Geology topics

Craig A. Johnson

Publications and source records attributed to Craig A. Johnson.

At least 37 records · Page 2Linked to original sources

USGS2017

Transgressive-regressive cycles in the metalliferous, oil-shale-bearing Heath Formation (Upper Mississippian), central Montana

The Upper Mississippian Heath Formation, which accumulated in the Big Snowy Trough of central Montana, has been known for three decades to contain mudrocks highly enriched in Zn, V, Mo, Ni and other metals, and source rocks for oil. The unit has more recently been recognized as a prospective tight oil play. Here we present petrographic, paleontologic, geochemical, and carbon and sulfur isotope data on seven continuous drill cores spanning ≤146 m of immature to marginally mature strata in order to improve understanding of the depositional setting of the Heath. The unit consists of five third-order transgressive-regressive cycles (C1–C5 from bottom to top) that were deposited during a fluctuating climatic regime. Cycles comprise thinly interbedded gray to black mudrock and carbonate strata capped by either coal, implying a humid climate (C1, C3 and C4), or gypsum, implying more arid conditions (C2); the upper part of C5 is not preserved in our study area. Mfs (maximum flooding surfaces) in C1, C2, C4, and C5 lie within black mudrock beds ~0.5–3-m thick with >10% TOC (total organic carbon), type I and type II kerogen (determined by programmed pyrolysis), high contents of Zn, V, Mo, and other metals, relatively low values of δ13CTOC and δ34Spyrite, and a limited-diversity fauna of locally abundant, thin-shelled pelecypods (Dunbarella? sp.). The mfs in C2 is within the Cox Ranch oil shale bed, which is known from previous studies to be metalliferous; new analyses reported here show ≤28 wt % TOC, 5140 ppm Zn, 1910 ppm V, 1590 ppm Mo, and 509 ppm Ni. Strata that contain the mfs of C1, C4, and C5 are shown here for the first time to also have high metal contents, with maximum values of 1030–7340 ppm Zn, 446–1980 ppm V, 72–859 ppm Mo, and 221–452 ppm Ni. Cycle C3, which contains more gray mudrock and carbonate beds than the other cycles, has lower TOC (≤4.2 wt %), lower metals, and mainly type III kerogen. Carbonate beds include normal-marine crinoidal mudstone to packstone and lesser (dolo)mudstone with fenestral fabric, peloids, intraclasts, and a euryhaline fauna. Mid-Chesterian (early Serpukhovian) foraminifers in C3, combined with previously published fossil data, suggest that third-order cycles in the Heath Formation were ~1–2 myr in duration. They formed during a time of active block faulting in the Big Snowy Trough and global cooling linked to Gondwanan glaciation. Tectonic, climatic, and paleogeographic factors shaped the cycles of the Heath Formation. Faunal and geochemical evidence indicate that conditions were most favorable for marine life during C3. Molybdenum concentrations >100 ppm and organic geochemical data suggest euxinic conditions during deposition of the black mudrock in C2, C4, and C5, but the presence of shell beds (1 mm–6-cm thick) within this mudrock requires bottom water with sufficient oxygen to support life, at least periodically. The apparent conflict between the geochemical and paleontologic observations likely reflects the different time scales of these two environmental proxies: 1000s of yrs vs <1–10s of yrs, respectively. Metal- and organic-rich strata in the Heath Formation formed by slow, condensed sedimentation from periodically anoxic or euxinic bottom waters in a marine basin. Fossil data indicate that anoxia was episodic, perhaps seasonal and/or linked to longer-duration climate shifts. On a millennial time scale, metal enrichments in the Heath reflect a balance between primary productivity that was high enough for oxygen to be consumed by sinking organic matter and oxic seawater inflow that was strong enough to maintain a supply of metals without compromising anoxia. Organic-rich mudrock in the Heath shares intriguing lithologic and geochemical similarities with mudrock in other Middle to Upper Paleozoic units such as the Devonian-Mississippian Bakken Formation and Pennsylvanian cyclothems (e.g., Excello Shale).

Montana↗

USGS2017

Origin and evolution of mineralizing fluids and exploration of the Cerro Quema Au-Cu deposit (Azuero Peninsula, Panama) from a fluid inclusion and stable isotope perspective

