The Crest iron deposit is the largest tested iron accumulation in the extensive Neoproterozoic Rapitan Iron Formation in the Yukon and Northwest Territories of northwestern Canada. The Crest deposit is located in the headwaters of the Snake and Bonnet Plume rivers, ~350 km northeast of Elsa, Yukon.
The Rapitan Iron Formation lies within the Neoproterozoic Rapitan Group, which is part of the lower Windermere Supergroup, and is widely exposed in the the Mackenzie Mountains of the northern Canadian Cordillera. The Mackenzie Mountains represent a Mesozoic to Tertiary fold-thrust belt, exposing sedimentary and lesser volcanic rocks deposited on the northwestern margin of the North America Shield from the mid-Neoproterozoic to the Permian (Gabrielse et al., 1973), with unexposed but presumably underlying Palaeo- and Mesoproterozoic sedimentary rocks similar to those preserved to the northwest in the Wernecke Mountains, Yukon (Thorkelson et al., 2005).
The oldest rocks in the region, the Palaeoproterozoic Wernecke Supergroup, is a metasedimentary suite that has been repeatedly deformed, including during the 1.6 Ga Racklan Orogeny, when the Palaeoproterozoic Bonnetia volcanic terrain was obducted over them (Furlanetto et al., 2013; Nielsen et al., 2013). The Wernecke Supergroup is unconformably overlain by the ~2.7 km thick clastic and carbonate succession of the Mesoproterozoic Pinguicula Group (Thorkelson et al., 2005), which is in turn unconformably followed by the Mackenzie Mountains Supergroup (Turner, 2011).
The Mackenzie Mountains Supergroup is ~4 to 5 km thick and represents the oldest Neoproterozoic unit in the Mackenzie Mountains, comprising the Dolores Creek, Tarn Lake, Black Canyon Creek, Tabasco and Tsezotene formations, and the overlying Katherine and Little Dal Groups (Turner, 2011; Long and Turner, 2012; Turner and Long, 2012). This succession is predominantly composed of shale, siltstone, shallow to deep water carbonate rocks, shallow marine sandstone and minor evaporitic rocks. The Supergroup was gently deformed in the Corn Creek 'orogeny', an extensional event that predated deposition of the unconformably overlying Windermere Supergroup (Thorkelson et al., 2005).
In the Mackenzie Mountains, thrust bound panels of the Windermere Supergroup forms an arcuate belt and comprises the basal Coates Lake Group, the Rapitan Group, the Hay Creek Group (Yeo 1978; James et al., 2001), and several ungrouped late Neoproterozoic formations (Aitken 1989; Narbonne and Aitken 1995). There is local evidence of a minor angular unconformity between the two groups, and within the Rapitan Group (Helmstaedt et al., 1979). The Coates Lake and Rapitan groups were both deposited in extensional rift settings presaging the break-up of the Rodinia supercontinent (Young 1992).
The basal 0 to 1500 m thick Coates Lake Group, is an intra-continental rift succession, comprising continental tholeiitic basalt, continental siltstone, evaporite, conglomerate and carbonate rocks, and is host to the Coates Lake stratabound copper deposits (Jefferson, 1983; Rose et al., 1986; Jefferson and Ruelle, 1987; Jefferson and Parrish, 1989). The basal tholeiitic basalts are interpreted to be the extrusive equivalents of the 780 to 770 Ma sills and dykes intruding the underlying Mackenzie Mountains Supergroup, making them part of the Gunbarrel mafic igneous event (Armstrong et al., 1982; Harlan et al., 2003).
The Rapitan Group, which is typically 1500 m thick, unconformably overlies the Coates Lake Group, and consists of the basal, regionally limited diamictites of the Mt. Berg Formation, overlain by the fine-grained turbidites of the Sayunei Formation, and massive diamictites of the Shezal Formation (Eisbacher, 1978). All three of these formations are glaciogenic in origin (Yeo, 1981), and presumed to belong to the global Sturtian glaciation event (Hoffman and Halverson, 2011). The Sayunei Formation is capped locally by an up to 120 m thick hematite-jasper iron formation, hosting the Crest iron deposit (Klein and Beukes, 1993). There is local evidence of a minor unconformities between the individual formations within the Rapitan Group (Helmstaedt et al., 1979). The Coates Lake Group is absent to the west in the Yukon, where the Rapitan Group directly overlies the Mackenzie Mountains Supergroup (Long et al., 2008). In sections of the Northwest Territories, the Rapitan Group is underlain by the 0 to 500 m thick Little Dal Group, which is similar to the Coates Lake Group with a basal flow basalts and overlying stromatolitic dolomite with lesser shale (Baldwin, 2014).
