Stratigraphic and Structural Setting of Sediment-Hosted Blackbird Gold-Cobalt-Copper Deposits, East-Central Idaho, U.S.A.
Published:January 01, 2007
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Karen I. Lund, Russell G. Tysdal, 2007. "Stratigraphic and Structural Setting of Sediment-Hosted Blackbird Gold-Cobalt-Copper Deposits, East-Central Idaho, U.S.A.", Proterozoic Geology of Western North America and Siberia, Paul K. Link, Reed S. Lewis
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The sediment-hosted gold–bearing cobalt–copper Blackbird deposits of east–central Idaho, U.S.A., are unique deposits hosted in enigmatic and poorly known Mesoproterozoic strata. New regional mapping studies elucidate the geographic distribution of units of the very thick Mesoproterozoic succession and the geometry of major structures. Blackbird deposits are hosted predominantly by the banded siltite unit of the Mesoproterozoic Apple Creek Formation; a few mineralized zones are in the underlying coarse siltite unit of the Apple Creek and the overlying basal Gunsight Formation. The depositional mechanism for hosting strata was predominantly turbidity– current flow. Blackbird deposits were long thought to be hosted in the Yellowjacket Formation, which was mapped as the dominant formation throughout central Idaho but, as originally defined, formed in a relatively shallow–water marine environment. Our conclusions about the host strata, in conjunction with previous interpretations that Blackbird deposits formed in a rift setting, resolves previous contradictions between models for environments of sediment deposition and origin of metals accumulation.
Along the west side of the Blackbird district, the Apple Creek and Yellowjacket formations are separated by the northwest–trending Late Cretaceous Iron Lake fault. The Yellowjacket Formation is confined to the Iron Lake thrust plate west of the district and does not contain sediment–hosted deposits. In contrast, the Blackbird deposits lie in the complexly deformed upper part of the underlying Poison Creek plate, specifically at the northeast–trending hinge zone of the Blackbird Mountain oblique ramp. Near the ramp, both hanging–wall and footwall rocks are strongly deformed, resulting in major overturned folds in the hanging wall and imbricate thrust plates in the footwall. The upper imbricate in the Poison Creek plate contains chloritoid–garnet schists that were long recognized as being above the garnet isograd and having a different mineralization style. The intermediate imbricate forms the lower to middle greenschist Blackbird subplate containing bedding–parallel and structurally remobilized cobalt–copper deposits. The structurally lowest imbricate forms the Haynes– Stellite subplate and includes the youngest rocks (lower Gunsight Formation) and mineralized tourmaline breccias. Subsequent to deformation related to the Iron Lake thrust fault, several normal faults reactivated the Iron Lake thrust fault, the Blackbird Mountain ramp, and the buried Great Falls tectonic zone in the basement. This complex deformation resulted in juxtaposition of metamorphic facies and stratigraphic units from different structural levels as well as from different parts of the stratigraphic section.
Present exposure of strata of the Apple Creek sedimentary basin, in which mineralized strata formed, is controlled by northwest– trending thrust faults and normal faults. Our study results in limiting the permissive stratigraphy that hosts the deposits in a meaningful way for future identification of related deposits. It also shows that the previous outline of an Idaho cobalt belt did not represent geometry of the rift basin per se, but instead is the expression of structural control of exposure of the hosting Apple Creek Formation, primarily those strata that lie in the Poison Creek plate.
The Blackbird gold–bearing cobalt–copper deposits are the most important of a unique type of sediment–hosted cobaltiferous massive sulfide deposits that were grouped into the “Idaho Cobalt Belt” (Hughes, 1983; Hahn and Hughes, 1984; Connor, 1990, 1991; Nold, 1990) in the Salmon River Mountains of east– central Idaho west–northwest of Salmon, Idaho (Fig. 1). The Blackbird deposits are considered to be a unique type of volcanogenic massive sulfide deposit (Earhart, 1986; Nash and Hahn, 1989) of international importance (Cox and Singer, 1986; duBray, 1995) or possibly a type of iron oxide–copper–gold deposit (Slack, 2006). The deposits are the only ready source of cobalt ore in the U.S.
The Blackbird deposits are hosted by a thick siltite– metasandstone succession. Early studies concluded that these strata were equivalent to the Mesoproterozoic Belt Supergroup (Umpleby, 1913; Ross, 1934). For more than 50 years, the hosting strata were included in the Mesoproterozoic Yellowjacket Formation (Vhay, 1948; Nash and Hahn, 1989; Nold, 1990; Evans and Connor, 1993) and correlated with the lower part of the Belt Supergroup of Montana and northern Idaho (Ruppel, 1975; Hahn and Hughes, 1984). More recent work documented that the stratigraphy near Blackbird was more complex and that rocks of several stratigraphic levels and environments of deposition were included in rocks mapped as the Yellowjacket Formation (Connor and Evans, 1986; Ekren, 1988; Evans and Connor, 1993; Link et al., 1993;, Winston et al., 1999). Regional mapping and stratigraphic studies resulted in recognition that units of the Lemhi Group and related strata (Ruppel, 1975, and see references therein for discussion and history of nomenclature) extend into the area of the Blackbird mining district (Tysdal, 2000a, 2003; Tysdal et al., 2000, 2003). This resulted in a new, but still incomplete, understanding of the relations between the Yellowjacket Formation and the Lemhi Group (Tysdal, 2000b). History of the nomenclature problems of Yellowjacket Formation (and related units) and Lemhi Group (and related units) is detailed in Link et al. (1993), Winston and Link (1993), Winston et al. (1999), and Tysdal (2000b). In several survey papers of the larger region, a continuous strati– graphic section among these units was assumed, and they were variously correlated with lower Belt Ravalli Group, middle Belt carbonate units, and lower Missoula Group (Link et al., 1993; Winston and Link, 1993; Winston et al., 1999; Link et al., 2003).
Metamorphic grade increases northwestward across the district (Vhay, 1948; Roberts, 1953; Nash and Hahn, 1989) and to the north (Cater et al., 1975; Lund et al., 1983a). The garnet isograd was mapped across the Blackbird mine area (Vhay, 1948; Purdue, 1975; Daggett and Smit, 1981; Nash and Hahn, 1989) where the increase of metamorphic grade was attributed to contact metamorphism related to granitic rocks (Vhay, 1948; Roberts, 1953; Cater et al., 1975; Purdue, 1975). The Blackbird deposits were modified by well–mapped post–mineralization normal faults (Vhay, 1948; Shenon et al., 1955; Bennett, 1977). Although local thrust faulting and complex folding in the district was documented by a number of workers (Vhay, 1948; Roberts, 1953; Purdue, 1975; Daggett and Smit, 1981), ductile deformation and thrust faulting were presented in recent models as having limited impact on mineral deposits (Nash and Hahn, 1989).
Gold and copper deposits were first prospected in the Blackbird district about 1893 and cobalt was discovered in 1901, although no cobalt was produced until 1917–1920 (Vhay, 1948). Early work suggested that the cobalt–copper deposits in the Blackbird district may have been epigenetic hydrothermal de– posits (Anderson, 1947; Purdue, 1975). In the most comprehensive mapping of the district (Vhay, 1948), hydrothermal replacement along shear zones was identified as the dominant mineralization process at Blackbird. More recent work resulted in models for the formation of the cobalt–copper deposits as synsedimen– tary exhalative and massive sulfide deposits that formed in a Mesoproterozoic rift basin (Hahn and Hughes, 1984; Nash and Hahn, 1989) with post–diagenetic processes deemphasized. Metals remobilization due to metamorphism and deformation is documented at some deposits in the highest metamorphic grade rocks (Nold, 1990).
