ABSTRACT Pinnacle reef tracts are geomorphic features of carbonate systems that originated in the early Silurian and display an episodic distribution into the Cenozoic. Detailed study of Silurian pinnacle reefs of the United States midcontinent demonstrates repeated motifs, but most enigmatic is the coincidence of carbonate carbon isotope (δ 13 C carb ) excursions and reef pulses. Silurian δ 13 C carb excursions were associated with environmental changes and extinctions, and reefs appear to mark a resurgence of conditions favorable to biomineralizers following those extinction events. Previous workers in the region identified six discrete reef origination events in the United States midcontinent during the Silurian. Our reevaluation of outcrops and cores, conodont collections, and the generation of considerable new chemostratigraphic data across the region are clarifying the age relations of these events and their relationships to perturbations of the global carbon cycle.

Abstract Times of metal-rich brine discharge into ancient ocean basins, associated with the formation of sedimentary-exhalative (sedex) Zn-Pb-Ba ore deposits, coincided with short-duration positive excursions (“spikes”) in the global marine Sr isotope record. While these spikes are unexplained by conventional oceanic models, chronostratigraphic correlations, combined with mass balance evidence and oceanographic modeling, suggest that the flux of radiogenic Sr from sedex brines during ore formation is sufficient to explain these previously enigmatic 87 Sr/ 86 Sr spikes. We review existing 87 Sr/ 86 Sr data and present new data as verification of these global 87 Sr/ 86 Sr spikes and their correlations with the formation of giant sedex ore deposits. Major events include an 1 ×10 −4 (~0.7078–~0.7079) excursion contemporaneous with formation of the Rammelsberg deposit at ~389 Ma; spikes on the order of 1 to 3 × 10 −4 , coeval with formation of the Meggen deposit at ~381 Ma, several ore deposits in the Macmillan Pass district at ~379 to 375 Ma, and the Silvermines deposits at ~352 Ma; and two >6 × 10 −4 spikes coincident with formation of the giant Navan deposit at ~346 Ma and Red Dog deposits at ~337 Ma. Moreover, the timing of peak 87 Sr/ 86 Sr spikes correlates with global δ 13 C and δ 18 O spikes, deposition of metal-rich black shales and ironstones, metal-induced malformation (teratology) of marine organisms, and mass extinctions. The relationships among these features were poorly understood, but our new model explains how the flux of key biolimiting nutrients and metals contained in sedex brines, demonstrably equivalent to or exceeding that of the total modern riverine flux to the ocean, spurred ocean eutrophication, which, ultimately, through a series of positive feedback mechanisms, may have triggered global chemical and biological events. If, as we hypothesize, sedex hydrothermal systems are recorded in the global marine isotopic, geologic, and biological records, our findings define a new approach to the study of and exploration for sedex deposits. We demonstrate that fluid inclusion solute chemistry and isotopic and stratigraphic studies of sedex deposits, coupled with chronostratigraphic correlation and high-resolution 87 Sr/ 86 Sr isotope chemostratigraphy, can be used to answer long-standing questions about geologic processes responsible for formation of these extraordinary deposits. This approach provides evidence for the age, duration, and fluxes of fluids and metals vented into the ocean by these giant hydrothermal systems. Accordingly, the marine 87 Sr/ 86 Sr curve constitutes a global exploration tool that could be applied to assess the mineral potential of sedimentary basins. To illustrate the potential of this tool to identify favorable stratigraphic ages and basins with potential for undiscovered giant sedex deposits, we highlight several spikes, on par with those characteristic of the Red Dog and Navan deposits, which have not been correlated with known metal deposits. Given these strong temporal correlations, mass balance estimates, and results of ocean chemistry modeling, our study suggests that further work is warranted to determine the extent to which periodic venting of hydrothermal basinal brines into the ocean has influenced the evolution of marine chemistry. Ultimately, these global signatures can be applied to the study of and exploration for sedex deposits.