The Hay Creek Group conformably overlies the Rapitan Group, and comprises mudstone, siltstone and wacke of the Twitya Formation, shallow-water limestone and sandstone of the Keele Formation, diamictite of the Ice Brook Formation, and cap carbonate of the Teepee dolostone (Yeo, 1978; Aitken, 1991; James et al., 2001; Day et al., 2004; James et al., 2005). This group is interpreted as the first of three 'grand cycles' that constitute the upper Windermere Supergroup, each with an upward transition from basinal shale to platformal carbonate, capped by a major flooding surface (Narbonne and Aitken, 1995; Yeo et al., 1978). The upper two cycles correspond to the unnamed 'upper group', comprising of the Sheepbed (shale), Gametrail (limestone), Blueflower (shale), and Risky (limestone) formations (Aitken, 1989). An unconformity that truncates the Risky Formation, marks the top of the Windermere Supergroup in the Mackenzie Mountains, where it is overlain by the lower Cambrian Ingta and Backbone Ranges formations and lower to middle Paleozoic rocks.
During the Palaeozoic, the region was divided into the eastern Mackenzie Platform, with predominantly shallow water carbonates, and the western deep marine Selwyn Basin, characterised by shales. These two regions host significant carbonate and shale hosted Zn-Pb mineralisation respectively (Baldwin, 2014).
All the rocks of the Mackenzie Mountains were variably thrust and folded during the Mesozoic-Tertiary cordilleran orogeny (Baldwin, 2014).
The Rapitan Group was most likely deposited between 755 and 730 Ma. Regionally, the Rapitan Group was deposited within two separate rift basins, the Snake River and Redstone basins, which are 135 km apart, separated by an interval in which the Rapitan Group is absent. Each of the two basins has a distinct sedimentological and stratigraphic character, particularly in the Sayunei Formation and the iron formation that caps it. Characteristics of the Shezal Formation are somewhat more uniform in these two basins. Sparse equivalents are also found 250 km to the west of the Snake River Basin, straddling the Yukon-Alaska border (Baldwin, 2014).
The Snake River Basin is the smaller of the two, being around 50 km in diameter, but contains the thickest known exposures, and the Crest deposit, where a non NI 43-101 inferred resource of ~5.6 Gt @ 47.2% Fe has been estimated within an area of 25.9 km2. The thicker development of the iron formation is distributed over an area of 50 x 13 km (Dustan, 2008). The Rapitan Group is largely surrounded by older, underlying rocks to the NW, NE and SW, and dips to the SE below younger Windermere Supergroup rocks.
Within this basin, the main iron formation that hosts significant mineralisation is a 100 to 120 m thick unit at the top of the Sayunei Formation. A section through this unit within the Crest deposit has been described by Baldwin (2014) and Klein and Beukes (1993) as follows.
The immediate footwall to the iron formation comprises a polymictic, clast-supported, channel-fill conglomerate, predominantly composed of pebble- to cobble-, and rare boulders-sized clasts of quartz arenite and carbonate sourced from the underlying Mackenzie Mountains Supergroup, as well as scattered basaltic clasts of uncertain origin. The intergranular space comprises carbonate cement and scattered quartz sand grains. The conglomerate has variable lateral thickness and continuity, and occurs towards the top of a 65.5 m interval of massive, dark red to purple matrix-supported clast-poor sandy diamictite with similar clast lithologies.
The transition from conglomerate/diamictite to iron formation includes a 2 m thick suite of siltstone and sandy siltstone, which are hematite-cemented, with weak normal grading, and faint cross-bedding, and contain scattered dropstones, predominantly of carbonate rocks. Similar units are found throughout the iron formation, and although the thickest is 21 m, the remainder are rarely more than a few tens of centimetres thick.
