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The Idaho cobalt belt
Abstract
The Idaho cobalt belt (ICB) is a northwest trending belt of cobalt (Co) ± copper (Cu)-bearing mineral deposits and prospects in the eastern Salmon River Mountains of east-central Idaho, U.S.A. (Fig. l).The Blackbird Co-Cu deposit is near the center of the ICB, where it is over 10 km wide. From the Blackbird area, the ICB extends
at least 25 kin to the southeast and northwest.
Ore zones and prospects of the ICB are hosted in Mesoproterozoic metasedimentary strata of the Lemhi sub-basin of the Belt-Purcell intracratonic basin. Most are within or near the banded siltite unit of the
Apple Creek Formation, which consists of about 2 km of interlayered gray siltite and argillite within a 14-km-thick section of predominantly gray fine-grained siliciclastic strata (Evans and Green,. 2003). Metamorphic
grades increase northwestward along the ICB, from greenschist facies at its southeast end, to .amphibolite facies ± garnet ± chloritoid in its central part, to upper amphibolite-facies± sillimanite at its at its northwest
end.
... These mineralized zones and their host strata generally strike northwest and dip about 60°northeast. Chalcopyrite, pyrite, and arsenopyrite are the main ore minerals, but quartz, siderite, cobaltian arsenopyrite, cobaltite, and magnetite are also present in some veins and mineralized breccias (Nold, 1990;Bookstrom, 2013). ...
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), Lehmi Sub-basin 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 200 mg. 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. 0.4‒4 ppb 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.
Induced Polarization (IP) surveys are commonly used to visualize the physical property of chargeability and are a widely applied tool to investigate subsurface geological structure. In this work we explore the petrological control on chargeability to better understand the underlying geological features that IP surveys may image. We report the results of petrophysics, automated mineralogy, and image analysis from a case study of eleven samples from the Salmon Canyon cobalt‑copper deposit in Idaho, USA with the aim of enhancing comprehension of the relationship between mineralogy and petrophysics. While total sulphide volume is positively related to chargeability, total number of sulphide grains and chargeability have a significantly stronger association. Of the sulphide minerals present in these samples, chalcopyrite is the predominant influence on chargeability response, followed by pyrite, while cobalt-bearing sulphides appear disassociated with response. Specifically, chalcopyrite grains less than 200 μm diameter yield the greatest association to chargeability while grain sizes larger than 200 μm have an association that is notably less significant. Grain shape parameters were examined for potential causation to this grain size drop in chargeability but do not yield any distinct relationship. However, chalcopyrite grain characteristics do exhibit strong correlations with the metamorphic minerals stilpnomelane and tourmaline, implying that chalcopyrite grain abundance and shape are modified during metamorphism. A strong relationship between the presence of stilpnomelane and total number of chalcopyrite grains implies that the process of stilpnomelane growth during metamorphism leads to enhanced conductive ore dissemination and hence chargeability. Furthermore, tourmaline presence correlates with chalcopyrite grain abundance and specifically greater tourmaline is linked with more elongate (less round) large chalcopyrite grains. The results of this case study implies that IP in part images metamorphic processes.
This field guide describes selected features of the Idaho cobalt belt and the Beartrack gold deposit, to be visited on a field trip in the eastern Salmon River Mountains, west of Salmon, in east-central Idaho. A simplified regional geologic map (Fig. 1, inset) shows the Belt-Purcell basin, and the Lemhi sub-basin, and the Idaho cobalt belt (ICB). The map in Figure 1 shows the field-trip route and stops, and locations of Cu-Co deposits and selected prospects of the ICB.
Cobalt-copper deposits of the Blackbird district are stratabound in the middle part of the Proterozoic Yellowjacket Formation and are closely associated with mafic rocks that are abundant only in a 10 km 2 area around the Blackbird mine. The mafic rocks and Co-Cu deposits are in local subbasins created by a combination of growth faults and clastic wedges; they are interpreted to have formed from tuffaceous and exhalative eruptions associated with a hypabyssal alkali basalt complex. Two varieties of stratabound As-rich Co-Cu ore occur in the Blackbird mine and adjacent prospects: 1) the stratiform type having more Co than Cu and having a significant amount of Au, and 2) the epigenetic type having more Cu than Co and more abundant sulphide minerals, which fill synsedimentary disruption structures. The Blackbird deposits are unusually rich in Fe, As, Bi, Au, and Co, but low in Ag, Pb, and Zn, compared with most clastic-hosted polymetallic sulphide deposits. -from Authors
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.