Cerro Quema is a high sulfidation epithermal Au-Cu deposit with a measured, indicated and inferred resource of 35.98 Mt. @ 0.77 g/t Au containing 893,600 oz. Au (including 183,930 oz. Au equiv. of Cu ore). It is characterized by a large hydrothermal alteration zone which is interpreted to represent the lithocap of a porphyry system. The innermost zone of the lithocap is constituted by vuggy quartz with advanced argillic alteration locally developed on its margin, enclosed by a well-developed zone of argillic alteration, grading to an external halo of propylitic alteration. The mineralization occurs in the form of disseminations and microveinlets of pyrite, chalcopyrite, enargite, tennantite, and trace sphalerite, crosscut by quartz, barite, pyrite, chalcopyrite, sphalerite and galena veins. Microthermometric analyses of two phase (L + V) secondary fluid inclusions in igneous quartz phenocrysts in vuggy quartz and advanced argillically altered samples indicate low temperature (140–216 °C) and low salinity (0.5–4.8 wt% NaCl eq.) fluids, with hotter and more saline fluids identified in the east half of the deposit (Cerro Quema area). Stable isotope analyses (S, O, H) were performed on mineralization and alteration minerals, including pyrite, chalcopyrite, enargite, alunite, barite, kaolinite, dickite and vuggy quartz. The range of δ 34 S of sulfides is from − 4.8 to − 12.7‰, whereas δ 34 S of sulfates range from 14.1 to 17.4‰. The estimated δ 34 S ΣS of the hydrothermal fluid is − 0.5‰. Within the advanced argillic altered zone the δ 34 S values of sulfides and sulfates are interpreted to reflect isotopic equilibrium at temperatures of ~ 240 °C. The δ 18 O values of vuggy quartz range from 9.0 to 17.5‰, and the δ 18 O values estimated for the vuggy quartz-forming fluid range from − 2.3 to 3.0‰, indicating that it precipitated from mixing of magmatic fluids with surficial fluids. The δ 18 O of kaolinite ranges from 12.7 to 18.1‰ and δD from − 103.3 to − 35.2‰, whereas the δ 18 O of dickite varies between 12.7 and 16.3‰ and δD from − 44 to − 30. Based on δ 18 O and δD, two types of kaolinite/dickite can be distinguished, a supergene type and a hypogene type. Combined, the analytical data indicate that the Cerro Quema deposit formed from magmatic-hydrothermal fluids derived from a porphyry copper-like intrusion located at depth likely towards the east of the deposit. The combination of stable isotope geochemistry and fluid inclusion analysis may provide useful exploration vectors for porphyry copper targets in the high sulfidation/lithocap environment.

Azuero Peninsula↗

USGS2017

Review: The size of the risk: Histories of multiple use in the Great Basin by Leisl Carr Childers

In The Size of the Risk , Leisl Carr Childers chronicles the changing ways in which public lands of the Great Basin have been managed from the latter half of the nineteenth century through the late 1970s. The main focus is the State of Nevada, which constitutes the core of the Great Basin. Rather than proceeding chronologically, the book is organized by the uses to which lands were put, including grazing of cattle and sheep, weapons testing by the military, parks and recreation, and grazing of wild mustangs. Review info: The size of the risk: Histories of multiple use in the Great Basin . By Leisl Carr Childers, 2015. ISBN: 9780806152530, 320pp.

Pacific Historical Review↗

USGS2016

Oxygen, hydrogen, sulfur, and carbon isotopes in the Pea Ridge magnetite-apatite deposit, southeast Missouri, and sulfur isotope comparisons to other iron deposits in the region

Oxygen, hydrogen, sulfur, and carbon isotopes have been analyzed in the Pea Ridge magnetite-apatite deposit, the largest historic producer among the known iron deposits in the southeast Missouri portion of the 1.5 to 1.3 Ga eastern granite-rhyolite province. The data were collected to investigate the sources of ore fluids, conditions of ore formation, and provenance of sulfur, and to improve the general understanding of the copper, gold, and rare earth element potential of iron deposits regionally. The δ 18 O values of Pea Ridge magnetite are 1.9 to 4.0‰, consistent with a model in which some magnetite crystallized from a melt and other magnetite—perhaps the majority—precipitated from an aqueous fluid of magmatic origin. The δ 18 O values of quartz, apatite, actinolite, K-feldspar, sulfates, and calcite are significantly higher, enough so as to indicate growth or equilibration under cooler conditions than magnetite and/or in the presence of a fluid that was not entirely magmatic. A variety of observations, including stable isotope observations, implicate a second fluid that may ultimately have been meteoric in origin and may have been modified by isotopic exchange with rocks or by evaporation during storage in lakes. Sulfur isotope analyses of sulfides from Pea Ridge and seven other mineral deposits in the region reveal two distinct populations that average 3 and 13‰. Two sulfur sources are implied. One was probably igneous melts or rocks belonging to the mafic- to intermediate-composition volcanic suite that is present at or near most of the iron deposits; the other was either melts or volcanic rocks that had degassed very extensively, or else volcanic lakes that had trapped rising magmatic gases. The higher δ 34 S values correspond to deposits or prospects where copper is noteworthy—the Central Dome portion of the Boss deposit, the Bourbon deposit, and the Vilander prospective area. The correspondence suggests that (1) sulfur either limited the deposition of copper or was cotransported with copper, and (2) sulfur isotope analysis may be useful in evaluating southeast Missouri iron deposits for copper and possibly for gold.

Missouri↗

USGS2016

A salt diapir-related Mississippi Valley-type deposit: The Bou Jaber Pb-Zn-Ba-F deposit, Tunisia: Fluid inclusion and isotope study