Abstract The Central African Copperbelt is the world’s premier sediment-hosted Cu province. It is contained in the Katangan basin, an intracratonic rift that records onset of growth at ~840 Ma and inversion at ~535 Ma. In the Copperbelt region, the basin has a crudely symmetrical form, with a central depocenter maximum containing ~11 km of strata positioned on the northern side of the border of the Democratic Republic of Congo and Zambia, and marginal condensed sequences <2 km in thickness. This fundamental extensional geometry was preserved through orogenesis, although complex configurations related to halokinesis are prevalent in central and northern parts of the basin, whereas to the south, relatively high-grade metamorphism occurred as a result of basement-involved thrusting and burial. The largest Cu ± Co ores, both stratiform and vein-controlled, are known from the periphery of the basin and transition to U-Ni-Co and Pb-Zn-Cu ores toward the depocenter maximum. Most ore types are positioned within a ~500-m halo to former near-basin-wide salt sheets or associated halokinetic structures, the exception being that located in extreme basin marginal positions, where primary salt was not deposited. Stratiform Cu ± Co ores occur at intrasalt (Congolese-type), subsalt (Zambian-type), and salt-marginal (Kamoa-type) positions. Bulk crush-leach fluid inclusion data from the first two of these deposit types reveal a principal association with residual evaporitic brines. A likely signature of the ore fluids, the brines were generated during deposition of the basin-wide salt-sheets and occupied voluminous sub and intrasalt aquifers from ~800 Ma. Associated intense Mg ± K metasomatism was restricted to these levels, indicating that capping and enclosing salt remained impermeable for prolonged periods of the basin’s history, isolating the deep-seated aquifers from the upper part of the basin fill. From ~765 to 740 Ma, the salt sheets in the Congolese part of the basin were halokinetically modified. Salt was withdrawn laterally to feed diapirs, ultimately leading to localized welding or breaching of the former hydrological seal. At these points, deeper-level residual brines were drawn into the intrasalt stratigraphy to interact with reducing elements and form the stratiform ores. It is probable that salt welding occurred diachronously across the northern and central parts of the basin, depending upon the interplay of original salt thickness, rates and volumes of sediment supply during accumulation of salt overburden, and tectonism. The variable timing of this fundamental change in hydrologic architecture is poorly constrained to the period of halokinetic onset to the earliest stages of orogenesis; however, the geometry of the ores and associated alteration patterns demands that mineralization preceded the characteristically complex fragmentation of the host strata. Thus, while an early orogenic timing is permissible, mineralization during the later stages of extensional basin development was more likely. In situ reducing elements that host Zambian-type stratiform Cu ± Co ores were in continuous hydrological communication with subsalt aquifers, such that ore formation could have commenced from the ~800 Ma brine introduction event. The nonhalokinetic character of the salt in this region allowed the intact seal to have maintained suprahydrostatic pore pressures, facilitating fluid circulation until late stages of basin growth and possibly early stage orogenesis. Leachate data from ores positioned in the depocenter maximum and southern parts of the basin that underwent relatively high grade metamorphism record mixing of residual and halite dissolution-related brines. Salt dissolution was likely triggered by emergence of diapirs or thermally and/or mechanically induced increased permeability of halite. While it is certain that halite dissolution occurred during and after orogenesis, conditions favorable for salt dissolution may have existed locally during extension in the depocenter maximum. The permeability of salt increased to a point where it became the principal aquifer. The salt’s properties as an aquiclude lost, originally deep-seated residual brine mixed with new phases of evaporite dissolution-related brine to produce ores at middle levels of the basin fill. During the final stages of ore formation, recorded by postorogenic Pb-Zn-Cu mineralization in the depocenter maximum, the salinity of fluids was dominantly derived from the dissolution of remnant bodies of salt.

