Idaho Cobalt Belt Research Articles

Copper Isotopes and Constraints on the Ore Genesis Process of Cu-Co Ore Deposits at the Idaho Cobalt Belt, USA

Quantifying and identifying the introduction of metal in ore deposits that have experienced multiple overprinting hydrothermal events remains an elusive yet essential goal in metallogenic studies. Here, we constrain the origin of Co in the Idaho Cobalt Belt (ICB) that experienced two distinct metal-rich events that introduced Co and Cu. We performed a detailed petrographic study of sulfide ore at Iron Creek in the ICB, in concert with the quantification of trace metal element concentrations and copper isotope values to identify the introduction of Co in the system. The pyrite displays various degrees of alteration, with the highest Co concentrations (up to 6 wt.%) in less-altered pyrite grains (e.g., sharp edges, absence of altered boundaries and fissures) and highest δ65Cu isotope value. The most-altered pyrite grains (e.g., corroded grains, round and altered boundaries) have lower Co contents and lower δ65Cu isotope values that match the copper isotope values of the chalcopyrite. The least-altered pyrite shows a narrow δ65Cu range between −0.39‰ to −0.58‰. In contrast, the most-altered pyrite grains are isotopically depleted, showing a δ65Cu range from −1.35‰ to −0.90‰. Chalcopyrite shows a δ65Cu range between −1.07‰ and −0.77‰. We interpret, from the Cu isotope compositions and Co concentrations in pyrite, that the Co was originally introduced into the siliciclastic host rock package in a Mesoproterozoic SEDEX environment. The heavier Cu was then preferentially leached in a second event, resulting in isotopically lighter Cu in the altered pyrite. Remobilization of the SEDEX cobalt was likely associated with CO2-rich metamorphic fluids present in the region during the Mesoproterozoic East Kootenay orogeny, the late Mesoproterozoic Grenville orogeny, and the Late Jurassic to Late Cretaceous Cordilleran orogeny.

Cobalt mineralogy at the Iron Creek deposit, Idaho cobalt belt, USA: Implications for domestic critical mineral production

Abstract Current U.S. policies aim to establish domestic supply chains of critical minerals for the energy transition. The Iron Creek deposit in the Idaho cobalt belt (ICB) is one of the most promising cobalt (Co) targets. Our case study illustrates the importance of mineralogy in strategic evaluations of critical mineral potential. Most of the Co at Iron Creek occurs as Fe substitution in pyrite, with lattice-bound and inclusion-hosted Ag, As, Bi, Ni, Pb, Se, Te ± trace Au and Sb. Cobalt also occurs in minor cattierite-vaesite. The Co minerals are intergrown with Co-poor chalcopyrite hosting Cu ± minor In and Zn. Worldwide, most Co is recovered from deposits mineralogically distinct from the ICB, and the United States currently lacks infrastructure to recover this Co and its associated metals. ICB ore minerals could be processed by autoclave, roaster, smelter, bioleach, or heap leach. Recovery of the Ag, As, Au, Bi, In, Pb, Se, Te, and Zn would be costly by autoclave, and construction of a custom smelter for ICB ores is likely uneconomic, so these elements would become waste irrespective of criticality. The Co-Fe and Co-As sulfide minerals are most suitable for Co and Ni recovery by a hydrometallurgical autoclave process, with potential pretreatment of cobaltiferous pyrite/arsenopyrite in an inert-atmosphere roaster, in new domestic or anticipated international facilities. The ICB is the second largest known Co resource in the United States. Consideration of ore mineralogy in the ICB is essential in strategies for domestic production.

