Geologic History of the Blackbird Co-Cu District in the Lemhi Subbasin of the Belt-Purcell Basin

OLD G

The Geological Society of America Special Paper 522 OPEN ACCESS

Geologic history of the Blackbird Co-Cu district in the Lemhi subbasin of the Belt-Purcell Basin

Arthur A. Bookstrom Stephen E. Box Pamela M. Cossette Thomas P. Frost U.S. Geological Survey, 904 W. Riverside Avenue, Spokane, Washington 99201, USA

Virginia S. Gillerman Idaho Geological Survey, 322 E. Front Street, Boise, Idaho 83702, USA

George R. King Formation Capital Corporation, 812 Shoup Street, Salmon, Idaho 83467, USA

N. Alex Zirakparvar American Museum of Natural History, New York, New York 10024, USA

ABSTRACT

The Blackbird cobalt-copper (Co-Cu) district in the Salmon River Mountains of east-central Idaho occupies the central part of the Idaho cobalt belt—a northwest-elongate, 55-km-long belt of Co-Cu occurrences, hosted in grayish siliciclastic metasedimentary strata of the Lemhi subbasin (of the Mesoproterozoic Belt-Purcell Basin). The Blackbird district contains at least eight stratabound ore zones and many discordant lodes, mostly in the upper part of the banded siltite unit of the Apple Creek Formation of Yellow Lake, which generally consists of interbedded siltite and argillite. In the Blackbird mine area, argillite beds in six stratigraphic intervals are altered to biotitite containing over 75 vol% of greenish hydrothermal biotite, which is preferentially mineralized. Past production and currently estimated resources of the Blackbird district total ~17 Mt of ore, averaging 0.74% Co, 1.4% Cu, and 1.0 ppm Au (not including downdip projections of ore zones that are open downward). A compilation of relative-age relationships and isotopic age determinations indicates that most cobalt mineralization occurred in Mesoproterozoic time, whereas most copper mineralization occurred in Cretaceous time. Mesoproterozoic cobaltite mineralization accompanied and followed dynamothermal metamorphism and bimodal plutonism during the Middle Mesoproterozoic East Kootenay orogeny (ca. 1379–1325 Ma), and also accompanied Grenville-age (Late Meso- proterozoic) thermal metamorphism (ca. 1200–1000 Ma). Stratabound cobaltite-biotite

Bookstrom, A.A., Box, S.E., Cossette, P.M., Frost, T.P., Gillerman, V.S., King, G.R., and Zirakparvar, N.A., 2016, Geologic history of the Blackbird Co-Cu district in the Lemhi subbasin of the Belt-Purcell Basin, in MacLean, J.S., and Sears, J.W., eds., Belt Basin: Window to Mesoproterozoic Earth: Geological Society of America Special Paper 522, p. 185–219, doi:10.1130/2016.2522(08).

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ore zones typically contain cobaltite1 in a matrix of biotitite ± tourmaline ± minor xenotime (ca. 1370–1320 Ma) ± minor chalcopyrite ± sparse allanite ± sparse microscopic

native gold in cobaltite. Such cobaltite-biotite lodes are locally folded into tight F2 folds

with axial-planar S2 cleavage and schistosity. Discordant replacement-style lodes of

cobaltite2-biotite ore ± xenotime2 (ca. 1320–1270 Ma) commonly follow S2 fractures and fabrics. Discordant quartz-biotite and quartz-tourmaline breccias, and veins contain

cobaltite3 ± xenotime3 (ca. 1058–990 Ma). Mesoproterozoic cobaltite deposition was followed by: (1) within-plate plutonism (530–485 Ma) and emplacement of mafic dikes (which cut cobaltite lodes but are cut by quartz-Fe-Cu-sulfide veins); (2) garnet-grade metamorphism (ca. 151–93 Ma); (3) Fe-Cu-sulfide mineralization (ca. 110–92 Ma); and (4) minor quartz ± Au-Ag ± Bi mineralization (ca. 92–83 Ma). Cretaceous Fe-Cu-sulfide vein, breccia, and replacement-style deposits contain various combinations of chalcopyrite ± pyrrhotite ± pyrite ± cobaltian arsenopyrite (not cobaltite) ± arsenopyrite ± quartz ± siderite ± monazite (ca. 144–88 Ma but mostly 110–92 Ma) ± xenotime (104–93 Ma). Highly radiogenic Pb (in these sulfides) and Sr (in siderite) indicate that these elements resided in Mesoproterozoic source rocks until they were mobilized after ca. 100 Ma. Fe-Cu-sulfide veins, breccias, and replacement deposits appear relatively undeformed and generally lack metamorphic fabrics. Composite Co-Cu-Au ore contains early cobaltite-biotite lodes, cut by Fe-Cu-sulfide veins and breccias, or overprinted by Fe-Cu-sulfide replacement-style deposits, and locally cut by quartz veinlets ± Au-Ag ± Bi minerals.

INTRODUCTION west-trending belt of Cu-Co mines, prospects, and geochemical anomalies. The Idaho cobalt belt (ICB) is ~55 km long and Blackbird Cobalt-Copper (Co-Cu) District extends ~27.5 km to the southeast and northwest of its centroid. Broad ridges are deeply weathered to saprolite, and hillsides Location are mostly covered with talus and periglacial colluvium. Bed- The Blackbird Co-Cu district is in the Salmon River Moun- rock outcrops are sparse, except along steep upper slopes and tains of east-central Idaho and is ~35 km west-southwest of the road cuts. town of Salmon, Idaho (Figs. 1A and 1B). The Salmon River As shown in Figure 1B, the geologically defined Blackbird Mountains are interpreted as an uplifted and dissected plateau district is within the legally defined Blackbird Mining District, with broad ridges at nearly concordant elevations, incised by which is wider but shorter than the Idaho cobalt belt, and includes steep-walled tributaries to Salmon River Canyon, which is epithermal gold-silver (Au-Ag) deposits outside of the Idaho ~1.5 km deep. The Blackbird district occupies the central, wid- cobalt belt. The Blackbird mine area is in the west-central part of est, and most-mineralized part of the Idaho cobalt belt, a north- the Blackbird district, where it is elongate northwest.

Figure 1. Simplified geologic map of the Salmon River Mountains with a tectonic index map (inset A) showing selected Precambrian features: (A) Tectonic index map (inset) showing the Idaho cobalt belt (ICB) in relation to the Mesoproterozoic Belt-Purcell Basin (Y) and the Lemhi subbasin (after O’Neill et al., 2007; Box et al., 2012; Burmester et al., 2013). Basement domains that surround or extend under different parts of the Belt-Purcell Basin include the Archean Wyoming Province (A), Medicine Hat block (A), and Paleoproterozoic-Neoarchean Clearwater block (Clwtr Block, XA) after Vervoort et al. (2015) and Wang (2015); Paleoproterozoic Selway terrane (X) after Foster et al. (2006). Major faults of the Great Falls tectonic zone (GFTZ [X] and thrust belt [XA]) are from O’Neill (1993) and Sims et al. (2004). The Perry line (PL) is a Belt Basin growth fault (after Winston, 1986). The eastern margin of the East Kootenay orogen (Y) is drawn to include sites where igneous or metamorphic minerals yield isotopic age determinations between ca. 1370 and 1320 Ma. Other features shown include the Great Divide megashear (GDM; after O’Neill et al., 2007), the Sheep Creek Cu-Co deposit (SC; after Graham et al., 2012), the rifted continental margin (RCM; after O’Neill et al., 2007), and accreted oceanic terranes (Pz-Mz—Paleozoic to Mesozoic). (B) Simplified geologic map of the Salmon River Mountains, after Evans and Green (2003), showing selected Co-Cu ore zones and prospects of the Idaho cobalt belt (after Johnson et al., 1998), all of which are in Mesoproterozoic strata of the Lemhi subbasin within the Poison Creek thrust plate. Stippling represents the southern boundary of the regional garnet zone (after Lewis et al., 2012). The Salmon Canyon fault is interpreted here as a thrust fault, which may have been a southeast-vergent thrust during the Nevadan orogeny, but may have been reactivated as a northwest-dipping lateral ramp during northeast-vergent thrusting related to the Sevier orogeny (as modeled by Skipp, 1987). Garnet-bearing rocks south of this fault are likely in its footwall, or in that of the Iron Lake thrust fault, or both. Also shown are boundaries of the legally defined Blackbird mining district (after Ross, 1941), the Blackbird Co-Cu-Au district (as defined geologically), and the Blackbird mine area, which is within the fault-bounded Blackbird structural block (Bsb) of Vhay (1948). Other abbreviated labels indicate the Salmon River (SR), the Big Deer Creek fault (BDf), and the Poison Creek fault (PCf).

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Regional Geologic Setting Province and the Medicine Hat block. As shown in Figure 1A, Co-Cu mines and prospects of the ICB and the Blackbird the western Belt-Purcell basin and most of the Lemhi subbasin district are hosted in Mesoproterozoic metasedimentary rocks are within the East Kootenay orogen, which underwent dynamo- of the Lemhi subbasin of the Belt-Purcell Basin (Fig. 1A). The thermal metamorphism and plutonism in Ectasian time. Lemhi subbasin probably is underlain by cratonized oceanic The Belt-Purcell Basin and the Lemhi subbasin also are rocks of the Paleoproterozoic Selway terrane along the Great within the Cordilleran orogen (Fig. 2A), which underwent oro- Falls tectonic zone between Archean terranes of the Wyoming genesis and plutonism from Late Jurassic through Cretaceous to

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BRITISH ALBERTA B Ylg 114°30'W Ylg COLUMBIA B Sa Yim ru A Tci Ylgn s Yla lm h ) on y ef . foreland-basin) Rive Cordilleran Orogenfold-thrust (K belt) system (KJ G CANADA Yig u + plutons (K Yig S r lc Yla 49°N C h c f In USA t Ylg RC metamorphi ru Nortch SL si fa -batholithi v ult M Fork C e Ylg hinterland Belt-Purcell le a C (KJ) rw o CdA basin (Y) m . a p R CCr t Ki le CM Ylgn e x IDAHO r WASHINGTON MONTANA SC WASHINGTON or Yig o Shoup OREGON MCr. Yig g Yig e AT(PzMz ) Salmon GDM Yim n CC ICB Lemhi ic sub-basin Yig z Yfa on WYOMING e Yla . Salmon Canyon 100 km W 111°W Copper Ts SCf . 114°W Yla Ylg Ylg 34

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Ys Yla? is Mine Ylgn o Arnett Creek r n F pluton a Tci Megacrystic C 4 e QTu Iron Lake granite pluton r h fau e O_i s lt of Big Deer Ck. e k Yla? k F4 Ylg MAP SYMBOLS e fau e Salmon bc lt r Idaho cobalt belt C . F4 (ICB) ma sr . LDf r tp. . e Mine f * sr th Tcv D Ylgb n . Prospect B Ylsq a Ys O_i P Tourmaline breccia Lsb Bsb Hsb* F Yyh 5 4 Na-scapolite F 24 Deep Ck. F * BLACKBIRD 4 pluton 45 BZg 4 BZ Biotite-rich zone Ylg * MINE . HS F2? Tcv OZ Fe-oxide zone 40 * QTu

45 Fold axis, showing ... plunge and F . F 2 cnc 2 fold-set number High-angle fault. Arrows show Ylac Tcv Tcv lateral offset; . Ylg bar and ball on Ylab QTu down-thown side BDf Black Pine Ylg BZ Thrust fault Yyh

? Thrust (inferred) 45°N I Ylab Shear zone ro n 45°N Town La Ylac ke

Yellowjacket Ys fa OZ Mine u . Iron Creek l Tcv Yellowjacket QTu t pluton O_i Tcv QTu Tcv Tci Yyh SOs EXPLANATION OF MAP UNITS QUATERNARY-TERTIARY MESOPROTEROZOIC Lemhi Group QTu Sediments ± Volcanics, Undivided Yig Granitic intrusions (1.37 ± 0.01 Ga) Ylgn Lemhi gneiss (+Grt+Sil) Tci Challis intrusions Yim Mafic intrusions (1.37 ± 0.01 Ga) Ylgsq Lemhi schist and quartzite Tcv Ylg Challis Volcanics Ys Swauger Fm. Gunsight Fm. (Ylg=quartzite facies) CRETACEOUS Ylgb=biotitic facies Yla Ki Granitoid intrusions Apple Cr. Fm. (of Yellow Lake) 0510 KM LOWER PALEOZOIC Ylab Banded siltite unit SOs Sedimentary strata (Silurian-Ordovician) 0 6 MI Ylac Coarse siltite unit

O_i Syenitoid intrusions (497 ± 6 Ma) (Ordovician-Cambrian) Yyh Yellowjacket and Hoodoo Fms.

Figure 2. Tectonic inset map and geologic map of the Salmon River Mountains. (A) Tectonic index map (inset) showing the Belt-Purcell Basin, the Lemhi subbasin, and the Idaho cobalt belt (ICB) in relation to the Cordilleran orogen, its western metamorphic-plutonic hinterland [(KJ) indicates Late Jurassic to Cretaceous metamorphism], and its eastern fold-and-thrust belt [(K) indicates Cretaceous deformation] (after Allmendinger and Jordan, 1981; Cowan and Bruhn, 1992; Burchfiel et al., 1992). Northeast-vergent thrust faults bound the northeastern margin of the Belt- Purcell Basin and project beneath it, such that rocks above these faults generally are displaced northeastward relative to the rifted continental margin (RCM) and oceanic accreted terranes (AT; PzMz—Paleozoic to Mesozoic). Metasedimentary rocks of the Belt-Purcell Basin and the Lemhi subbasin host several Cu ± Co ± Au ± Ag districts, deposits, and prospects, including those labeled as follows: CC—Copper Camp (Cu- Au district), CCr—Copper Creek (Cu + Co + Ni prospect), CdA—Coeur d’Alene district, silver-belt (Cu-Ag ± Ni-Co), MCr—Meadow Creek (Co-Cu prospect), SC—Sheep Creek (Cu-Co deposit), SL—Spar Lake (Cu-Ag district ± Co), W—Wells Co prospect. (B) Geologic map of the Salmon River Mountains (modified from Evans and Green, 2003). Fold axes are from Shockey (1957), Connor and Evans (1986), and Evans and

Connor (1993). Fold sets are numbered from oldest (F1) to youngest (F4). Locations of selected Cu ± Co sites in and around the Idaho cobalt belt are from Johnson et al. (1998). Names of most mines and some prospects are spelled fully, but others are abbreviated as follows: HS—Haynes- Stellite (Co), bc—Bonanza Copper (Cu-Co-Au), cnc—Conicu (Co-Cu), ec—Elkhorn Creek (Co-Cu), ma—Mary Ann (Co-Cu), sr—Sweet Repose (Cu-Co), tp—Tinkers Pride (Co-Cu), and ef—East Fork (Co-Cu, ~40 km NE of the Salmon Canyon Copper prospect). Selected faults are labeled as follows: BDf—Big Deer fault and SCf—Salmon Canyon fault. Fault-bounded structural blocks: Bsb—Blackbird structural block, Hsb—Haynes-Stellite structural block, Lsb—Lookout structural block. A weak rock-chip geochemical anomaly for Fe, Co, As, and Y (labeled BZg) is in biotite-scapolite hornfels of the banded siltite unit, southeast of the megacrystic-granite pluton of Big Deer Creek (Yig) and the syenitoid Deep Creek pluton (OCi).

early Tertiary time. Within the Cordilleran orogen, the ICB strad- Although the Blackbird deposit contains nearly twice as dles the boundary between a western metamorphic-batholithic much Cu as Co, unit prices for Co generally are 5 to 10 times hinterland and an eastern fold-thrust belt, and the Blackbird dis- higher than unit prices for Cu. Thus, Co is the primary product of trict is within a zone of transition between biotite-grade rocks to the Blackbird district, whereas Cu is a co-product, and Au is a by- the southeast and garnet-grade rocks to the northwest (Fig. 1B). product. Slack (2006) suggested that minerals such as monazite The entire ICB also is within the Poison Creek thrust plate, and xenotime might also be recoverable as by-products for their between the Cordilleran Iron Lake and Poison Creek thrust faults rare-earth-element (REE) contents. (Figs. 1B and 2B). Grade-tonnage diagrams by Slack et al. (2013) for 13 Co-Cu- Co-Cu mines and prospects of the ICB are all hosted in Au deposits in metasedimentary rocks indicate that the Blackbird metasedimentary rocks of the Mesoproterozoic Lemhi Group deposit has a higher average Co grade and contains more Co than (Fig. 2B). These metasedimentary rocks are intruded by plutons any other known deposit of that type. The average Co grade of of Mesoproterozoic, Cambrian–Ordovician, Cretaceous, and Ter- the Blackbird deposit also is higher than those of most sediment- tiary ages. The Blackbird Co-Cu district is near a granitoid pluton hosted Cu deposits, and much higher than those of selected Co- of a bimodal mafic-felsic intrusive suite of Mesoproterozoic age, bearing volcanogenic massive sulfide deposits (VMS), iron-oxide and the Yellowjacket Cu-Au deposit and gold placers of Arnett Cu-Au deposits (IOCG), and magmatic Ni-Cu deposits. Globally, Creek are near syenitoid plutons of Cambrian–Ordovician age. however, most Co is recovered as a by-product from Co-bearing However, none of the exposed plutons is known to contain any Cu deposits in sedimentary rocks, and from Co-bearing Cu-Ni cobalt-bearing mineral occurrences, and Cu-Co prospects in the deposits in mafic-ultramafic rocks (Vhay et al., 1973). southern part of the ICB are far from any exposed plutons. Previous Work Production and Resources Previous publications on the geology and mineral deposits The Blackbird district contains at least eight stratabound of the Blackbird Co-Cu district are listed in Table 1, with a brief ore zones, several stratabound prospects, and many discordant note about each. Vhay (1948) provided the first geologic maps veins and breccias—all hosted in metasedimentary rocks of the of the Blackbird district, and Anderson (1947) provided the first Lemhi Group. Past production from the Haynes-Stellite, Uncle summary of the sequence of deposition of hydrothermal miner- Sam, and Blackbird mines totals ~2.4 Mt of ore with average als there. Geologic maps of the Blackbird Mountain quadrangle grades of 0.82% Co, 2.1% Cu, and 3.3 ppm Au, according to data by Evans and Connor (1993), the Salmon River Mountains by from Anderson (1943, 1947; Bennett, 1977; and Prenn, 2006). Tysdal et al. (2003), and the Salmon National Forest by Evans The total of production and estimated resources of the Blackbird and Green (2003) provide stratigraphic and structural context for district was reported by Slack (2013) as 16.8 Mt of ore, averaging the geology of the Idaho cobalt belt and the Blackbird district. 0.735% Co, 1.37% Cu, and 1.04 ppm Au. This estimate included In addition to published work listed in Table 1, a tremendous measured, indicated, and inferred resources of eight stratabound amount of geologic work has been done by mineral-exploration ore zones, five stratabound prospects, and one breccia pipe. How- and mine geologists of several companies, and much of that is ever, this estimate did not include downward extensions of ore documented in unpublished reports, archived in the office of the zones that are open down-dip. Geologic Division of the U.S. Geological Survey, in Spokane,