Abstract The Kalahari Copperbelt in northwestern Botswana is characterized by structurally controlled, stratabound, mineralogically zoned copper-silver deposits hosted along a major redox boundary within a late Mesoproterozoic rift succession. Copper-silver mineralized rocks occur on the limbs and in the hinge positions of regional-scale folds that characterize the Pan-African Ghanzi-Chobe zone fold-and-thrust belt. Regional facies changes along the base of the transgressive marine D’Kar Formation, the host to the majority of mineralized rocks, delineate a series of synsedimentary basin highs and lows. The facies changes were identified through both lithostratigraphic analysis of drill holes and along-strike variations in magnetic lithostratigraphy, a technique that correlates the magnetic fabrics of second vertical derivative aeromagnetic maps with changes in lithostratigraphy. Basin highs controlled the development and distribution of favorable lithostratigraphic and lithogeochemical trap sites for later sulfide precipitation. Major facies changes across the Ghanzi Ridge area straddle a significant crustal structure identified in gravity datasets that appears to have influenced extensional activity during basin development. During basin inversion, the basin highs, cored by rheologically stronger bimodal volcanic rocks, localized strain within mechanically weaker rock types of the Ghanzi Group metasedimentary rocks, leading to the development of locally significant permeability and the formation of structural trap sites for mineralization by hot (250°–300°C), oxidizing, metalliferous Na-Ca-Cl brines. Structural permeability was maintained within trap sites due to silicification and/or feldspar alteration during progressive deformation and associated hydrothermal mineralizing events.

Abstract Each year an estimated 56,000 metric tons (t) of rare earth elements (REEs), including 23,000 t of heavy REEs (HREEs), are mined, beneficiated, and put into solution, but not recovered, by operations associated with the global phosphate fertilizer industry. Importantly, the REEs in sedimentary phosphorites are nearly 100% extractable, using technologies currently employed to meet global phosphate fertilizer needs. Our evaluation suggests that by-product REE production from these phosphate mines could meet global REE requirements. For example, the calculated REE flux accompanying phosphate production in the United States is approximately 40% of the world’s total and, alone, could supply 65% of global HREEs needs. Moreover, recognition that the tonnages and HREE concentrations of some unmined phosphorite deposits dwarf the world’s richest REE deposits suggests that these deposits might constitute stand-alone REE deposits. The hypothesized genesis of these REE-rich occurrences strongly supports the long-debated suggestion that oceanic REE contents vary in a secular fashion and that associated high-grade REE abundances reflect oceanic redox state transitions during specific time periods. Here, we use this new process-based model, based on observed variations in global-secular REE abundances, to identify phosphorite horizons deposited during periods favorable for highgrade REE accumulation.

ABSTRACT New data and review of classic sections from the Middle and Upper Ordovician North American Midcontinent in the Upper Mississippi Valley provide a refined picture of the age, stable isotope geochemistry, faunal composition, and—ultimately—origin of this epeiric ramp succession. Sequence stratigraphic analysis reveals a series of unconformity-bounded, genetically related facies packages. Shallowing and deepening trends are sometimes difficult to resolve due to a paucity of hydrodynamic indicators, yet unconformity surfaces are well marked by hardgrounds and confirmed by negative C-isotope spikes. Recent conodont biostratigraphy, new U-Pb radioisotopic ages for K-bentonites, and correlation of C-isotope profiles to global trends suggest that the succession spans the Darriwilian to Hirnantian epochs. Focus on Platteville to lower Galena Group strata (Sandbian to early Katian) provides a temporally high-resolution look at the onset and evolution of a long-term (>2 m.y.) positive carbon-isotope excursion, short-term perturbations in that record, and relationship to the preservation and diversity of the enclosed fauna and strata. Major changes in authigenic mineral suites and organic carbon content throughout the Upper Ordovician Upper Mississippi Valley suggest at least three major redox cycles. The combined evidence for globally recognized, positive carbon-isotope excursions coincident with these redox cycles, as well as high-frequency, sea-level fluctuations and successive faunal turnover events, suggests far-field responses to multiple global oceanic anoxic events.