Re-Os systematics and geochemistry of cobaltite (CoAsS) in the Idaho cobalt belt, Belt-Purcell Basin, USA: Evidence for middle Mesoproterozoic sediment-hosted Co-Cu sulfide mineralization with Grenvillian and Cretaceous remobilization

We report the first study of the Re-Os systematics of cobaltite (CoAsS) using disseminated grains and massive sulfides from samples of two breccia-type and two stratabound deposits in the Co-Cu-Au Idaho cobalt belt (ICB), Lemhi subbasin to the Belt-Purcell Basin, Idaho, USA. Using a 185Re+190Os spike solution, magnetic and non-magnetic fractions of cobaltite mineral separates give reproducible Re-Os analytical data for aliquot sizes of 150 to 200mg. Cobaltite from the ICB has highly radiogenic 187Os/188Os ratios (17–45) and high 187Re/188Os ratios (600–1800) but low Re and total Os contents (ca. 0.4–4ppb and 14–64 ppt, respectively). Containing 30 to 74% radiogenic 187Os, cobaltite from the ICB is amenable to Re-Os age determination using the isochron regression approach.Re-Os data for disseminated cobaltite mineralization in a quartz-tourmaline breccia from the Haynes-Stellite deposit yield a Model 1 isochron age of 1349±76Ma (2σ, n=4, mean squared weighted deviation MSWD=2.1, initial 187Os/188Os ratio=4.7±2.2). This middle Mesoproterozoic age is preserved despite a possible metamorphic overprint or a pulse of metamorphic-hydrothermal remobilization of pre-existing cobaltite that formed along fold cleavages during the ca. 1190–1006Ma Grenvillian orogeny. This phase of remobilization is tentatively identified by a Model 3 isochron age of 1132±240Ma (2σ, n=7, MSWD=9.3, initial 187Os/188Os ratio of 9.0±2.9) for cobaltite in the quartz-tourmaline breccia from the Idaho zone in the Blackbird mine.All Mesoproterozoic cobaltite mineralization in the district was affected by greenschist- to lower amphibolite-facies (garnet zone) metamorphism during the Late Jurassic to Late Cretaceous Cordilleran orogeny. However, the fine- to coarse-grained massive cobaltite mineralization from the shear zone-hosted Chicago zone, Blackbird mine, is the only studied deposit that has severely disturbed Re-Os systematics with evidence for a linear trend of mixing with (metamorphic?) fluids.The new Re-Os ages and extremely high initial 187Os/188Os ratios of cobaltite reported here favor a magmatic-hydrothermal genetic model for a multi-stage REE-Y-Co-Cu-Au mineralization occurring at ca. 1370 to 1349Ma, and related to the emplacement of the Big Deer Creek granite pluton at ca. 1377Ma. In our model, deposition of paragenetically early xenotime and gadolinite was followed by an influx of Mesoproterozoic evaporitic brines and magmatic-hydrothermal fluids containing metals and reduced sulfur derived from mafic and oceanic island-arc Archean to Paleoproterozoic rocks in the Laurentian basement. Cobaltite mineralization occurred upon cooling of these fluids at an inferred temperature of 300°C or below.

ORE GENESIS CONSTRAINTS ON THE IDAHO COBALT BELT FROM FLUID INCLUSION GAS, NOBLE GAS ISOTOPE, AND ION RATIO ANALYSES—A DISCUSSION

Sir: In their paper, “Ore genesis constraints on the Idaho cobalt belt from fluid inclusion gas, noble gas isotope, and ion ratio analyses,” Landis and Hofstra (2012) report on the analysis of hydrogen gas within fluid inclusions using mass spectrometry. They indicate surprisingly high hydrogen contents in fluid inclusions (~25,000 ppm) in this and their other, previous work that they reference. But it is highly unlikely that hydrogen is really that common in geofluids and even less likely that it could remain trapped in fluid inclusions for geological time periods because hydrogen diffuses easily through most materials, including quartz. The laser Raman method for analysis of gas contents of fluid inclusions can also measure hydrogen contents, but such analyses never find the presence of hydrogen within inclusions. The only detection of hydrogen in fluid inclusions by laser Raman that I am aware of was by Dubessy et al. (1988) and the hydrogen was present due to radiolytic dissociation of water within inclusions in a uranium deposit. These contradictions in the measurement of hydrogen in fluid inclusions are most likely a problem within the method of mass spectrometric analysis for hydrogen in aqueous fluids, as occur in fluid …

Ore Genesis Constraints on the Idaho Cobalt Belt from Fluid Inclusion Gas, Noble Gas Isotope, and Ion Ratio Analyses