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TABLE 1. PREVIOUS PUBLICATIONS ABOUT THE BLACKBIRD DISTRICT Reference Summary note* Subject Umpleby (1913) Ore replaces schist (pC) near gabbro (KT?).Blackbird district

Ross (1941) Hypogene ore minerals occur in schistose rocks. Blackbird district Anderson (1947) Ore is epigenetic, mafi c-magmatic-hydrothermal and (T?). Mineral sequence Vhay (1948) Ore is epigenetic, felsic-magmatic-hydrothermal and (K?). Geologic maps Canney et al. (1953) Co and Cu contents in soil correlate with those in bedrock. Bennett (1977) Ore is stratabound and remobilized, or epigenetic. Geol., geochem. Lopez (1981) Ore may be remobilized from low-grade syngenetic protore. Stratigraphy Hughes (1983) Ore is stratabound, submarine-volcanic-related, and (pC). Stratabound ore Modreski (1985) Ore is stratabound, but variously remobilized. Co-Cu deposits Earhart (1986) Ore is volcanigenic massive-sulfi de of Besshi sub-type (pC). Geologic model Eiseman (1988) Sunshine ore is silicic-exhalative with garnet overprint. Sunshine ore Nash and Hahn (1989) Ore is of volcanic-related submarine-exhalative type (pC). Blackbird district Connor (1990) OZ strata contain Fe-oxides, BZ strata are biotite-rich. Geochem. survey Nash and Connor (1993) OZ and BZ strata contain submarine chemical exhalites.OZ, BZ origin Nold (1990) ICB deposits are like those of the Zambian copper belt. Nold (1990) Proterozoic ore is increasingly metamorphosed northwestward. Idaho cobalt belt Kissin (1993) Ore minerals resemble those of epigenetic 5-element veins. Classifi cation Bending and Scales (2001) ICB deposits have many similarities to SEDEX deposits. Idaho cobalt belt Slack (2006) Blackbird deposits contain REE minerals. REE mineralogy Bookstrom et al. (2007) Blackbird includes both stratabound and discordant ores.Blackbird Cu-Co Lund and Tysdal (2007) Ore is in deformed banded siltite below the Iron Lake thrust. Blackbird Au-Co-Cu Zirakparvar et al. (2007) Post-cobaltite garnets yield Lu-Hf ages of 151 to 113 Ma. Lu-Hf geochron. Slack et al. (2013) Blackbird is one of many more-or-less similar deposits.Geologic model Lund et al. (2011) Structures related to Cretaceous thrusting host Blackbird ore. Structural geology Tr umbull et al. (2011) Source of B in tourmaline is sedimentary. B isotopes Slack (2012) Blackbird deposits are variants of IOCG-type deposits.Minerals, geochem. Johnson et al. (2012) Sources of S and O are metamorphic ± magmatic and reduced.S, O isotopes Landis and Hofstra (2012) Fluid inclusions indicate hypersaline brine at 279°–347 °C. Fluid inclusions Aleinikoff et al. (2012) Hydrothermal xenotime yields Y-aged cores and K-aged rims.U-Pb geochron. Panneerselvam et al. (2012) Pb was derived from Y-aged sedimentary strata after 100 Ma. Pb isotopes Box et al. (2012) Belt-Purcell strata host mineral deposits of Y to KT ages. Belt-Purcell ores Bookstrom (2013) Mafi c dikes cut Co ore but are cut by Cu veins and breccias. Idaho cobalt belt Bookstrom et al. (2014) Cobaltite-biotite ore is Y-age, polymetallic veins are K-age.Blackbird Co-Cu *Abbreviations: pC—Precambrian, Y—Mesoproterozoic, K—Cretaceous, T—Tertiary; BZ—biotitic Blackbird zone, OZ—oxide zone, ICB—Idaho cobalt belt, IOCG—iron-oxide-copper-gold, NW—northwest, REE—rare-earth-element, SEDEX—sedimentary exhalative.

Washington. Nevertheless, the age, origin, deposit-type, and geo- rocks were metamorphosed during the Big Sky orogeny (1.78– logic history of Blackbird Co-Cu deposits have long been contro- 1.72 Ga), which contributed to assembly of the supercontinent versial (as indicated by notes in Table 1). of Nuna (or Columbia), upon which the Belt basin subsided and Recent articles on the mineralogy, geochemistry, geochro- filled with sediments and mafic sills. nology, fluid inclusions, and isotopic characteristics of the Black- As shown in Figure 1A, the Idaho cobalt belt and the Sheep bird Co-Cu deposits are included in a collection of papers on Creek Cu-Co deposit are both hosted in Beltian strata that overlie mineral deposits of the Belt-Purcell Basin edited by Box et al. the projected trend of the Great Falls tectonic zone, a broad zone (2012). These studies provide many new constraints and clues to of northeast-trending shear zones (interpreted as thrust faults) in guide interpretation of Co-Cu deposits in the Blackbird district basement rocks between Archean crustal blocks (the Wyoming and the Idaho cobalt belt. Province and the Medicine Hat block), according to O’Neill (1993) and Sims et al. (2004). This Study In north-central Idaho, basement rocks of the Clearwater block consist of Neoarchean orthogneiss (2.67–2.65 Ga) intruded The purpose of this study was to compile and integrate cur- by Paleoproterozoic orthogneiss (1.88–1.84 Ga), according to rently available information about the geology of the Blackbird Vervoort et al. (2015). The location of the boundary between the Co-Cu district and its geologic settings through geologic time in Clearwater block and the Selway terrane is mostly covered or the Lemhi subbasin of the Belt-Purcell Basin, the East Kootenay intruded by younger rocks, so basement rocks beneath the Lemhi orogen, a regional Grenville-age metamorphic belt, and the Cor- subbasin could include rocks of either or both of these terranes. dilleran orogen. The goal of this paper is to present a regionally to locally consistent history of the geology of the Blackbird Co-Cu STRATIGRAPHIC SETTING district. This story proceeds from Mesoproterozoic to Cenozoic time, from regional to local settings within each time interval, Belt-Purcell Basin (Ca. 1470–1250 Ma) and from observed relative-age relationships and isotopic age determinations to interpretations of their implications for the As shown in Figure 3, the lower 12 km of mostly grayish geologic history of the Blackbird Co-Cu deposits and their host siliciclastic sediments and mafic sills accumulated very rapidly rocks. Names of minerals and rocks are abbreviated as listed in from ca. 1470 to 1454 Ma, during rift-stage subsidence of the Table 2. Belt Basin. Flows of basaltic Purcell lava erupted at ca. 1413 Ma, after which rates of sedimentary accumulation slowed during BASEMENT ROCKS sag-stage deposition of the upper 5 km of varicolored strata in shallow-water settings from ca. 1454 to 1250 Ma. According to Foster et al. (2006), cratonized oceanic-island- A paleocontinental reconstruction by Sears (2007) suggests arc–type basement rocks of the Paleoproterozoic Selway terrane a predominantly Australian source for rift-stage detrital sedi- (ca. 1.86–1.7 Ga) probably underlie most of the southern part of ments, transported via grabens across the Siberian craton to the the Belt-Purcell Basin and most of the Lemhi subbasin, which rapidly subsiding Belt-Purcell Basin. Subsidence associated with hosts Co-Cu deposits of the Idaho cobalt belt and the Black- the eruption of the Purcell lava at 1443 Ma cut off the pathway bird district (Fig. 1A). According to Harms et al. (2004), these for delivery of sediments from Australia and opened a new route

TABLE 2. DEFINITIONS OF ABBREVIATED LABELS IN FIGURES AND CAPTIONS Label Mineral Label Mineral Label Rock type AlnAllanite GrtGarnet am Amphibolite Am Amphibole Kfs K-feldspar arg Argillite Bt Biotite NaPl Na-plagioclase btt Biotitite* CcpChalcopyrite Pl Plagioclase bx Breccia ChlChloritePo PyrrhotitehfHornfels CldChloritoidPy PyritembMaﬁ c dike, biotitic CoApyCobaltian arsenopyrite QtzQuartz mg Maﬁ c dike, gabbroic CobCobaltite NaScp Sodic scapolitembx Microbreccia CoPy Cobaltian pyrite Sd SideritephPhyllite Cth Chalcanthite TurTourmaline qtzt Quartzite Er Erythrite Xtm XenotimeslttSiltite *Biotitite—a metamorphic rock with at least 75% of biotite (Schmid et al., 2004).

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20 1370 Ma type Apple Creek Phyllite

quartzite of Jahnke Lake d

colore Lawson Creek Fm

- 15 vari Swauger Fm .

Gunsight Fm 238 m/m.y 10 muddy, deltaic facies Lemhi sub-basin 1409 Ma banded siltite unit

verage sediment-accumulation rate coarse siltite unit ellow Lk) A Belt basin diamictite unit (of Y gray

Lemhi Gp Pilcher Fm. Apple Creek Fm Gp 17 5 fine siltite unit 1336 Ma 12 m/m.y. metasandstone unit 1378 Ma Garnet Range Hoodoo Fm 1445 Ma McNamara Fm 15 1448 Ma 1401 Ma Bonner Fm Mt. Shields Fm Yellowjacket Fm Shepard Fm vari-colored

sag stage 53 m/m.y 53 1454 Ma 1443 Ma Snowslip Fm 0

1454 Ma n Missoula base not exposed 1469 Ma Wallace Fm iega 10

1457 Ma St Regis Fm mixe d iP Revett Fm

Burke Fm Ravall

H mbr Rift stage 5 719 m/m. y. G mbr . F mbr verage sediment-accumulation rates 1469 Ma gray E mbr A C mbr D mbr Thickness (km)

Sample types dated by B mbr Prichard Fm Lower Belt A mbr U-Pb geochronology 0 Detrital muscovite base not exposed Seismically imaged sills Detrital zircons, youngest set

Volcanic ash-fall tuff 1450 1400 1350 1300 1250 Igneous intrusive rock AGE (MILLIONS OF YEARS)

Figure 3. Graphs of stratigraphic thickness (km) vs. age (Ma) for the Belt Basin (after Sears, 2007) and the Lemhi subbasin: Belt age determinations are 1469 Ma for a mafic sill at Plains, Montana, and 1457 Ma for a mafic sill at Paradise, Montana (Sears et al., 1998); 1454 Ma for upper Helena tuff, 1443 Ma for Purcell Lava, and 1401 Ma for Bonner tuff (Evans et al., 2000); 1378 Ma and 1330 Ma for the Garnet Range Formation (Ross and Villeneuve, 2003). Estimated thicknesses of stratigraphic units in the Belt Basin are from Sears (2007), and those for the Lemhi subbasin in the Salmon River Mountains (Yellowjacket Formation to Swauger Formation) are from Evans and Green (2003). Estimated thicknesses of units above the Swauger Formation are from a columnar section for strata of the Lemhi and Beaverhead Ranges by Lonn et al. (2013, p. 206). Mean ages of youngest sets of detrital zircons in samples from key stratigraphic units in the Lemhi subbasin are from Link et al. (2007) and Aleinikoff et al. (2012b). The mean age of 1370 ± 10 Ma for megacrystic granite is from Evans and Zartman (1990).

for transport of sediments to the Belt-Purcell Basin from a south- the Salmon River Mountains. Labels to the left of the column eastern source region, as indicated by the age-distribution pro- indicate previous stratigraphic nomenclature (according to Evans files of detrital zircons in strata of the upper Belt succession and and Connor, 1993), whereas labels to the right indicate current the Lemhi subbasin. nomenclature (according to Evans and Green, 2003). Current Ross and Villeneuve (2003, p. 1191) interpreted the Belt- stratigraphic nomenclature is based on correlations of lithostrati- Purcell Basin as “an extensional domain along a collisional/con- graphic units in the Salmon River Mountains with those mapped vergent plate margin, analogous to Tethyan sedimentary basins previously by Tysdal (2003) in the Lemhi Range. Thus, Tysdal such as the Black and Caspian Seas.” This is consistent with a et al. (2003) applied nomenclature from the Lemhi Range to western (non-Laurentian) source for detrital zircons (dated 1610– lower, middle, and upper subunits of the Yellowjacket Formation 1480 Ma) and with evidence for dynamothermal metamorphism (as previously mapped by Evans and Connor, 1993). Evans and and magmatism along the western margin of the Belt-Purcell Green (2003) provided lithologic descriptions of these renamed Basin during the Ectasian East Kootenay orogeny. stratigraphic units in the Salmon River Mountains. Age determinations shown in Figure 4A are maximum ages, indicated by the Lemhi Subbasin (Ca. 1454–1370 Ma) youngest set of detrital zircons in samples from each stratigraphic unit dated by Link et al. (2007) and Aleinikoff et al. (2012b). The Lemhi subbasin constitutes the southwestern part of the Burmester et al. (2013) reported that the Swauger and Law- Belt-Purcell Basin (Fig. 1A). Prior to 1999, most metasedimen- son Creek Formations are overlain by a very thick quartzite unit tary strata of the Lemhi subbasin in the Salmon River Mountains in the Lemhi Range and the Beaverhead Mountains. They named were assigned to the broadly defined Yellowjacket Formation, this unit the quartzite of Jahnke Lake, which they assigned to which Ruppel (1975) perceived as similar to and correlative with the stratigraphic succession of the Lemhi subbasin. This extends deep-water strata of the lower Belt Prichard Formation in the strata assigned to the Lemhi subbasin eastward into Montana, as Belt Basin. By contrast, Winston et al. (1999) suggested that the shown in Figures 1A and 2A (Lonn et al., 2013; McDonald and broadly defined Yellowjacket Formation was similar to and cor- Lonn, 2013). relative with shallow-water strata of the Ravalli, Piegan, and Mis- Unfortunately, there are at least two Apple Creek Forma- soula Groups in the Belt Basin. However, subsequent dating of tions—the greenish Apple Creek Phyllite of Anderson (1961), detrital zircons by Link et al. (2007) and Aleinikoff et al. (2012b) and the Apple Creek Formation of the Yellow Lake reference indicated that grayish metasedimentary strata of the Yellowjacket section, established by Ruppel (1975) near Golden Trout Lake through Apple Creek Formations are age-equivalent to vari- (now known as Yellow Lake). A columnar section by Burmester colored, mostly terrestrial strata of the Missoula Group, which et al. (2013) indicated that the Apple Creek Phyllite overlies the overlie strata of the Ravalli and Piegan Groups in the Belt Basin. quartzite of Jahnke Lake. However, the Apple Creek Formation As shown in Figure 3, a total thickness of nearly 20 km of silici- of Yellow Lake underlies the Gunsight Formation and contains a clastic strata accumulated in the Lemhi subbasin between ca. 1454 banded siltite unit, similar to that of the Blackbird district. Thus, and 1370 Ma, at an average rate of ~238 m/m.y., while average the banded siltite unit of the Blackbird district is here assigned rates of sediment accumulation decreased from ~719 m/m.y. to to the Apple Creek Formation of Yellow Lake (as labeled in ~53 m/m.y. in the Belt Basin (Fig. 3). The base of the Yellowjacket Fig. 4A). Formation is not exposed, so a very thick stratigraphic section, Banded siltite unit. Where it is relatively unaltered, coeval with strata of the lower-to-middle parts of the Belt Super- the banded siltite unit of the Apple Creek Formation (of Yel- group, might be hidden below present levels of exposure. low Lake) in the Salmon River Mountains consists of cycli- The lower 11.5 km of strata exposed in the Lemhi subbasin cally repeated couplets, in which each relatively resistant bed consists mostly of grayish siltite, argillite, and quartzite. Lopez of pale-gray siltite is overlain by a recessive bed of dark-gray (1981) identified abundant quartz, common plagioclase and argillite, thus giving outcrops a rhythmically banded appear- orthoclase, and accessory zircon, garnet, tourmaline, and opaque ance (Fig. 5A). Aleinikoff et al. (2012b) reported an age of 1409 minerals, as detrital constituents of argillaceous quartzite from ± 10 Ma for the youngest set of detrital zircons in metasandstone the lower part of the Yellowjacket Formation. interlayered with biotite phyllite to schist in the upper part of the Bed forms, sedimentary structures, and the grayish appear- banded siltite unit in the Blackbird mine area. ance of these metasedimentary strata are interpreted to indicate Argillic rocks within ~2 km of the megacrystic granite plu- rift-style deposition in shallow-marine settings that were rela- ton of Big Deer Creek are commonly metamorphosed to dark- tively reducing. By contrast, the upper 8.5 km of strata in the brown biotite hornfels with round porphyroblasts of white sodic Lemhi subbasin are varicolored and were probably deposited in scapolite (Fig. 5B), and argillite beds near Blackbird ore zones sag-style terrestrial settings that were relatively oxidizing. are altered to biotitite containing ~75 vol% (or more) of greenish- gray to greenish-black biotite (Fig. 5C). Such biotitite is prefer- Stratigraphic Units of the Lemhi Subbasin entially mineralized in stratabound zones of cobaltite-biotite ore, The columnar stratigraphic section in Figure 4A illustrates which consists of disseminated to semimassive concentrations Mesoproterozoic stratigraphic units of the Lemhi subbasin in of cobaltite grains in a matrix of biotitite ± tourmaline. In the