Abstract Newly recognized gold-rich sedimentary-exhalative (sedex) mineralization in Nevada, with an average gold grade of 14 g/tonne (t), and the occurrence of significant amounts of gold in classic sedex deposits like Rammelsberg, Germany (30 Mt at 1 g/t), Anvil, Canada (120 Mt at 0.7 g/t), and Triumph, Idaho (? at 2.2 g/t) demonstrate that basin brines can form gold ore. The sedex Au mineralization in Nevada represents a previously unrecognized end member in a spectrum of sedex deposits that also includes large Zn-Pb, intermediate Zn-Pb-Ba ± Au, and barite deposits. Study of ore deposits, modern brines, and chemical modeling indicates that variation in metal ratios and their abundance in sedex deposits are dominantly controlled by the concentration and redox state of sulfur in brines. For example, Au and Ba solubilities are highest in H 2 S-rich, SO 4 -poor fluids, whereas base metal solubilities are highest when H 2 S is not present. Chemical modeling indicates a typical reduced brine (15 wt % NaCl equiv, pH = 5.5, H 2 S = 0.01 m) at 200°C is capable of transporting as much as 1 ppm Au in solution. The H 2 S content in brines is controlled by the rate of its production through thermochemical reduction of sulfate by organic matter and the rate of its removal from the fluid through the sulfidation of reactive Fe in the sediments. Thus, sedimentary basins with high organic carbon and sulfate in rocks low in reactive Fe, such as carbonates and shales, are most likely to produce H 2 S-rich brines that may form gold-rich sedex deposits. Because of the tremendous scale of sedex hydrothermal systems, evidence that basin fluids can transport gold identifies a new mechanism for concentrating gold in sedimentary basins and opens extensive areas to further gold exploration.

Abstract This report presents evidence for a previously unrecognized type of Au mineralization on the Barrick Goldstrike property that is geochemically, mineralogically, and temporally distinct from classic Carlin-type mineralization (Table 1). This newly recognized mineralization is characterized by a mineral assemblage that includes barite, sphalerite, minor boulangerite, pyrite, galena, minor tetrahedrite, tennantite, chalcopyrite and native gold. Gold occurs as native inclusions in pyrite, chalcopyrite, tetrahedrite, bitumen, dolomite, and barite. In contrast, features recognized in Carlin-type deposits include a mineral asemblage of As-rich pyrite, realgar, orpiment, and arsenopyrite, with a distinct absence of base metal sulfides. Gold occurs as sub-micron inclusions in As-rich pyrite and alteration consists of decalcification, silicification, and argillization. Carlin-type mineralization is younger than a 39 Ma dike (Emsbo et al. , 1996). Recognition of the base metal mineralization resulted from a broad study of the breccia-hosted Carlin-type deposit in the Meikle Mine. Examination of the stratigraphic section adjacent to Meikle revealed that these base metal sulfides and barite are stratiform and occur in unaltered rocks of the Popovich Formation. In Meikle, the base metal-rich assemblage also occurs as discordant veins which were brecciated and cut by Carlin-type mineralization (Lamb, 1995). The association of low but significant gold content with this base metal mineralization outside Meikle prompted a study of its geological and spatial characteristics, its age and significance within the geologic evolution of the Goldstrike property, and the relationship of its gold to its other sulfides. In this abstract we describes textural, stratigraphic/spatial, and geochemical evidence that this base

Abstract Mississippi Valley-type (MVT) Zn-Pb deposits are generally thought to have formed from hot basinal brines. Much has been learned about the origin of modern basinal brines through studies of their fluid chemistry. The chemical composition of ancient brines preserved in fluid inclusions in MVT deposits is not well known. Published data are mainly from deposits in the United States Mississippi Valley. The purpose of this study is to characterize the chemical composition of inclusion fluids in sphalerite from major MVT districts in North America and Europe: Northern Arkansas, Tri-State, Viburnum Trend, Central Missouri, Southern Illinois Fluorspar, Upper Mississippi Valley and Central Tennessee from the U.S. Mississippi Valley; East Tennessee from the U.S. Appalachian region; Pine Point, Polaris, and Nanisivik from Northwest Territories, Canada; Daniel's Harbor, Newfoundland, Canada; Cracow-Silesia, southern Poland; and Treves, southern France. This data is used to determine the origin of the brines, map brine provinces, and identify subsequent compositional modifications resulting from mineral precipitation, fluid-rock reactions, and