Above this basal clastic interval, the iron formation comprises variably interbedded banded iron formation (BIF) and nodular iron units, with continuous jasper beds being common in the nodular intervals, particularly where these units are thicker. Dropstones, typically of carbonate rocks, are ubiquitous and generally only penetrate and deform no more than 1 to 2 cm into the underlying iron formation.
Some iron formation intervals, referred to as â€˜dropstone-rich IFâ€™, containing >70% dropstones (from granules to large pebbles) of mixed, but mostly carbonate composition in a predominantly hematitic matrix, with dropstone grain sizes ranging.
The iron formation also contains some intervals with irregular bedding and local small to large-scale slump folds where there is considerable reworking and folding. Within these intervals of slumping, the primary relationships between the folded and disjointed jasper beds, hematite beds, and jasper nodules are not clearly discernible, and are referred to as â€˜slumped or reworked IFâ€™.
The iron formation layers comprise a mix of bedded jasper and hematite (BIF) and nodular iron formation, where jasper nodules occur within a massive to bedded hematite matrix, but is dominantly the latter. The relative proportion of BIF to nodular iron formation increases in the uppermost 15 m of the iron formation, although nodular iron formation remains abundant, as do nodular intercalations within the BIF. In contrast to the lower sections of the iron formation, the upper part contains jasper nodules (generally not zoned) within the jasper beds, which commonly have a fairly subtle colour contrast to the host jasper. A greater proportion of the jasper in these intervals is contained within the nodules relative to the bands. The uppermost jasper bed, at the contact between the iron formation and overlying diamictite, contains greenish-tan chert nodules in a jasper matrix, and is the only non-jaspilitic chert in the section.
The uppermost 2.5 m of the iron formation, in the transition zone with the overlying mixtites of the Shezal Formation, is characterised by granular iron formation (GIF) i.e., iron-formation arenites, which is composed of medium-sand sized grains of jasper, hematite and subordinate angular volcaniclastic particles, deposited above wave base.
The hanging wall to the iron formation is composed of at least 15 m of texturally and compositionally variable diamictite, dominated by grey-green to tan, carbonate-rich, sandy clast-rich pebble diamictite, commonly with a fissile, scaly weathering style that is typical of the Shezal Formation diamictite.
The Shezal Formation consists of massive, recessive-weathering grey or tan diamictite with minor sandstone and siltstone intervals, and is interpreted to be a glaciomarine outwash diamictite, with possible localised terminal moraines (Eisbacher 1978). Locally, the base of the Shezal Formation consists of reworked material from the top of the Sayunei Formation, including iron formation material.
Some 16 km to the NE at Discovery Creek, east of the Crest deposit, the Sayunei Formation overlies the Little Dal Group, and is composed of a basal 340 m of purple to red clast-poor and clast-rich diamictite with muddy sand matrix, with lesser interbeds of matrix- and clast-supported conglomerate. In this area, the iron formation is split in three separate stratigraphic intervals. The lowermost is 1 m thick and dominantly BIF, the middle is 0.1 m of nodular iron formation, and the uppermost is 20.2 m thick, with equal parts of BIF and nodular iron formation. The top band is unconformably overlain by lower Palaeozoic rocks. The Rapitan Group pinches out 10 km to the northwest, with a clearly erosional upper surface. Several small copper occurrences, consisting of chalcopyrite weathering to malachite and chrysocolla are present in the Sayunei Formation, in both the iron formation and the underlying diamictite.
On the SE margin of the basin at Cranswick River, the iron formation is 30 m thick and contains approximately equal proportions of BIF and nodular iron formation, with several thin (<50 cm) interbeds of siltstone and diamictite. It is overlain by a tan clast-poor diamictite that includes a single bed containing remobilised jasper clasts 5 m above the iron formation (Baldwin, 2014).
The iron-formation and siliciclastics undergo lateral interfingering with the diamictites becoming more prominent and coarser grained, toward the east over a strike distance of some 41 km. Nevertheless, although individual siliciclastic units, display marked facies variations, they tend to be laterally persistent, and have relatively sharp vertical contacts with iron formation units, which drape over virtually every available siliciclastic facies (from finest shale to coarsest diamictite) along any one stratigraphic level.
