J. Richard Kyle

Rb-Sr dating of sphalerites from Mississippi Valley-type (MVT) ore deposits

Abstract Mississippi Valley-type (MVT) ore deposits are epigenetic carbonate-hosted Pb-Zn deposits that contain galena, sphalerite, fluorite, barite, 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 ± 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 Daniels Harbour mine, New foundland, 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 361 ± 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 87 Sr 86 Sr 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.

Na-Cl-Br systematics of mineralizing brines in Mississippi Valley–type deposits

New and published analyses of fluid-inclusion leachates from Mississippi Valley–type (MVT) deposits can be divided into two groups on the basis of Na/Br and Cl/Br ratios. MVT leachates from the Illinois-Kentucky and Cincinnati arch districts and from cubic galena in the Viburnum Trend have Na/Br and Cl/Br ratios that extend to values significantly above that of seawater, which are characteristic of evaporite-dissolution brines. MVT leachates from Polaris and octahedral galena in the Viburnum Trend have Na/Br and Cl/Br ratios that plot below seawater and along the compositional trend formed by evaporation. Solubility-volume constraints require that brines formed by seawater evaporation had high dissolved metal contents. Preliminary correlation of leachate compositions allows delineation of two brine provinces in the midcontinent United States: (1) an early, high-Br province found only in southeastern Missouri, and (2) a later, low-Br province of probable Permian age that extended from the Cincinnati arch to the Tri-State district. These observations, along with Na-Cl-Br data for modern brines from the Illinois basin, argue against models for single-stage midcontinent MVT brine flow based on recharge from late Paleozoic (Arkoma) foreland basins to the south.

Stable chlorine isotopes in halite and brine from the Gulf Coast Basin: brine genesis and evolution

Stable chlorine isotope data, used in conjunction with other geochemical parameters, are useful in determining the origin of solute in formation waters with salinity different from that of seawater. Jurassic salt in the Gulf Coast Basin has δ37Cl ranges of −0.5‰ to 0.3‰ (bedded) and 0.0‰ to 0.5‰ (diapiric). The values for bedded salt are consistent with a δ37Cl value of 0.0‰ for Jurassic seawater chloride, as in modern oceans. The slightly higher values for diapiric salt are possibly caused by incongruent solution of halite. Formation waters have a δ37Cl range of −1.9‰ to 0.7‰. Waters with δ37Cl −0.2‰. Low-salinity water from geopressured aquifers appears to have resulted from shale dewatering. Low δ37Cl values in such water are related to low mCl/mNa (a parameter indicating dewatering) but show no general relationship to Cl/Br. They are consistent with diffusion of chloride from allochthonous brine. Diffusion can generate domains of water with negative δ37Cl on a scale of hundreds of meters in 104–106 years, and may also generate positive-δ37Cl chloride in residual source brine.

Fluid-rock interactions in the salt dome environment: An introduction and review

Abstract The salt dome environment in the Gulf of Mexico Coast, U.S.A, provides one of the most diverse and long-lived records of fluid-rock interactions in an active sedimentary basin dominated by saline formation waters. The geochemical record, most of which is isotopic, appears to span a temperature range between Earth surface temperatures and lower greenschist facies. Most mineral-forming processes in the salt dome cap rocks appear to involve mixtures of warm saline formation fluids from deep basin sources and cool dilute meteoric waters. Papers in this issue of Chemical Geology document some of the current research on fluid-rock interactions in the salt dome environment. These papers highlight the diversity of opinion about salt dome processes, offering clues for further research. The present paper reviews some of the key and more current literature on salt dome geochemistry, fluid convection around salt domes, and fluid-rock interactions within salt and within the salt dome environment.

Upper Proterozoic carbonate stratigraphy, diagenesis, and stromatolitic phosphorite formation, Irece Basin, Bahia, Brazil