The present study is based on our recent regional strati– graphic, structural, and mapping studies (Lund et al., 1983a; Lund et al., 1983b; Tysdal et al., 2000; Tysdal, 2000a, 2000b, 2002, 2003; Lund et al., 2003; Tysdal et al., 2003; Lund, 2004; Lund, 2001,2002, unpublished mapping) that resulted in new areal distribution of Mesoproterozoic strata and identification of regional bounding structures. This changed the region–wide understanding of Mesoproterozoic stratigraphy, the influence of Meso– proterozoic orogeny, the history of metamorphism, and the location and style of Cretaceous compressional deformation. The purpose of this report is to present new interpretations of the distribution of units, describe the field relations among the strata, document the metamorphic and structural history, and clarify the geologic setting of the Blackbird mining district.
Interpretations for environment of deposition of the strata in the region are divergent. Although based on interpretation of sedimentary structures, the water depth and mode of deposition are apparently in dispute: compare Winston and Link (1993), Winston et al. (1999), and Link et al. (2003) with Tysdal (2000a, 2000b, 2003). Some of the differences in interpretation started because of earlier incomplete mapping that resulted in confusion over which rocks and sediment types belonged to which formations. Additionally, before the regional mapping was completed, the relative importance of and relationships among different facies within units was unclear. However, in cases where the same sedimentary structures were studied and the unit is agreed upon, there are basic disagreements about the interpretation of sedimentary structures. Because interpretation of environment of deposition is not central to and is beyond the scope of this study and because the conclusions of Tysdal (2000a, 2000b, 2003) are tied directly to the units as mapped in this study, these conclusions on depositional environment and those of other studies that centered on the Blackbird district (Lopez, 1981; Sobel, 1982; Hughes, 1983) are generalized herein. Further research into some of these questions is warranted.
Additional controversy relates to correlation of the widely separated units of east–central Idaho with those of the Belt Supergroup of western Montana and northern Idaho as advocated by Ruppel (1975), Hahn and Hughes (1984), Link et al. (1993), Winston et al. (1999), and Link et al. (2003). Strata at Blackbird were earlier correlated with the Prichard (Aldrich) Formation, thus the oldest part of the Belt Supergroup (Ruppel, 1975), and the setting and timing of Blackbird deposits were related to the lead– zinc deposits at Sullivan, British Columbia. More recently, the sedimentary environment of deposition was interpreted to be similar to that of the middle Belt carbonate units and a much younger correlation with the Wallace Formation was preferred (Winston et al., 1999; Link et al., 2003). Preliminary U–Pb SHRIMP detrital zircon studies (Link and Fanning, 2003; Link et al., this volume) suggest that much of the stratigraphic succession of east– central Idaho is younger than previously was supposed and younger than all but the youngest units of the Belt Supergroup (see ages in Evans et al., 2000). In the absence of continuity among the units and given the preliminary detrital zircon data, arguments about correlation are not well substantiated. Resolution rests on future geochronology or other methods. Correlation with the Belt Supergroup is not directly relevant to the subject of this study and is not further addressed.
Yellowjacket Formation and Related Units
The Yellowjacket Formation is composed predominantly of interbedded fine–grained quartzitic metasandstone with lesser thin–bedded siltite and of interlayered metasandstone and carbonate– or calc–silicate–bearing strata low in the exposed section (Fig. 4A). The quartz–rich metasandstones and siltites are thin bedded and composed predominantly of quartz with about 15 percent each of feldspar and biotite (Ekren, 1988). Millimeter– to centimeter–scale sedimentary structures are climbing ripples, ripple cross lamination, herringbone cross lamination, mudchips, mudcracks, and loadcasts (Ekren, 1988; Winston et al., 1999; Tysdal, 2000b). The sedimentary structures are interpreted to indicate a tidal environment (Tysdal, 2000b) or subaqueous and subaerial sheet floods and playas (Winston et al., 1999; Link et al., 2003). Scapolite–bearing carbonate or calc–silicate gneiss zones originated from evaporite horizons (Tysdal and Desborough, 1997). These interpretations are consistent with the previous interpretations for a shallow–water environment of deposition in the reference section (Ross, 1934; Ekren, 1988). The more than 2,250–m–thick Yellowjacket Formation grades upward into the overlying Hoodoo Quartzite (Fig. 4B), but the base of the unit is not exposed (Ross, 1934; Tysdal, 2000b).
The Hoodoo Quartzite is white to light–gray, medium–grained quartzite to feldspathic quartzite. Well–rounded quartz forms 80 to 90 percent of the rock, feldspar forms 5 to 10 percent, and micas and iron oxide form the remaining 5 to 10 percent (Ekren, 1988). Minor marble and calcareous quartzite are present as discontinous layers near the base of the unit. Bedding is thin to thick with local high–angle cross lamination, but the unit is commonly massive. Oscillatory ripples and current ripples are present throughout the unit. The sedimentary environment is interpreted as shallow subtidal to intertidal where high–energy water movement resulted in the high–angle cross lamination and winnowing of the sand without the presence of silt and clay interbeds (Evans and Green, 2003) or as sheetfloods on flat alluvial aprons (Link et al., 2003). Total thickness of the unit is about 1,100 m (Ekren, 1988). The lower contact is gradational with the Yellowjacket Formation, and the upper contact is gradational with the overlying unnamed argillaceous quartzite unit (Ekren, 1988; Evans and Connor, 1993).
The argillaceous quartzite unit was recognized by Ross (1934), but it has not been formally named (Ekren, 1988). The unit is predominantly dark gray, thin– and thick–bedded, fine to very fine–grained, feldspathic and micaceous quartzite interlayered with lesser amounts of medium–gray argillaceous siltite (Ekren, 1988; Tysdal, 2000b; Evans and Green, 2003). It includes abundant crossbeds, ripple cross lamination, and local rip–up clasts. An intertidal depositional environment is suggested by sedimentary structures and alternating lithologies, which reflect contrasting energy conditions (Evans and Green, 2003). The unit is more than 500 m thick. The base is gradational with the underlying Hoodoo Quartzite, but the top is not exposed (Tysdal et al., 2000).
Coarse Siltite Unit of the Apple Creek Formation.—
Near Blackbird, strata previously mapped as the lower unit of the Yellowjacket Formation (Evans and Connor, 1993) and interpreted as storm–influenced shallow–water deposits (Winston et al., 1999) are identified in this study as the coarse siltite unit of the Apple Creek Formation, the lowest member of the formation (Tysdal, 2000a). The unit is characterized by 10– to 30– cm–thick beds of coarse–grained quartz–rich siltite or fine–grained sandstone that grades into medium–grained siltite. The beds have erosional bases and Bouma sequences (Tb–c, Tb–c–d, and local Ta). Soft–sediment structures are common and include convolute lamination, dish, pillar, and flame structures, syneresis cracks, and ball–and–pillow structures. Herein, the unit is interpreted as a turbidite deposit (Tysdal, 2000a, 2000b; Evans and Green, 2003). The upper part of the coarse siltite unit of the Apple Creek Formation (Tysdal, 2000a) crops out south of the Blackbird district (Fig. 2). Bedded magnetite layers are found in the Iron Creek area (Nash, 1989) in this unit (Fig. 1). The coarse siltite unit grades upward into the banded siltite unit of the Apple Creek Formation.
Banded Siltite Unit of the Apple Creek Formation.—
At the Blackbird mine, low–metamorphic–grade exposures were previously mapped as the middle part of the Yellowjacket Formation (Connor and Evans, 1986; Evans and Connor, 1993), and some strata thought to be part of that unit were interpreted as storm–influenced shallow–water or sheet–flood deposits (Winston et al., 1999). Our new mapping documents that the dominant stratigraphic unit in the Blackbird district is the banded siltite unit of the Apple Creek Formation, which is the upper member of the formation (Tysdal, 2000a). The unit is characterized by 0.5– to 10– cm–thick layers of light–gray siltite to very fine–grained metasandstone alternating with black argillite (Fig. 4C). The couplets are interpreted to have formed by turbidity flows (Sobel, 1982; Tysdal, 2003). Most exposures contain downward–penetrating “dikelets” of coarser sediment from overlying layers (Fig. 4D). Dikelets commonly display ptygmatic–like folds interpreted to be due to compaction of originally water–rich layers. This unit is more than 2,000 m thick near Blackbird. The banded siltite unit gradationally overlies the coarse siltite unit and has a thin transition at the top into the Gunsight Formation.