The Idaho cobalt belt is a 60-km-long alignment of deposits composed of cobaltite, Co pyrite, chalcopyrite, and gold with anomalous Nb, Y, Be, and rare-earth elements (REEs) in a quartz-biotite-tourmaline gangue hosted in Mesoproterozoic metasedimentary rocks of the Lemhi Group. It is the largest cobalt resource in the United States with historic production from the Blackbird Mine. All of the deposits were deformed and metamorphosed to upper greenschist-lower amphibolite grade in the Cretaceous. They occur near a 1377 Ma anorogenic bimodal plutonic complex. The enhanced solubility of Fe, Co, Cu, and Au as chloride complexes together with gangue biotite rich in Fe and Cl and gangue quartz containing hypersaline inclusions allows that hot saline fluids were involved. The isotopes of B in gangue tourmaline are suggestive of a marine source, whereas those of Pb in ore suggest a U ± Th-enriched source. The ore and gangue minerals in this belt may have trapped components in fluid inclusions that are distinct from those in post-ore minerals and metamorphic minerals. Such components can potentially be identified and distinguished by their relative abundances in contrasting samples. Therefore, we obtained samples of Co and Cu sulfides, gangue quartz, biotite, and tourmaline and post-ore quartz veins as well as Cretaceous metamorphic garnet and determined the gas, noble gas isotope, and ion ratios of fluid inclusion extracts by mass spectrometry and ion chromatography. The most abundant gases present in extracts from each sample type are biased toward the gas-rich population of inclusions trapped during maximum burial and metamorphism. All have CO 2 /CH 4 and N 2 /Ar ratios of evolved crustal fluids, and many yield a range of H 2 -CH 4 -CO 2 -H 2 S equilibration temperatures consistent with the metamorphic grade. Cretaceous garnet and post-ore minerals have high R H and R S values suggestive of reduced sulfidic conditions. Most extracts have anomalous 4 He produced by decay of U and Th and 38 Ar produced by nucleogenic production from 41 K. In contrast, some ore and gangue minerals yield significant SO 2 and have low R H and R S values of a more oxidized fluid. Three extracts from gangue quartz have high helium R/R A values indicative of a mantle source and neon isotope compositions that require nucleogenic production of 22 Ne in fluorite from U ± Th decay. Two extracts from gangue quartz have estimated 40 K/ 40 Ar that permit a Precambrian age. Extracts from gangue quartz in three different ore zones are biased toward the hypersaline population of inclusions and have a tight range of ion ratios (Na, K, NH 4 , Cl, Br, F) suggestive of a single fluid. Their Na, Cl, Br ratios suggest this fluid was a mixture of magmatic and basinal brine. Na-K-Ca temperatures (279°–347°C) are similar to homogenization temperatures of hypersaline inclusions. The high K/Na of the brine may be due to albitization of K silicate minerals in country rocks. Influx of K-rich brines is consistent with the K metasomatism necessary to form gangue biotite with high Cl. An extract from a post-ore quartz vein is distinct and has Na, Cl, Br ratios that resemble metamorphic fluids in Cretaceous silver veins of the Coeur d’Alene district in the Belt Basin. The results show that in some samples, for certain components, it is possible to “see through” the Cretaceous metamorphic overprint. Of great import for genetic models, the volatiles trapped in gangue quartz have 3 He derived from a mantle source and 22 Ne derived from fluorite, both of which may be attributed to nearby ~1377 Ma basalt-rhyolite magmatism. The brine trapped in gangue quartz is a mixture of magmatic fluid and evaporated seawater. The former requires a granitic intrusion that is present in the bimodal intrusive complex, and the latter equatorial paleolatitudes that existed in the Mesoproterozoic. The results permit genetic models involving heat and fluids from the neighboring bimodal plutonic complex and convection of basinal brine in the Lemhi Group. While the inferred fluid sources in the Idaho cobalt belt are similar in many respects to those in iron oxide copper-gold deposits, the fluids were more reduced such that iron was fixed in biotite and tourmaline instead of iron oxides.