ABSTRACT Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece Basin of east-central Brazil. The Irece Basin contains a basal siliciclastic sequence (the Bebedouro Formation) composed of diamictites dominantly of glacial origin that is overlain by lacustrine carbonate sediments. These are succeeded by the Una Group, a marine carbonate sequence that totals several hundred meters thick. The marine units consist of a basal laminated limestone and dolomitic limestone sequence that grades upward into dolostone and cherty dolostone; a middle sequence dominated by gray argillaceous limestones, shales, and siltstones; and an upper sequence of black organic-rich lime grainstones. Stratabound phosphate- and sulfide-rich units are present within a 50-m-thick tid l flat sequence of dolomitic limestone and cherty dolostone in the lower Una Group. Shallow-water sedimentary structures include stromatolitic laminae, columnar stromatolites, mudcracks, teepee structures, collapse breccias, length-slow quartz nodules, and other pseudomorphs after evaporitic sulfates. Conventional petrography supported by cathodoluminescence and scanning electron microscopy of the carbonate strata reveals a complex diagenetic history representing four major diagenetic environments: Marine, Reflux, Meteoric, and Burial. Carbon and oxygen isotope analyses of diagenetic carbonate generations yield distinct populations that support the environmental interpretations. Concentrations of fine-grained carbonate fluorapatite are present in stromatolitic carbonate units in the Irece-Lapao area. Three types of primary phosphate concentrations have been defined: columnar stromatolitic, laminar stromatolitic, and intraclastic. The presence of resedimented phosphatic clasts and of phosphatic units interbedded with barren dolostones suggest that the formation of stromatolitic phosphorite was an early-diagenetic process. Carbon and oxygen isotope data for calcite, dolomite, and carbonate fluorapatite, as well as sulfur isotope data from sulfates and sulfides, suggest the presence of an anoxic environment during the formation of the Irece phosphorites. Postglacial enrichment of the basin waters in phosphate is considered to be a critical aspect of phosphatizat on. Bacterial degradation of organic matter in the stromatolites probably was responsible for local phosphate enrichment of pore waters and resulted in concentration of carbonate fluorapatite by direct precipitation or by replacement of calcium carbonate.

Deposition of magnetic pyrrhotite during alteration of crude oil and reduction of sulfate

Abstract Crude oil seepage is widespread in carbonate cap rock and in overlying sediments of the Damon Mound salt dome, Brazoria County, Texas. Scanning electron microscopy of calcite-lined cavities provides insight to the latest stage of mineral deposition in this salt dome environment. Solid crude oil residues and microbes occur in association with surface minerals such as gypsum and barite. Deposition of pyrrhotite, pyrite, marcasite, sphalerite, and elemental sulfur is attributed to microbial sulfate reduction, and deposition of aragonite and calcite is related to microbial oxidation of crude oil hydrocarbons. The C1—C4 hydrocarbons in pyrrhotite-bearing cap rock samples are a biodegraded residue derived from crude oil. Higher molecular weight crude oil components in cap rock are altered by water washing and to a lesser extent by biodegradation. Hydrocarbon compositions could be explained by limited aerobic biodegradation, but a component of anaerobic biodegradation cannot be excluded. Some Damon Mound samples are highly magnetic because of abundant ferrimagnetic pyrrhotite. This study provides new evidence of a link between crude oil migration, microbial activity, and diagenetic magnetic anomalies in rocks.

Multiple fluid components of salt diapirs and salt dome cap rocks, Gulf Coast, U.S.A.

Abstract Salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions. Reported 87Sr/86Sr ratios of these salt-hosted and cap rock anhydrites in the Gulf Coast, U.S.A., indicate their derivation from Middle Jurassic seawater. However, a much wider range of 87Sr/86Sr ratios, incorporating a highly radiogenic component in addition to the Middle Jurassic component, has been found in several Gulf Coast salt domes. This wide range of 87Sr/86Sr ratios of anhydrite within the salt stocks records Sr contributions from both marine water and formation water that has equilibrated with siliciclastics. During cap rock formation this anhydrite either recrystallized in the presence of, or was cemented by, a low-Sr fluid with a Late Cretaceous seawter-type Sr isotope ratio or simply lost Sr during recrystallization. Later, the cap rock was invaded by warm saline brines with high Sr isotope ratios from which barite and metal sulfides were precipitated. Subsequently, low-salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth. The progenitor of these salt diapirs, the Louann Formation, is generally thought to have formed from marine water evaporated to halite and, rarely, higher evaporite facies. Salt domes in the East Texas, North Louisiana, and Mississippi Salt Basins have 87Sr/86Sr and δ34S values that corroborate a Mid-Jurassic age for the mother salt. However, salt domes in the Houston and Rio Grande Embayments of the Gulf Coast Basin have 87Sr/86Sr ration ranging to values higher than both Middle Jurassic seawater and all Rb-free marine Phanerozoic rocks. These anomalous 87Sr/86Sr ratios are probably derived from radiogenic Sr-bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to, or during, diapirism. Potential sources of the radiogenic 87Sr component include clay and/or feldspar (located either in older units beneath the Louann Formation or younger units flanking the salt diapirs) and K-salts within the Louann evaporites. Because partial Sr exchange in anhydrite had to take place in a fluid medium, admittance of radiogenic 87Sr-bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt. The slight number of anomalous 87Sr/86Sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the Gulf Coast Basin.