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/980372/spe522-08.pdf by guest on 28 September 2021 194 ) biotite biotitite (> 75% Bt) quartz TION interlayered with

biotite-rich breccia q / c chlorite st siltite g garnet p phyllite x cross-bedded ms metasandstone b bb bt t bx Short-label parts cd chloritoid EXPLANA ourmaline-matrix Grt-Bt-rich phyllite interlayered with T breccias Banded siltite unit with biotite-rich beds = BZ Thrust fault of Little Deer Creek (LDf) Banded siltite unit with interlayered siltite and argillite Coarse siltite unit with iron oxides = OZ middle Grt-Cld ms Bt phyllite / ms (cross-bedded) Grt-Chl phyllite / quartzite ourmaline-matrix breccia

btt interlayered with metasandstone (ms) Bt-rich phyllite interlayered with siltite upper Grt-Bt metasandstone (ms lower Grt-Bt ms T 2 Bt-Qtz metasandstone (ms) ms, (massive to laminated) Bt-rich phyllite interlayered with siltite

Bt phyllite interlayered with siltite ms, (rippled and graded) Blackbird structural block

Haynes-Stellite structural block Salmon River Mountains 3 1 Metasedimentary

Igneous Metasedimentary Igneous megacrystic monzogranite megacrystic monzogranite chlorite chloritoid tourmaline garnet biotite quartzite Yab Yabt ur bx metasiltstone (mst) bqms btt/ms msm msr bp/msx T Yig gbms gbms gcp/gcq gbbp/mst btt/st bbp/st bp/st gcdms Yi g ur

IUGS standard abbreviations Bt Chl Cld Grt Qtz T Mesoproterozoic (Y) Mesoproterozoic (Y)

Gunsight Fm Gunsight

D deltaic facies deltaic uni t

banded-siltite

P Repose Sweet Haynes-Stellite Haynes-Stellite bqms btt/ms bqms msm bp/msx msr bp/msx bp/st ur bx

P Conicu Yi g D

f f 3

D D 1 L Haynes-Stellite structural block L gbms gcdms gbms bbp/st btt/st gbbp/mst bp/st Yig pluton truncates Gunsight Fm

Yi g

g a r n e t

z o n

e (scale = A-scale) t uni

Blackbird structural block

Gunsight Fm Gunsight banded-siltite Yig pluton truncates Gunsight Fm

oronto

1 km 1 Prospects - not bold Chelan Horseshoe, T Blacktail, Iowa Merle, Idaho, Chicago, Brown Bear Burl Mushroom , St. Joe Katherine, Ella Ore zones - bold tex t East Chelan Ram Sunshine East Sunshine

g a r n e t

z o n e B

btt/st btt/st btt/st btt/st bbt/st

gcp/gcq gcp/gcq

b a n d e d

s i l t i t e

w i t h

g r e e n i s h - Blackbird mine area b l a c k

b i o t i t i t e

( b + t )

(scale = 10 x A-scale)

L i t h o l o g y fine-siltite unit

coarse-siltite unit banded-siltite unit

Sandstone and siltite Siltite Sandstone (hematitic) Sandstone (feldspathic) Sandstone (feldspathic) Calc-silicate hornfels and siltite Siltite (chloritic) Sandstone and siltite Fine sandstone and siltite Sandstone

U n i t

l a b e l Ys Ylgq Ylab Ylac Yyh

Ylgms

Q u a r t z i t e (of Yellow Lk) Yellow (of

G S u w n a s u i g g h e t r

F F m m . . C u r r e n t

N a m e Hoodoo Y e l l o w j a c k e t

F m . A p p l e

C r e e k

F m .

L e m h i

siltite

ltite

uni

mictite unit mictite uni 1445 ± 12 Ma 1454 ± 9 Ma

Tinkers Pride

Blackbird Blackbird dia

Black Pine Iron Creek

fine-si coarse-

BZ OZ

Salmon River Mtns.

5,000m

M i d d l e

Y e l l o w j a c k e t

F m . P r e

U v p i o p u e s r

L N Y o e a w l m l o e e w r

Y j a e c l l k o e w t

j F a m c k . e t

F m 15,000m 10,000m . 1409 ± 10 Ma

Figure 4. Columnar stratigraphic sections for Mesoproterozoic strata in the Salmon River Mountains, the Blackbird mine area, and the Black- bird and Haynes-Stellite structural blocks: (A) Columnar stratigraphic section for stratigraphic units of the Lemhi subbasin in the Salmon River Mountains (modified from Link et al., 2007). Alternative labels indicate previous and current stratigraphic nomenclature. (B) Columnar stratigraphic section showing lithologic subunits of the banded siltite stratigraphic unit in the Blackbird mine area (modified from Nash and Hahn, 1989; Bending and Scales, 2001; enlarged to 10 times the scale of A). Mineralized stratigraphic intervals are indicated by names of contained ore zones (bold italic) and prospects (italic). The garnet zone is indicated by a coarsely stippled pattern. (C) Columnar stratigraphic section showing metasedimentary lithologic units in the upper part of the banded siltite unit (of the Apple Creek Formation) and the overlying Gunsight Formation in the northern part of the Blackbird structural block—west of and above the reverse-to-thrust Little Deer Creek fault (LDf on Fig. 2). The scale matches that of part A. (D) Columnar stratigraphic section showing metasedimentary lithologic subunits in the upper part of the banded siltite unit (of the Apple Creek Formation) and the overlying Gunsight Formation in the Haynes-Stellite structural block (east of and below the reverse- to-thrust fault of Little Deer Creek [LDf]). The scale matches that of part A. IUGS—International Union of Geological Sciences.

Blackbird mine area, biotitite beds are concentrated within six Instead, spidergrams in Figure 5 are interpreted here as stratigraphic intervals in the upper part of the banded siltite unit, manifestations of chemical and mineralogical changes associated and concentrations of cobaltite are preferentially concentrated with hydrothermal biotitization of argillite to biotitite. Relative to in biotitite layers within three of these stratigraphic intervals argillite and siltite in banded siltite, biotitite samples are depleted (Figs. 4B and 5C). in Sr but enriched in Rb, which can be attributed to hydrother- Vhay (1948) noted that biotite in and around ore zones is mal replacement of plagioclase by biotite. Also, most biotitite greenish, and he interpreted such biotite as a product of hydro- samples are enriched in Y and heavy REEs, but some samples thermal alteration. Lee (1958) reported that greenish biotite from are enriched, while others are depleted in light REEs. This would the Blackbird district is unusually rich in ferrous Fe (29 wt%) seem consistent with differential hydrothermal transport and and Cl (1.1 wt%). Biotitite, composed of such greenish biotite, deposition of the chemical constituents of xenotime, monazite, is commonly metamorphosed to biotite phyllite to schist, but is gadolinite, and allanite (identified in Blackbird ore samples by locally recrystallized to biotitite granofels with randomly ori- Slack, 2012). ented biotite plates that sparkle in reflected light. Biotite also is Gunsight Formation. In the Haynes-Stellite structural block locally altered to chlorite. (Hsb in Fig. 2B), the lower part of the Gunsight Formation con- Siltite interlayers generally are less biotitized and less miner- sists of quartzite beds (0.2–2 m thick) with thin interbeds of alized than biotitite layers. Nevertheless, some siltite, which may black biotite phyllite. Northward, this passes into massive biotitic have had a muddy component, is moderately biotitized, and metasandstone, which is predominant in the drainage basin of locally, beds of siltite-to-quartzite beds are partly replaced by Little Deer Creek (Figs. 6 and 7). Such biotitic metasandstone is tourmaline. More commonly, however, siltite is silicified and interpreted here as a muddy facies of the Gunsight Formation, in recrystallized to hard, sugary-textured micromosaic quartz, which which the muddy component is replaced by black biotite. Around Eiseman (1988) interpreted as metamorphosed siliceous exhalite. the Sweet Repose prospect, a subfacies of the Gunsight Forma- Hahn and Hughes (1984) interpreted biotite-rich layers in tion consists of scapolitic metasandstone, interlayered with scap- rocks of the Blackbird district as beds of metamorphosed aqua- olitic biotitite. Above the Sweet Repose adit, a thick biotitite bed gene mafic tuff. However, these biotite-rich layers display no is internally folded, and the folds are cut by a quartz vein, con- remnant volcaniclastic textures, even where sedimentary struc- taining chalcopyrite and cobaltian arsenopyrite. tures are well preserved in adjacent siltite beds. For example In the northern part of the Blackbird structural block (Bsb these biotite-rich rocks do not contain euhedral or angular-frag- in Fig. 2B), banded siltite grades up section into biotitic siltite to mental mineral grains, typical of tuffs. Instead, they characteristi- metasandstone (± garnet ± chloritoid), overlain by quartzite with cally contain rounded detrital grains of resistate minerals, typical subordinate interlayers of biotite schist (±garnet). These rocks of siliciclastic sediments. Furthermore, mafic dikes of the Black- are mapped as quartzite and schist of the Lemhi Group (undi- bird district cut banded siltite that was lithified and tightly folded vided) in Figure 2B, but on larger-scale maps in Figures 7 and 8, before the dikes intruded, so such mafic dikes could not have they are mapped as metasedimentary lithologic subunits of the produced syn-sedimentary mafic tuffs. Gunsight Formation. Spidergrams of chondrite-normalized abundances of incompatible elements in samples of banded siltite, biotite-scapolite Co and Cu Contents of Metasedimentary Country Rocks hornfels, and biotitite show that these rocks are depleted in Nb Connor (1990) collected rock-chip samples from strata and Ta relative to La (Fig. 5), as is characteristic of continentally equivalent to Yellowjacket-Hoodoo unit, the Apple Creek For- derived marine shale (according to Thompson et al., 1984). If mation, and the Gunsight Formation, southeast of the Blackbird Blackbird biotitites were to represent metamorphosed aquagene district. For 111 analyzed samples, he reported median concen- mafic-alkalic tuff, however, they should be enriched in Nb and La trations of 8 ppm Co and 6.5 ppm Cu. relative to Ta, as are post-sedimentary mafic dikes of the Black- In the upper part of the coarse siltite unit and the lower bird district (this paper), and as are mantle-derived mafic intru- part of the banded siltite unit, Connor (1990, 1991) mapped sions (according to Thompson et al., 1984). and sampled a more-or-less stratabound zone characterized by

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1000 A D banded siltite B Ylab 100 tops Co, Cu)

arg 10 sltt rock/chondrite (except Rb, K, P, 1 Sltt, Sweet Repose TF110B Sltt-Arg, Hawkeye SB145B 5 cm Sltt-Arg, Hawkeye SB145C Arg, Deep Ck SB106B Arg, Deep Ck SB106C 0.1 Co Ba Th Nb La Sr P Zr Ti Y Yb Bt hf Cu Rb K Ta Ce Nd Sm Hf Tb Tm B 1000 E sltt biotite-scapolite Bt hf hornfels 100 Bt hf

lichen Scp Co, Cu) 10 rock/chondrite

(except Rb, K, P, Btt-Scp, Sweet Bt-Scp, Deep Ck TF101A sltt 1 Repose TF110A 3 cm Ylab Btt-Scp, Sweet Bt-Scp, Deep Ck Bt hf Repose TF110C TF217 Btt-Scp, Sweet Bt-Scp, Deep Ck Repose TF110D SB106A 0.1 Co Ba Th Nb La Sr P Zr Ti Y Yb C Cu Rb K Ta Ce Nd Sm Hf Tb Tm

tops F Cth 1000 biotitite Btt Btt Er 100

sltt Co, Cu) Btt 10 rock/chondrite (except Rb, K, P, sltt Btt Hawkeye TF109 Btt Merle core M81- 5 cm 1 Btt Btt Blacktail TF120 12A 836.5 Btt Blacktail F850 Btt S. Idaho core S7- Btt Merle core M80-1A 80-1A-406 45-50 0.1 Co Ba Th Nb La Sr P Zr Ti YYb Cu Rb K Ta Ce Nd Sm Hf Tb Tm

Figure 5. Photos of and corresponding spidergrams for banded siltite, contact-metamorphosed banded siltite, and biotitized banded siltite: (A) Banded siltite (Ylab) exposure on the southeast rim of Blacktail pit of the Blackbird mine, showing interlayered siltite (sltt) and biotitic argillite (arg). Siltite beds pinch and swell, with basal scours ± mud chips, and planar tops. Upward-fining graded beds and upward-concave cross- bedding indicate an upright structural orientation. (B) Dark-brown biotite-scapolite hornfels (Bt hf) interlayered with siltite in banded siltite (Ylab), exposed in road cuts near Deep Creek. This outcrop is within a biotite-scapolite (Scp) aureole around the megacrystic monzogranite pluton of Big Deer Creek (dated 1370 ± 4 Ma by Aleinikoff et al., 2012b). (C) Biotitite (Btt) interlayered with siltite in banded siltite (sltt), exposed along the lower northeast wall of Blacktail pit. Biotitite contains at least 75 vol% of greenish-black biotite (after argillite beds). Erythrite (Er) and chalcanthite (Cth) are weathering products of sparsely disseminated cobaltite and chalcopyrite. Basal scours, cross-bedding, and upward-fining beds indicate upright bedding. (D) Spidergrams for siltite and argillite of the banded siltite unit, compared to a set of spidergrams for marine shale (gray pattern) after Thompson et al. (1984). (E) Spidergrams for biotite-scapolite hornfels after argillite, interlayered with siltite in contact- metamorphosed banded siltite, compared to a set of spidergrams for marine shale (gray pattern) after Thompson et al. (1984). (F) Spidergrams of biotitite after argillite, interlayered with siltite in hydrothermally biotitized banded siltite (stained with erythrite and chalcanthite after cobaltite and chalcopyrite), compared to a set of spidergrams for marine shale (gray pattern) after Thompson et al. (1984).

C A a protomylonitic microbrecci Yig LDf NaPl S Btqtzt am C Kfs Qtz

Bt 1cm Pl 10 cm

BC 1000 Megacrystic monzogranite (Yig) Figure 6. Photo of megacrystic granite (Yig in Fig. 2B), spidergrams and amphibolite (Yam) for megacrystic granite and amphibolite (Yim in Fig. 2B), and a photo showing amphibolite clasts in protomylonitic breccia from the Little Deer fault zone (LDf in Fig. 2B): (A) Rapakivi texture in a sample 100 btt from the megacrystic monzogranite of Big Deer Creek (Yig). A large phenocryst of pinkish K-feldspar (Kfs) is rimmed by white sodic plagioclase (NaPl) in a phaneritic matrix of quartz (Qtz), plagioclase (Pl),

Co, Cu) and biotite (Bt). An inclusion of biotitic quartzite (Btqtzt) is similar to 10 nearby metasedimentary rocks of the Gunsight Formation. Contact re- Mafic sill, lationships indicate that metasedimentary host strata were folded before Antarctica granite was emplaced at ca. 1370 ± 10 Ma, according to Evans (1981, rock/chondrite Yig-92TF175 1986), and Evans and Zartman (1990). (B) Spidergrams for megacrys-

(except Rb, K, P, Yig-92TF126 Yig-SB131A tic granite and meta-mafic amphibolite of the Salmon River Mountains 1 Yig-92TF149 Yig-SB157C compared to spidergrams by Thompson et al. (1984) for continental leu- Yig-92TF140A Yig-SB161A cogranites (gray pattern) and a mafic sill, similar to continental flood basalts (dashed line). Chemical data for 92TF samples are from Lewis and Yig-92TF142 Yam-92TF130 Frost (2005). (C) Protomylonitic breccia with amphibolite clasts (am), 0.1 found downslope from the Little Deer reverse-to-thrust fault (LDf). The Co Ba Th Nb La Sr P Zr Ti Y cataclastic matrix contains fine- to microfragmental quartz > biotite Cu Rb K Ta Ce Nd Sm Hf Tb Tm > amphibolite. Two foliations (S and C) define protomylonitic fabric in the matrix. Quartz grains near clasts are elongate and aligned with flow foliation, which is deflected around amphibolite clasts.

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114°20'0"W Pant 114°17'30"W EXPLANATION OF MAP UNITS (in Figs. 7 and 8) A Tf Yig h Eocene er Cr Yig 40 Challis Gp Tf Felsic dike 45 bc 80 . YbB 25 mb Lower Paleozoic? mg Mafic dikes 50 tp 65 42 Megacrystic granite of Big Deer Creek 50 38 Mesoproterozoic Yig 50 39 30 (1370 ± 10 Ma) (Evans and Zartman, 1990) 50 sr 45°10'0"N 30 . 25 58 Lemhi Gp r Cr 36 40

Dee 40 60 Gunsight Fm r Yg BG ? YgB

YbB 3 C 59 YgB—Biotitic metasandstone (massive)±Na-scapolite (near Yig).

Yg B 3 ? Includes: Tf ? 45 r 50 38 e e Yg2BCd Yg3B—Quartzite-biotite schist+Grt 40 30 55 63 D YgB

F Yg BCd—Quartzite-biotite phyllite+Cld+Grt Yb 2 67 Yg Btt 2 Yg BCd tle 1 2 it Yg Btt—Biotitic metasandstone with interlayered biotitite±Grt L Yg1QC 1 Yig Blackbird Yg1B Yg QC—Biotitic metasandstone+Grt+Qtz+Chl 57 1 block Yg B 46 Yg B—Biotitic metasandstone±Grt 1 60 Yg 1 YbQC F 4 Yg—Quartzite >> argillite 63 Apple Creek Fm (of Yellow Lake) YgB 65 Lookout block Lookout Yg LDf b 50 YbQc—Quartzite and chlorite schist+Grt±Cld±Cob±Ccp 45°7'30"N YbQC YbBQC F 48 YbBQC—Siltite+Qtz and biotite phyllite+Grt+Chl 2 43 YbB c Haynes-Stellite block YbBtt Cr YbBtt—Siltite and biotitite±Grt±Cld±Chl±Cob±Ccp ird 24 50 f Blackb L 85 YbB YbB—Siltite and biotite phyllite±Grt W Yb F 39 3 HS

YbB Yb Yb—Siltite and argillite to phyllite±Na-scapolite (near Yig).