The Bou Jaber Ba-F-Pb-Zn deposit is located at the edge of the Bou Jaber Triassic salt diapir in the Tunisia Salt Diapir Province. The ores are unconformity and fault-controlled and occur as subvertical column-shaped bodies developed in dissolution-collapse breccias and in cavities within the Late Aptian platform carbonate rocks, which are covered unconformably by impermeable shales and marls of the Fahdene Formation (Late Albian–Cenomanian age). The host rock is hydrothermally altered to ankerite proximal to and within the ore bodies. Quartz, as fine-grained bipyramidal crystals, formed during hydrothermal alteration of the host rocks. The ore mineral assemblage is composed of barite, fluorite, sphalerite, and galena in decreasing abundance. The ore zones outline distinct depositional events: sphalerite-galena, barite-ankerite, and fluorite. Fluid inclusions, commonly oil-rich, have distinct fluid salinities and homogenization temperatures for each of these events: sphalerite-galena (17 to 24 wt% NaCl eq., and Th from 112 to 136 °C); ankerite-barite (11 to 17 wt% NaCl eq., and Th from 100 to 130 °C); fluorite (19 to 21 wt% NaCl eq., Th from 140 to 165 °C). The mean temperature of the ore fluids decreased from sphalerite (125 °C) to barite (115 °C) and increased during fluorite deposition (152 °C); then decreased to &sim;110 °C during late calcite precipitation. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of fluid inclusions in fluorite are metal rich (hundreds to thousands ppm Pb, Zn, Cu, Fe) but the inclusions in barite are deficient in Pb, Zn, Cu, Fe. Inclusions in fluorite have Cl/Br and Na/Br ratios of several thousand, consistent with dissolution of halite while the inclusions analysed in barite have values lower than seawater which are indicative of a Br-enriched brine derived from evaporation plus a component of halite dissolution. The salinity of the barite-hosted fluid inclusions is less than obtained simply by the evaporation of seawater to halite saturation and requires a dilution of more than two times by meteoric water. The higher K/Na values in fluid inclusions from barite suggest that the brines interacted with K-rich rocks in the basement or siliciclastic sediments in the basin. Carbonate gangue minerals (ankerite and calcite) have δ 13 C and δ 18 O values that are close to the carbonate host rock and indicate fluid equilibrium between carbonate host rocks and hydrothermal brines. The δ 34 S values for sphalerite and galena fall within a narrow range (1 to 10 &permil;) with a bulk value of 7.5 &permil;, indicating a homogeneous source of sulfur. The δ 34 S values of barite are also relatively homogeneous (22 &permil;), with 6 &permil; higher than the δ 34 S of local and regional Triassic evaporites (15 &permil;). The latter are believed to be the source of sulfate. Temperature of deposition together with sulfur isotope data indicate that the reduced sulfur in sulfides was derived through thermochemical sulfate reduction of Triassic sulfate via hydrocarbons produced probably from Late Cretaceous source rocks. The 87 Sr/ 86 Sr ratio in the Bou Jaber barite (0.709821 to 0.711408) together with the lead isotope values of Bou Jaber galena ( 206 Pb/ 204 Pb = 18.699 to 18.737; 207 Pb/ 204 Pb = 15.635 to 15.708 and 208 Pb/ 204 Pb = 38.321 to 38.947) show that metals were extracted from homogeneous crustal source(s). The tectonic setting of the Bou Jaber ore deposit, the carbonate nature of the host rocks, the epigenetic style of the mineralization and the mineral associations, together with sulfur and oxygen isotope data and fluid inclusion data show that the Bou Jaber lead-zinc mineralization has the major characteristics of a salt diapir-related Mississippi Valley-type (MVT) deposit with superimposed events of fluorite and of barite deposition. Field relations are consistent with mineral deposition during the Eocene–Miocene Alpine orogeny from multiple hydrothermal events: (1) Zn-Pb sulfides formed by mixing of two fluids: one fluid metal-rich but reduced sulfur-poor and a second fluid reduced sulfur-rich; (2) barite precipitation involved the influx of a meteoric water component that mixed with a barium-rich fluid; and (3) fluorite precipitated from a highly saline fluid with higher temperatures.

Mineralium Deposita↗

USGS2016

The F'derik-Zouerate iron district: Mesoarchean and Paleoproterozoic iron formation of the Tiris Complex, Islamic Republic of Mauritania

High-grade hematitic iron ores (of HIF, containing 60-65 wt%Fe) have been mined in Mauritania since 1952 from Superior-type iron deposits of the F'derik-Zouerate district. Depletion of the high-grade ores in recent years has resulted in new exploration projects focused on lower-grade magnetite ores occurring in Algoma-type banded iron formation (of BIF, containing ca. 35 wt% Fe). Mauritania is the seventeenth largest iron producer in the world and currently has about 1.1 Gt of crude iron ore reserves.

Book chapter↗

USGS2015

Depositional conditions for the Kuna Formation, Red Dog Zn-PB-Ag-Barite District, Alaska, inferred from isotopic and chemical proxies

Water column redox conditions, degree of restriction of the depositional basin, and other paleoenvironmental parameters have been determined for the Mississippian Kuna Formation of northwestern Alaska from stratigraphic profiles of Mo, Fe/Al, and S isotopes in pyrite, C isotopes in organic matter, and N isotopes in bulk rock. This unit is important because it hosts the Red Dog and Anarraaq Zn-Pb-Ag ± barite deposits, which together constitute one of the largest zinc resources in the world. The isotopic and chemical proxies record a deep basin environment that became isolated from the open ocean, became increasingly reducing, and ultimately became euxinic. The basin was ventilated briefly and then became isolated again just prior to its demise as a discrete depocenter with the transition to the overlying Siksikpuk Formation. Ventilation corresponded approximately to the initiation of bedded barite deposition in the district, whereas the demise of the basin corresponded approximately to the formation of the massive sulfide deposits. The changes in basin circulation during deposition of the upper Kuna Formation may have had multiple immediate causes, but the underlying driver was probably extensional tectonic activity that also facilitated fluid flow beneath the basin floor. Although the formation of sediment-hosted sulfide deposits is generally favored by highly reducing conditions, the Zn-Pb deposits of the Red Dog district are not found in the major euxinic facies of the Kuna basin, nor did they form during the main period of euxinia. Rather, the deposits occur where strata were permeable to migrating fluids and where excess H 2 S was available beyond what was produced in situ by decomposition of local sedimentary organic matter. The known deposits formed mainly by replacement of calcareous strata that gained H 2 S from nearby highly carbonaceous beds (Anarraaq deposit) or by fracturing and vein formation in strata that produced excess H 2 S by reductive dissolution of preexisting barite (Red Dog deposits).