fluid mixing. This information places important constraints on genetic models for MVT deposits. Samples consisted of grains of sphalerite, 2-4 mm in size, with an aggregate weight of about 200 mg. Inclusion fluids were released for analysis by crushing in pure water and the leachate analyzed using a new ion chromatographic method. The detection limit for most ionic species is 1 nanogram or less. Forty three samples were analyzed. The following ionic species were determined: Na +1 , NH 4 +1 , K +1 , Ca +2 , Mg +2 , Sr +2 , Cl -1 , Br -1 , F -1 , CO 3 -2 , S 2 O 3 -2 , SO 4 -2 , and acetate. Analytical results are reported in nanograms and molar ratios (e.g. Cl/Br). Absolute concentrations are not reported because the method cannot measure the amount of inclusion water released from the samples. The source of the brines and subsequent compositional modifications are evident from comparison of solute ratios of the samples with those of evaporated seawater and halite. Comparison of the solute ratios of samples from different districts are used to map brine provinces. Solute ratios also provide evidence for multiple fluids and/or fluid mixing within individual districts. The Cl/Br-Na/Cl systematics show that the solutes in the ore fluids are derived from evaporated seawater and water that has dissolved halite. However, halite dissolution water accounts for less than ten percent of the solutes in the samples analyzed. Although meteoric water is probably also a component of the ore fluids, its presence cannot be evaluated using solute ratios. All of the samples have low Na/CI and high Ca/Cl ratios when compared to seawater, which is typical of most basinal brines. The Cl/Br-Na/Cl systematics of the samples indicate that the primary reason for the low Na/CI ratios is precipitation of halite from evaporated seawater, although albitization of plagioclase may have played a minor role. The Cl/Br-Ca/Cl-Mg/Cl systematics of the samples indicate that the main cause of the high Ca/Cl ratios and low Mg/Cl ratios is dolomitization of limestone by Mg enriched evaporated seawater. Albitization is less important than dolomitization in controlling Ca composition of the fluids. Most samples have SO 4 /C1 ratios far below that of evaporated seawater. The low SO 4 /C1 ratios result from gypsum and anhydrite precipitation and sulfate reduction. Precipitation of gypsum and anhydrite may have resulted from the Ca increases in the fluid associated with dolomitization of limestone and albitization of plagioclase. The K/Cl ratios of the samples are both greater and less than that of evaporated seawater, reflecting respectively dissolution and precipitation of K-bearing minerals. Many samples with high K/Cl ratios are from districts where ore fluids migrated through arkosic sandstone aquifers, such as the Lamotte sandstone in the Viburnum Trend. The high NH 4 /Cl and acetate/Cl ratios of some samples reflect interactions of the ore fluids with organic-rich lithologies, such as coal measures in the Cracow-Silesia district. The C1/Br ratios are also unusually high in some samples from the Cracow-Silesia district, suggesting that Br was also leached from the coal measures. In all of the other districts, Br appears to have behaved conservatively. The fluid inclusion solute data shows that MVT districts in the United States were deposited from four compositionally distinct brines derived from different sources. Brine provinces are distinguished on the basis of the degree of evaporation of the brine and the relative proportion of halite dissolution water. Ore fluids in the Northern Arkansas, Tri-State, Central Missouri districts, and later stages in the Viburnum Trend of the Ozark region are derived from halite-saturated evaporated seawater with a component of halite-dissolution water of less than 10 percent. These fluids are thought to be from the adjacent Arkoma basin. Ore fluids in the Southern Illinois Fluorspar, Central Tennessee, and Upper Mississippi Valley are derived from halite-saturated evaporated seawater with a very minor component of halite dissolution water. These fluids are most likely from the adjacent Illinois basin. The early ore fluids in the Viburnum Trend are derived from halite-saturated seawater that migrated through the Reelfoot Rift from the Arkoma or Black Warrior basins. Ore fluids in East Tennessee are derived from more highly evaporated halite-saturated seawater and are probably from basins of the Appalachian region. All of the districts in Canada and Europe were deposited from ore fluids derived from variably evaporated halite-saturated seawater. Samples from the Viburnum Trend and Cracow-Silesia exhibit temporal compositional variations and samples from Pine Point exhibit spatial compositional variations.