Yb Youngest set of detrital Zrn indicates age of 1409 ± 10 Ma 01,000 2,000 3,000 METERS for deposition of the upper part of Yb (Aleinikoff et al., 2012) Area of Fig. 8 For explanation of structural symbols see Fig. 8

B b N78W c Elev. (m)

Grt 1

1 Grt 2,500 Grt

Yg YbBtt+Cob

B YbBtt 1 1

B YbBtt+Cob Cubx

YbBtt Grt YbQC+Cob Cob Yg Yb 2 E 2 YbBtt M YbQC+Cob E 2 E Cob 2,000 YbB 2 U YbB YbB LDf YbB YbQC YbB WLfD Cob 2 LDf MC f 1,500

YbBtt NFf

YbB SEf Yb Yb Ygb Ygb

YbBt HGf YbB

t YbB Ygb Yg YbBt t YbBt t YbBt t Db x 1,000 M Elev. of deepest mining 0 0.5 1 KILOMETER Elev. of deepest Apparent displacement E away from viewer exploration drilling

Figure 7. Diagrammatic geologic map and cross section of the Blackbird mine area and its surroundings: (A) Geologic map, showing the Black- bird, Lookout, and Haynes-Stellite structural blocks, and the southwest margin of the megacrystic granite pluton of Big Deer Creek. Sources of information include Vhay (1948), Evans and Connor (1993), Johnson et al. (1998), and Tysdal et al. (2003). Labeled mines and prospects are the Bonanza Copper prospect (bc), Haynes-Stellite mine (HS), Sweet Repose prospect (sr), and Tinkers Pride prospect (tp). Labeled faults are the White Ledge fault (Wlf), and the Little Deer fault (LDf). The explanation of map units here also applies to Figures 7B and 8. (B) Geologic cross section of the Blackbird mine area. The line of section (b–c) is shown on the geologic maps in Figure 7A and Figure 8. A line (labeled M) at the 2088 m elevation indicates the deepest level of previous mining (the 6850 level of the Blackbird mine, which was the main haulage level below the stopes). Dashed horizontal lines (labeled E) indicate the deepest levels of exploration beneath the Blackbird mine, the Merle zone, and the Sunshine and East Sunshine zones. Mineral abbreviations from International Union of Geological Sciences. Labeled faults are (from west to east) the White Ledge fault (WLf), Sunshine East fault (SEf), Meadow Creek fault (MCf), Hawkeye Gulch fault (HGf), Little Deer fault (LDf), and Northfield fault (NFf).

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/980372/spe522-08.pdf by guest on 28 September 2021 199 114°21'40"W 114°20'50"W 114°20'0"W 55 Yg BCd ?

Yb Tf 2 r

2250

? ?

Ram

T 30 B 50

YbQC Yg BCd 57 2 ? YbBQC 2400Yg B Yg B

1 1

2300 YgB

? ?

U C a ech 39 D le

r EBf ? a ? ch 50 45°8'15"N a n t D 38 ic Yg Btt 80 l U 1 in B Yg e la c 35 Yg Btt 2100 1 kt LD Cr QC ail hs 1 ? S 40 Yg B LDf u 1 n ess 50 s YbBtt Yg Btt h 50 YbBQC 1 i n 42 e B 65 r ia tor 85 brl o 84 w YbB Yg B n ?Yg B b YbB B 1 1 e YbBtt ar YbB YgB 55 Idaho 65 s Yb 50 y Yg Btt

Sunshine syncline 55 c 1 l

i bk n nf mr e ? YbBtt C YbB k h YbB Yb i rt

c NFf ? 45 a 65 ? g 40 M ? 7100L o e 45°7'30"N r U M Cr le YbBtt D U D e YbBQC ? Dandy YbB YbBtt D c U 85 sj N.Idaho YbQC mb YbB 80 mg YbB ? 35 S.Idaho WLf YbB Yb YbB 35 Hawkeye ? SEf 85

MCf F1 or 2 6850L ? mb Dsz U. Sam 40 BB Cr2150 Yg

Yb Millsite 85 35 f HG U 2300 0 500 1,000 METERS D U Yb D 24 2100 D 44 Contour interval 50 meters U F3 2300 SGf EXPLANATION OF MAP SYMBOLS Open pit Axis of overturned anticline, Grt zone— Garnets are common where 40 Low-grade stockpile from open pit pattern is dense but sparse where pattern is showing plunge Metamorphic quartz pods diffuse. Boundaries are gradational. Co-Cu prospect Adit Adit closed Co-Cu ore-zone centroid Drainage Average of fold axes, or pole to Contact showing dip 35 Polymetallic quartz veins 52 best-fit fold girdle, showing plunge Polymetallic breccias ? Thrust fault, inferred 40 Strike and dip of beds (dashed where uncertain) U Fault, steeply dipping (dashed where approx.), 70 Cob-Tur breccias D U = Up-thrown side, D = down-thrown side Strike and dip of overturned beds Cob-Qtz-Chl ore 40 Axis of anticline, showing plunge Strike and dip of foliation, 35 70 Cob-Bt ore 40 Axis of syncline, showing plunge showing plunge of lineation (dashed where uncertain) Axis of overturned syncline, Vertical foliation 40 showing plunge

For Explanation of map-unit labels, see Fig. 7.

Figure 8. Diagrammatic geologic map of the Blackbird structural block, showing ore zones and prospects of the Black- bird mine area: Line b–c is the section line for the diagrammatic cross section in Figure 7B. Labels for ore zones are spelled fully, but labels for selected prospects are abbreviated as follows: Buckeye (bk), Burl (brl), Chelan (ch), East Chelan (ech), Ella (e), Horseshoe (hs), Iowa (ia), Katherine (k), Mushroom (mr), Ridgetop (rt), St. Joe (sj), and Toronto (tor). Labeled faults are (from west to east): the White Ledge reverse-right-lateral fault and shear zone (WLf), Sunshine East normal fault (SEf), East Blacktail normal fault (EBf), Meadow Creek fault zone (MCf), Dandy shear zone (Dsz), Hawkeye Gulch fault (HGf), Northfield fault (NFf), on strike with the Slippery Gulch fault (SGf), and Little Deer reverse-to-thrust fault (LDf). Stippled patterns indicate garnet-bearing rocks. Sparse stipple indicates rocks with small, sparsely scattered garnets; dense stipple indicates rocks with relatively abundant and easily visible porphyroblasts of garnet ± chloritoid. Principal sources of information include Vhay (1948) and Nash and Hahn (1989).

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iron-oxide-bearing siltite beds (OZ in Fig. 2B). For 14 samples Middle Mesoproterozoic Metamorphism (1379–1325 Ma) from the OZ, he reported median concentrations of 21 ppm Cu and 9 ppm Co. Isotopic age determinations on metamorphic minerals bracket In the upper part of the banded siltite unit and the lower part of the time span of the East Kootenay orogeny between ca. 1379 and the Gunsight Formation, Connor (1990, 1991) mapped and sampled 1325 Ma. A maximum age constraint of ca. 1379 Ma is based on a a more-or-less stratabound zone characterized by biotite-rich beds Lu-Hf age determination of 1379 ± 8 Ma, reported by Zirakparvar between beds of siltite to quartzite (BZ in Fig. 2B). For 24 samples et al. (2010) for garnets from lower Belt mica schist exposed in of biotite-rich rocks from the BZ, he reported median concentra- the Priest River metamorphic complex in northern Idaho. A mini- tions of more-or-less stratabound zone 19 ppm Co and 5 ppm Cu. mum age constraint of ca. 1325 Ma is based on Pb-Pb age deter- Lopez (1981) had suggested that low-grade syngenetic Co and minations on titanite in samples from chloritic rocks beneath the Cu in metasedimentary country rocks may have been remobilized Sullivan Zn-Pb deposit (dated 1325 ± 21 Ma) and from region- and concentrated to form Co-Cu ores of the Blackbird district. ally metamorphosed mafic sills in southeastern British Columbia According to that suggestion, the somewhat elevated Co and Cu (dated 1329 ± 29 Ma to 1325 ± 25 Ma), according to Schandl and contents of rocks from OZ and BZ could be interpreted as precur- Davis (2000). sors to higher-grade Co and Cu deposits of the Blackbird district. McFarlane (2015) reported an age of 1365 ± 10 Ma for Nash and Connor (1993) interpreted rocks of the OZ and monazite in sillimanite metapelite in metasedimentary rocks of BZ as siliciclastic sedimentary rocks with chemical-exhalative the lower Belt-Purcell Supergroup in the Matthew Creek meta- components, derived from submarine hot springs. That hypoth- morphic zone (MCMZ). McFarlane and Pattison (2000) esti- esis seems consistent with the presence of rounded pebbles to mated that peak metamorphism occurred at ~3.5 ± 0.5 kbar and cobbles of strongly magnetic mudstone in a matrix of moderately 610 ± 30 °C in the Matthew Creek metamorphic zone. Accord- magnetic mudstone in diamictite near Tobias Creek in the Lemhi ing to depth-pressure-temperature curves by Kern and Weisbrod Range (southeast of the OZ). In the Salmon River Mountains, (1967, p. 90), this indicates that peak metamorphism occurred however, rocks of the OZ contain only detrital magnetic mud at a depth of ~12 km in a thermal gradient that was abnormally chips (possibly derived from the diamictite of Tobias Creek), but hot for either continental or oceanic crust. McFarlane (2015) not of local submarine-exhalative origin in rocks of the OZ in the noted that sillimanite in the Matthew Creek metamorphic zone is Salmon River Mountains. strongly lineated to the northeast. He suggested that removal of Furthermore, those samples from the OZ that contain somewhat Cretaceous clockwise rotation would restore this into rough par- elevated concentrations of Cu (but not Co) are from the vicinity of allelism with the basin margin, and that this orientation would be the Iron Creek Cu-Co prospect area, where semi-massive magnetite consistent with transpressive to transtensional tectonic settings deposits are stratabound, but are interpreted here as epigenetic brec- during East Kootenay orogeny. cia, vein, and replacement-style deposits, hosted in phyllitic biotitic In the Lemhi subbasin, Doughty and Chamberlain (1996) siltite that was metamorphosed to biotite grade before being chlo- reported that migmatitic pelites near the base of the Salmon River ritized, veined, and partly replaced by early hydrothermal pyrite intrusive complex contain leucosomes dated at 1370 Ma, and they ± magnetite, and later chalcopyrite ± cobaltian pyrite. noted that the leucosomes contain metamorphic minerals consis- Likewise, most samples from the BZ that contain anomalous tent with metamorphism and partial melting at 6.5 ± 0.5 kbar and Co (but not Cu) are from an area labeled BZg in Fig. 2B, where 690 ± 50 °C. According to depth-pressure-temperature curves by the geochemically anomalous samples are from beds of silici- Kern and Weisbrod (1967, p. 90), this indicates that peak meta- fied scapolite-biotite hornfels (after argillite) between siltite beds morphism occurred at a depth of ~22.5 km in a thermal gradient in banded siltite within the contact-metamorphic aureole of the that was higher than normal for continental crust. nearby granitic pluton of Big Deer Creek. O’Neill et al. (2007) mapped the Great Divide megashear (GDM in Fig. 1A) as a 3–8-km-wide mylonitic shear zone of EAST KOOTENAY OROGENY (Ca. 1379–1325 Ma) Mesoproterozoic age with more than 50 km of left-lateral offset. This is consistent with a transpressive to transtensional setting for The East Kootenay orogeny affected rocks of the western the East Kootenay orogeny in the Lemhi subbasin as suggested margin of the Belt-Purcell Basin and much of the Lemhi subbasin by McFarlane (2015) and with dynamothermal metamorphism, from ca. 1379 to 1325 Ma. McMechan and Price (1982) applied bimodal plutonism, and hydrothermal mineralization during the the term “East Kootenay orogeny” to a Mesoproterozoic episode East Kootenay orogeny in the Lemhi subbasin. of metamorphism and granitic plutonism in the Purcell Basin of southeastern British Columbia. The southern part of the East Koo- Middle Mesoproterozoic Plutonism (Ca. 1370–1335 Ma) tenay orogen is imposed on the Lemhi subbasin (Fig. 1A), where Evans and Zartman (1990) noted that plutons of a bimodal gab- McFarlane (2015) reported a zircon U-Pb age determina- bro-granite suite (dated 1370 ± 10 Ma) intruded metasedimentary tion of 1365 ± 10 Ma for the peraluminous granite of Hellroaring strata that had been previously folded and metamorphosed to bio- Creek (in southern British Columbia). This pluton intruded an tite grade during an earlier stage of the East Kootenay orogeny. open anticline in low-grade metasedimentary strata of the lower

Belt-Purcell succession. McFarlane suggested that this granite crystallization of the nearby megacrystic granite pluton of Big intruded to a relatively high structural level, while peak meta- Deer Creek. morphism occurred at lower-crustal levels in the nearby Matthew Landis and Hofstra (2012, p. 1189) suggested that volatiles Creek metamorphic zone. He suggested that late-stage pegma- trapped in Blackbird gangue quartz permit models for ore gen- tites (1335 ± 5 Ma) were emplaced during transtensional uplift esis involving evaporative connate brine from metasedimentary and exhumation of the Matthew Creek metamorphic zone, which strata, heated by and mixed with fluids from the nearby bimodal he interpreted as a Mesoproterozoic metamorphic core complex. plutonic complex. With Co and Cu concentrations nearly an In the Lemhi subbasin, a bimodal suite of gabbroic and order of magnitude higher than those of metasedimentary coun- granitic plutons (1370 ± 10 Ma) intruded previously folded try rocks or megacrystic monzogranite, metamafic amphibolite metasedimentary strata, according to Evans and Zartman (1990). would seem a possible source rock for the Co and Cu that are The Salmon River intrusive complex (Fig. 2B) consists of a large concentrated in Blackbird Co-Cu ore zones. However, metamafic pluton of megacrystic monzogranite (locally metamorphosed to amphibolite has not been found in the Blackbird district, except augen gneiss or orthogneiss) underlain by gabbroic sills (mostly as clasts in mylonitic breccia along the Little Deer Creek reverse- metamorphosed to amphibolite). The monzogranitic pluton to-thrust fault (LDf in Figs. 6C, 7, and 8). Nevertheless, abundant locally contains bulbous masses of mafic rock, hybrid dioritic clasts of metamafic amphibolite in this fault breccia indicate that rocks, and sparse mafic dikes, the ages of which are all ca. 1370 metamafic amphibolite was present below the Blackbird struc- ± 10 Ma, according to Evans and Zartman (1990), Doughty and tural block before it was displaced upward and eastward along Chamberlain (1996), and Aleinikoff et al. (2012b). the Little Deer Creek fault.

Megacrystic Monzogranite and Metamafic Amphibolite GRENVILLE-AGE METAMORPHISM AND Megacrystic monzogranite typically contains large K-feld- MINERALIZATION (Ca. 1200–1000 Ma) spar phenocrysts in a coarse-grained matrix of plagioclase, quartz, and biotite (Fig. 6). Evans and Zartman (1990) reported Late Mesoproterozoic Metamorphism (Ca. 1190–1006 Ma) minor primary muscovite in some samples and noted that available chemical analyses indicate a slightly peraluminous compo- Anderson and Davis (1995) documented geologic and iso- sition. Some K-feldspar phenocrysts are rimmed by oligoclase to topic evidence of a thermal disturbance that affected rocks of the form rapakivi texture, so megacrystic monzogranite was charac- Belt-Purcell Basin, northwestern Canada, and southern Australia, terized as A-type rapakivi granite by Schulz (2013), who noted at ca. 1120–1030 Ma (during the time span of the Grenville orog- that such granites typically have high contents of incompatible- eny along the eastern margin of Laurentia). Vervoort et al. (2005) element contents (such as REEs) but low contents of Co, Sc, Cr, reported Lu-Hf age determinations of 1149–1006 Ma on cores Ni, Ba, Sr, and Eu. of garnets in Belt rocks of northern Idaho, and they suggested Spidergrams for samples of megacrystic granite (Fig. 6B) that these indicate an episode of Grenville-age crustal thickening generally match those for a set of leucocratic continental granit- along the western margin of Laurentia. oid rocks, and a spidergram for metamafic amphibolite is similar Aleinikoff et al. (2015) also reported U-Pb ages of 1160– to that of a mafic sill belonging to a set of continental flood basalts 1050 Ma on metamorphic xenotime from older rocks of the analyzed by Thompson et al. (1984). Chemical data reported by Revett, McNamara, and Garnet Range Formations in the Belt Lewis and Frost (2005) indicate that five samples of megacrystic basin (Fig. 3). They noted an apparent absence of pervasive defor- monzogranite contain ~6 ppm Co and ~6 ppm Cu, whereas a mational fabrics in the dated samples, which they interpreted to sample of metamafic amphibolite from the Salmon River intru- indicate that Grenville-age metamorphism was predominantly sive complex contains 51 ppm Co and 65 ppm Cu. thermal, and possibly caused by regional mafic underplating of Vhay (1948) and Slack (2012) suggested that the Black- continental crust. bird Co-Cu deposits are genetically related to the megacrystic Grenville-age metamorphism also affected rocks in the monzogranite pluton of Big Deer Creek (which Vhay interpreted Lemhi subbasin, according to Panneerselvam et al. (2012, as a manifestation of the Cretaceous Idaho batholith, but which p. 1185), who reported that “Whole-rock samples of the Apple Evans and Zartman dated as Mesoproterozoic). That pluton is Creek Formation define a highly radiogenic common lead iso- exposed ~6 km northeast of the Blackbird mine, and it likely dips chron with a slope corresponding to 1190 ± 60 Ma.” southwestward beneath the Blackbird mine, as indicated by an associated low-magnetic-potential anomaly that extends toward Late Mesoproterozoic Mineralization

the Blackbird district (Mankinen et al., 2004). Aleinikoff et al. (Stage Y3, Ca. 1200–990 Ma) (2012b) reported isotopic age determinations of 1378 ± 4 Ma for megacrystic monzogranite of Big Deer Creek, and 1370 ± 4 Ma Hydrothermal mineralization accompanied Grenville-age for early hydrothermal xenotime, surrounded by cobaltite in metamorphism of rocks in the Belt-Purcell Basin and the Lemhi cobaltite-biotite ore. This indicates that early hydrothermal subbasin, as indicated by the following age determinations on activity in the Blackbird mine area began during or soon after hydrothermal minerals from the Coeur d’Alene and Blackbird