Alaska↗

USGS2015

The fate of cyanide in leach wastes at gold mines: an environmental perspective

This paper reviews the basic chemistry of cyanide, methods by which cyanide can be analyzed, and aspects of cyanide behavior that are most relevant to environmental considerations at mineral processing operations associated with gold mines. The emphasis is on research results reported since 1999 and on data gathered for a series of U.S. Geological Survey studies that began in the late 1990s. Cyanide is added to process solutions as the CN − anion, but ore leaching produces numerous other cyanide-containing and cyanide-related species in addition to the desired cyanocomplex of gold. These can include hydrogen cyanide (HCN); cyanometallic complexes of iron, copper, zinc, nickel, and many other metals; cyanate (CNO − ); and thiocyanate (SCN − ). The fate of these species in solid wastes and residual process solutions that remain once gold recovery activities are terminated and in any water that moves beyond the ore processing facility dictates the degree to which cyanide poses a risk to aquatic organisms and aquatic-dependent organisms in the local environment. Cyanide-containing and cyanide-related species are subject to attenuation mechanisms that lead to dispersal to the atmosphere, chemical transformation to other carbon and nitrogen species, or sequestration as cyanometallic precipitates or adsorbed species on mineral surfaces. Dispersal to the atmosphere and chemical transformation amount to permanent elimination of cyanide, whereas sequestration amounts to storage of cyanide in locations from which it can potentially be remobilized by infiltrating waters if conditions change. From an environmental perspective, the most significant cyanide releases from gold leach operations involve catastrophic spills of process solutions or leakage of effluent to the unsaturated or saturated zones. These release pathways are unfavorable for two important cyanide attenuation mechanisms that tend to occur naturally: dispersal of free cyanide to the atmosphere and sunlight-catalyzed dissociation of strong cyanometallic complexes, which produces free cyanide that can then disperse to the atmosphere. The widest margins of environmental safety will be achieved where mineral processing operations are designed so that time for offgassing, aeration, and sunlight exposure are maximized in the event that cyanide-bearing solutions are released inadvertently.

Applied Geochemistry↗

USGS2014

Carbonate margin, slope, and basin facies of the Lisburne Group (Carboniferous-Permian) in northern Alaska

The Lisburne Group (Carboniferous-Permian) consists of a carbonate platform that extends for >1000 km across northern Alaska, and diverse margin, slope, and basin facies that contain world-class deposits of Zn and Ba, notable phosphorites, and petroleum source rocks. Lithologic, paleontologic, isotopic, geochemical, and seismic data gathered from outcrop and subsurface studies during the past 20 years allow us to delineate the distribution, composition, and age of the off-platform facies, and to better understand the physical and chemical conditions under which they formed. The southern edge of the Lisburne platform changed from a gently sloping, homoclinal ramp in the east to a tectonically complex, distally steepened margin in the west that was partly bisected by the extensional Kuna Basin (~200 by 600 km). Carbonate turbidites, black mudrocks, and radiolarian chert accumulated in this basin; turbidites were generated mainly during times of eustatic rise in the late Early and middle Late Mississippian. Interbedded black mudrocks (up to 20 wt% total organic carbon), granular and nodular phosphorite (up to 37 wt% P 2 O 5 ), and fine-grained limestone rich in radiolarians and sponge spicules formed along basin margins during the middle Late Mississippian in response to a nutrient-rich, upwelling regime. Detrital zircons from a turbidite sample in the western Kuna Basin have mainly Neoproterozoic through early Paleozoic U-Pb ages (~900-400 Ma), with subordinate populations of Mesoproterozoic and late Paleoproterozoic grains. This age distribution is similar to that found in slightly older rocks along the northern and western margins of the basin. It also resembles age distributions reported from Carboniferous and older strata elsewhere in northwestern Alaska and on Wrangel Island. Geochemical and isotopic data indicate that suboxic, denitrifying conditions prevailed in the Kuna Basin and along its margins. High V/Mo, Cr/Mo, and Re/Mo ratios (all marine fractions [MF]) and low MnO contents (<0.01 wt%) characterize Lisburne black mudrocks. Low Qmf/Vmf ratios (mostly 0.8-4.0) suggest moderately to strongly denitrifying conditions in suboxic bottom waters during siliciclastic and phosphorite sedimentation. Elevated to high Mo contents (31-135 ppm) in some samples are consistent with seasonal to intermittent sulfidic conditions in bottom waters, developed mainly along the basin margin. High d 15 N values (6-120) imply that the waters supplying nutrients to primary producers in the photic zone had a history of denitrification either in the water column or in underlying sediments. Demise of the Lisburne platform was diachronous and reflects tectonic, eustatic, and environmental drivers. Southwestern, south-central, and northwestern parts of the platform drowned during the Late Mississippian, coincident with Zn and Ba metallogenesis within the Kuna Basin and phosphogenesis along basin margins. This drowning was temporary (except in the southwest) and likely due to eutrophication associated with upwelling and sea-level rise enhanced by regional extension, which allowed suboxic, denitrifying waters to form on platform margins. Final drowning in the southcentral area occurred in the Early Pennsylvanian and also may have been linked to regional extension. In the northwest, platform sedimentation persisted into the Permian; its demise there appears to have been due to increased siliciclastic input. Climatic cooling may have produced additional stress on parts of the Lisburne platform biota during Pennsylvanian and Permian times.