Five-element veins are open-space fillings, which characteristically occur as features developed late in the tectonic history of older crystalline or sedimentary-volcanic terranes. Their major characteristics include: 1) occurrence in areas of continental crust, where a spatial and temporal relation to rifting or other extensional tectonics is often prominent; 2) development from the early Proterozoic to the Tertiary; 3) association of mild propylitic alteration with sequential deposition of distinct vein assemblages; 4) deposition at initially high (up to 450°C) temperatures from highly saline solutions, which decreased in temperature and became more reducing through the depositional sequence; 5) isotopic characteristics generally suggestive of a non-magmatic origin and derivation of components from diverse sources; and 6) frequent association of ore lodes with structural traps, reductants, and evidence of intermittent boiling at shallow depths. The weight of evidence seems to suggest that the non-magmatic model, in which the driving mechanism is continental rifting, and the solution is mobilized formational water, provides the best explanation for a widely scattered group of deposits. -from Author
Xenotime occurs as epitaxial overgrowths on detrital zircons in the Mesoproterozoic Revett Formation (Belt Supergroup) at the Spar Lake red bed-associated Cu-Ag deposit, western Montana. The deposit formed during diagenesis of Revett strata, where oxidizing metal-bearing hydrothermal fluids encountered a reducing zone. Samples for geochronology were collected from several mineral zones. Xenotime overgrowths (1-30 μm wide) were found in polished thin sections from five ore and near-ore zones (chalcocite-chlorite, bornitecalcite, galena-calcite, chalcopyrite-ankerite, and pyrite-calcite), but not in more distant zones across the region. Thirty-two in situ SHRIMP U-Pb analyses on xenotime overgrowths yield a weighted average of 207Pb/ 206Pb ages of 1409 ± 8 Ma, interpreted as the time of mineralization. This age is about 40 to 60 m.y. after deposition of the Revett Formation. Six other xenotime overgrowths formed during a younger event at 1304 ± 19 Ma. Several isolated grains of xenotime have 207Pb/ 206Pb ages in the range of 1.67 to 1.51 Ga, and thus are considered detrital in origin. Trace element data can distinguish Spar Lake xenotimes of different origins. Based on in situ SHRIMP analysis, detrital xenotime has heavy rare earth elements-enriched patterns similar to those of igneous xenotime, whereas xenotime overgrowths of inferred hydrothermal origin have hump-shaped (i.e., middle rare earth elements-enriched) patterns. The two ages of hydrothermal xenotime can be distinguished by slightly different rare earth elements patterns. In addition, 1409 Ma xenotime overgrowths have higher Eu and Gd contents than the 1304 Ma overgrowths. Most xenotime overgrowths from the Spar Lake deposit have elevated As concentrations, further suggesting a genetic relationship between the xenotime formation and Cu-Ag mineralization.
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 34S 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 2S. 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.
The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The
origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades.
In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained
using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample
of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons
from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at
Blackbird.
Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to
1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks
of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen
gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between
1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally
juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved
in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides.
In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects,
plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of
Mesoproterozoic and Cretaceous events. On the basis of textural relationships observed in thin section, xenotime and cobaltite formed during multiple episodes. The oldest age for xenotime (1370 ± 4 Ma), determined
on oscillatory-zoned cores, may date the time of initial cobaltite formation, and provides a minimum age for
the host metasedimentary rocks. Additional Proterozoic xenotime growth events occurred at 1315 to 1270 Ma
and ca. 1050 Ma. Other xenotime grains and rims grew in conjunction with cobaltite during Cretaceous metamorphism. However, ages of these growth episodes cannot be precisely determined due to matrix effects on 206Pb/238U data for xenotime. Monazite, some of which encloses cobaltite, uniformly has Cretaceous ages that mainly are 110 ± 3 and 92 ± 5 Ma. These data indicate that xenotime, monazite, and cobaltite were extensively mobilized and precipitated during Middle to Late Cretaceous metamorphic events.