Chloritoid–garnet–feldspar–quartz–muscovite–biotite schist of the Indian Creek subplate (Fig. 4E), in topographically higher parts of Blackbird, have been considered to be a younger part of the mineralized section (Nash and Hahn, 1989). In our mapping (Figs. 2, 3), we interpret the schists as a higher–grade equivalent of the banded siltite unit. Alternating porphyoblast–bearing schist, biotite schist, and feldspar–quartz–rich layers are controlled by original strongly interlayered composition of the unit. However, although some compositional layering is probably parallel to primary layering, sedimentary features are nearly universally destroyed by metamorphic mineral growth, remobilization, and transposition.
In the Salmon River Mountains near the Blackbird mine, strata previously mapped as the upper unit of the Yellowjacket Formation (Connor and Evans, 1986; Evans and Connor, 1993) and interpreted as a tongue of sand into the Yellowjacket turbid– ite basin (Connor and Evans, 1986; Nash and Hahn, 1989), are now correlated with the Gunsight Formation (Tysdal, 2000b; Tysdal et al., 2000; Tysdal et al., 2003).
The Gunsight Formation is light– to dark–gray, very fine– to medium–grained, feldspathic metasandstone. Bedding is typically 10 to 100 cm thick, displaying decimeter–thick trough and planar crossbeds and lesser hummocky cross–stratification near the base of the unit. Metasandstone was deposited dominantly in a fluvial environment, but hummocky cross–stratified beds indicate transition from the marine turbidites of the underlying Apple Creek Formation upward into nonmarine beds (Evans and Green, 2003) north and east of Blackbird mine (Fig. 3).
Relations among Stratigraphic Packages
The history of stratigraphic nomenclature near Blackbird is complicated and has caused much confusion over the distribution of units, the host strata for the Blackbird deposits, and the environment of deposition of strata thought to be in the same unit. Confusion was initiated by correlations and mapping that extended the use of Yellowjacket–Hoodoo terminology across much of central Idaho (Ruppel, 1975; Bennett, 1977; Lopez, 1981). Evans and Connor (1993) and Hahn and Hughes (1984) extended use of their “lower Yellowjacket” to include rocks to the east that are now interpreted to be units of the Lemhi Group and their “upper Yellowjacket” to include rocks now interpreted to be part of the Gunsight Formation. Winston et al. (1999) and Link et al. (2003) considered all these strata to be in a continuous strati– graphic section from Yellowjacket Formation–Hoodoo Quartz– ite, to Apple Creek Formation (by including the argillaceoous quartzite as part of the Apple Creek Formation), to Gunsight Formation. However, Bennett (1977) and Ekren (1988) had recognized a normal fault that locally separated the Yellowjacket– Hoodoo–argillaceous quartzite succession from rocks of the Lemhi Group. Subsequent work indicates that the succession of Yellowjacket Formation–Hoodoo Quartzite–argillaceous quartzite (as defined by Ross, 1934) is fault bounded on a regional basis and, as shown in Figure 2, is structurally isolated from Lemhi Group strata with which it was previously correlated. Our data show that the Yellowjacket package is a genetically related package of shallow–water strata. It is exposed in a thrust plate bounded by a thrust fault that was modified significantly by younger normal faulting (now Iron Lake fault; see following structure section). Tysdal (2000b) provided a detailed account of the geology and history of correlations relevant to this strati– graphic and structural relation.
In addition to the great thickness of the stratigraphic units and the lack of age control, metamorphic overprint and unidentified structures made mapping these strata nearly intractable. Regional mapping (shown in Evans and Green, 2003) resulted in mapping structures in lower–grade strata that, when traced into the higher–grade rocks around the Blackbird district, elucidated stratigraphic units as well as metamorphic and structural patterns.
Composition of the original sedimentary rocks was not conducive to formation of index minerals at the metamorphic grades represented at Blackbird. Thus, metamorphic grade as described in this study is somewhat approximate and is described with respect to structural domains.
Mesoproterozoic low–grade regional metamorphism was recognized across most of the region (Evans, 1981; Evans and Zartman, 1990), and more than one Proterozoic metamorphic event may have taken place in some parts of the region (Lund et al., 2004). However, a marked increase in grade northwestward across the district (Vhay, 1948; Roberts, 1953; Nash and Hahn, 1989) and to the north (Cater et al., 1975; Lund et al., 1983a) was first attributed to contact metamorphism related to granitic rocks assumed to be Cretaceous (Vhay, 1948; Roberts, 1953; Cater et al., 1975; Purdue, 1975). The granitic rocks subsequently were recognized as Mesoproterozoic (Bennett, 1977; Lund et al., 1983a) and genetically related to other granitic rocks dated at about 1.37 Ga (Evans, 1981; Evans and Zartman, 1990) so the metamorphism was considered to be a Mesoproterozoic event that affected an intact stratigraphic section (Nash and Hahn, 1989). Although minor contact metamorphism related to the Mesoproterozoic granitic rocks may overprint previously metamorphosed rocks, Cretaceous metamorphism, related to Sevier thrust faulting, is the dominant event that overprinted earlier sedimentary features and metamorphic fabric(s) in the region west of the Salmon River. Our structural, stratigraphic, and mapping work provide a necessary foundation for future studies of overlapping metamorphic events.
Most previous workers mapped a number of different compositional and metamorphic units (then assigned to the Yellowjacket Formation) within the Blackbird mining district (Daggett and Smit, 1981; Purdue, 1975) and interpreted the high– est–grade rocks, as caused by contact metamorphism, to be the uppermost stratigraphic unit (Nash and Hahn, 1989). Alternatively, Vhay (1948) suggested that minor original compositional differences, which may have existed in strata across the area, were enhanced by structural and metamorphic effects.
Iron Lake Plate
The Iron Lake plate (Fig. 3) contains rocks of the Yellowjacket Formation, as redefined by Tysdal (2000a) and delimited in this study; these are interlayered metasandstone and siltite metamorphosed to lower greenschist facies. Metamorphic minerals are dominantly quartz, feldspar, and biotite. Bedding is generally preserved except where strong axial–plane schistosity formed in fold hinges, as in the strongly overturned folds in the area north of Blackbird Mountain and where sheared near the Iron Lake fault. In these areas, the rocks were converted to biotite–feldspar–quartz schist containing minor fine–grained garnets (Lund, unpublished mapping, 2001–2002). The Hoodoo Quartzite and argillaceous quartzite are metamorphosed to lower greenschist facies and are not generally near enough to the major structures to have developed upgraded metamorphic mineralogy or fabrics.
Poison Creek Plate
Indian Creek Subplate.—
Upper–greenschist–facies metamorphic rocks of the banded siltite unit of the Apple Creek Formation underlie the upland areas from north of the Blackbird mine to the contact with Mesoproterozoic porphyritic granite on the ridges north of the mouth of Indian Creek and on Sunshine ridge west of the main Blackbird mine area (but east of the White Ledge shear zone; Fig. 3). Compositionally, chloritoid and garnet porphyroblasts in biotite–rich layers alternate with quartz–feldspar–rich layers (Fig. 4E, F). Porphyroblasts of garnet and chloritoid are fine to coarse grained and form knots in a fine– to medium–grained quartz– feldspar–biotite matrix; cordierite and sillimanite have been reported in some areas (Vhay, 1948; Daggett and Smit, 1981; Nash and Hahn, 1989). Garnets formed across an early foliation, as shown by inclusions in the garnets that are oriented parallel to the early dominant foliation and were rotated during later deformation in which the earlier foliation was folded and the newly formed fabric wrapped around the rotated garnets (Fig. 4G). Chloritoid porphyroblasts formed across the deformation fabrics. Both types of porphyroblasts grew over fine–grained cobal– tite–bearing layers (Eiseman, 1988; Nash and Hahn, 1989), indicating that metamorphism was post–mineralization.