Alaska↗

USGS2013

Descriptive and geoenvironmental model for Co-Cu-Au deposits in metasedimentary rocks

Introduction This report is a revised model for a specific type of cobalt-copper-gold (Co-Cu-Au) deposit that will be evaluated in the next U.S. Geological Survey (USGS) assessment of undiscovered mineral resources in the United States (see Ferrero and others, 2012). Emphasis is on providing an up-to-date deposit model that includes both geologic and geoenvironmental aspects. The new model presented here supersedes previous USGS models by Earhart (1986) and Evans and others (1995), which are based solely on deposits in the Blackbird mining district of central Idaho. This report is a broader synthesis of information on 19 Co-Cu-Au deposits occurring in predominantly metasedimentary successions worldwide (table 1–1) that generally share common geologic, mineralogical, and geochemical features; preliminary summary versions were presented in Slack and others (2010) and Slack and others (2011), which are superseded by this report. As defined herein, the individual Co-Cu-Au deposits are located more than 500 meters from similar deposits and contain 0.1 percent or more by weight of Co in ore or mineralized rock; some deposits included in the database lack reported average Co grades, but they contain high Co concentrations, at least locally. Most of the deposits also have high As contents, present in Co arsenide and sulfarsenide minerals. Type examples of the Co-Cu-Au deposits are those in the Blackbird district, Skuterud in Norway, and Kouvervarra and Juomasuo in Finland. Some deposits in the database have low grades for Cu (for example, NICO in Canada) or Au (for example, Lemmonlampi in Finland), but these deposits are included because their geological, mineralogical, and alteration features are similar to those of the type examples. Several deposits included in the model are partly hosted by metavolcanic or metaigneous rocks (including granite), but regionally these deposits are within metasedimentary successions; no deposits are wholly within granite or other plutonic igneous intrusions. Despite having a lower average Co grade, the Mt. Cobalt deposit in Australia is included here because it has past Co production from higher-grade ore zones (Nisbet and others, 1983). The Black Pine deposit in the Idaho cobalt belt is included because it contains mineable Co- and Au-rich lenses within Cu-rich mineralized zones (Formation Metals, Inc., 2012). Six deposits that lack data for average Co grades are also included because each reportedly contains abundant Co (>0.1 weight percent Co), at least locally. Many of the deposits are noteworthy as possible resources of Ag, Bi, W, Ni, Y, REE, and (or) U. Detailed data on the deposits listed in table 1–1, including references, are available in appendix 1. Significantly, the grouping in this report of Co-Cu-Au deposits in metasedimentary rocks into a single model includes deposits that other workers have previously classified in different ways. For background information, a global overview of different types of Co deposits worldwide is given in Smith (2001). Additional geologically and compositionally similar deposits are known, but have average Co grades less than 0.1 percent. Most of these deposits contain cobalt-rich pyrite and lack appreciable amounts of distinct Co sulfide and (or) sulfarsenide minerals. Such deposits are not discussed in detail in the following sections, but these deposits may be relevant to the descriptive and genetic models presented below. Examples include the Scadding Au-Co-Cu deposit in Ontario, Canada; the Vähäjoki Co-Cu-Au deposit in Finland; the Tuolugou Co-Au deposit in Qinghai Province, China; the Lala Co-Cu-UREE deposit in Sichuan Province, China; the Guelb Moghrein Cu-Au-Co deposit in Mauritania; and the Great Australia Co-Cu, Greenmount Cu-Au-Co, and Monakoff Cu-Au-Co-UAg deposits in Queensland, Australia. Detailed information on these deposits is presented in appendix 2.

Scientific Investigations Report↗

USGS2012

Sulfur, carbon, hydrogen, and oxygen isotope geochemistry of the Idaho cobalt belt

Cobalt-copper ± gold deposits of the Idaho cobalt belt, including the deposits of the Blackbird district, have been analyzed for their sulfur, carbon, hydrogen, and oxygen isotope compositions to improve the understanding of ore formation. Previous genetic hypotheses have ranged widely, linking the ores to the sedimentary or diagenetic history of the host Mesoproterozoic sedimentary rocks, to Mesoproterozoic or Cretaceous magmatism, or to metamorphic shearing. The δ 34 S values are nearly uniform throughout the Blackbird dis- trict, with a mean value for cobaltite (CoAsS, the main cobalt mineral) of 8.0 ± 0.4‰ ( n = 19). The data suggest that (1) sulfur was derived at least partly from sedimentary sources, (2) redox reactions involving sulfur were probably unimportant for ore deposition, and (3) the sulfur was probably transported to sites of ore for- mation as H 2 S. Hydrogen and oxygen isotope compositions of the ore-forming fluid, which are calculated from analyses of biotite-rich wall rocks and tourmaline, do not uniquely identify the source of the fluid; plausible sources include formation waters, metamorphic waters, and mixtures of magmatic and isotopically heavy meteoric waters. The calculated compositions are a poor match for the modified seawaters that form vol- canogenic massive sulfide (VMS) deposits. Carbon and oxygen isotope compositions of siderite, a mineral that is widespread, although sparse, at Blackbird, suggest formation from mixtures of sedimentary organic carbon and magmatic-metamorphic carbon. The isotopic compositions of calcite in alkaline dike rocks of uncertain age are consistent with a magmatic origin. Several lines of evidence suggest that siderite postdated the emplacement of cobalt and copper, so its significance for the ore-forming event is uncertain. From the stable isotope perspective, the mineral deposits of the Idaho cobalt belt contrast with typical VMS and sedimentary exhalative deposits. They show characteristics of deposit types that form in deeper environments and could be related to metamorphic processes or magmatic processes, although the isotopic evidence for magmatic components is relatively weak.