The Redstone Basin is characterised by excellent exposure of the Rapitan Group in the central Mackenzie Mountains. This area is centred approximately 170 km south of Norman Wells, NWT, and spans a NW-SE trending strike-length of roughly 320 km and exposed over widths of 0 to <40 km, before dipping to the SW below younger Windermere Supergroup rocks (Baldwin, 2014).
In the central section of the basin, at Boomerang, which is 280 km south of the Snake River Basin, the Sayunei Formation unconformably overlies the Coppercap Formation of the Coates Lake Group, which contains patchy copper mineralisation directly below the contact. The Sayunei Formation comprises a basal 0.8 m thick, orange-weathering maroon clast-supported diamictite, the clasts of which are all derived from the underlying Coppercap Formation. This is overlain by 91 m of maroon siltstone and mudstone, and blue-grey medium-grained sandstone, with scattered dropstones and diamictite interbeds, then by 28 m of maroon pebble diamictite with siltstone interbeds. The uppermost unit of the formation is a silty- to sandy-matrix pebble diamictite, which contains up to 30 cm clasts of reworked and plastically deformed jasper and hematite. This jasper-bearing diamictite is overlain by grey clast-rich muddy diamictite of the Shezal Formation (Baldwin, 2014).
At Hayhook North, 36 km to the south Boomerang, the Sayunei Formation is underlain by grey-weathering limestone of the Coppercap Formation, and comprises 120 m of dominantly maroon siltstone with interbeds of sandy siltstone and sandstone, and rare interbeds of green mudstone and sandstone up to 1.4 m thick. In its upper sections this section becomes finer-grained, and is dominated by maroon to pink mudstone and siltstone, and towards the top contains a 70 cm thick interval of bedded jasper with minor bedded hematite. This thin ironstone is immediately overlain by interbedded pale green mudstone and grey to tan diamictite (variable clast content) of the Shezal Formation. At Hayhook South, a further 6 km south, the Sayunei Formation has thickened to >250 m, with 6.5 m of iron formation towards the top, almost entirely composed of BIF, with flaggy-weathering beds of jasper and hematite. Nodules of black chert are found in the basal jasper bed, but not in the remainder of the iron formation. Traces of chalcopyrite, malachite and chrysocolla occur along jasper-hematite bed contacts at the top of the iron formation, which is immediately overlain by 4.5 m of purple to maroon siltstone, followed by Shezal Formation talus (Baldwin, 2014).
The mineralogy of the iron-formation essentially comprises hematite cemented by micro-quartz, with the texture of a fine hematite mud containing, one or more of silt-sized hematite granules and shard-like, bladed, quadrangular, triangular, disc-like or stringer-like micro-clasts (Klein and Beukes, 1993). Sedimentary structures range from massive, to faintly streaky laminated, to very finely and evenly laminated. Streaky laminations are most common in nodular iron-formation units, whilst fine, evenly laminated types are restricted to a few thin units defined as finely laminated iron-formation.
Five lithofacies of iron formation have been recognised within the Rapitan Iron Formation, specifically:
• Laminated jasper-hematite - or laminated hematite felutite (after Beukes, 1983) is the predominant facies in the Sayunei Formation, where it is characterised by alternating laminae of hematite and jasper. The laminae are composed of micron sized irregular blebs and lenticular aggregates of jasper and hematite in varying proportions, with either predominating to form jasper- or hematite-rich bands. Laminations may also be caused by slight variations in the crystallinity of hematite and quartz. Contacts between the bands are generally sharp and even, coincident with lamination (Yeo, 1984; Klein and Beukes, 1993).
• Nodular jasper-hematite - ovoid and irregular nodules of quartz, red jasper, chert or specular hematite, typically <5 mm thick, may be scattered or clustered in laminated or massive hematite in beds that are from 1 to 20 cm thick. Jasper or chert nodules have sharp margins and commonly contain two or three concentric shells which reflect internal variation in silica and iron content. Nodules may exhibit simple or complex zoning. The former typically consists of an iron-rich rim becoming chert-rich toward the centre of the nodule. In contrast, more complex zoning may produce concentric, alternating iron-rich and iron-poor bands (Yeo, 1984; Klein and Beukes, 1993).