Banded siltite unit of the Apple Creek Formation at lower– to middle–greenschist facies metamorphic grade also crops out in the Blackbird subplate: (1) in the Meadow Creek drainage (central part of Fig. 3) surrounding the Blackbird mine and (2) in the area between the White Ledge shear zone and the Big Deer Creek fault, from south of upper West Fork of Blackbird Creek to lower Indian Creek. Rocks in this subplate are of metamorphic grade intermediate between those in the Indian Creek subplate above and the Haynes–Stellite subplate to the east. These rocks are composed of quartz, alkali feldspar, biotite, and minor muscovite; fine–grained garnets are in rocks directly below the Iron Lake subplate (Vhay, 1948; Daggett and Smit, 1981). Bedding is commonly preserved except in the many fold hinges and shear zones, where strong axial–plane schistosity developed and the rocks were converted to biotite– feldspar–quartz schist with quartz lenses and nodules (Vhay, 1948; Roberts, 1953).
In the eastern part of the study area (Haynes–Stellite structural block of Vhay, 1948), exposed rocks include basal Gunsight Formation and the underlying banded siltite unit of the Apple Creek Formation. Bedding character and mineralogy of the Gunsight Formation are not conducive to showing metamorphic effects. Apple Creek strata are lower greenschist facies and retain primary sedimentary features, except where schistosity is locally developed in fold hinges (Vhay, 1948). The minerals are dominantly fine–grained quartz, feldspar, biotite, and minor muscovite.
Regional Pattern of Metamorphic Zonation
A northwestward increase in metamorphic grade was noted near Blackbird from the map view of the garnet isograd (Vhay, 1948) and was borne out in a general way in the region (Lund et al., 1983a; Nold, 1990). Previous workers generally attributed this pattern of metamorphism to contact effects from granitic intrusions; however, some workers recognized that contact metamorphism could not account for fabrics seen north of Big Deer Creek closest to the intrusive rocks (Daggett and Smit, 1981). Our study shows that, in three dimensions, the patterns of metamorphic isograds form a consistent geometric pattern wherein higher– grade rocks overlie lower–grade rocks (Fig. 3). This is well documented in the Meadow Creek drainage, north of Big Deer Creek, and along the west wall of Little Deer Creek (Fig. 3) by previous workers (Vhay, 1948; Purdue, 1975; Daggett and Smit, 1981) and forms the Indian Creek subplate.
Previous workers noted increase in metamorphic mineral growth and alignment along and near shear zones and in the axial areas of folds in the well–studied areas near the mine (Vhay, 1948; Roberts, 1953; Daggett and Smit, 1981). Our study shows that this area of increase in metamorphic grade in the axial areas of minor and major folds and along reverse faults lies mainly in the Blackbird subplate and to a lesser degree in the Haynes–Stellite subplate (Lund, unpublished data, 2001–2002).
Published information in the Blackbird district suggests much structural disruption (Vhay, 1948; Purdue, 1975; Daggett and Smit, 1981). Early workers concluded that outcrop–scale ductile structures played a part in the mineralization history (Anderson, 1947; Vhay, 1948; Roberts, 1953), but there was no regional structural context and no structural pattern was identified. More recent studies (Hahn and Hughes, 1984; Nash and Hahn, 1989) asserted stratigraphic continuity in the heart of the Blackbird district and emphasized this continuity over later metamorphic and structural disruption. Thus, the roles of metamorphism, folding, and reverse faulting, described by early workers, were minimized and three–dimensional structural understanding of the Blackbird district lagged behind investigations of the mineral deposits. Because of the early work, a large amount of valuable detailed structural information on the Blackbird mine area is available. Our new context for geometry and sequence of structures in the Blackbird area results from combining these older detailed data with the more recent regional structural observations made during geologic mapping (Ekren, 1988; Tysdal et al., 2000, 2003; Tysdal, 2003; Lund, unpublished mapping, 2001–2002).
East–central Idaho, near Blackbird, lies in the metamorphic hinterland of the Late Cretaceous Cordilleran thrust belt and was recently shown to be underlain by a series of thrust plates (Evans and Green, 2003) that were transported to the northeast during the Late Cretaceous. The Poison Creek thrust fault, a major fault of the region, trends northwest across the western part of the Lemhi Range and into the eastern part of the Salmon River Mountains (Fig. 1; Tysdal, 2002). Strata of the hanging wall (directly southwest of this fault) constitute the Poison Creek plate (Fig. 3), which contains the rootless sediment– hosted cobalt–bearing rocks of the Blackbird mining district (Tysdal, 2003). The southwestern limit of exposed rock of the Poison Creek plate is defined by the Iron Lake thrust fault, originally a thrust fault with related tear faults, and subsequent down–to–the–west normal faults that reactivated segments of the thrust. Rocks of the hanging wall to (southwest of) the Iron Lake fault are part of the Iron Lake plate (Fig. 3; Tysdal, 2000b, 2003; Tysdal et al., 2003).
Iron Lake Fault
Segments of the Iron Lake fault were described south and southeast of the study area by previous workers (Bennett, 1977; Ekren, 1988). Recent mapping efforts connected the fault segments into a regional feature (Fig. 1) interpreted to have originated as a major thrust fault that bounded different stratigraphic packages and structural domains (Tysdal and Desborough, 1997; Tysdal, 2000b; Tysdal et al., 2000, 2003; Lund, unpublished mapping, 2001–2002). Recognition of a thrust fault at this horizon allows us to address questions regarding: (1) extent of individual formations in the thick Mesoproterozoic sections, (2) identification of imbricate thrust faults, (3) extent of reactivation of thrust faults by later normal faults and evaluation of the amount of offset along these and other normal faults, and (4) impact of ductile deformation on the Blackbird deposits.
The Iron Lake fault has a regional northwest trend but makes an important deviation in the area near Blackbird. South of the study area and southeast of the Quartzite Mountain fault, the Iron Lake fault strikes northwest (Fig. 1). From the northwest end of this segment, at the junction with the Quartzite Mountain fault, the Iron Lake fault follows the northeast–trending Blackbird Mountain zone for about 5 km (Figs. 2, 3). The change in trend of the Iron Lake fault, from regional northwest to northeast near Blackbird, is an important area of structural complication and is herein called the Blackbird Mountain oblique ramp. North of the zone, the edge of the Iron Lake plate swings north–south where the Iron Lake fault is reactivated by the Big Deer Creek normal fault and the plate is preserved to the west. In the northwestern part of the study area, northwest from the confluence of Indian Creek with Big Deer Creek (Fig. 2), the Iron Lake thrust fault regains a northwest trend.
Iron Lake Plate
The Mesoproterozoic Yellowjacket Formation, Hoodoo Quartzite, and overlying informally named argillaceous quartz– ite unit are known only west of and above the Iron Lake fault; thus they occur only in the Iron Lake plate (Fig. 3). Recognition of the difference in the stratigraphic section on this thrust plate compared to the Poison Creek plate to the east is critical to understanding of the regional geology.