Reconnaissance Lead Isotope Characteristics of the Blackbird Deposit: Implications for the Age and Origin of Cobalt-Copper Mineralization in the Idaho Cobalt Belt, United States

Strata-bound cobalt-copper deposits occur in the metasedimentary rocks of the Apple Creek Formation (Mesoproterozoic) in a linear belt called the Idaho cobalt belt, near Salmon, Idaho. We report the first set of lead isotope measurements on silicate rocks and sulfide ores and new sulfur isotope data on sulfide ores. The Pb isotope compositions of cobalt-copper (Co-Cu) ores (206Pb/204Pb = 30.8–40.4; 207Pb/204Pb = 16.8–17.6; 208Pb/204Pb = 49.7–63.9) are very radiogenic, much more radiogenic than any known sedimentary exhalative (sedex) deposits. The data plot well beyond the crustal growth model curves, a feature shared by the Mississippi Valley-type (MVT) deposits which is characteristic of an upper crustal source. It is unlikely that the ore lead could have come from any mafic igneous source of mantle origin (Proterozoic Moyie sill and its equivalent, or the Tertiary Challis Volcanic Group). The Cretaceous felsic igneous rocks of the region (the Idaho batholith and related rocks) also have much lower Pb isotope ratios than the Co-Cu ores. The Pb isotope ratios of Proterozoic crystalline rocks partially overlap Pb isotope ratios of the Apple Creek Formation but would have been much less radiogenic than the ores at an assumed mineralization age of about 1400 Ma. Only the host Apple Creek Formation is known to have appropriate Pb isotope compositions (206Pb/204Pb = 26.8–86.7; 207Pb/204Pb = 16.3–21.1; 208Pb/204Pb = 47.9–64.8) to be the source of Co-Cu ores. Leaching of metals from the host sedimentary sequence by hydrothermal fluids and subsequent deposition of ores at chemically and structurally favorable sites could have resulted in the formation of Co-Cu deposits in the Idaho cobalt belt.

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 district, 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 formation 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 volcanogenic 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.

Strata-Bound Fe-Co-Cu-Au-Bi-Y-REE Deposits of the Idaho Cobalt Belt: Multistage Hydrothermal Mineralization in a Magmatic-Related Iron Oxide Copper-Gold System

Mineralogical and geochemical studies of strata-bound Fe-Co-Cu-Au-Bi-Y-rare-earth element (REE) deposits of the Idaho cobalt belt in east-central Idaho provide evidence of multistage epigenetic mineralization by magmatic-hydrothermal processes in an iron oxide copper-gold (IOCG) system. Deposits of the Idaho cobalt belt comprise three types: (1) strata-bound sulfide lenses in the Blackbird district, which are cobaltite and, less commonly, chalcopyrite rich with locally abundant gold, native bismuth, bismuthinite, xenotime, allanite, monazite, and the Be-rich silicate gadolinite-(Y), with sparse uraninite, stannite, and Bi tellurides, in a gangue of quartz, chlorite, biotite, muscovite, garnet, tourmaline, chloritoid, and/or siderite, with locally abundant fluorapatite or magnetite; (2) discordant tourmalinized breccias in the Blackbird district that in places have concentrations of cobaltite, chalcopyrite, gold, and xenotime; and (3) strata-bound magnetite-rich lenses in the Iron Creek area, which contain cobaltiferous pyrite and locally sparse chalcopyrite or xenotime. Most sulfide-rich deposits in the Blackbird district are enclosed by strata-bound lenses composed mainly of Cl-rich Fe biotite; some deposits have quartz-rich envelopes. Whole-rock analyses of 48 Co- and/or Cu-rich samples show high concentrations of Au (up to 26.8 ppm), Bi (up to 9.16 wt %), Y (up to 0.83 wt %), ∑REEs (up to 2.56 wt %), Ni (up to 6,780 ppm), and Be (up to 1,180 ppm), with locally elevated U (up to 124 ppm) and Sn (up to 133 ppm); Zn and Pb contents are uniformly low (≤821 and ≤61 ppm, respectively). Varimax factor analysis of bulk compositions of these samples reveals geochemically distinct element groupings that reflect statistical associations of monazite, allanite, and xenotime; biotite and gold; detrital minerals; chalcopyrite and sparse stannite; quartz; and cobaltite with sparse selenides and tellurides. Significantly, Cu is statistically separate from Co and As, consistent with the general lack of abundant chalcopyrite in cobaltite-rich samples. Paragenetic relations determined by scanning electron microscopy indicate that the earliest Y-REE-Be mineralization preceded deposition of Co, Cu, Au, and Bi. Allanite, xenotime, and gadolinite-(Y) commonly occur as intergrowths with and inclusions in cobaltite; the opposite texture is rare. Monazite, in contrast, forms a poikiloblastic matrix to cobaltite and thin rims on allanite and xenotime, reflecting a later metamorphic paragenesis. Allanite and xenotime also show evidence of late dissolution and reprecipitation, forming discordant rims on older anhedral allanite and xenotime and separate euhedral crystals of each mineral. Textural data suggest extensive deformation of the deposits by folding and shearing, and by pervasive recrystallization, all during Cretaceous metamorphism. Sensitive high resolution ion microprobe U-Pb geochronology by Aleinikoff et al. (2012) supports these paragenetic interpretations, documenting contemporaneous Mesoproterozoic growth of early xenotime and crystallization of megacrystic A-type granite on the northern border of the district. These ages are used together with mineralogical and geochemical data from the present study to support an epigenetic, IOCG model for Fe-Co-Cu-Au-Bi-Y-REE deposits of the Idaho cobalt belt. A sulfide facies variant of IOCG deposits is proposed for the Blackbird district, in which reducing hydrothermal conditions favored deposition of sulfide minerals over iron oxides. This new model includes Mesoproterozoic vein mineralization and related Fe-Cl metasomatism that formed the biotite-rich lenses, a predominantly felsic magmatic source for metals in the deposits, given their local abundance of Y, REEs, and Be, and a major sedimentary component in the hydrothermal fluids based on independent sulfur isotope and boron isotope data for sulfides and ore-related tourmaline, respectively.