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districts (Fig. 2A). For arsenopyrite from a quartz-siderite- in that it is concentrated along shear zones that transect previ- tetrahedrite vein cutting previously folded quartzite and siltite of ously folded beds of banded siltite. Nevertheless, ore zones also the Ravalli Group in the Coeur d’Alene Ag-Pb-Zn district, Arka- are stratigraphically controlled, in that argillitic layers in the dakskiy et al. (2009, p. 83) reported a Re-Os isochron age of upper banded siltite unit are preferentially sheared, biotititized, ca. 1200 Ma with a very high initial 187Os/188Os ratio, interpreted and mineralized. to indicate a crustal source of Os. Aleinikoff et al. (2012b) reported 207Pb/206Pb age determina- Mineral Depositional Sequence tions of 1058 ± 11 Ma and 990 ± 12 Ma for unzoned hydrother-

mal xenotime3 from the Merle ore zone in the Blackbird deposit. Anderson (1947) tabulated and described mineral deposi- These age determinations indicate that a pulse of hydrothermal tional sequences for two stages of mineralization in cobaltite- mineralization occurred at Blackbird in association with Gren- biotite lodes, cobaltite-tourmaline lodes, and cobaltite-quartz ville-age regional thermal metamorphism along the western lodes; and for three stages of mineralization in gold-copper- margin of Laurentia (ca. 1200–1000 Ma). Quartz-biotite and cobalt lodes. He suggested that all stages of mineralization quartz-tourmaline breccias ± cobaltite, which appear to cut older occurred in association with mafic magmatism in early Tertiary

deposits of cobaltite1 and cobaltite2, are here assigned to this Late time. Mineral-depositional sequences described by Vhay (1948) Mesoproterozoic episode of regional metamorphism and local and tabulated by Lund et al. (2011) are mostly consistent with hydrothermal mineralization. those of Anderson (1947). However, Vhay (1948) suggested a Cretaceous age, based on the spatial association of the Blackbird COMPOSITE BLACKBIRD Co-Cu DEPOSITS district with the monzogranite pluton of Big Deer Creek, which he mistakenly correlated with the Cretaceous Idaho batholith. The Blackbird Co-Cu deposit is composite in that it includes Lund et al. (2011) also suggested a Cretaceous age, but on at least eight stratabound ore zones and many discordant lodes the basis of perceived structural control, related to Cretaceous (Figs. 7 and 8). These are hosted by hydrothermally altered and thrust-faulting. metamorphosed rocks in the upper part of the banded-siltite unit, The mineral depositional sequence summarized in Table 3 where it consists of interlayered siltite and biotitite (interpreted as is constrained by observed relative age relationships, and biotitized argillite that is preferentially mineralized). Stratabound calibrated by currently available isotopic age determinations ore zones also are composite, in that they contain Mesoprotero- on rocks and minerals associated with Co-Cu deposits of the zoic cobaltite-biotite and cobaltite-tourmaline ore (of Mesopro- Blackbird district. Such age constraints bracket an episode of terozoic age), which is variably overprinted by Fe-Cu-sulfide Co-dominant mineralization in Mesoproterozoic time (episode breccias, veins, and replacement-style deposits containing vari- Y), and an episode of Fe-Cu-dominant mineralization in Cre- ous combinations of quartz, pyrrhotite, pyrite, chalcopyrite, and taceous time (episode K). The time span of Mesoproterozoic cobaltian arsenopyrite ± siderite. episode Y (ca. 1370–1000 Ma) includes that of the East Koote- In the Blackbird mine area, argillite beds in six stratigraphic nay orogeny along the western margin of the Belt-Purcell Basin intervals are altered to biotitite, containing at least 75 vol% of (ca. 1379–1325 Ma), and that of Grenville-age metamorphism greenish biotite of probable metamorphic-hydrothermal origin, along the western margin of Laurentia (ca. 1200–1000 Ma). which is preferentially mineralized (Figs. 4B, 5C, 7, and 8). The time span of Cretaceous episode K (ca. 110–83 Ma) cor- Some of these intervals contain multiple stacked horizons of min- responds to that of the Sevier orogeny in the Cordilleran orogen eralized biotitite interbeds (up to nine mineralized horizons in the (ca. 112–85 Ma). Merle zone, six in the Ram zone, and four in the Sunshine zone). However, such mineralized horizons are too closely spaced to be MESOPROTEROZOIC COBALTITE DEPOSITS shown in Figures 7 and 8, and not all of them are of sufficient (Ca. 1370–1000 Ma) grade, thickness, or continuity to be considered ore. From southeast to northwest, stratabound ore zones with Cobaltite is the principal cobalt-bearing mineral of the past production or estimated resources include the Northfield Blackbird district. Although cobaltite [(Co>>Fe)AsS] forms a and Merle zones (east of the axis of the Idaho syncline), and the solid solution series with glaucodot [(Co>Fe)AsS)], the cobalt- north Idaho, Chicago, Brown Bear, Horseshoe, Blacktail, and ite-glaucodot series is undivided here, and is simply regarded Ram ore zones along the southwest limb of the Idaho syncline, as cobaltite, which is gunmetal gray with a pinkish-violet tar- where they strike northwest and dip ~50°–65° northeast (Fig. 8). nish. Major cobaltite is commonly associated with minor quartz Although the Chicago and Brown Bear ore zones appear nearly + major greenish-black biotite ± minor to major black tourma- concordant at the scale of the geologic map in Figure 8, larger- line ± minor xenotime ± minor chalcopyrite in disseminated to scale geologic maps of underground workings by Vhay (1948) semi-massive replacement-style deposits, breccias, and veinlets. and Blackbird mine geologists, indicate that these ore zones cut Textural relationships indicate that early hydrothermal quartz is

across broad F1 folds in bedding at low angles. This is inter- largely replaced by biotite, which is partly replaced by cobaltite preted to indicate that mineralization is structurally controlled, ± minor late chalcopyrite.

TABLE 3. TECTONISM AND MINERALIZATION IN THE BELT- PURCELL BASIN AND BLACKBIRD Co-Cu DISTRICT c Tectono-stratigraphic Key geologic Alteration & Age (Ma)* Ore and gangue mineralsa features and episodes features metam minsb Stage

Ccp, Py, Mrc, Bn, CoPy, Sg, Cob, Gn, Sp, Tn (Sheep >1470– Brt, Dol, Qtz, Belt basin rift Creek, sed-hosted diagenetic repld deposit in lower Belt 1454 Very thick Chl, Cal sedimentary strata)e1 successions, local

Nuna— Belt basin sag, maﬁ c sills and Ccp, Cc, Bn, Dg, Py, Gn, Sp, Po, Tet, Mag, Hem, CoPy, Ank, Chl, Ab, 1454– Lemhi subbasin ﬂ ows Cob (Spar Lake, sed-hosted diagenetic repl deposit in Cal, Qtz, Ksp, 1379 e2 Belt-Purcell basin rift-sag Revett Fm) wMca

f F1 folds Open folds (>1370 Ma)bBt

Granite (ca. 1370 Ma), Co-Cu-Ygeochem. (BZg)g East Kootenay Bimodal plutonic bBt, Scp hf Y orogeny in suite 0 1379– Xtm1 (ca. 1370–1320 Ma) (BB) gBt

y orogen western Belt- 1325 Purcell basin and Cob Stratabound Cob —dissem, repl, bx (BB)g Qtz, bx, gBt Lemhi sub-basin 1 1

ootena Y F folds in Cob Cob dissem, repl—Cob -Bt ore locally folded into F 1 2 1 1 1 2 gBt > Tur ore folds (BB) Nuna to Rodinia— 1325– East K Post-orogenic Cob along S Cob vn, repl + Xtm (ca. 1320–1270 Ma) in Qtz-gBt bx 2 2 2 2 gBt > TurY 1270 extension? cleavage (BB) 2

e3 1200– West margin, Grenville-age Metamorphic homogenization of U/Pb (ICB)Grt (N.ID) Y 1000 Rodinia (?) metam. 3 Cob3 + Xtm3 (ca. 1060–990 Ma) in Qtz-Tur bx Tur > gBt Laurenti a 665–650 Rodinia to Windermere rift Within-plate Qtz, Mag, Ccp, Au (Cu Camp, vn, repl)e4 Bt plutons and dikes Rifted margin, that cut Cob ore Qtz, Cal, Sid, Py, Ccp, Gn, Tet, Hem, Au (Yellowjacket, 550–485 Qtz, Chl, Ser, Cla Laurentia (?) (in BB district) vn, vnlt, dissem)e5 ange a

550–155 Laurentia Westward-thickening sedimentary wedge across the rifted continental margin. to P

155–142 Nevadan orogeny—Salmon Canyon thrust, F folds, Grt (ca. 150–93 Ma) Grt, Bt

3 K

0 142–112 Lull in orogenesis Mnz1 (ca. 144–110 Ma) (BB) Grt, Bt, Ctd

ica— Mnz (ca. 110–92 Ma), Xtm (ca. 104–93 Ma) (BB) Grt, Sil (SC)f Metamorphic- 2 4 f

Amer plutonic Py, Mag, CoPy, Ccp vn, repl (IC) Chl (IC)

112–85 Sevier orogeny hinterland and K1 fold-thrust belt Po, Py, Ccp, CoApy bx (BB)h Grt, Bt (BB) with F folds 4 Ccp, Qtz, Py, Po, Sid, CoApy vn (BB, BP)g Ms, Chl Cordilleran Orogen

Laurasia to N. Qtz, Au-Au-Bi vnlt + Ser (ca. 83 Ma) (BB) Ser Basement-cored 85–55 Laramide orogeny K uplifts 2 Qtz-Py-Au-Ag vnlt, dissem (Beartrack)e6 Ser

*See text and Figures 3 and 4 for sources of age determinations and age ranges summarized in this table. a. Ore- and gangue-mineral abbreviations: Apy—arsenopyrite, Au—gold, Bi—bismuth, or bismuthinite, Bn—bornite, Cc—chalcocite, Ccp—chalcopyrite, CoApy—cobaltian arsenopyrite, Cob—cobaltite, CoPy—cobaltian pyrite, Dg—digenite, Gn—galena, Hem—hematite, Mag—magnetite, Mnz— monazite, Mrc—marcasite, Po—Pyrrhotite, Py—pyrite, Qtz—quartz, Sg—siegenite, Sid—siderite, Sp—sphalerite, Tet—tetrahedrite, Tn—tennantite, Xtm—xenotime. b. Alteration- and metamorphic-mineral abbreviations: Ank—ankerite, bBt—brownish biotite, Brt—barite, Bt—biotite, Cal—calcite, Chl—chlorite, Cla— clay minerals, Ctd—chloritoid, Dol—dolomite, gBt—greenish biotite, Grt—garnet, Ms—muscovite, Qtz—quartz, Scp—scapolite, Ser—sericite, Sil— sillimanite, Tur—Tourmaline.

c. Episode Y—Mesoproterozoic mineralization. Stage Y0—pre-ore dynamothermal metamorphism, bimodal plutonism (1370 ± 10 Ma), and contact metamorphism + Xtm1 deposition (1370 ± 4 Ma); Stage Y1—stratabound Cob1-biotite ore (ca. 1370 to 1320 Ma); Stage Y2—Xtm2 (ca. 1320 to 1270 Ma) with Cob2 inclusions, and Cob2 along S2 cleavage of F2 folds; Stage Y3—Xtm3 (ca. 1060 to 990 Ma) + Cob3 deposition in Qtz-Tur bx. Episode K— Cretaceous mineralization. Stage K0—metamorphic Bt, Grt, and Ctd (ca. 150 to 93 Ma) and early Mnz (ca. 144 to 110 Ma). Stage K1—Fe-Cu-sulﬁ de deposition in quartz veins and breccias with associated Mnz (mostly ca. 110 to 92 Ma) and Xtm4 (ca. 104 to 93 Ma). Stage K2—Qtz vnlts, some with electrum + Bi minerals + Ser (ca. 83 Ma). d. Deposit-style abbreviations: bx—breccia, dissem—disseminated, hnﬂ s—hornfels, vn—vein, vnlt—veinlet, repl—replacement. e. References: 1. Graham et al. (2012), 2. Aleinikoff et al. (2012a), 3. Panneerselvam et al. (2012), 4. Cater et al. (1973), 5. Johnson et al. (1998), 6. Hawksworth et al. (2003).

f. Fold-set and cleavage-set abbreviations: F1, F2, F3, F4—numbered fold sets, S2—axial-plane cleavage of the F2 fold set. S2 cleavage is transitional to shear-slip cleavage, and to transposed layering, related to the F2 fold set. g. Place-name abbreviations: (BB)—Blackbird district, (BP)—Black Pine, (BZg)—biotite-zone geochemical anomaly near Deep Creek, (IC)—Iron Creek, (ICB)—Idaho cobalt belt, (N. ID)—northern Idaho, (SC)—Salmon Canyon Copper.

h. In high-grade composite Co-Cu ore zones of the Blackbird district, minerals of the K2 and K3 assemblages are superimposed on minerals of the Y1 and Y2 mineral assemblages.

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Xenotime in Cobaltite Deposits (Ca. 1370–1000 Ma) ite deposition during Mesoproterozoic time, as follows: stage

Y0 (ca. 1370 Ma)—geochemically anomalous Co and Y in

Aleinikoff et al. (2012b) reported U-Pb age determinations silicified scapolitic biotite hornfels (BZg in Fig. 2B), stage Y1

on xenotime in cobaltite-biotite ore from the Merle ore zone. (ca. 1370–1320 Ma)—cobaltite1-biotite ore + xenotime1 (in strat-

Backscattered-electron (BSE) images in Figure 9 show cores abound replacement-style ore that is locally F2-folded), stage Y2

and rims of selected xenotime grains dated by Aleinikoff et al. (ca. 1320–1270 Ma)—cobaltite2-biotite ore + xenotime2 (along

(2012b). Most of the samples of xenotime in cobaltite-biotite ore axial planar S2 cleavage of F2 folds), and stage Y3 (ca. 1060–

yielded Middle Mesoproterozoic age determinations between 990 Ma)—cobaltite3 + xenotime3 (in discordant quartz-tourma- ca. 1370 and 1270 Ma—a time span which overlaps with that line breccias). of the East Kootenay orogeny (ca. 1379–1325 Ma). However,

Aleinikoff et al. (2012b) reported two Late Mesoproterozoic age Stratabound Cobaltite1-Biotitite Ore ± Xenotime

determinations of 1058 ± 11 Ma and 990 ± 12 Ma for grains (Stage Y1, Ca. 1370–1320 Ma)

of hydrothermal xenotime3 from the Merle ore zone. This time span overlaps with that of Grenville-age metamorphism along Textural relationships between xenotime (dated 1370 Ma and

the western margin of Laurentia. surrounded by cobaltite1, rimmed by cobaltite2) indicate that cobalt-

In Figure 9A xenotime1 (1370 ± 4 Ma) is surrounded by ite1 is younger than 1370 Ma but older than cobaltite2 (Fig. 9A).

cobaltite1, which is rimmed by cobaltite2. In Figure 9B xeno- However, xenotime grains with cobaltite inclusions appear similar

time2 (1316 ± 6 Ma) is rimmed by Cretaceous xenotime4 (101 to xenotime grains dated 1316 ± 6 to 1271 ± 7 Ma (Figs. 9A–9D

± 2 Ma). Figure 9C shows a grain of xenotime2 that contains and captions). These relationships are interpreted to indicate that

abundant cobaltite inclusions. That xenotime grain was not dated, cobaltite1 probably is older than ca. 1316 Ma, so its approximate but its vaguely splotchy zoning appears similar to that shown age probably is bracketed between ca. 1370 and 1320 Ma.

in grains of xenotime2 that were dated 1316 ± 6 Ma and 1271 Cobaltite1-biotite ore of stage Y1 is concentrated in strata-

± 7 Ma (Figs. 9B and 9D). The core of the xenotime grain dated bound ore zones of disseminated to semimassive cobaltite1 sur-

ca. 1271 Ma (Fig. 9D) contains a white inclusion, which may be rounded by greenish biotite ± tourmaline ± xenotime1 (Figs. 9E

older cobaltite. A surrounding rim of xenotime4 resembles that and 9F). Such ore is preferentially hosted in layers of greenish- dated ca. 101 Ma (Fig. 9B). black biotitite (between layers of siltite) in the upper part of the

If an episode of cobaltite deposition accompanied or closely banded siltite unit. In relatively undeformed cobaltite1-biotite ore,

followed each episode of xenotime deposition, these xenotime cobaltite-rich bands (or layers) appear stratiform, and cobaltite1 age determinations would indicate multiple stages of cobalt- grains generally appear subcubic to equant but rounded.

Figure 9. Backscattered electron (BSE) images of cobaltite and xenotime grains (reproduced from Aleinikoff et al. [2012b] with permission from

J.N. Aleinikoff), a photo of banded cobaltite-biotite ore, scanning electron microscope (SEM) image of disseminated cobaltite1 in biotite phyllite + tourmaline, two photos of F2-folded cobaltite-biotite ore, and a photo of nearly isoclinal F2 folds in banded siltite (Ylab) cut by a mafic dike (mb): (A) BSE image showing an inclusion of xenotime1 in cobaltite1, rimmed by younger cobaltite2 (labeled Cob2). The inclusion of xenotime1 belongs to a set of nine xenotime1 cores that yielded an average age of 1370 ± 4 Ma (Aleinikoff et al., 2012b). The surrounding cobaltite is therefore younger than ca. 1370 Ma—image contrast was adjusted to display compositional zoning in cobaltite. (B) BSE image showing weakly

zoned xenotime2, dated 1316 ± 6 Ma, rimmed by unzoned xenotime4, dated 101 ± 2 Ma by Aleinikoff et al. (2012b)—image contrast was adjusted to show compositional zoning in xenotime in Figures 9B, 9C, and 9D, where cobaltite appears bright white, and the black background

is mostly biotite. (C) BSE image showing several cobaltite inclusions in xenotime2 with weak splotchy zoning, which fits the description of weakly zoned and unzoned xenotime core and splotchy zoned grains that were dated ca. 1315 to 1270 Ma by Aleinikoff et al. (2012b, p. 1163).