Three structural domains in the Iron Lake plate relate to four main segments of the Iron Lake fault (Fig. 2), described from southeast to northwest. (1) Southeast of the Quartzite Mountain fault where the Iron Lake fault strikes northwest, the stratigraphy also strikes northwest and the Yellowjacket Formation and Hoodoo Quartzite of the Iron Lake plate were deformed into a large northwest–trending syncline (or pair of synclines), now broken by normal faults (Ekren, 1988; Tysdal et al., 2000). The northwest–trending folds are open, upright features. (2) North of the Quartzite Mountain fault, where the edge of the Iron Lake plate turns northeast (controlled by a steep fault that is a reactivation along the Blackbird Mountain zone) and continues with a north trend (controlled by the Big Deer Creek fault) is an important area of structural complication in the Iron Lake plate. Where the Iron Lake fault intersects the Quartzite Mountain fault and the exposure of the Iron Lake plate steps to the northeast, the strike of the Yellowjacket Formation and the Hoodoo Quartzite and the trend of fold axes in the Iron Lake plate swing from northwest through north to northeast (Figs. 2, 3). Northward and eastward, east–vergent overturned folds dominate. Curved fold axes in the Iron Lake plate near the junction of Iron Lake and Quartzite Mountain faults (Tysdal et al., 2000) indicate two overprinted fold events having different orientations. A number of northeast–trending, open, upright macroscopic fold axes are mapped west of Blackbird Mountain (Tysdal et al., 2000). (3) Northeast of Blackbird Mountain, a complex of northeast–trending southeast–overturned macroscopic folds is the dominant structure (Fig. 3) and axial–plane schistosity is widely developed in the cores of folds and along sheared fold limbs (Lund, unpublished data, 2001–2002). (4) West of the Big Deer Creek fault, a paired set of map–scale northwest–vergent overturned folds control exposure of formations between Blackbird Mountain and Big Deer Creek (Figs. 5, 6). The paired overturned folds mark a zone of major constriction in deformation of the Iron Lake plate along the Blackbird Mountain oblique ramp. The overturned syncline cored by Hoodoo Quartzite marks the change in trend of strata, minor structures, and return of the Iron Lake fault to the regional northwest strike. All of these folds in the Iron Lake plate document regional northeast–directed motion of the plate disrupted by an oblique ramp in the area of the Blackbird district.
Blackbird Mountain Zone
During thrust faulting, a northwest–facing ramp formed in the Poison Creek plate (Figs. 5, 6D) facilitating a tear fault in the Iron Lake plate above the upper hinge zone of the ramp (Figs. 2, 3, 5). The orientation and position of the tear is marked by the zone of northeast–trending minor structures in the Iron Lake plate (described in preceding section) where the regional northwest trend of units and structures swings northward and northeastward. The ramp is marked in the Iron Lake plate by the map– scale overturned paired anticline and syncline of Yellowjacket Formation and Hoodoo Quartzite from Blackbird Mountain to Big Deer Creek (Figs. 2, 6).
Poison Creek Plate
The Poison Creek plate, east of and structurally below the Iron Lake fault (Fig. 1) is composed of the upper part of the Apple Creek Formation and, to a lesser extent, lower part of the Gunsight Formation. The Mesoproterozoic porphyritic granite and Cam– brian–Ordovician Deep Creek pluton in the northeastern part of the study area are also in this plate. Metamorphism and parasitic deformation related to the Iron Lake thrust fault are responsible for the local metamorphic and ductile deformation in the Apple Creek Formation near the thrust fault and particularly in three locally preserved footwall imbricate thrust faults in the upper part of the Poison Creek plate (Figs. 5, 6).
Indian Creek Subplate
Sheared chloritoid–garnet–biotite schist of the Indian Creek subplate is exposed as a fault sliver from the northwestern corner of the study area to near the mouth of Indian Creek and in a thin klippe across the topographically higher terrain north of Blackbird Creek (Fig. 3). This Indian Creek imbricate fault is coincident with the garnet isograd. Although the garnet isograd was described as gradational (“stratigraphic”) by Nash and Hahn (1989) in places where only occurrence of garnet was considered, mapping by Vhay (1948) and Daggett and Smit (1981) showed that the garnet isograd is oblique to bedding in underlying rocks and to compositional layering in the overlying rocks. Compositional layering in the garnet–bearing rocks is oblique to bedding in non–garnetiferous rocks, demonstrating that the “garnet isograd” is a structure separating different structural domains. Additionally, details of the map view together with cross–section views (Figs. 3, 6) demonstrate that the schist consistently overlies lower–grade, less deformed rocks. Map expression indicates that the Indian Creek subplate dips north across most of the study area and is as much as about 700 m thick (Fig. 6).
The Indian Creek subplate north of Big Deer Creek contains many shear zones and tight minor folds in schistosity plunging 15° to 25° northeast to north (Vhay, 1948). Kink banding is common. In exposures of the Indian Creek plate south of Big Deer Creek, compositional layering is folded into a broad north–plunging megascopic syncline with a steep western limb. There is a southward change in plunge of minor folds from northward plunges in the north to converging west– northwest plunges on the east side and east–northeast plunges on the west side in the southern part of this domain (Vhay, 1948). This change in orientation is similar to that of folds in the Iron Lake plate directly west and suggests noncylindrical folds or two generations of folding near the Blackbird Mountain ramp system.
Chloritoid–garnet–biotite schist, contained in this subplate (Fig. 4E, F), formed as a result of shearing in the Poison Creek plate as the Iron Lake plate was thrust over it. Thus, the Indian Creek subplate is a fault–bounded shear zone having unknown translation and is preserved as a remnant of a footwall imbricate at the Blackbird Mountain ramp and to the northwest (Figs. 5, 6). Similar effects are seen beneath the Poison Creek thrust fault in the area of Copper Mountain (8 kilometers north of the northeast corner of the study area), where chloritoid–garnet–feldspar– quartz–biotite schist formed in the upper footwall rocks to the Poison Creek thrust fault (Lund, unpublished data, 2001) in a similar structurally complex zone (see patterned areas in Tysdal et al., 2003). Although arguments about estimates of movement are important to the nature and geometry of the mineralized horizons, the enormous thickness of the stratigraphic units and the unresolved original stratigraphic relationship between Yellowjacket Formation and Lemhi Group packages makes it difficult to estimate the amount of translation across the Indian Creek imbricate thrust fault.
In the Blackbird subplate, the lower– to middle–greenschist– facies rocks of the banded siltite unit of the Apple Creek Formation are deformed into many macroscopic and outcrop–scale folds (Fig. 6B, C) that are divided into two domains by the White Ledge shear zone (see “Normal Faulting”). In this central part of the mining district east of the White Ledge shear zone, macroscopic to megascopic folds plunge 30–40° N and trend N12° W with diverging senses of overturning (Vhay, 1948; Daggett and Smit, 1981); these are parasitic to a large east–vergent syncline that is only partially preserved in the study area (Figs. 3, 6B, C). Axial– plane cleavage or schistosity (Fig. 4H) are common in the cores of the minor and outcrop–scale folds (Vhay, 1948). A fault that was mapped striking north–northwest along Meadow Creek is probably an axial–plane shear zone (Vhay, 1948). Tight, east–vergent folds on the east side of the subplate (Fig. 6C) near the Slippery Creek fault (Fig. 3) implies that this normal fault reactivated an earlier reverse fault (Vhay, 1948) that originally juxtaposed the Blackbird and Haynes–Stellite subplates.
West of the White Ledge shear zone (Fig. 3), rocks of the banded siltite unit are of lower to middle greenschist facies and form another domain in the Blackbird subplate. Overturned bedding on the east side of the block (Vhay, 1948) indicates a large tightly eastward–overturned anticline with north–trending axis (Fig. 3). Tight minor folds are south–plunging in the southern part and north– or northeast–plunging in the northern part (Vhay, 1948), indicating gentle refolding of the anticline in the area of the Blackbird Mountain ramp. Schistosity also formed in the axial area of folds and in minor shear zones in this domain.