Distribution of Cu, Co, As, and Fe in mine waste, sediment, soil, and water in and around mineral deposits and mines of the Idaho Cobalt Belt, USA

► Stream water contains elevated Cu, Co, As, and Fe compared to backgrounds. ► Sediment and water guidelines were exceeded in some mine runoff samples. ► Mine waste and stream sediment exceeded the probable effect concentration for Cu. ► Stream water exceeded an LC50 for Co in rainbow trout by more than two orders of magnitude. ► Mine waste and stream sediment exceeded the probable effect concentration for As. The distribution of Cu, Co, As and Fe was studied downstream from mines and deposits in the Idaho Cobalt Belt (ICB), the largest Co resource in the USA. To evaluate potential contamination in ecosystems in the ICB, mine waste, stream sediment, soil, and water were collected and analyzed for Cu, Co, As and Fe in this area. Concentrations of Cu in mine waste and stream sediment collected proximal to mines in the ICB ranged from 390 to 19,000 μg/g, exceeding the USEPA target clean-up level and the probable effect concentration (PEC) for Cu of 149 μg/g in sediment; PEC is the concentration above which harmful effects are likely in sediment dwelling organisms. In addition concentrations of Cu in mine runoff and stream water collected proximal to mines were highly elevated in the ICB and exceeded the USEPA chronic criterion for aquatic organisms of 6.3 μg/L (at a water hardness of 50 mg/L) and an LC50 concentration for rainbow trout of 14 μg/L for Cu in water. Concentrations of Co in mine waste and stream sediment collected proximal to mines varied from 14 to 7400 μg/g and were highly elevated above regional background concentrations, and generally exceeded the USEPA target clean-up level of 80 μg/g for Co in sediment. Concentrations of Co in water were as high as in 75,000 μg/L in the ICB, exceeding an LC50 of 346 μg/L for rainbow trout for Co in water by as much as two orders of magnitude, likely indicating an adverse effect on trout. Mine waste and stream sediment collected in the ICB also contained highly elevated As concentrations that varied from 26 to 17,000 μg/g, most of which exceeded the PEC of 33 μg/g and the USEPA target clean-up level of 35 μg/g for As in sediment. Conversely, most water samples had As concentrations that were below the 150 μg/L chronic criterion for protection of aquatic organisms and the USEPA target clean-up level of 14 μg/L. There is abundant Fe oxide in streams in the ICB and several samples of mine runoff and stream water exceeded the chronic criterion for protection of aquatic organisms of 1000 μg/L for Fe. There has been extensive remediation of mined areas in the ICB, but because some mine waste remaining in the area contains highly elevated Cu, Co, As and Fe, inhalation or ingestion of mine waste particulates may lead to human exposure to these elements.