(D) BSE image showing a weakly zoned xenotime2 core (dated 1271 ± 7 Ma by Aleinikoff et al., 2012b). Near its outer margin, this xenotime2 contains a semi-rectangular inclusion that appears bright white (like the cobaltite in Fig. 9C). This probable cobaltite is therefore older than

about 1270 Ma. The xenotime2 core is rimmed by xenotime4, similar to that shown in Figure 9B, which was dated 101 ± 2 Ma by Aleinikoff et al. (2012b). (E) Photo of a polished slab of banded (semistratiform) cobaltite1-biotite ore of stage Y1 from the Blacktail ore zone, showing layer-like bands of disseminated to semimassive cobaltite1 in a matrix of greenish biotitite. Relatively large cobaltite grains (such as those labeled Cobrxt) may be recrystallized. Late veinlets of pyrite and chlorite ± chalcopyrite cut this banded cobaltite1-biotite ore. (F) SEM image of schistose cobaltite1-biotite ore + tourmaline in sample 1BB-SP-79-803 from the Brown Bear zone. This sample also contains minor chalcopyrite (Ccp) and very fine-grained allanite (Aln) in a schistose matrix of biotite (Bt) and tourmaline (Tur). The chalcopyrite appears pseudomorphic

after schistose biotite. Qtz—quartz; Cob—cobaltite. (G) Photos showing F2 folds in folded cobaltite1-biotite ore, overprinted by postfold Fe-Cu- sulfides. (a) A surface cut along 2F fold axes, viewed from above, showing lines of deformed cobaltite grains, subparallel to F2 fold axes. (b) A surface cut across F2 fold axes, showing point-like cobaltite grains, as the lines of grains appear in this end-on view, looking along fold axes. Irregular patches of pyrite (Py), pyrrhotite (Po), and chalcopyrite are overprinted across the lineated fabric of this F2-folded cobaltite-biotite ore. (H) Northeast-looking photo of a road cut through a crumpled zone of nearly isoclinal F2 folds in banded siltite (Ylab). These F2 folds plunge ~45° north-northeast and have nearly vertical axial-planar cleavage (S2). A mafic dike (mb) cuts the folded strata, and the dike is neither folded nor foliated. This road cut is between the portals of the 6850 level of the Blackbird mine (to the left) and the caved-in workings of the Uncle Sam mine (to the upper right).

A B D C Xtm Cob Xtm2 4 Xtm1 Cob2 Xtm2 Cob1 Xtm2 Xtm4 Cob Cob?

G(a) E Ccp Py Ccp +Ccp btt Po+Ccp Po+Ccp Cob1 Ccp Cob1 Cob1 Bt+Tur Chl Cob Po +Ccp rxt Cob1 Cob1 Py Cob1 1 cm Cob1 Cob1 Bt Bt

G(b) Py Bt Po+Py F F Aln Ccp Cob1 Cob1

1cm Cob1 Tur H Ccp Bt

Qtz

Ylab mb

2 S

F2 ° 45 NNE plunge 1 m

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However, cobaltite1-biotite ore is locally folded and lineated, Quartz-Biotite and Quartz-Tourmaline Breccias

with lines of cobaltite grains (Fig. 9G[a]) that appear point-like ± Cobaltite3 (Stage Y3, Ca. 1200–1000 Ma) as viewed along fold axes (Fig. 9G[b]). Likewise, Vhay (1948)

noted that in crumpled zones of tight folds (like the F2 folds Quartz-biotite and quartz-tourmaline breccias and veins

shown in Fig. 9H), high-grade cobaltite-biotite ore is concen- ± cobaltite3 typically contain partly resorbed remnants of whit- trated in cigar-shaped ore shoots that plunge north-northeast ish quartz clasts in a matrix of biotitite or tourmalinite (after

(parallel to F2 fold axes). Thus, cobaltite1 was deposited before microbreccia). Cobaltite in such breccias is tentatively classified

or during F2 folding, and was locally deformed during F2 folding, as cobaltite3, because these breccias are regarded as later than

to form cigar-shaped lodes with lineated fabrics that parallel fold replacement-style deposits of cobaltite2, and this supports a logi-

axes in crumpled zones of F2 folds. cal correlation with hydrothermal xenotime3 (dated ca. 1058 Ma and 990 Ma by Aleinikoff et al., 2012b). This corresponds to a

Cobaltite2-Biotite Ore along S2 Cleavage (Stage Y2, Late Mesoproterozoic episode of Grenville-age metamorphism

Ca. 1320–1270 Ma) in the western Belt-Purcell Basin (Vervoort et al., 2005) and the Lemhi subbasin (Panneerselvam et al., 2012).

Cobaltite2-biotite ore of stage Y2 commonly follows S2

cleavage of F2 folds, as shown in Figure 10A. In crumpled zones Quartz-Biotite Breccias and Veins ± Cobaltite3

of tight F2 folds in banded siltite, S2 cleavage is transitional to S2 Quartz-biotite breccias commonly contain clasts of whit-

shear-slip cleavage, which is transitional to S2 transposed layer- ish quartz (± clasts of biotitic siltite and phyllite) in a matrix

ing, in which siltite boudins are interlayered with phyllitic bio- of biotitite ± disseminated to semi-massive cobaltite3. Matrix

titite (Fig. 10B). and clasts are either cut by veinlets of cobaltite3 (Fig. 10D), or

Cobaltite2-biotite lodes of the South Idaho ore zone are in partly replaced by semi-massive to massive cobaltite3 (Fig. 10E). the tight, north-plunging hinge zone of the Idaho syncline, where Some quartz-biotite breccias are not notably foliated, but others they are preferentially hosted in biotitite phyllite between sil- are well foliated, with aligned lenticular clasts of quartz ± siltite itite boudins. The Sunshine and Sunshine East ore zones are on in a matrix of biotitite ± disseminated cobaltite ± semi-massive opposite limbs of the nearly isoclinal and west-vergent Sunshine cobaltite bands or lenses, which commonly are elongate subpar-

syncline (Fig. 7B), where cobaltite2 is preferentially hosted in allel to lenticular clasts, or conform with swirly flow foliation chloritized biotitite phyllite between boudins of sugary-textured in the matrix. Tabular quartz-biotite veins contain biotite books quartzite, interpreted here as silicified and recrystallized siltite of in a matrix of whitish quartz but have not been found to contain the banded-siltite unit (Fig. 10C). cobaltite.

Inasmuch as xenotime2, dated ca. 1320 to 1270 Ma is simi-

lar to xenotime that contains cobaltite inclusions, this is inter- Quartz-Tourmaline Breccias ± Cobaltite3

preted to indicate that cobaltite2 probably was deposited between Quartz-tourmaline breccias typically contain whitish quartz ca. 1320 and 1270 Ma, a time interval that overlaps with late to clasts in a black matrix of microcrystalline tourmalinite (± cobalt-

post-metamorphic stages of the East Kootenay orogeny (ca. 1379 ite3 ± minor chalcopyrite). Dike-like to pipe-like bodies of to 1325 Ma). quartz-tourmaline breccia and stratabound bodies of tourmalin-

Figure 10. Photos of cobaltite2 along S2 cleavage in F2-folded banded siltite, and of cobaltite3 in quartz-biotite and quartz-tourmaline breccias: (A) F2 syncline with axial-planar S2 slip-shear cleavage in banded siltite. Biotitized argillite beds ± cobaltite1 are folded, and S2 shear-slip fractures host biotite ± cobaltite2 (photo by J.F. Slack, 6850 level, Blackbird mine). (B) Structurally transposed layering, defined by siltite boudins in a matrix of biotitite ± local cobaltite2 after argillite (as seen along the northeast rib of the Hawkeye adit, which transects the tight hinge zone of the Idaho syncline, as indicated on Fig. 8). On the right side of the image, a black mafic dike (mb) follows transposed layering. Chalcopyrite-quartz veinlets generally follow but locally cut into the dike, which also contains a pod of white quartz (probably of late metamorphic-hydrothermal ori-

gin). Oxidized surfaces are coated with limonite (Lm—appearing medium gray on black). (C) Cobaltite2-quartz-chlorite ore from the Sunshine zone (sample GHBB18). Cobaltite2 is concentrated in chloritic bands between lenses of micromosaic quartz. Such ore is typical of the Sunshine zone, which is in the tight hinge zone of the nearly isoclinal Sunshine syncline. Quartz lenses are interpreted as siltite boudins that are silicified and recrystallized to micromosaic quartz. The chloritic matrix contains local remnants of biotite, and is therefore interpreted as chloritized

biotitite (after the argillite component of banded siltite). Cobaltite stringers locally coalesce into semimassive cobaltite2. Strings of cobaltite grains (Cob2rxt) that are inclined at ~65° to S2 (lower-left) are interpreted to indicate recrystallization, possibly during Cretaceous metamorphism. (D) Quartz-biotite breccia cut by a swarm of anastomosing veinlets of cobaltite3—Minor clasts of white vein quartz appear rounded and partly replaced by the surrounding biotitite matrix, which contains disseminated cobaltite and veinlets of cobaltite3. (E) Quartz-biotite breccia, cut by quartz veinlets, and partly replaced by disseminated to massive cobaltite3. Boundaries between quartz clasts, matrix biotitite, and disseminated to massive cobaltite3 are gradational. This is interpreted to indicate that clasts of early vein quartz were partly replaced by biotitite, which was cut by discontinuous quartz veinlets before being replaced by cobaltite3 across a gradational replacement front. (F) Quartz-tourmaline breccia cut by cobaltite3 veinlets in sample (GHBB23) from the Haynes-Stellite mine (HS in Figs. 2 and 7).

A B Lm sltt in F btt ±Cob Yb F2 mb Qtz-Ccp 2 folded be Qtz Bt ± Cob

2 2

s along S

sltt sltt Yb mb

clvg

Ccp>Qtz 20 cm btt btt ±Cob 5 cm D C Qtz

Cob3 Cob2 sltt Cob2 Qtz

S btt + Cob3 2 Cob btt + Cob3

2rxt 1 cm Cob Chl 1 cm

E FF

Cob Tur +Cob Tur1 Qtz 2 3 btt+Tur Qtz Qtz

Tur2+Cob3 Cob3 3 Cob2 b 1 cm 5 mm Co

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ite in quartzite are widely scattered in a broad belt that extends A gabbroic dike that is ~20 m thick trends north to northwest east-southeastward across the south end of the Blackbird district along the western margin of the Idaho ore zone (mg in Fig. 8).

(Fig. 2B). Some of these quartz-tourmaline breccias are envel- That dike cuts F2 folds, and its margins are flow-foliated par- oped by biotitized host rocks. According to Trumbull et al. (2011), allel to its contacts, but its interior is not foliated. Vhay (1948) tourmaline of the Blackbird district is aluminous schorl-dravite noted that gabbroic dikes consist of plagioclase (partly replaced with B-isotopic ratios that indicate an isotopically heavy, saline by albite and zoisite), clinopyroxene (mostly replaced by dark- marine-fluid source for the contained boron. brown biotite), and amphibole (mostly replaced by chlorite and The sample of quartz-tourmaline-cobaltite breccia shown in clinozoisite). The outcropping gabbroic dike contains traces of Figure 10F is from the Haynes-Stellite mine. That sample exhib- very fine-grained pyrrhotite ± pyrite, and it is cut by a quartz vein. its at least two generations of breccia, with clasts of the older Similar dikes were encountered underground in the Uncle Sam breccia in the younger breccia. Clasts of both breccias are elon- where they cut cobaltite lodes, but are cut by mineralized faults, gate, streamlined, and similarly aligned, as are cobaltite veinlets, according to Vhay (1948).

which are deflected around clasts of the older breccia. These A thinner mafic dike cuts across F2 folds in banded siltite, relationships are interpreted to indicate multiple upward pulses as shown in Figure 9H. This dike consists mostly of fine-grained of high-velocity fluid flux, resulting in multiple episodes of brec- intergrowths of plagioclase, amphibole, and biotite. It contains ciation and tourmalinization, followed by cobaltite deposition sparse inclusions of similar but coarser-grained rocks with higher in fractures along flow foliation. Thus, quartz-tourmaline and proportions of plagioclase. It also contains sparsely disseminated

quartz-biotite breccias (± cobaltite3) are interpreted to indicate grains and discontinuous veinlets of cobaltian arsenopyrite. Some cyclical upward release and forceful injection of overpressured biotite is altered to white mica, and lenticular vugs are rimmed by hydrothermal fluid, as modeled by Sibson (2004). limonite-stained chlorite. Although most quartz-tourmaline breccias are peripheral Biotitic mafic dikes (Figs. 11A and 11B) are widely distrib- to the Blackbird mine area, Vhay (1948) mapped a dike-like uted throughout the Blackbird mine area, but most are less than body of quartz-tourmaline breccia that strikes north and dips ~3 m thick and are too narrow to be represented in Figure 8. Bio- steeply through the South Idaho ore zone (Fig. 8). That quartz- titic mafic dikes consist mostly of reddish brown biotite, but some tourmaline breccia appears to cut previously biotitized rocks, contain small phenocrysts of sodic plagioclase. Although biotitic containing cobaltite lodes, but it also contains abundant cobalt- mafic dikes are variously altered and metamorphosed, they prob- ite veinlets. ably were originally lamprophyric. Some biotitic dikes contain In the northwestern part of the Blackbird Co-Cu district, inclusions of siltite to quartzite, and one contained an inclusion of Vhay (1948) mapped a long quartz tourmaline vein west of the calcite-tremolite rock with carbon- and oxygen-isotopic compo- Tinkers Pride prospect, and black tourmalinite also is scattered sitions consistent with a magmatic history (either as carbonatite on the dump of a collapsed adit near the Bonanza Copper pros- magma, or as limestone that reacted with mafic magma, accord- pect. Quartz-tourmaline veins locally contain pyrite and chalco- ing to Johnson et al., 2012). pyrite but have not been found to contain cobaltite. Hahn and Hughes (1984) described ultramafic diatremes and breccia dikes containing fragments of carbonatite, perido- MAFIC-ALKALIC MAGMATISM (Ca. 530–485 Ma) tite, pyroxenite, and anorthosite. Drill-core logs indicate that an ultramafic breccia dike cuts cobaltite-bearing quartz-tourmaline Gillerman (2008) and Lund et al. (2010) reported U-Pb zir- breccia at the Conicu prospect (cnc in Fig. 2B). con age determinations of ca. 530 to 485 Ma for an assemblage of Blackbird mafic dikes have not yielded U-Pb zircon age gabbroid to syenitoid plutons in east-central Idaho. Samples from determinations, because they have not been found to contain zir-

these intrusions have incompatible-element profiles indicative of con (ZrSiO4). However, an effort is under way to find and date

mantle-derived within-plate magmatism (Lund et al., 2010). An baddeleyite (ZrO2), which is more likely to be present in such episode of such magmatism occurred east of the rifted continen- silica-poor rocks. Until mafic dikes can be directly dated, a prob- tal margin, in association with early subsidence and growth of the able Late Cambrian to Early Ordovician age is indicated by cor- Cordilleran miogeocline in Cambrian–Ordovician time, as sug- relation with nearby intrusions that are geochemically similar gested by Evans (1984) and Lund et al. (2010). and have been dated at 530–485 Ma by Evans (1984), Gillerman (2008), and Lund et al. (2010). Spidergrams in Figure 11C show Mafic Dikes of the Blackbird District that patterns of chondrite-normalized concentrations of incompatible elements in mafic dikes of the Blackbird district closely Mafic dikes are common in the Blackbird Co-Cu district, match those of a mafic dike associated with the syenitoid intru- and they also occur at other mines and prospects in the Idaho sive complex of Deep Creek (dated 485 Ma by Lund et al., 2010). cobalt belt. At least four types of mafic dikes are present in the These spidergrams also indicate that Blackbird mafic dikes are Blackbird district—gabbroic dikes, amphibole-biotite-plagio- enriched in Nb and Ta relative to La, as is typical for ocean-island clase dikes, biotite-rich dikes ± plagioclase, and ultramafic-brec- basalts and alkalic continental basalts, according to Thompson cia dikes and diatremes. et al. (1984). Thus, Blackbird mafic dikes lack the depletion of