The Haynes–Stellite subplate contains the lowest–grade rocks in the study area, wherein bedding defines broad open folds with amplitudes of about 1.5 km. Near the Slippery Creek fault (Fig. 3), which defines the western margin of the subplate, the rocks are more tightly folded and west–vergent, supporting the suggestion of Vhay (1948) that the Slippery Creek fault had earlier movement as an east–directed, west–dipping reverse fault. This earlier manifestation of the Slippery Creek fault, herein identified as a footwall imbricate fault separating the Blackbird and Haynes–Stellite subplates (Fig. 6C), put slightly higher–grade, older rocks of the Blackbird subplate above the lower–grade, younger rocks of the Haynes–Stellite subplate (Figs. 3, 5C).
Along much of its known length, the Iron Lake thrust fault was reactivated during the Late Cretaceous and Tertiary by normal faults. South of the study area, both northeast–directed reverse and down–to–the–southwest normal movements along the fault zone are coincident, and the name Iron Lake fault is used. In the southwestern and central parts of the study area, the Iron Lake thrust fault is reactivated along northeast–trending normal fault segments originally considered part of the Quartzite Mountain fault (Tysdal et al., 2000). Recent mapping (Tysdal et al., 2003; Lund, unpublished mapping, 2001–2002) resulted in reconsideration of the normal–fault offset along these segments and led to renaming faults on the basis of kinematics and history. As shown in Figures 2 and 3, (1) the southwest–trending fault segments southwest of Quartzite Mountain retain the name Quartzite Mountain fault, (2) the northeast–trending segment (of the original Quartzite Mountain fault of Tysdal et al., 2000) that extends northeast of Quartzite Mountain is herein referred to as the Blackbird Mountain zone, and (3) the north–trending segment (of the original Quartzite Mountain fault of Tysdal et al., 2000) that extends across Big Deer Creek is herein called the Big Deer Creek fault.
Quartzite Mountain Fault
The Quartzite Mountain fault extends southwest from Quartzite Mountain and moved down to the southeast (Figs. 2, 3). It formed along a zone of overprinted structures (refolded folds) that extends southwest from the Blackbird Mountain zone and that cuts off a set of northwest–trending map–scale synclines south of the fault trace. It is a southwestern extension of the complex Blackbird Mountain oblique ramp that offsets the Iron Lake fault to the northeast but is linked kinematically with the southeastern segment of the Iron Lake fault. The Quartzite Mountain and Iron Lake faults bound a triangle–shaped down–to–the– south block that, together with several faults parallel to and west of the Iron Lake fault, manifest the post–thrust–faulting structural disintegration of the Iron Lake plate southwest of the Blackbird Mountain ramp zone.
Blackbird Mountain Zone
The southern (structurally higher) hinge zone of the Blackbird Mountain oblique ramp/tear zone (Figs. 2, 3) underwent complex reactivated fault movements. The structure originated as a Cretaceous strike–slip tear fault during contractional deformation at the top of the lateral/oblique ramp in the Iron Lake fault system. Tertiary normal–fault movements along this zone previously were included with the Quartzite Mountain fault and shown as down to the southeast (Tysdal et al., 2000). Our reinterpretation indicates that this segment probably moved down to the northwest, a possibility discussed by Tysdal et al. (2000). This interpretation indicates that, although along trend with the Quartz– ite Mountain fault, normal movement along the Blackbird Mountain segment had opposite sense of relative displacement. This northeast–trending normal fault trace is parallel to normal faults of the Panther Creek graben but oblique to the north–trending normal faults that are so strongly expressed in the Blackbird mining district (Fig. 2).
Big Deer Creek Fault
Throughout the northern part of the study area, the Big Deer Creek fault cut off and reactivated the Iron Lake thrust fault except at the northern edge of the study area, where the Big Deer Creek fault diverges from the Iron Lake fault. The Big Deer Creek fault is steeply west dipping, west–side–down, and apparently has more displacement to the south, resulting in scissorslike movement. The Big Deer Creek fault juxtaposes the Yellowjacket Formation of the Iron Lake plate to the west against the banded siltite unit of the Apple Creek Formation of the Poison Creek plate to the east (Fig. 6A, B, C). Because the Big Deer fault juxtaposes the Iron Lake plate against both the Blackbird and Indian Creek subplates, we conclude that this fault truncated the Iron Lake plate at the Blackbird Mountain oblique ramp and that movement on this fault is post thrust faulting. Eocene Challis Volcanic Group–age granite porphyry and porphyritic rhyolite dikes are located in the hanging wall, following the trace of the north–trending Big Deer Creek fault for most of its length (Fig. 7) and indicating pre–Challis movement. The Big Deer Creek fault is the most important of a series of north–striking faults (including the White Ledge shear zone and Sunshine and Slippery Creek faults) that truncated the Iron Lake plate and the Blackbird Mountain zone (Fig. 1) and that are oblique to the Blackbird Mountain zone and Panther Creek graben.
White Ledge Shear Zone
The White Ledge shear zone (Shenon et al., 1955; Bennett, 1977) is a broad zone of faults and shears as much as 300 m wide (Vhay, 1948). Along most of its length, the White Ledge shear zone separates the lower–greenschist–facies banded siltite unit on the west from chloritoid–garnet–biotite schist of the Apple Creek Formation on the east (Fig. 2). The outcrop pattern suggests that the zone is primarily steeply east dipping and that the last movement was normal and brought the west side up relative to the east (Fig. 6B, C). A number of Eocene granite porphyry and porphyritic rhyolite dikes also follow this fault zone. These are all within a few hundred meters west of the fault trace. Map–view offset of the garnet isograd (from near the Sunshine prospect to lower Indian Creek) led Vhay (1948) to suggest a component of left–lateral movement. However, this apparent separation probably has more to do with the location and geometry of tearing or oblique ramping in the Iron Lake thrust fault. In addition to warping and tearing during thrust faulting, there may have been rotational motion on the White Ledge shear zone during extensional faulting because rocks in the Indian Creek subplate north of the Blackbird mine seem to have been rotated to a general northward dip from a regional south– westward dip.
The White Ledge shear zone follows the overturned eastern limb of a large anticline (in the Poison Creek plate) and may have started out as a fault cutting the stretched limb zone. This interpretation explains the large amount of shearing reported along the zone (Vhay, 1948; Bennett, 1977). Remnants of the anticline lie west of the White Ledge shear zone, and part of the related syncline is east of the White Ledge fault in the Blackbird subplate (Fig. 6A, B, C).
The minor Sunshine normal fault truncates the Indian Creek plate and offsets the garnet isograd such that high–metamor– phic–grade mineralized rocks of the Indian Creek subplate are downdropped and exposed along the ridge west of Meadow Creek (Fig. 3). Thus, the down–to–the–west movement of the Sunshine fault lifted the Blackbird subplate on the east side, within which lie the important mineralized strata of the Blackbird subplate (Fig. 6C).
Slippery Creek Fault
The Slippery Creek fault (Shenon et al., 1955; Bennett, 1977) is a down–to–the–west steep normal fault that juxtaposes the Blackbird subplate beside the Haynes–Stellite subplate (fig. 6C). To the north, the Slippery Creek fault offsets the Indian Creek imbricate thrust fault (Figs. 3, 5). The geometry of folding on both sides suggests that, near Blackbird Creek, the normal fault cuts near the site of a preexisting reverse fault. The Slippery Creek fault was thought to define the eastern limit of most of the mineralization in the Blackbird mining district (Bennett, 1977), but, in this new structural model, it juxtaposes different–age strata and different styles of mineralization. Thus, the Slippery Creek fault defines the eastern limit of the style of mineralization found in the Blackbird subplate.
Character of Blackbird Deposits
In the Blackbird district, early workers inferred that deposits were related to biotite– and tourmaline–impregnated zones rich in fine–grained cobaltite (Anderson, 1947). We interpret these zones to be in three related stratigraphic and structural settings.