New production in the Idaho Cobalt Belt: a unique metallogenic province

The Idaho Cobalt Belt (ICB) is a unique example of sea-floor hydrothermal brines that vented along an ancient rift within a deep-water setting, providing for an interesting and distinctive geochemistry. The ICB is a northwest-trending zone of Co-Cu-Au occurrences, at least 64 km long and up to 10 km wide, centred on the Blackbird mine. It is an exceptional metallogenic province in which cobalt occurs in sufficiently high concentrations to make it the primary metal in the deposits. The deposits are mostly strata-bound and appear to have formed during a mafic volcanogenic-exhalative mineralizing event on a palaeo-sea-floor ca 1600 m.y. ago. Regional metamorphic events have remobilized Cu and, to a lesser extent, Co in varying degrees throughout the belt. The deposits are unusually rich in Co, Cu, Fe, As, Au, B, Bi and light rare earths, but low in Ni (relative to Co), Ag, Pb, and Zn. The synsedimentary controls, mineralogy and geochemistry are unusual among ore deposits, though similar features in some other deposits are known. Sediment-hosted deposits are major ore producers in the world, but most tend to be Pb- and Zn-rich. Some sediment-hosted deposits that contain Cu also contain Co—notably, the deposits of the Zambian Copperbelt. Significant differences exist between deposits in the Zambian Copperbelt and the ICB, whereas many intriguing similarities are found between ICB deposits and Sullivan, Canada, and Mount Isa and Broken Hill, Australia.The ICB is currently known to host 18 Co-Cu deposits within eight sequences of metamorphosed, mafic volcaniclastic rocks. Additional exploration is expected to discover more deposits as only a small portion of the belt has undergone systematic exploration. Formation Capital's Idaho cobalt project encircles the inactive Blackbird mine and encompasses the Ram and Sunshine deposits as well as more than 30 prospects and six deposits. The project's total resource base exceeds 5 000 000 t in four deposits, all of which remain open in at least two directions. To date, only two of the deposits have been included in pre-feasibility studies—Ram and Sunshine. The project is in the final feasibility, permitting and pre-development phase.

Iron and chlorine as guides to stratiform Cu-Co-Au deposits, Idaho Cobalt Belt, USA

The Cu-Co-Au deposits of the Idaho Cobalt Belt are in lithostratigraphic zones of the Middle Proterozoic Yellowjacket Formation characterized by distinctive chemical and mineralogical compositions including high concentrations of Fe (15- > 30 wt. percent Fe2O3), Cl (0.1–1.10 wt. percent), and magnetite or biotite (> 50 vol. percent). The Cu-Co-Au deposits of the Blackbird mine are stratabound in Fe-silicate facies rocks that are rich in biotite, Fe, and Cl, but stratigraphically equivalent rocks farther than 10 km from ore deposits have similar compositions. A lower lithostratigraphic zone containing magnetite and small Cu-Co-Au deposits extends for more than 40 km. The Fe-rich strata are probably exhalative units related to mafic volcanism and submarine hot springs, but the origin of the high Cl concentrations is less clear. Former chlorine-rich pore fluids are suggested by the presence of supersaline fluid inclusions, by Cl-rich biotite and scapolite (as much as 1.87 percent Cl in Fe-rich biotite), and by high Cl concentrations in rock samples. Chlorine is enriched in specific strata and in zones characterized by soft-sediment deformation, thus probably was introduced during sedimentation or diagenesis. Unlike some metasedimentary rocks containing scapolite and high Cl, the Yellowjacket Formation lacks evidence for evaporitic strata that could have been a source of Cl. More likely, the Cl reflects a submarine brine that carried Fe, K, and base metals. Strata containing anomalous Fe-K-Cl are considered to be a guide to sub-basins favorable for the occurrence of stratiform base-metal deposits.

