphic information about the hydrothermal origin of the deposit, whereas pyrite and barite occurring together equilibrated to record the metamorphic conditions. Preliminary¯uid inclusion data from sphalerite are consistent with a modi®ed seawater source for the mineralizing¯uids, but data from quartz and barite may re¯ect later metamorphic and (or) more recent meteoric water input. Lead isotope values from pyrites range for 206 Pb/ 204 Pb from 18.005±18.294, for 207 Pb/ 204 Pb from 15.567±15.645, and for 208 Pb/ 204 Pb from 37.555± 38.015. The data indicate derivation of the ore leads from the country rocks, which themselves show evidence for contributions from relatively unradiogenic, mantlelike lead, and more evolved or crustal lead. Geological relationships, and stable and radiogenic isotopic data, suggest that the Barite Hill gold deposit formed on the Late Proterozoic sea¯oor through exhalative hydrothermal processes similar to those that were responsible for the massive sul®de deposits of the Kuroko district, Japan. On the basis of similarities with other gold-rich massive sul®de deposits and modern sea¯oor hydrothermal systems, the gold at Barite Hill was probably introduced as an integral part of the formation of the massive sul®de deposit.
Economic Geology, 2013
New U-Pb zircon SHRIMP geochronology confirms that the Coles Hill uranium deposit in Pittsylvania County, Virginia, is hosted within the Late Ordovician to Silurian Martinsville Intrusive Complex. The meta-igneous host rocks at Coles Hill consist of two units of the Martinsville Intrusive Complex: the felsic Leatherwood Granite and the mafic Rich Acres Formation. Two samples of unmineralized Leatherwood Granite orthogneiss yield 206 Pb/ 238 U ages between 444.5 ± 2.5 and 447.5 ± 1.9 Ma. A third sample of unmineralized Leatherwood Granite orthogneiss shows a wider range in 206 Pb/ 238 U ages, possibly due to Pb loss, and a 206 Pb/ 207 Pb age of 452 ± 18 Ma. Unmineralized Rich Acres Formation amphibolite that cuts the Leatherwood gives a mean 206 Pb/ 238 U age (426.2 ± 7.0 Ma), slightly younger than the Leatherwood age. Samples of mineralized orthogneiss and mineralized amphibolite give similar 206 Pb/ 207 Pb ages of 419 ± 19 and 426 ± 21 Ma, respectively. A biotite gneiss unit that underlies the mineralized zone yields a 206 Pb/ 207 Pb age of 415 ± 21 Ma, indicating that it is part of the Martinsville Intrusive Complex and not a member of the early Cambrian Fork Mountain Schist, as has been previously reported. A genetic model for the Coles Hill uranium deposit has not yet been developed, although age constraints indicate that mineralization is either late or postmagmatic, and this is consistent with the epigenetic, fracture-controlled nature of the mineralization. Results obtained here do not preclude either the igneous host rocks (or similar rocks at depth) or the sedimentary units in the adjacent Triassic basin as possible sources for the uranium.