The northern margin of the Helena Embayment contains extensive syngenetic to diagenetic massive pyrite horizons that extend over 25 km along the Volcano Valley-Buttress fault zone and extend up to 8 km basinward (south) within the Mesoproterozoic Newland Formation. The Sheep Creek Cu-Co deposit occurs within a structural block along a bend in the fault system, where replacement-style chalcopyrite mineralization is spatially associated mostly with the two stratigraphically lowest massive pyrite zones. These mineralized pyritic horizons are intercalated with debris flows derived from synsedimentary movement along the Volcano Valley-Buttress fault zone. Cominco American Inc. delineated a geologic resource of 4.5 Mt at 2.5% Cu and 0.1% Co in the upper sulfide zone and 4 Mt at 4% Cu within the lower sulfide zone. More recently, Tintina Resources Inc. has delineated an inferred resource of 8.48 Mt at 2.96% Cu, 0.12% Co, and 16.4 g/t Ag in the upper sulfide zone. The more intact upper sulfide zone displays significant thickness variations along strike thought to represent formation in at least three separate subbasins. The largest accumulation of mineralized sulfide in the upper zone occurs as an N-S-trending body that thickens southward from the generally E trending Volcano Valley Fault and probably occupies a paleograben controlled by normal faults in the hanging wall of the Volcano Valley Fault. Early microcrystalline to framboidal pyrite was accompanied by abundant and local barite deposition in the upper and lower sulfide zones, respectively. The sulfide bodies underwent intense (lower sulfide zone) to localized (upper sulfide zone) recrystallization and overprinting by coarser-grained pyrite and minor marcasite that is intergrown with and replaces dolomite. Silicification and paragenetically late chalcopyrite, along with minor tennantite in the upper sulfide zone, replaces fine-grained pyrite, barite, and carbonate. The restriction of chalcopyrite to inferred synsedimentary E- and northerly trending faults and absence of definitive zonation with respect to the Laramide Volcano Valley Fault in the lower sulfide zone suggest a diagenetic age related to basin development for the Sheep Creek Cu-Co-Ag deposit.
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 2S 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 4He produced by decay of U and Th and 38Ar produced by nucleogenic production from 41K. 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 22Ne in fluorite from U ± Th decay. Two extracts from gangue quartz have estimated 40K/ 40Ar 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 meta - somatism 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 3He derived from a mantle source and 22Ne 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.
The Cu-Co ± Au (± Ag ± Ni ± REE) ore deposits of the Blackbird district, east-central Idaho, have previously been classified as Besshi-type VMS, sedex, and IOCG deposits within an intact stratigraphic section. New studies indicate that, across the district, mineralization was introduced into the country rocks as a series of structurally controlled vein and alteration systems. Quartz-rich and biotite-rich veins (and alteration zones) and minor albite and siderite veinlets maintain consistent order and sulfide mineral associations across the district. Both early and late quartz veins contain chalcopyrite and pyrite, whereas intermediate-stage tourmaline-biotite veins host the cobaltite. Barren early and late albite and late carbonate (generally siderite) form veins or are included in the quartz veins. REE minerals, principally monazite, allanite, and xenotime, are associated with both tourmaline-biotite and late quartz veins. The veins are in mineralized intervals along axial planar cleavage, intrafolial foliation, and shears.
Mineralized intervals are hosted by a variety of metasedimentary rocks, including three phyllitic units of Mesoproterozoic age and two schistose units. All of these units are S-tectonites in the footwall of a regional thrust fault. Specifically, the district lies within an oblique thrust ramp containing a series of structural horses (three domains) in a duplex system. The deposits span the three domains and are hosted by metamorphic rocks
that range from lower amphibolite facies in the structurally upper domain to lower-middle greenschist facies in the lower domain (an inverted metamorphic sequence). Early quartz and biotite veins were introduced during progressive folding and prolonged peak metamorphic conditions and they underwent late-tectonic retrograde recrystallization and metamorphic mineral growth, to the same extent as the country rocks in each domain. Where little subsequent deformation occurred, early veins are discordant to bedding but, where folding was polyphase and fabrics are penetrative, mineralized zones are concordant with metamorphic compositional layering. Late quartz veins in the zones are associated with retrograde minerals and textures and are only locally deformed. 40Ar/39Ar dating of unoriented muscovite from the selvage of a late quartz vein yields a Late Cretaceous age of about 83 Ma, the time of retrograde metamorphism associated with introduction of late
quartz veins.
Textural data at all scales indicate that the host sites for veins and the tectonic evolution of both host rocks and mineral deposits were kinematically linked to Late Cretaceous regional thrust faulting. Heat, fluids, and conduits for generation and circulation of fluids were part of the regional crustal thickening. The faulting also juxtaposed metaevaporite layers in the Mesoproterozoic Yellowjacket Formation over Blackbird district host rocks. We conclude that this facilitated chemical exchange between juxtaposed units resulting in leaching of
critical elements (Cl, K, B, Na) from metaevaporites to produce brines, scavenging of metals (Co, Cu, etc) from rocks in the region, and, finally, concentrating metals in the lower-plate ramp structures. Although the ultimate source of the metals remains undetermined, the present Cu-Co ± Au (± Ag ± Ni ± REE) Blackbird ore deposits formed during Late Cretaceous compressional deformation.