The Idaho cobalt belt, northwestern United States ? A metamorphosed Proterozoic exhalative ore district

In the Idaho cobalt belt, originally exhalative, stratiform mineralization within the Proterozoic Yellow-jacket Formation has become increasingly coarse-grained and remobilized toward the northwest in the direction of increasing regional metamorphic grade. The Idaho cobalt belt is located about 40 km west of Salmon, Idaho in the northwestern United States. The most important deposit in the district is the Blackbird mine which produced copper-cobalt ore sporadically from the early 1900's until about 1960. The Iron Creek deposit at the southeast end of the belt has undergone greenschist fades, biotite zone metamorphism; zones of disseminated, veinlet and massive sulfides lie more or less parallel to bedding of quartzites and phyllites. The main ore minerals are chalcopyrite and cobaltiferous pyrite. Toward the northwest at the Blackpine mine, remobilization has concentrated most of the mineralization into relatively thin concordant and discordant veins containing chalcopyrite, pyrite and arsenopyrite. The cobalt is reported to occur within arsenopyrite. Further northwest at the Blackbird mine where the Yellowjacket formation has been metamorphosed to the lower amphibolite facies, zones of disseminated and coarse-grained vein ores lie approximately along the same stratigraphic zone. Chalcopyrite, cobaltite, arsenopyrite, pyrite and pyrrhotite are the dominant ore minerals. Up to 0.22 oz. Au/ton was present in some of the ore. In addition, tourmaline-bearing sedimentary rocks (tourmalinites) are associated with some of the Blackbird ores. The Salmon Canyon deposit at the northwest end of the belt has undergone upper amphibolite facies, sillimanite zone metamorphism. In these garnet-sillimanite gneisses, chalcopyrite is found as coarse blebs and cobaltite as large porphyroblastic crystals. Gold occurs in amounts up to 0.02 oz. Au/ton. Elsewhere in the world the two most similar districts are the cobalt-bearing portion of the Zambian-Zairian Copperbelt of central Africa where Proterozoic Roan sedimentary rocks contain stratiform copper-cobalt ore-bodies over a distance of more than 500 kilometers, and the Sheep Creek district of Meagher County, Montana, which contains strata-bound copper-cobalt mineralization. The Idaho cobalt belt is a strata-bound copper-cobalt district hosted by the Proterozoic Yellowjacket Formation and located in east-central Idaho within Lemhi County, approximately 40 kilometers west of Salmon, Idaho, northwestern United States (Fig. 1). Of the four main deposits described here (from southeast to northwest, the Iron Creek, Blackpine, Blackbird, and the Salmon Canyon deposits), the Blackbird mine is the most important in the district. It was discovered in 1893 and sporadically produced copper and cobalt until about 1960. The Yellowjacket Formation has undergone an increasing degree of metamorphism toward the northwest. The deposits are largely strata-bound in a belt over 50 km in length, strongly suggesting a syngenetic mode of origin. However, the proximity of the district to satellitic granitic plutons of the Idaho batholith has prompted many investigators to suggest an epigenetic hydrothermal origin (Anderson 1947 and Purdue 1975). Remobilization of some of the mineralization into veins at the Blackbird mine, where most of the previous work has been concentrated, has also suggested an epigenetic origin. A more district-wide view of the mineralization points to a strong degree of stratigraphic control.

Copper and cobalt in aquatic mosses and stream sediments from the Idaho Cobalt Belt

Abstract Samples of stream sediments and aquatic mosses were collected from nine sites across several mineralized zones at the southeasternmost extension of the Idaho Cobalt Belt. Because the steepness of the terrain and the attendant high flow rate of the streams made it difficult to obtain adequate sediment samples, mosses were considered as an alternative sampling medium. The results not only showed that the Cu and Co content of the mosses correlated almost perfectly with that of the sediments, but that the contrast between samples taken from mineralized and background areas was greater in mosses, especially for Co. Maximum concentrations of 35,000 μg/g Cu and 2000 μg/g Co were observed in the ash of mosses, compared to maximum concentrations of 1700 μg/g and 320 μg/g, respectively, in the associated sediments. Species identification was considered unimportant, which should dispel some reluctance to use mosses in mineral exploration.