Idaho↗

USGS2012

Genesis of the Touissit-Bou Beker Mississippi Valley-type district (Morocco-Algeria) and its relation to the Africa-Europe collision

The Mississippi Valley-type deposits of the Touissit-Bou Beker district are hosted by a 25 m thick sequence of diagenetically and hydrothermally dolomitized carbonate platform rocks of Aalenian-Bajocian age. The sulfide mineralization consists principally of galena and sphalerite and occurs as open-space fillings of voids and moderate to massive replacement of the medium- to coarse-grained host dolostone. Five types of dolomite have been distinguished, two of which (D 1 and D 2 ) are of replacement origin, whereas HD 1 , HD 2 , HD 3 occurring as open-space filling are of hydrothermal affiliation. Main ore controls include stratigraphy and lithology, carbonate dissolution, paleogeography, faults or faulted rocks, and availability of organic matter. Fluid inclusion data, along with Na-Cl-Br leachate, indicate that the ore-forming fluids correspond to evolved NaCl-CaCl 2 -KCl-MgCl 2 basin-derived hot (100° ± 20°C) saline brines (>20 wt % NaCl equiv) that acquired their high salinities and Ca/Na ratios through evaporation of seawater, and subsequent dolomitization and fluid-rock interactions. Stable isotope data for replacement and hydrothermal dolomites are tightly clustered and overlapping, with δ 18 O and δ 13 C values from 20.5 to 21.2 and 0.2 to 0.7&permil;, respectively. Similarly, sulfides yield δ 34 S values between 11.2 and 1.9&permil;, whereas those corresponding to the nearby Triassic gypsum cluster yield around 14&permil;. Altogether, these isotopic compositions are consistent with a basinal-type fluid with reduced sulfur very likely being derived through thermochemical reduction of dissolved sulfate, resulting in metal precipitation, and carbon of mainly marine Aalenian-Bajocian carbonate origin with a minor biogenic component. 87 Sr/ 86 Sr values of replacement dolostone are similar to those of ore-related hydrothermal dolomites, ranging from 0.70746 to 0.70833 and from 0.70769 to 0.70828, respectively, and are different from those of the Visean rhyodacite (0.71849–0.72167). Lead isotope ratios ( 206 Pb/ 204 Pb = 18.319–18.390; 207 Pb/ 204 Pb = 15.620–15.680; 208 Pb/ 204 Pb = 38.452–38.650) of sulfides are consistent with Pb being derived from the Visean rhyodacite and associated volcaniclastic rocks. The intimate link between faults and mineralization suggests the strong possibility of brine flow along both ENE-trending regional-scale faults and NW-SW-trending local-scale faults. The data suggest that MVT mineralization was emplaced during the late Neogene-Quaternary (i.e., ca. 15–0 Ma), possibly as a result of subsurface gravity-driven fluid flow in response to the collision between the African and Eurasian plates.

Touissit-bou Beker District↗

USGS2011

Phosphorite-hosted zinc and lead mineralization in the Sekarna deposit (Central Tunisia)

The Sekarna Zn–Pb deposit is located in Central Tunisia at the northeastern edge of the Cenozoic Rohia graben. Mineralization comprises two major ore types: (1) disseminated Zn–Pb sulfides that occur as lenses in sedimentary phosphorite layers and (2) cavity-filling zinc oxides (calamine-type ores) that crosscut Late Cretaceous and Early Eocene limestone. We studied Zn sulfide mineralization in the Saint Pierre ore body, which is hosted in a 5-m-thick sedimentary phosphorite unit of Early Eocene age. The sulfide mineralization occurs as replacements of carbonate cement in phosphorite. The ores comprise stratiform lenses rich in sphalerite with minor galena, Fe sulfides, and earlier diagenetic barite. Laser ablation–inductively coupled plasma mass spectrometry analyses of sphalerite and galena show a wide range of minor element contents with significant enrichment of cadmium in both sphalerite (6,000–20,000 ppm) and galena (12–189 ppm). The minor element enrichments likely reflect the influence of the immediate organic-rich host rocks. Fluid inclusions in sphalerite give homogenization temperatures of 80–130°C. The final ice melting temperatures range from −22°C to −11°C, which correspond to salinities of 15–24 wt.% NaCl eq. and suggest a basinal brine origin for the fluids. Sulfur isotope analyses show uniformly negative values for sphalerite (−11.2‰ to −9.3‰) and galena (−16‰ to −12.3‰). The δ 34 S of barite, which averages 25.1‰, is 4‰ higher than the value for Eocene seawater sulfate. The sulfur isotopic compositions are inferred to reflect sulfur derivation through bacterial reduction of contemporaneous seawater sulfate, possibly in restricted basins where organic matter was abundant. The Pb isotopes suggest an upper crustal lead source.