In the heart of the district, seven bedding–parallel, laterally continuous stacked, tourmaline– and biotite–rich siliceous zones (tourmalinite and biotite) have been mapped in detail, largely from drill core (Nash and Hahn, 1989). Fine–grained cobaltite is found both disseminated in the biotitite and tourmalinite layers and defining cobalt–rich fine layering (Anderson, 1947; Nash and Hahn, 1989). Analysis of the biotite–rich layers indicates they are B–, Fe–, and Cl–rich (Lee, 1955) sedimentary rocks (possibly Fe–silicate facies iron–formation as used by Spry et al., 2000) and interpreted to be mafic volcaniclastic rocks related both to mafic volcanism and to synsedimentary hot–springs activity in the sedimentary basin (Nash and Hahn, 1989; Nash and Connor, 1993). Local depositional features of the host rock, chemical data, and unoxygenated character of the deposits were used to conclude that Blackbird deposits formed as the relatively deep–water, rift–related type of massive sulfide deposit rather than the type formed from evaporite beds (Nash and Hahn, 1989; Nash and Connor, 1993). Our interpretation that the host rocks are the deeper–water Apple Creek Formation rather than the shallower–water Yellowjacket Formation supports the previous interpretations of the deposit genesis.
In the Blackbird subplate, sulfide minerals are locally remobilized along discrete shear zones and in the axial zones of folds (Anderson, 1947; Vhay, 1948; Nash and Hahn, 1989). Quartz veins, veinlets, and nodules formed in these zones, and some open–space quartz filling is reported (Anderson, 1947; Vhay, 1948; Roberts, 1953). Hydrothermal activity in these zones, also produced a more complex paragenesis of sulfide minerals, including remobilization of copper in many places and deposition of secondary gold mineralization at the Uncle Sam mine (Anderson, 1947; Vhay, 1948).
In structurally higher parts of the district, gold–cobalt– copper deposits are hosted in chloritoid–garnet–quartz–feld– spar–biotite schist of the Indian Creek subplate. In these schists, most primary sedimentary features and original relationships of mineralized layers to bedding have been destroyed and primary versus secondary processes of deposit formation cannot be discussed separately. These topographically higher deposits are associated with biotitite, tourmalinite layers, and (or) fine–grained quartz–rich layers (“metachert”) that have been interpreted as exhalative chemical sediments (Nash and Hahn, 1989; Nash and Connor, 1993; and see Spry et al., 2000). Previously they were interpreted to be hosted by stratigraphic layers that were younger than or equivalent to the mineralized strata in the Blackbird subplate (Daggett and Baer, 1981; Toth and Hahn, 1982; Nash and Hahn, 1989). However, details of the structures of the Blackbird and Indian Creek subplates are different, and, more importantly, mineralized marker units (as shown by Vhay, 1948; Nash and Hahn, 1989) do not continue along strike northwest of the main district from the Blackbird subplate into the schist of the Indian Creek subplate.
Tourmaline–cemented breccias were recognized by early workers (Vhay, 1948), and more than a hundred of these have been identified by subsequent study (Ater, 1981) in the Haynes– Stellite subplate. The tourmalinized fissures and disseminations formed in upper strata of the banded siltite unit of the Apple Creek Formation and in the transition to the basal strata of the Gunsight Formation (Fig. 3). They are in a zone that lies south and east of the main deposits and that crosses a considerable thickness of stratigraphy. Some may be related to a late basinal structure rather than to a particular set of stratigraphic horizons; tourmaline was also remobilized into tourmaline– bearing breccia zones by Cretaceous or Tertiary deformation events.
Outside of the main Blackbird district, a number of similar or related prospects and deposits are hosted by structure–delimited exposures of the banded siltite unit of the Apple Creek Formation, by undivided rocks of the Apple Creek Formation, or by higher–grade rocks correlated with rocks of the Apple Creek Formation (Fig. 1). These sediment–hosted deposits are both in laterally equivalent and stratigraphically lower settings than Blackbird and may have formed from related processes. The distribution of these related deposits supports the interpretation of both stratigraphic control of mineralization and structural control of present exposure.
Structural Juxtaposition of Deposits
The different styles of gold–cobalt–copper mineralization are presently telescoped in the foreshortened upper part of the Poison Creek plate. They are located in the zone that contains several footwall imbricates and are in a partially inverted relationship. The thrust–fault geometry developed such that, within the Blackbird mining district, the strata young to the east in structurally lower positions. (1) The structurally highest rocks in the Poison Creek plate and in the Blackbird mining district lie within the Indian Creek subplate, sandwiched between the Iron Lake fault and the Indian Creek imbricate fault. Topographically higher prospects in the main part of the district (Daggett and Baer, 1981; Toth and Hahn, 1982; Nash and Hahn, 1989; Bending and Scales, 2001) and deposits west of the White Ledge shear zone in lower Indian Creek (Snyder, 1957) are all located along the base of the Indian Creek subplate. The host rocks are transposed, and the mineral deposits are recrystallized and remobilized. (2) Deposits of the main part of the Blackbird mining district are in the Blackbird subplate at middle structural levels. These mostly lower– to middle–greenschist–facies rocks are subdivided into two structural domains and mineralization styles by the White Ledge shear zone. Cobalt–bearing strata of the Blackbird subplate are cut off at the structural contact with the overlying Indian Creek subplate and to the east by the Slippery Creek fault (Fig. 6). (3) Tourmaline breccia deposits in the Haynes–Stellite subplate are associated with the structurally lowest, but youngest, rocks in the district, in which the most primary bedding and mineralization features are preserved.
The three different structural levels and the different mineral deposit styles subsequently were readjusted and juxtaposed side by side during Late Cretaceous to Tertiary normal faulting. This late normal faulting complicates restoration of original geographic and stratigraphic separation of the deposit types relative to each other, and the amount of translation on these footwall imbricate faults cannot be determined at present.
The Mesoproterozoic stratigraphy of east–central Idaho is being revised by recent regional studies. These studies indicate that the synsedimentary Blackbird gold–cobalt–copper deposits are hosted by the upper units of the Apple Creek Formation and strata of the lowest Gunsight Formation. Although hosting strata were previously mapped as Yellowjacket Formation, none of the deposits occurs in the Yellowjacket Formation, as it was originally defined by Ross (1934) and as recently confirmed by Tysdal (2000a).
Recent mapping (Tysdal, 2002, 2003; Tysdal et al., 2003; Lund, 2004) and our resultant stratigraphic and structural interpretations place the most important deposits and groups of deposits in exposures of the banded siltite unit of the Apple Creek Formation (Fig. 1). The Yellowjacket Formation, as restricted, was deposited in a shallow–water environment (Ross, 1934; Ekren, 1988; Tysdal, 2000b). In contrast, cobalt–bearing tourmaline– and biotite–rich siliceous layers, related tourmalinite breccias, and the sedimentary rocks that host them have been interpreted as having formed in a deep–water, rift–basin environment (Hahn and Sweide, 1981; Hahn and Hughes, 1984; Nash, 1989; Nash and Hahn, 1989; Nash and Connor, 1993). The banded siltite unit of the Apple Creek Formation, which hosts the main Blackbird–type deposits, is interpreted to be a deep– water turbidite sequence (Lopez, 1981; Sobel, 1982; Tysdal, 2000a, 2000b, 2003). The result of the new stratigraphic revisions is, at long last, agreement between the interpretation for environment of deposition of host strata on a regional basis and for the Blackbird–type deposit model (Earhart, 1986).
Without direct information on the age of the thick Mesoproterozoic sedimentary units that host various deposit types in the northern Cordillera and with large areas of metamor– phic rocks in central Idaho largely unmapped, it is not possible at present to make direct correlations among units that host the Blackbird deposits and units in the Belt–Purcell Supergroup that host other sedimentary metal deposits. However, detrital–zircon studies (Link and Fanning, 2003; Lund et al., 2004; Link and others, this volume) and direct dating studies (Evans et al., 2000) of the Proterozoic units are suggesting that the Apple Creek Formation is younger than units that host sedimentary deposits in the Belt–Purcell basin. Thus, these Blackbird deposits may have an independent origin.