Geological and Isotopic Constraints on the Metallogenic Evolution of the Proterozoic Sediment-Hosted Pb-Zn (Ag) Deposits of Brazil

Abstract Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Parana state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irece and Nova Redencao (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur. Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.

Skarn CuAu orebodies of the Gunung Bijih (Ertsberg) district, Irian Jaya, Indonesia

Abstract The major CuAu skarn deposits of the Gunung Bijih (Ertsberg) district in central Irian Jaya are products of hydrothermal systems that developed in association with Pliocene magma emplacement in an active continental margin. The CuAu skarn orebodies occur within a Cretaceous to Tertiary sedimentary sequence that was deformed as the northern Australian continental margin entered a north-dipping subduction zone at ∼ 12 Ma. The intermediate-composition intrusions consist of fine-grained porphyritic stocks, dikes, and sills that have K-Ar ages ranging from 2.7 to 4.4 Ma. Most intrusions are slightly potassic, but these data could be affected by alteration. The skarn orebodies in the Ertsberg district are hosted in deformed lower Tertiary New Guinea Group carbonate strata along the periphery of the Pliocene Ertsberg intrusion. Major skarn orebodies include the Ertsberg (GB), the Ertsberg East (GBT) complex, including the GBT, the Intermediate Ore Zone (IOZ) and the Deep Ore Zone (DOZ), and the Dom. Chalcopyrite is the dominant ore mineral in the GB and Dom orebodies, whereas bornite dominates in the GBT complex. Native Au occurs within bornite and chalcopyrite in GB and GBT ores. The district calc-silicate alteration assemblages are characterized by high-temperature skarn minerals, including forsterite, monticellite, and minor melilite. Diopsidic clinopyroxene is common, particularly in GBT. Anhydrite and phlogopite are abundant in the GBT complex, and the anhydrite:calcite ratio increases with depth from GBT to DOZ where anhydrite is ubiquitous and calcite rare. At least three types of garnets have been identified at the Dom and show a progressive increase in ferric iron content. Garnet decreases with depth in the GBT complex. Talc, serpentine, tremolite-actinolite, and chlorite are common retrograde minerals. Copper sulfide mineralization is texturally associated with early retrograde alteration. Differences among the skarn orebodies are related in part to variable protolith composition that affected skarn development within different stratigraphic positions. Distinctive fossil replacement textures preserved within skarn indicate that the Oligocene-Miocene Ainod Formation is the most likely protolith for the GB and Dom orebodies. The GBT and upper IOZ orebodies probably are hosted by the Eocene Faumai Formation. The DOZ and lower IOZ orebodies, dominated by magnesian skarn alteration, appear to be developed in a dolomitic unit within the lower New Guinea Limestone Group, which probably is equivalent to the Paleocene Waripi Formation.

Supergene geochemistry and crystal morphology of gold in a semiarid weathering environment: application to gold exploration

Abstract Oxidation of sulfide- and carbonate-rich vein gold deposits under semiarid conditions can be represented as a three-stage process, each creating supergene environments conducive to dissolution and reprecipitation of gold-silver alloys. The three-stage weathering process of sulfide-carbonate gold veins is depth-dependent, and develops from the relatively young, lowermost weathering horizon just below the water table, through an intermediate weathering horizon in the oxidation zone above the water table, and culminating in the oxide-rich upper saprolite and oxisol. Neoformed gold crystals in the weathering profile have distinct composition and morphologic characteristics from the hypogene gold crystals associated with the sulfide- and carbonate-rich ores. Two distinct types of secondary gold are present in the weathering profile: (1) gold crystals associated with sulfates and arsenates; and (2) gold crystals associated with iron and aluminum oxides/hydroxides, or with kaolinite. The distinct crystal morphologies and mineralogical associations of primary and secondary gold are useful in prospecting for gold deposits in weathered terrains.