Nb and Ta relative to La, which is typical of subduction-related Blackbird Garnet Zone ± Chloritoid igneous rocks, according to Thompson et al. (1984) and Tatsumi (2005), and also is typical of mafic rocks of Cretaceous to Ter- Garnet porphyroblasts are common in metasedimentary tiary age in northern Idaho (according to chemical data from rocks of the upper-northern and down-faulted western parts of Lewis and Frost, 2005). the Blackbird mine area (Fig. 8), where subhedral porphyro- Although Blackbird mafic dikes are geochemically simi- blasts of dark-red almandine are preferentially hosted in layers lar to nearby syenitoid plutons in many ways, they are variably of biotite phyllite to schist. Eiseman (1988) estimated peak meta- depleted in Sr and enriched in Rb, relative to those plutons. This is morphic temperatures of 400–520 °C, based on biotite-garnet interpreted to indicate that Blackbird mafic dikes underwent vari- geothermometry. ous degrees of differentiation by fractional crystallization of Sr- Chloritoid porphyroblasts are locally present in rocks of the bearing plagioclase to deplete Sr while enriching Rb in residual garnet zone, where they are preferentially hosted in layers of bio- magmas. Samples from Blackbird mafic dikes are also variably titic siltstone and biotitic metasandstone. Garnet and chloritoid

enriched in Co and Cu relative to the mafic dike of Deep Creek. porphyroblasts generally transect S2 schistosity, and in ore zones, Although some of the Co contained in mafic dikes may they commonly contain cobaltite inclusions (Figs. 11D and 11E). have been assimilated from cobaltite-biotite ore (which the dikes Some mafic dikes in the garnet zone also contain garnets, so gar-

cut locally), most of the Co and Cu in these dikes probably was net and chloritoid porphyroblasts are younger than cobaltite, S2 added during post-dike mineralization. Mafic dikes are locally schistosity, and mafic dikes. cut by veins and veinlets, containing minerals such as pyrite, pyrrhotite, chalcopyrite, and cobaltian arsenopyrite (Fig. 11A). Age Determinations on Metamorphic Garnet and Biotite These minerals also are sparsely disseminated in hydrothermally (Ca. 151–93 Ma) altered mafic dikes, and minerals of this assemblage are typi- Zirakparvar et al. (2007) reported Lu-Hf age determina- cal of Fe-Cu-sulfide breccias and veins of Cretaceous age in the tions of 151 ± 32 Ma for garnets from the Blacktail pit, and Blackbird district. 112.8 ± 7.7 Ma for garnets from the Salmon Canyon Copper mine. An additional Lu-Hf age determination of 93 ± 8.3 Ma is CORDILLERAN OROGENESIS (Ca. 155 to 55 Ma) reported here for garnets from a schistose biotitic mafic dike near the Tinkers Pride prospect (Fig. 1B) in the Lookout structural Cordilleran orogenesis affected rocks of the Belt-Purcell block (Fig. 7A). Basin and the Lemhi subbasin from ca. 155 to 55 Ma. According 40Ar/39Ar step-heating plateau ages of 151 ± 1 Ma and 122 to DeCelles (2004), northwest-southeast compression occurred ± 1 Ma also are reported here for metamorphic biotite in schis- in the northwestern part of the North American plate from ca. 155 tose biotitic mafic dikes that cut garnet-bearing metasedimen- to 142 Ma (during the Nevadan orogeny). This was followed by a tary rocks in the Ram zone (Fig. 8). These step-heating plateau lull and westward retreat of subduction-related magmatism from ages are interpreted to indicate when biotite last cooled below ca. 142 to 112 Ma, and then by the Cretaceous Sevier orogeny, ~350 °C. Figure 11D shows relict biotite in retrograde chlorite, during which there was widespread metamorphism and pluto- which also rims a garnet porphyroblasts. nism in the hinterland (Armstrong and Ward, 1993), and northeast-vergent folding and thrusting in the fold-thrust belt of the Gradational Base of the Garnet Zone Cordilleran orogen (Fig. 2A and Table 3). The Insular Superter- The base of the garnet zone is gradational, as previously rane was accreted to the western margin of North America from mapped by Vhay (1948) and Nash and Hahn (1989), and as rep- ca. 105 to 90 Ma (Giorgis et al., 2008), and the Idaho batholith resented in Figures 2B, 7B, and 8. The gradational base of the and associated plutons were emplaced mostly between ca. 90 and garnet zone is well exposed on the upper slope of the northeast 55 Ma (Lewis et al., 2012). Cordilleran orogenesis was then fol- wall of the Blacktail pit, and it has also been observed in drill core lowed by extensional tectonism and bimodal magmatism from from the Ram zone. On the geologic map in Figure 8, the garnet- ca. 55 to 0 Ma (Armstrong and Ward, 1991). in transition is manifested by different versions of the garnet-in boundary, as mapped by different geologists. An outer garnet-in Regional Garnet Zone line represents the lower boundary of rocks with small, sparsely scattered garnets, whereas an inner garnet-in line represents the Lewis et al. (2012) outlined a regional zone of garnet-bearing lower boundary of rocks with plenty of easily visible garnets. metamorphic rocks in the hinterland of the Cordilleran orogen in As indicated on Figure 8, the gradational base of the garnet central Idaho. The northwestern part of the ICB and the upper zone does not offset ore zones, biotitite-rich intervals, or mapped northwestern part of the Blackbird structural block are within folds, so it is not a thrust fault (as previously mapped by Tysdal that regional zone of garnet-grade metamorphic rocks (Fig. 1B). et al., 2003; Lund and Tysdal, 2007; and Lund et al., 2011). Nev- However, lower-grade metasedimentary rocks of the biotite zone ertheless, the garnet zone is locally truncated and displaced by characterize the southeastern parts of the Blackbird district and post-garnet faults, such as the thrust fault of Little Deer Creek, the ICB (as noted by Nold, 1990). and the Sunshine East normal fault (Figs. 7 and 8).

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A B 72

47 85 Cob1 Cob1 70

B Qtz la c 40 37 Ca kb Lm le ir 45 ra d 55 7 min mb Btph 44 10 e F 54 0 L 49 2

44 85 58 Cob 5 mm F2 crumpled zone 2

S2 82 mb C 87 1000 BB mafic dikes: biotitic (mb) gabbroic (mg)

100 Qtz-Fe-Cu-sulfide vein 75

EXPLANATION N Co, Cu) Qtz-Fe-Cu-sulfide vein 70 10 58 mb—Biotitic mafic dike, showing dip OCmd-Sb143A 72 84 rock/chondrite mb-AB-06-HK1 Shear zone with semi-massive Cob2 3 m (except Rb, K, P, mb-07SB145A DisseminatedDissemi Cob 1 1 mb-07SB126A Fault mg-BBMD1 7100 level, underground workings mg-07SB140A 7070 StrikeStrike anandd dip of bedding mg-07SB147A 35 Plunge of F fold axes 0.1 2 Co Ba Th Nb La Sr P Zr Ti Y Yb S shear-slip cleavage, showing Cu Rb K Ta Ce Nd Sm Hf Tb Tm 65 2 strike and dip

E D Cld

Cld btt Chl Cob Cob

bttph Grt Chl bttph Cob 3 mm 5 mm

Figure 11. Geologic map, photomicrograph, and a spidergram to illustrate key characteristics of Blackbird mafic dikes (relative to cobaltite deposits and quartz-Fe-Cu-sulfide veins), and two photomicrographs to show porphyroblasts of garnet and chloritoid porphyroblasts with cobaltite inclusions: (A) Geologic map of the east-southeastern part of the Calera (7100) level of the Blackbird mine (after Vhay, 1948), showing a mafic

dike (mb) that cuts disseminated cobaltite1 ore but is cut by a quartz-Fe-Cu-sulfide vein. Also shown is a cobaltite2 lode along S2 shear-slip cleavage of an F2 fold. (B) Biotite phyllite (Btph) from a metamorphosed biotitic mafic dike in the Ram zone (as seen in a thin section viewed in plane-polarized light). This dike cuts cobaltite-biotite ore in the Ram zone, where schistose fabric is typical of biotitic mafic dikes within or near the garnet zone. Lm—Limonite. (C) Spidergrams for Blackbird (BB) mafic dikes (mb—biotitic, and mg—gabbroic), compared to a spidergram for a mafic dike (OCmd) in the syenitoid intrusive complex of Deep Creek (dated 485 Ma by Lund et al., 2010). Also shown is a gray pattern that envelopes a set of spidergrams for continental (within-plate) basic rocks (after Thompson et al., 1984, p. 560). (D) Garnet porphyroblast with cobaltite inclusions in cobaltite-biotite ore from the Ram zone. Biotite is altered to chlorite, and chlorite rims the garnet. This image from J.F. Slack shows sample R05-03-102 ft (31.1 m) as seen in a thin section viewed in plane-polarized light. (E) A cluster of chloritoid porphyroblasts

transects S2 schistosity and contains a foliation-parallel train of cobaltite inclusions. This image from S.G. Peters shows a thin section (viewed in plane-polarized light) of a sample from the Brown Bear ore zone (subjacent to the garnet zone).

The garnet-in transition is interpreted as a gradational plate, did not mention garnets. However, a schistose mafic dike metamorphic isograd along the base of a downward-decreasing in the Lookout structural block (also in the footwall of the Iron metamorphic gradient, probably below a hotter thrust plate of Lake thrust plate) contains large and abundant garnets (dated higher-grade metamorphic rocks from a deeper crustal level. ca. 93 Ma). A thin-section of a sample from that dike shows a The observation that biotitic mafic dikes in the garnet zone are rolled garnet with a crenulated quartz-inclusion train that is off- schistose, whereas biotitic dikes well below the garnet zone are set from crenulated schistosity in the dike. This is interpreted to not schistose, is consistent with the hypothesis that dynamother- indicate that deformation to form crenulated schistosity began mal metamorphism was imposed from above by an over-riding before, and continued after ca. 93 Ma. Orientations and asym- thrust plate. metries of two complementary sets of crenulations in schistose As shown in Figures 1B and 2B, biotite gneiss of the Clear- rocks of the Blackbird structural block are consistent with up-to- water orogenic zone lies north of and above the inferred Salmon the northeast compression, which also is consistent with regional

Canyon thrust fault. This Clearwater thrust plate, which is com- northeast-vergent F4 folds and thrust faults of Cretaceous age in posed largely of amphibolite-facies metasedimentary rocks, may the Cordilleran orogen (Fig. 2A) and the Salmon River Moun- have been thrust upward and southeastward over the northwest- tains (Fig. 2B). ern part of the ICB, causing garnets and biotite to form in its foot-

wall, beginning at ca. 151 Ma. Northeast-plunging F3 folds may IRON-COPPER (Fe-Cu)-SULFIDE DEPOSITS

also have formed in response to northwest-southeast compression (STAGE K1, Ca. 110–92 Ma) during the Late Juarssic to Cretaceous Nevadan orogeny (ca. 155

to 142 Ma). Such F3 folds are common in metasedimentary rocks Fe-Cu-sulfide deposits of the Blackbird district contain min- of the Haynes-Stellite block (Fig. 2B), and are superimposed on erals of a characteristic suite that includes: early-to-late quartz

the southwest limb of a major F2 fold (the Idaho syncline), where ± monazite ± sparse magnetite ± locally abundant pyrrhotite it is exposed along the upper northeast rim of the Blacktail pit. + pyrite ± late chalcopyrite ± siderite ± later chlorite ± cobal- Younger garnets (dated ca. 113 Ma) from sillimanite-garnet- tian arsenopyrite (rather than cobaltite). Cobaltian arsenopyrite quartz-biotite gneiss in the footwall of the Salmon River thrust occurs as silver-white-metallic rhombic prisms with a pale pink- fault at the Salmon Canyon Copper prospect may indicate that ish-silver tarnish (as compared to gun-metal gray cobaltite with Salmon River fault was reactivated during regional northeast-ver- violet-tinted tarnish, and silver-white arsenopyrite, which does

gent F4 folding and thrusting during the Sevier orogeny (ca. 112 to not tarnish noticeably). 85 Ma). Thus, the Salmon River fault may have been reactivated as a lateral ramp, along which the Clearwater thrust plate may Fe-Cu-Sulfide Breccia, Vein, and Replacement Deposits

have been displaced northeastward during the Sevier orogeny. (Stage K1) The Iron Lake thrust plate probably was thrust northeastward over the northwestern part of the ICB during the Sevier orogeny, Fe-Cu sulfide-bearing breccias and veins cut cobaltite and it too may have metamorphosed rocks in its footwall. How- lodes, and veins of this assemblage also cut mafic dikes, which ever, metasedimentary rocks of the Iron Lake thrust plate appear cut cobaltite lodes (Fig. 11A). In composite Co-Cu ore zones, to contain few garnets. This may indicate that quartzitic rocks of Fe-Cu sulfides of this assemblage rim and partly replace cobalt- the Iron Lake thrust plate were either less chemically favorable ite. Aleinikoff et al. (2012b) reported U-Pb age determinations to garnet growth, or were less hot than sillimanite-garnet-quartz- of ca. 144 to 88 Ma but mainly between ca. 110 and 92 Ma for biotite gneiss of the Clearwater thrust plate. hydrothermal monazite, and 104 to 93 Ma for Cretaceous xeno- A description by Johnson et al. (1998) of the Mary Ann pros- time rims on cores of Mesoproterozoic xenotime from the Merle pect, which is in the proximal footwall of the Iron Lake thrust and Sunshine Co-Cu ore zones (Fig. 9B).

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Panneerselvam et al. (2012) reported highly radiogenic Pb Dandy Fe-Cu-Sulfide Breccia in pyrite, chalcopyrite, pyrrhotite, and cobaltite (which may have The Dandy Fe-Cu-sulfide breccia zone contains multiple been cobaltian arsenopyrite, rather than cobaltite, as indicated by tabular bodies of Fe-Cu-sulfide breccia, which generally strike its accompanying Fe-Cu sulfides). Johnson et al. (2012) reported north and dip steeply, along the Dandy shear zone (Fig. 8). This highly radiogenic Sr in siderite from a quartz-chalcopyrite-sider- shear zone appears to truncate a gabbroic dike and offset the Chi- ite vein in the composite Chicago Co-Cu ore zone. These results cago ore zone relative to the north Idaho ore zone (Fig. 8). were interpreted to indicate Cretaceous extraction of Pb and S A sample of Dandy breccia, shown in Figure 12B, contains after long-term evolution of Pb and Sr in metasedimentary source clasts of vein quartz and greenish-black biotitite-phyllite in a rocks of Mesoproterozoic age. massive-sulfide matrix of intergrown pyrrhotite and pyrite. Thin Fluid-inclusion studies by Nash and Hahn (1989) and Landis chalcopyrite veinlets cut the biotitite clast, which also is partly and Hofstra (2012) indicate that gangue quartz was deposited at rimmed by lacy intergrowths of quartz and chalcopyrite. Small temperatures between ca. 375 and 250 °C, probably as tempera- subhedral crystals of cobaltian arsenopyrite are disseminated tures decreased after garnet-grade metamorphism. Fe-Cu-sulfide throughout the biotitite clast and its pyrrhotite-pyrite matrix. breccias, veins, and replacement-style deposits are not folded, Some cobaltian-arsenopyrite rhombs crosscut chalcopyrite and their sulfide minerals are not foliated. veinlets, and some occur in drusy quartz, which rims and lines open vugs. Minor marcasite follows rough fractures through Merle Fe-Cu-Sulfide Breccia brittle pyrrhotite and pyrite. Such marcasite may be of super- Stratabound Fe-Cu-sulfide breccia in the Merle zone is gene origin. chalcopyrite-rich. The breccia contains clasts of quartz and siderite in a matrix of biotitite. The biotitite surrounds remnants of Quartz-Fe-Cu-Sulfide Veins microbreccia with consistently oriented foliation (interpreted as Quartz-Fe-Cu-sulfide veins, veinlets, and pods typically flow foliation). Both matrix and clasts contain minor pyrrhotite contain various combinations of early quartz ± pyrite ± pyrrhotite and abundant late chalcopyrite (Fig. 12A). Irregularly shaped ± late chalcopyrite ± cobaltian arsenopyrite ± siderite (Figs. 12C aggregates of Fe-Cu sulfides curve around the clasts, but also and 12D). Anderson (1947) also reported traces of sphalerite, penetrate them. These relationships are interpreted to indicate enargite, galena, and native silver in samples from the Uncle that biotitized microbreccia and clasts are partly replaced by later Sam mine (now collapsed). He considered these minerals to be ore minerals. younger than pyrrhotite but older than chalcopyrite.

Figure 12. Photos of samples of late Fe-Cu-sulfide breccias, quartz-Fe-Cu-sulfide veins, and composite Co-Cu ores: (A) Chalcopyrite-mineralized breccia from the Merle zone (drill-core sample M7A-174-193 ft, or 53–59 m) contains clasts of hydrothermal quartz and siderite in a flow-foliated microbreccia matrix that is deflected around clasts and is mostly altered to greenish-black biotitite. Chalcopyrite and pyrrhotite replace the biotitized matrix and fill cracks in clasts. Although sulfides follow foliation, the sulfides appear undeformed, indicating that they were deposited after dynamic emplacement of the breccia. (B) Dandy breccia, containing clasts of hydrothermal quartz (Qtz), quartz-breccia (Qtz bx), and biotitite (btt) in a massive-sulfide matrix of intergrown pyrrhotite (Po) and pyrite (Py), locally cut and partly replaced by lacy networks of chalcopyrite (Ccp), and also containing sparsely disseminated cobaltian arsenopyrite (CoApy). Open vugs are lined with drusy quartz and rimmed by quartz, containing disseminated cobaltian arsenopyrite. This sample was collected by M.L. Zientek on the 6850 level of the Blackbird

mine. (C) Chalcopyrite-quartz vein and veinlet, both along S2 cleavage in F2-folded banded siltite, containing older biotitite ± cobaltite1 in F2- folded banded siltite, and biotite ± cobaltite2 along S2 cleavage (underground photo by J.F. Slack, 6850 level of the Blackbird mine). (D) Close-up photo showing part of a small quartz-Fe-Cu-sulfide vein, containing chalcopyrite, pyrrhotite, quartz, pyrite, and cobaltian arsenopyrite. This vein belongs to a set of small veins and veinlets that follow and locally transect the margin of a mafic dike in structurally transposed banded siltite in the Hawkeye adit (Fig. 10B). (E) Composite Co-Cu-ore from the Chicago zone (northwest of the Dandy breccia zone). This sample (GHBB08) contains a tabular body of hydrothermal breccia with lenticular quartz clasts in a biotitic matrix with alternating bands of disseminated to semimassive cobaltite that is locally cut, rimmed, and partly replaced by pyrite and chalcopyrite. Variously cobaltite-rich bands generally parallel vein walls but are deflected around quartz clasts. By contrast, some large grains and strings of cobaltite grains are elongate nearly perpendicular to depositional banding. Such cross-fabric is attributed to cobaltite recrystallization, possibly during or after Cretaceous metamorphism. Some large grains of recrystallized cobaltite have cores of bright bluish-white safflorite (Saf), possibly exsolved during recrystallization of cobaltite with excess As. Along the right margin of this sample, a discontinuous pyrite veinlet cuts cobaltite but is cut by chalcopyrite, which locally cuts, rims, and partially replaces recrystallized cobaltite. Thus, cobaltite recrystallized before chalcopyrite was deposited to form composite Co-Cu ore. (F) Composite Co-Cu ore from the Blacktail zone. This sample (GHBB16) contains scattered to semimassive cobaltite, much of which is localized along discontinuous biotitic streaks (possibly after argillite) between quartz bands and lenses (possibly after siltite that is silicified and recrystallized) in structurally transposed banded siltite. Cobaltite is present in both biotite and quartz, but is preferentially concentrated along biotitic streaks. Strings of cobaltite grains are locally aligned at ~70° to the banded fabric of the sample. Such cross-fabric is interpreted to

indicate recrystallization of cobaltite, possibly during Cretaceous metamorphism. Such recrystallized cobaltite (Cobrxt) is locally cut, invaded, surrounded, and partly replaced by pyrrhotite + pyrite and later chalcopyrite, which lack metamorphic fabric. Thus, cobaltite appears to have recrystallized before chalcopyrite was deposited to form composite Co-Cu ore.