Known major occurrences of Blackbird–type cobalt–copper deposits lie along two northwest–trending zones more than 50 km long (Fig. 1). Part of these trends was called the Idaho cobalt belt, based on the interpretation that the deposits lay along a paleo–rift that formed the hosting sedimentary basin (Hughes, 1983; Hahn and Hughes, 1984; Nold, 1990). Our data show that the mineralized rocks are within the banded siltite unit and the upper part of the conformably underlying coarse siltite unit of the Apple Creek Formation (Connor, 1990, 1991; Tysdal, 2003; Tysdal et al., 2003) near Blackbird and, regionally, within these units as well as within undivided Apple Creek strata. Because the exposures of the Apple Creek are controlled by regional structural geometries, the cobalt–bearing rocks, including the most important Blackbird–type cobalt–copper deposits, are limited to two northwest– trending outcrop belts, more than 50 km long, in central Idaho. Thus, it is the structural belts that determine exposures of the host strata and, therefore, the sediment–hosted deposits. Location of other sediment–hosted prospects and deposits, outside of the Blackbird area, support the interpretation of stratigraphic control of mineralization but also of structural control of exposure. Thus, the phrase “cobalt belt” is not used here to describe the distribution of the cobalt deposits.
In the broader region, location of probable Apple Creek country rocks (Lund, 2004), cobalt–copper–rich stream sediment, and cobalt–copper mineral occurrences in prospects (Cater et al., 1973; Lund et al., 1990) define a stratigraphic belt that trends northwest across central Idaho. That belt extends northwest from the Blackbird area (west of Fig. 1), and exposure of strata correlated with the upper Apple Creek Formation is also structurally controlled (Lund et al., 1990; Lund, 2004).
Structural analysis shows that the Blackbird mining district lies in an intensely deformed zone. The most complex deformation is in the upper part of the Poison Creek plate, particularly in the Blackbird Mountain oblique ramp in that system, where the Blackbird mine is located. The spatial correlation among the Iron Lake thrust fault, related imbricate faults, and particularly the ramp zone in the system may be due to preferential failure occurring in the mineralized rock. Additionally, the oblique ramp in the Poison Creek plate in the Blackbird area lies about 10 km west–southwest of an oblique ramp interpreted by Tysdal (2003) to lie in the footwall of the Poison Creek plate (northeast corner of Figure 2). The two oblique ramps probably reflect a deep–seated structural or basinal feature of unknown character. Identification of the Blackbird Mountain oblique ramp and its parallel relationship to the Panther Creek graben, together with the crosscutting nature of the graben with respect to regional thrust faults, suggest that the oblique ramp in the Iron Lake fault, the location of the Tertiary Panther Creek graben, and the location of most intense mineralization in the Apple Creek basin may all have origins related to the underlying Great Falls tectonic zone, a buried basement suture zone (O’neill and Lopez, 1985; Sims et al., 2005).
Steep normal faults offset metamorphic facies, low–angle compressional structures, and mineralized rocks (Figs. 3, 5) and are younger than the metamorphism and ductile structures. Eocene dikes were emplaced along the normal–fault zones and indicate that the faults predate the Challis volcanic–plutonic episode, probably forming between the Late Cretaceous and the early Eocene. The western edge of the Panther Creek graben (Bennett, 1986; Janecke et al., 1997; Tysdal and Desborough, 1997) may be defined by the Quartzite Mountain fault (Fig. 1). However, the set of north–trending normal faults, which include the Big Deer Creek fault, the White Ledge shear zone, the Sunshine fault, and the Slippery Creek fault (Figs. 2, 5), are strongly oblique to the Panther Creek graben and are an uncommon orientation in the region (Fig. 2).
Our study results in the following conclusions about the stratigraphy and structure of rocks hosting Blackbird–type deposits:
Blackbird–type sediment–hosted gold–cobalt–copper deposits are predominantly in the banded siltite unit of the Mesoproterozoic Apple Creek Formation. Related deposits are also hosted by the underlying coarse siltite unit of the Apple Creek and the overlying basal Gunsight Formation.
These strata of the Apple Creek Formation are interpreted to have formed as turbidites in relatively deep water, thus matching the proposed origin of the Blackbird deposits. Previously the host strata were correlated with the Yellowjacket Formation, which, as originally defined, formed in relatively shallow water. Our stratigraphic conclusions resolve the previous conflict between interpretations for genesis of the Blackbird deposits and their immediate hosting strata by matching the environments of deposition of mineral deposits and host strata on a regional basis.
The Yellowjacket Formation is limited to exposures on the hanging wall of the Iron Lake thrust fault and is not known to contain the sediment–hosted gold–cobalt–copper mineral deposits.
Mineral deposits in the Blackbird district formed in three related sedimentary environments that now lie in the complexly imbricated upper Poison Creek plate. Specifically, the district is located at the northeast–striking hinge zone of an oblique ramp in the underlying Poison Creek plate. Both hanging–wall and footwall rocks are strongly deformed, resulting in major overturned folds in the hanging wall and imbricate thrust plates in the footwall. The upper rocks of the footwall, which form part of a thrust–related shear zone, are strongly metamorphosed and transposed.
Northwest– and north–striking normal faults reactivated the Iron Lake thrust fault and dismembered the related imbricate thrust faults. This resulted in juxtaposition of different strati– graphic units, regional metamorphic facies, structural levels, and styles of mineralization within this complex district.
The stratigraphy that hosts the deposits is now sufficiently understood and regionally limited for future identification of related mineral deposits. The stratigraphic and structural features of the Blackbird mining district, as determined by this study, integrated with our other recent work, allow reconstruction of origin and present configuration of known Blackbird gold–cobalt–copper deposits. We confirm that mineralized strata formed in a rift–related basinal setting and document that the hosting strata lie in two northwest–trending belts, more than 50 km long, in which the distribution of exposed formations is controlled by northwest–trending thrust faults and normal faults. That geometry of the original Mesoproterozoic basin as well as the geometry of the later Cretaceous thrust faulting may have been controlled by features of the basement is the subject of ongoing study. These conclusions are ultimately useful for (1) better development of exploration models and geoenvironmental assessment of Blackbird–type deposits; (2) reconstructing the sedimentary basin in which these deposits formed; and (3) relating Blackbird deposits in time, space, and tectonic setting to sediment–hosted deposits in the Belt–Purcell basin to the north.
Access into the isolated parts of the study area was skillfully provided by J. Wilson, G.E. Wilson, and R. Tucker. G. King and W. Scales of Formation Capital, K.V. Evans of USGS, and J. Zieg of Cominco provided insights into the deposits and discussed preliminary results. E. DeWitt of USGS provided insightful discussion of the character of deformed sediment–hosted mineral deposits. Helpful reviews by P.K. Link and K.V. Evans greatly improved the paper.
Figures & Tables
Proterozoic Geology of Western North America and Siberia
This volume is a compendium of research on the Belt Supergroup. It is an outgrowth of Belt Symposium IV, held in Salmon, Idaho, in July, 2003, in conjunction with the Tobacco Root Geological Society annual field conference. Because of the geographic extent and great thickness of the Belt Supergroup, years of work have been required before conclusions are “bona fide”. The Mesoproterozoic Belt Supergroup of western Montana and adjacent areas is geologically and economically important, but it has been frustratingly hard to understand. The previous Belt Symposium volumes offer an historical view of the progress of the science of geology in the western United States. The advent of U-Pb geochronology, especially using the ion microprobe (SHRIMP) and laser-ablation ICPMS, has injected geochronometric reality into long-standing arguments about Belt stratigraphy. Several papers in this volume utilize these new tools to provide constraints on age and correlation of Belt strata (Chamberlain et al., Lewis et al., Link et al., and Doherty et al.)