Mineralium Deposita↗

USGS2011

A 50-year record of NOx and SO2 sources in precipitation in the Northern Rocky Mountains, USA

Ice-core samples from Upper Fremont Glacier (UFG), Wyoming, were used as proxy records for the chemical composition of atmospheric deposition. Results of analysis of the ice-core samples for stable isotopes of nitrogen (δ 15 N, ) and sulfur (δ 34 S, ), as well as and deposition rates from the late-1940s thru the early-1990s, were used to enhance and extend existing National Atmospheric Deposition Program/National Trends Network (NADP/NTN) data in western Wyoming. The most enriched δ 34 S value in the UFG ice-core samples coincided with snow deposited during the 1980 eruption of Mt. St. Helens, Washington. The remaining δ 34 S values were similar to the isotopic composition of coal from southern Wyoming. The δ 15 N values in ice-core samples representing a similar period of snow deposition were negative, ranging from -5.9 to -3.2 ‰ and all fall within the δ 15 N values expected from vehicle emissions. Ice-core nitrate and sulfate deposition data reflect the sharply increasing U.S. emissions data from 1950 to the mid-1970s.

Wyoming↗

USGS2010

Testing the limits of Paleozoic chronostratigraphic correlation via high-resolution (<500 k.y.) integrated conodont, graptolite, and carbon isotope (δ 13 C carb ) biochemostratigraphy across the Llandovery–Wenlock (Silurian) boundary: Is a unified Phanerozoic time scale achievable?

The resolution and fidelity of global chronostratigraphic correlation are direct functions of the time period under consideration. By virtue of deep-ocean cores and astrochronology, the Cenozoic and Mesozoic time scales carry error bars of a few thousand years (k.y.) to a few hundred k.y. In contrast, most of the Paleozoic time scale carries error bars of plus or minus a few million years (m.y.), and chronostratigraphic control better than ??1 m.y. is considered "high resolution." The general lack of Paleozoic abyssal sediments and paucity of orbitally tuned Paleozoic data series combined with the relative incompleteness of the Paleozoic stratigraphic record have proven historically to be such an obstacle to intercontinental chronostratigraphic correlation that resolving the Paleozoic time scale to the level achieved during the Mesozoic and Cenozoic was viewed as impractical, impossible, or both. Here, we utilize integrated graptolite, conodont, and carbonate carbon isotope (??13Ccarb) data from three paleocontinents (Baltica, Avalonia, and Laurentia) to demonstrate chronostratigraphic control for upper Llando very through middle Wenlock (Telychian-Sheinwoodian, ~436-426 Ma) strata with a resolution of a few hundred k.y. The interval surrounding the base of the Wenlock Series can now be correlated globally with precision approaching 100 k.y., but some intervals (e.g., uppermost Telychian and upper Shein-woodian) are either yet to be studied in sufficient detail or do not show sufficient biologic speciation and/or extinction or carbon isotopic features to delineate such small time slices. Although producing such resolution during the Paleozoic presents an array of challenges unique to the era, we have begun to demonstrate that erecting a Paleozoic time scale comparable to that of younger eras is achievable. ?? 2010 Geological Society of America.

Geological Society of America Bulletin↗

USGS2010

Evolution of Ore Deposits and Technology Transfer Project: Isotope and Chemical Methods in Support of the U.S. Geological Survey Science Strategy, 2003-2008

Principal functions of the U.S. Geological Survey (USGS) Mineral Resources Program are providing assessments of the location, quantity, and quality of undiscovered mineral deposits, and predicting the environmental impacts of exploration and mine development. The mineral and environmental assessments of domestic deposits are used by planners and decisionmakers to improve the stewardship of public lands and public resources. Assessments of undiscovered mineral deposits on a global scale reveal the potential availability of minerals to the United States and other countries that manufacture goods imported to the United States. These resources are of fundamental relevance to national and international economic and security policy in our globalized world economy. Performing mineral and environmental assessments requires that predictions be made of the likelihood of undiscovered deposits. The predictions are based on geologic and geoenvironmental models that are constructed for the diverse types of mineral deposits from detailed descriptions of actual deposits and detailed understanding of the processes that formed them. Over the past three decades the understanding of ore-forming processes has benefited greatly from the integration of laboratory-based geochemical tools with field observations and other data sources. Under the aegis of the Evolution of Ore Deposits and Technology Transfer Project (referred to hereinafter as the Project), a 5-year effort that terminated in 2008, the Mineral Resources Program provided state-of-the-art analytical capabilities to support applications of several related geochemical tools to ore-deposit-related studies. The analytical capabilities and scientific approaches developed within the Project have wide applicability within Earth-system science. For this reason the Project Laboratories represent a valuable catalyst for interdisciplinary collaborations of the type that should be formed in the coming years for the United States to meet its natural-resources and natural-science needs. This circular presents an overview of the Project. Descriptions of the Project laboratories are given first including descriptions of the types of chemical or isotopic analyses that are made and the utility of the measurements. This is followed by summaries of select measurements that were carried out by the Project scientists. The studies are grouped by science direction. Virtually all of them were collaborations with USGS colleagues or with scientists from other governmental agencies, academia, or the private sector.

Circular↗

USGS2010

Geology, geochemistry, and genesis of the Greens Creek massive sulfide deposit, Admiralty Island, southeastern Alaska