B A vug mbx t Ccp o btt Po Qtz vug Ccp Sd Po Ccp Po Ccp Po Qtz btt Po Ccp Qtz Py btt CoApy Ccp Ccp Ccp CoApy 1 cm Qtz bx vug Qtz 1 cm C Ccp-Qtz vnlt D Qtz sltt Po sltt Ccp Ccp CoApy

Ccp-Qtz vn B Py bx t t ± C ob 1 Btt CoApy Ccp F2 Qtz 5 cm Bt±Cob2 1 cm Ccp S2

E F Py Qtz Bt Qtz Saf Py Ccp d Ccp n Ccp Qtz a Qtz b Cob btt Cob b rxt o

C Cob Py Po+Py Cob Qtz rxt rxt

d Cob b n o

a Ccp Cob C

Qtz Bt Bt Bt 1 cm 1 cm

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Quartz-Fe-Cu-sulfide veins cut cobaltite-biotite ore, F2 mostly replaced by biotitite and cobaltite during Mesoprotero- folds, mafic dikes, and garnet-biotite phyllite. Such veins are zoic mineralization. relatively undeformed, and they lack metamorphic fabric. This Some cobaltite grains and strings of smaller cobaltite grains is interpreted to indicate that they probably formed after the peak are elongate nearly perpendicular cobaltite-rich bands, quartz of Cretaceous dynamothermal metamorphism in the Blackbird clasts, and wall-rock contacts. Such cross-aligned cobaltite fab-

district. rics are labeled “Cobrxt” in Figure 12E, and they are interpreted to indicate recrystallization of cobaltite in response to metamor-

Au-BEARING QUARTZ VEINLETS (STAGE K2, phism in a post-depositional stress field. Safflorite [(Co, Fe)As2] Ca. 92–83 Ma) in the cores of some large cobaltite grains may have exsolved during recrystallization of cobaltite with excess As. According to Anderson (1947), Au recovered from the Late pyrite and chalcopyrite are interpreted as products of Uncle Sam mine was from electrum-bearing quartz veinlets, Cretaceous mineralization, superimposed on Mesoproterozoic which cut quartz-chalcopyrite veins. In the Ram zone, Au- cobaltite after it was metamorphosed and recrystallized. A dis- enriched intercepts coincide with intervals containing late continuous pyrite veinlet cuts cobaltite along the right-hand mar- quartz veinlets ± bismuthinite ± Bi ± electrum (Au-Ag amal- gin of the cobaltite-mineralized breccia shown in Figure 12E, and gam). A quartz-magnetite-chalcopyrite vein was prospected for a lenticular body of chalcopyrite cuts both cobaltite and pyrite gold at the Anaconda prospect, and a similar quartz-magnetite along the upper-right margin of the sample. Chalcopyrite also veinlet was found on a prospect dump near the Sunshine ore surrounds and partly replaces recrystallized cobaltite grains in zone. Late quartz veinlets probably are of Late Cretaceous age, the lower-right quarter of the sample. In addition to pyrite and as indicated by a 40Ar/39Ar age determination of 82.9 ± 1.1 Ma, chalcopyrite, some samples from the Chicago and Brown Bear reported by Lund et al. (2011) for muscovite around a late zones also contain pyrrhotite (and some contain associated mar- quartz veinlet. casite, which may be of supergene origin).

COMPOSITE Co-Cu-Au ORES Blacktail Zone Figure 12F shows a sample of a composite replacement- Composite Co-Cu-Au ores consist of cobaltite-biotite ore style vein from the Blacktail zone. This vein is bounded by of probable Mesoproterozoic age, in which cobaltite grains are interlaminated siltite and phyllitic biotitite (probably after tightly cut, surrounded, or partly replaced by Fe-Cu ore minerals of folded and sheared banded siltite). Within this composite vein, Cretaceous age ± later Au-bearing quartz veinlets. In compos- discontinuous lenses of quartz (interpreted as silicified siltite) are ite Co-Cu ores of the Chicago, Brown Bear, Blacktail, Ram, and set in a subordinate matrix of discontinuous stringers and pods of Sunshine ore zones, Cu grades and abundances of chalcopyrite, biotitite phyllite (interpreted as biotitized argillite). Biotitite phyl- pyrrhotite, and pyrite generally decrease with increasing lateral lite preferentially hosts cobaltite, but quartz also contains clus- distance from the Cretaceous Dandy breccia zone. Thus, average ters and strings of cobaltite grains. In the lower-right quarter of Cu grades decrease laterally from 2.4% in the Dandy zone, to the sample, a lens of semimassive cobaltite nearly parallels vein 1.2% in the Chicago zone, to 0.82% in the Brown Bear zone, to walls, but within it, a cross-fabric is defined by strings of cobalt- 0.74% in the Ram zone, and to 0.3% in the Sunshine zone. The ite grains at ~65° to the cobaltite-rich lens. This cross-fabric is Blacktail zone, however, averaged an exceptionally high 1.7% interpreted to indicate metamorphic recrystallization of cobaltite

Cu, probably because it included supergene-enriched Cu ore that (labeled Cobrxt). was mined from the open pit. Along the right-hand margin of the sample shown in Fig- ure 12F, a massive intergrowth of pyrite and pyrrhotite cuts Examples of Ore from Composite Co-Cu Ore Zones cobaltite in quartz. In the upper-left corner of the sample, a pyrite veinlet, which cuts cobaltite in quartz, is cut and partly replaced Chicago Zone by chalcopyrite. Pyrite, pyrrhotite, and chalcopyrite lack the Figure 12E shows an example of composite ore from the secondary metamorphic fabric of recrystallized cobaltite. These Chicago zone (northwest of the Dandy zone). This is a tabular, observations are interpreted to indicate that cobaltite was depos- vein-like body of composite ore, in which early quartz-biotite- ited and metamorphosed before pyrite, pyrrhotite, and chalcopy- cobaltite breccia is bounded by wall rocks of interlaminated rite were deposited. micromosaic quartz and biotitite ± very fine-grained tourmaline and cobaltite. Within this breccia, alternating bands of dissemi- CENOZOIC HISTORY nated to semimassive cobaltite are surrounded by greenish-black biotite (after microbreccia). Variously cobaltite-rich bands gener- Uplift and Erosion ally parallel the breccia walls but curve around elongate quartz clasts, aligned parallel to the centerline of the breccia. This band- The Cenozoic history of central Idaho is one of regional ing probably follows flow foliation in a microbreccia matrix, uplift and erosion, accompanied by Eocene magmatism and

Paleogene to Neogene block faulting. Eocene dikes are mostly (but in different forms) during Mesoproterozoic and Cretaceous quartz-phyric felsites (Tf in Figs. 7A and 8). Such dikes weather episodes of mineralization in the Blackbird district. nearly white, and most of them strike north and dip steeply along Slack (2006, 2012) identified xenotime, monazite, gadolin- the White Ledge fault or shear zone (WLf in Figs. 7 and 8). ite, and allanite as minor minerals in Blackbird deposits, and he Most normal faults in the Blackbird district strike nearly north suggested these minerals might be recoverable as by-products for

and dip steeply (subparallel to preexistent S2 cleavage). They their contained REEs. However, the distribution of these minerals are classified as normal faults on the basis of features that are is not sufficiently known to support estimation of REE resources offset across tabular zones of fault gouge ± breccia ± slicken- in Blackbird Co-Cu-Au ore zones or mill tailings. sides. Examples shown in Figures 7B and 8 include the Sunshine East fault (SEf), Meadow Creek fault (MCf), and East Blacktail CONCLUSIONS fault (EBf). The Hawkeye Gulch fault (HGf) and the Northfield fault (NFf), which projects southward to the Slippery Gulch fault Co-Cu-Au ore zones of the Blackbird district are classified (SGf), probably have longer and more complicated histories but as Co-Cu-Au deposits in metasedimentary rocks, according to a were most recently active as normal faults. Such faults probably descriptive model, edited by Slack (2013). Such deposits in the were active during Cenozoic time, in response to regional post- Blackbird district are interpreted as composite, epigenetic, pre- orogenic uplift and extensional tectonism. dominantly metamorphic-hydrothermal deposits (± subordinate magmatic hydrothermal input), hosted in metasedimentary rocks Weathering and Supergene Enrichment of Composite Ores of Mesoproterozoic age (ca. 1454–1370 Ma). These Co-Cu-Au deposits underwent multiple episodes of metamorphism ± plu- Common products of oxidative weathering of composite tonism ± metamorphic-hydrothermal mineralization, as follows:

ores include widespread limonite and even wider-spread man- Mesoproterozoic episode Y, stage Y0 involved F1 folding, ganese wad. Pink erythrite (after cobaltite and cobaltian arseno- biotite-grade metamorphism, bimodal mafic-felsic plutonism pyrite), and chalcanthite, tenorite, malachite, and azurite (after (1370 ± 10 Ma), and deposition of hydrothermal xenotime (1370 chalcopyrite) are common locally (especially near ore zones and ± 4 Ma).

prospects). Vhay (1948) reported that depths of oxidation vary Stages Y1 (post-F1, pre-F2 folding) and Y2 (post-F2 fold- from 50 to 100 ft (15 to 30 m) beneath hillsides but are deeper ing) involved metamorphic-hydrothermal deposition of replace- beneath deeply weathered upland surfaces. ment-style deposits of cobaltite-biotite ore ± tourmaline According to Vhay (1948), supergene ore minerals, includ- ± minor xenotime (ca. 1370–1270 Ma) ± traces of microscopic ing chalcocite, covellite, cuprite, and native copper were locally gold (in cobaltite) ± minor late chalcopyrite. Such mineraliza- deposited just below the vadose zone of oxidative weathering of tion occurred during and after dynamothermal metamorphism

hydrothermal ore minerals. Supergene vivianite (Fe3P2O8·8H2O), (ca. 1379–1325 Ma) and bimodal mafic-felsic plutonism a mineral prized by collectors, was found underground, in vugs (ca. 1370–1335 Ma), related to the East Kootenay orogeny along within fault gouge. Most supergene-enriched ore was removed the western margin of the Belt-Purcell Basin.

during mining before 1963. However, supergene ore minerals Stage Y3 involved emplacement of hydrothermal quartz- continue to precipitate from mine-drainage water beneath stopes biotite and quartz-tourmaline breccia, vein, and replacement-style that were back-filled with the sand fraction of mill tailings, which deposits ± cobaltite ± minor xenotime (ca. 1058–990 Ma) ± minor are now oxidizing in the vadose zone. chalcopyrite. This mineralization probably occurred during regional thermal metamorphism (ca. 1200–1000 Ma), possibly POTENTIAL BY-PRODUCTS related to regional mafic underplating of the western Laurentian margin, while Rodinia was being assembled during the distant About 3.8 t of Au were recovered from the Uncle Sam mine Grenville orogeny (far to the northeast, east, and southeast). (Anderson, 1947), but only ~1.6 t of Au were recovered from the Mafic dikes of probable Cambrian–Ordovician age (ca. 530– larger Blackbird mine. Remaining Au resources of the Blackbird 485 Ma) were emplaced after Mesoproterozoic deposition of

patented claim block have not been estimated for lack of suf- cobaltite-biotite ore, after F2 folding, and after emplacement and ficient assay data. However, Kunter and Prenn (2007) estimated mineralization of quartz-biotite and quartz-tourmaline breccias, that Co-Cu ore of the Ram zone contains ~1 t of Au at an average but before Late Jurassic–Cretaceous garnet-growth, and before concentration of 0.45 ppm Au. deposition of Fe-Cu-sulfide minerals in breccias, veins, and Nash et al. (1987) reported that bulk samples of cobaltite ore replacement-style deposits of Cretaceous age.

from the Sunshine zone contain 1–6 ppm of Au as microscopic Cretaceous episode K, stage K0 involved metamorphic grains of native gold within cobaltite grains (here considered to garnet-growth (151–93 Ma), cooling of metamorphic biotite be of Mesoproterozoic age). However, Anderson (1947) reported (ca. 151–122 Ma), and deposition of minor metamorphic mona- that Au from the Uncle Sam mine was recovered from electrum zite (144–110 Ma).

in quartz veinlets that cut quartz-chalcopyrite veins (here consid- Stage K1 involved deposition of hydrothermal pyrrhotite, ered to be of Cretaceous age). Thus, minor gold was deposited pyrite, chalcopyrite, and minor cobaltian arsenopyrite ± quartz

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± siderite ± monazite ± xenotime (mostly ca. 110–92 Ma) in asso- A future MT survey across the Lemhi subbasin from the ciation with the Cretaceous Sevier orogeny (ca. 112 to 85 Ma) in Idaho batholith to the Beaverhead Mountains might indicate the hinterland of the Cordilleran orogen. whether exposed strata of the Lemhi subbasin (which are time-

Stage K2 involved deposition of hydrothermal quartz ± elec- equivalent to upper-Belt strata) are underlain by a thick section trum ± Bi minerals in veinlets ± sericite (ca. 83 Ma) in host rocks of sedimentary rocks (possibly time-equivalent to lower-Belt around veinlets. strata). If relatively conductive horizons were indicated, these might be interpreted as sulfide-bearing or thermal-fluid-bearing SUGGESTIONS FOR ADDITIONAL RESEARCH zones, which might have served as source rocks for elements that are concentrated in hydrothermal deposits of the Blackbird Exploration and Resource Estimation Co-Cu district and the Idaho cobalt belt.

Deeper drilling will be required to test for downward exten- ACKNOWLEDGMENTS sions of ore zones that are open down-dip. Results of such drilling are likely to support significant increases in estimated Co-Cu-Au Gordon Hughes donated many polished slabs of ore from under- resources of the Blackbird district. ground workings that are no longer accessible. Tom Nash and Mike Zientek also provided samples from the Blackbird mine Mineralogy and core-drill holes. Dan Meyers and George Lusher, of the Blackbird Mine Group, granted access to the Blackbird mine Distribution of potential by-products, such as Au and REE- area, and Brett Riggan and Bruce Hull led tours to temporarily bearing minerals could be better defined through systematic re- accessible parts of the Blackbird mine. Bill Scales, of Forma- examination and analysis of drill core from within and around tion Metals Inc., granted permission to sample drill core. Greg intercepts of mineralized rocks. Hahn donated unpublished reports, and John Aleinikoff, John A systematic analysis of mineral species of the Co-Fe-As- Slack, and Steve Peters contributed photos, photomicrographs, S solid-solution-series in different types of ore throughout the and SEM images. Jeff Vervoort supervised Lu-Hf dating of Blackbird district would support better definition of composi- garnets, and Paul Layer and Jeff Drake provided 40Ar/39Ar age tional variations within, and distinctions between such mineral determinations on metamorphic biotite. John Wallis helped pre- species. This would also better define the spatial distributions and pare digital illustrations, and student interns from Eastern Wash- paragenetic relationships of cobaltite, glaucodot, cobaltian arse- ington University helped with various parts of this study. John nopyrite, arsenopyrite, and safflorite, relative to those of associ- Slack and Karl Evans reviewed a first draft, and Doug Causey ated ore and gangue minerals. Ultimately, this could contribute to and Niki E. Wintzer reviewed a revised manuscript. We thank optimal extraction and concentration of ore minerals. these reviewers, editors J.S. MacLean and J.W. Sears, and editors of the U.S. Geological Survey and the Geological Society Geochronology of America for many helpful comments and suggestions. Any use of trade, firm, or product names is for descriptive purposes A project is underway to obtain Re-Os age determinations only and does not imply endorsement by the U.S. government. on ore minerals of the Blackbird district and the ICB, and an effort also is underway to find and date baddeleyite in a sample REFERENCES CITED from a mafic dike near the Blackbird mine portal. Aleinikoff, J.N., Hayes, T.S., Evans, K.V., Mazdab, F.K., Pillers, R.M., and Fanning, C.M., 2012a, SHRIMP U-Pb ages of xenotime and monazite Geophysics from the Spar Lake red bed-associated Cu-Ag deposit, western Mon- tana: Implications for ore genesis: Economic Geology and the Bulletin A magnetotelluric (MT) survey across west-central Idaho of the Society of Economic Geologists, v. 107, no. 6, p. 1251–1274, doi:10.2113/econgeo.107.6.1251. by Rodriguez et al. (1996) indicated a very conductive zone, Aleinikoff, J.N., Slack, J.F., Lund, K., Evans, K.V., Fanning, C.M., Mazdab, which extends from 10 to 30 km deep, east of the Idaho batho- F.K., Wooden, J.L., and Pillers, R.M., 2012b, Constraints on the timing of lith, beneath an area that contains hot springs and mineral Co-Cu+Au mineralization in the Blackbird district, Idaho, using SHRIMP deposits in Precambrian metasedimentary rocks, Cretaceous U-Pb ages of monazite and xenotime plus zircon ages of related Mesopro- terozoic orthogneisses and metasedimentary rocks: Economic Geology intrusive rocks, and Tertiary volcanic and intrusive rocks. This and the Bulletin of the Society of Economic Geologists, v. 107, p. 1143– conductive zone was interpreted to represent a very thick sec- 1175, doi:10.2113/econgeo.107.6.1143. tion of sedimentary rocks, containing thermal fluids. More Aleinikoff, J.N., Lund, K., and Fanning, C.M., 2015, SHRIMP U-Pb and REE data pertaining to the origins of xenotime in Belt Supergroup rocks: Evi- recently, an MT survey across northern Idaho and western Mon- dence for ages of deposition, hydrothermal alteration, and metamorphism: tana by Bedrosian and Box (this volume) traced a zone of con- Canadian Journal of Earth Sciences, v. 52, p. 722–745, doi:10.1139/cjes ductive horizons (interpreted as sulfide-bearing strata) beneath -2014-0239. Allmendinger, R.W., and Jordan, T.E., 1981, Mesozoic evolution, hinterland lower-Belt strata of the Prichard Formation in the central of the Sevier orogenic belt: Geology, v. 9, p. 308–313, doi:10.1130/0091 Belt basin. -7613(1981)9<308:MEHOTS>2.0.CO;2.

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