In 1996, a memorandum of understanding was signed by representatives of the U.S. Geological Survey and Kennecott Greens Creek Mining Company to initiate a cooperative applied research project focused on the Greens Creek massive sulfide deposit in southeastern Alaska. The goals of the project were consistent with the mandate of the U.S. Geological Survey Mineral Resources Program to maintain a leading role in national mineral deposits research and with the need of Kennecott Greens Creek Mining Company to further development of the Greens Creek deposit and similar deposits in Alaska and elsewhere. The memorandum enumerated four main research priorities: (1) characterization of protoliths for the wall rocks, and elucidation of their alteration histories, (2) determination of the ore mineralogy and paragenesis, including metal residences and metal zonation within the deposit, (3) determination of the ages of events important to ore formation using both geochronology and paleontology, and (4) development of computer models that would allow the deposit and its host rocks to be examined in detail in three dimensions. The work was carried out by numerous scientists of diverse expertise over a period of several years. The written results, which are contained in this Professional Paper, are presented by 21 authors: 13 from the U.S. Geological Survey, 4 from Kennecott Greens Creek Mining Company, 2 from academia, and 2 from consultants. The Greens Creek deposit (global resource of 24.2 million tons at an average grade of 13.9 percent zinc, 5.1 percent lead, 0.15 troy ounce per ton gold, and 19.2 troy ounces per ton silver at zero cutoff) formed in latest Triassic time during a brief period of rifting of the Alexander terrane. The deposit exhibits a range of syngenetic, diagenetic, and epigenetic features that are typical of volcanogenic (VMS), sedimentary exhalative (SEDEX), and Mississippi Valley-type (MVT) genetic models. In the earliest stages of rifting, formation of precious-metal-rich silica-barite-carbonate white ores began at low temperature in a shallow, subaqueous setting, probably a thin carbonate shelf on the flanks of the Alexander landmass. Epigenetic carbonate replacement textures in the footwall dolostones are overlain by stratiform silica-carbonate-barite-rich ores and indicate that early mineralization formed at and just beneath the paleo sea floor by mixing of a reduced, precious-metal-rich, base-metal-poor hydrothermal fluid with oxygenated seawater. As rifting intensified, the shelf was downfaulted and isolated as a graben. Isolation of the basin and onset of starved-basin shale sedimentation was concurrent with emplacement of mafic-ultramafic intrusives at shallow levels in the rift, resulting in an increasingly higher temperature and progressively more anoxic ore-forming environment. The formation of the main stage of massive sulfide ores began as the supply of bacterially reduced sulfur increased in the accumulating shales. As the main-stage mineralization intensified, shale sedimentation inundated the hydrothermal system, eventually forming a cap. Biogenic sulfate reduction supplied reduced sulfur to the base of the shales where mixing occurred with hot, base-metal-rich hydrothermal fluids. Ore deposition continued by destruction and epigenetic replacement of the early white ores in proximal areas and by inflation and diagenetic replacement of unlithified shale at the interface between the white ores and the base of the shale cap. Ore deposition waned as the shales became lithified and as the supply of bacterially reduced sulfur to the site of ore deposition ceased. The final stages of rifting resulted in the emplacement of mafic-ultramafic intrusive rocks into the Greens Creek system and extrusion of voluminous basaltic flows at the top of the Triassic section. Greenschist facies metamorphism during the Jurassic-Cretaceous accretion of the Alexander terrane to the continental margin resulted in recrystalli

Professional Paper↗

USGS2009

The δ 15 N and δ 18 O values of N 2 O produced during the co-oxidation of ammonia by methanotrophic bacteria

In order to determine if the δ 15 N and δ 18 O values of N 2 O produced during co-oxidation of NH 4 + by methanotrophic (methane oxidizing) bacteria can be isotopically distinguished from N 2 O produced either by autotrophic nitrifying or denitrifying bacteria, we conducted laboratory incubation experiments with pure cultures of methanotrophic bacteria that were provided NH 4 Cl as an oxidation substrate. The N 2 O produced during NH 4 + oxidation by methanotrophic bacteria showed nitrogen isotope fractionation between NH 4 + and N 2 O ( ε N 2 O–NH 4 + ) of − 48 and − 55&permil; for Methylomonas methanica and Methylosinus trichosporium , OB3b respectively. These large fractionations are similar to those previously measured for autotrophic nitrifying bacteria and consistent with N 2 O formation by multiple rate limiting steps that include NH 4 + oxidation by the methane monooxygenase enzyme and reduction of NO 2 − to N 2 O. Consequently, N 2 O formed by NH 4 + oxidation via methanotrophic or autotrophic nitrifying bacteria might generally be characterized by lower δ 15 N N 2 O values than that formed by denitrificaiton, although this also depends on the variability of δ 15 N of available nitrogen sources (e.g., NH 4 + , NO 3 − , NO 2 − ). Additional incubations with M. trichosporium OB3b at high and low CH 4 conditions in waters of different δ 18 O values revealed that 19–27% of the oxygen in N 2 O was derived from O 2 with the remainder from water. The biochemical mechanisms that could explain this amount of O 2 incorporation are discussed. The δ 18 O of N 2 O formed under high CH 4 conditions was ~ + 15&permil; more positive than that formed under lower CH 4 conditions. This enrichment resulted in part from the incorporation of O 2 into N 2 O that was enriched in 18 O due to an isotope fractionation effect of − 16.1 ± 2.0&permil; and − 17.5 ± 5.4&permil; associated with O 2 consumption during the high and low methane concentration incubations, respectively. Therefore, N 2 O formed by NH 4 + oxidation via methanotrophic or autotrophic nitrifying bacteria can have very positive δ 18 O N 2 O values if the O 2 incorporated is previously enriched in 18 O from high rates of respiration. Nitrous oxide was collected from various depths in soils overlying a coal-bed methane seep where methanotrophic bacteria are naturally enriched. In one sampling when soil methane concentrations were very high, the δ 18 O VSMOW values of the N 2 O were highly enriched (+ 50&permil;), consistent with our laboratory experiments. Thus, soils overlying methane seeps could provide an 18 O-enriched source of atmospheric N 2 O.

Chemical Geology↗