Progressive nodule growth has been preserved, with the earliest normally lensoid or podded in form, with a marked compaction of laminae around it. This stage can be seen to be overgrown by a second generation ovoid, typically more spheroidal and replacive, i.e., crosscutting laminations and with little or no surrounding compaction. In the latter case, laminae are usually abruptly terminated against, and deflected around the nodules, suggesting the nodules formed prior to compaction. However, in the preserved early stage, laminae extend into the nodules, where they are markedly thickened, although never to the centre, which is commonly replacive, i.e., massive, with a total absence of original laminations. Flattening and smearing of the nodules has been observed above and below dropstones, while elsewhere, nodules range from ovoid to discoid shapes, with flattened parallel to bedding. Hematitic pressure shadows are found adjacent to many nodules. Infrequent dark grey chert nodules occur within laminated red jasper (Yeo, 1984; Klein and Beukes, 1993).
Some nodules have hematite or calcite nucleii. Many nodules contain plate-like or irregular shard-like concentrations of hematite in their cores. These shards are typically surrounded by iron-poor massive chert and may represent 'septarian' infill of the core of the nodule by hematite. However, in samples where shard-like microclasts occur in the iron formation, it appears as if the nodules preferentially grew around the shard. Specular hematite nodules are typically massive (Klein and Beukes, 1993).
In some sections of the iron formation, nodule-rich and nodule-poor bands alternate, defining a facies described as nodular banded iron-formation. In these bands nodules may coalesce to form jasper band and become a banded iron-formation. Commonly, nodular iron-formation grades into nodular banded iron-formation, which in turn, passes into banded iron-formation forming cyclical repetitions. Nodules cause laminations to be differentially compacted around them and may also disrupt laminae by cutting across them. Preservation of finely laminated iron-formation without jasper nodules and/or banding is relatively rare, only preserved in a small number of thin bands (Klein and Beukes, 1993).
• Irregular jasper-hematite - has many features in common with the nodular iron-formation, occurring as lenticular patches up to several cm thick. Irregular iron-formation has many features in common with the nodular iron-formation. Irregularly jasper lenses commonly form masses and intergrowths in massive or laminated hematitic matrix, commonly with compositional zoning. Laminae within the matrix may be traced into the lenses, while enveloping laminae flow around the lenses, implying that like nodules they are early diagenetic features. Less commonly, hematite lenses occur in a jasper matrix, occurring as patchy zones with typically diffuse and irregular contacts, although Iaminae may be traced across these patches, which may reflect local epigenetic iron-enrichment. Hematite depletion in laminae and haloes surrounding hematita-rich lenses and patches appear as brighter, reddish-orange, iron-poor jasper (Klein and Beukes, 1993).
• Hematitic argillite - appear to be preferentially developed along transitions near the bottom and top of the iron-formation sequence, and associated with argillaceous interbeds within the iron formation, as well as in association with iron-formation arenites (granular iron-formation) where they become sufficiently enriched in hematite to warrant the term 'iron-formation' (Klein and Beukes, 1993).
• Hematitic diamictite and arenites - are composed of poorly sorted angular jasper particles and hematite felutite particles with highly irregular shapes. The felutite fragments appear to have been plastic during transportation. The sandstones are lithic arenites and greywackes composed of well-rounded to angular quartz, carbonate, chert, jasper, hematite and lava grains in a matrix of chlorite and carbonate cement. Hematite comprises part of the matrix and cement in the red iron-rich varieties, whilst volcaniclastic arenites contain abundant angular chloritised lava grains. One major characteristic of the diamictites is the presence of abundant carbonate clasts derived from carbonate sequences underlying the Rapitan Group. Red varieties contain hematite in the matrix and grey varieties contain chlorite (Klein and Beukes, 1993).
The latter two lithofacies are generally mutually closely associated, as well as with the jasper-hematite iron formations. Siliciclastic components in the silt-rich iron-formations are of two types, namely as distinctive single or composite interbeds within the jasper-hematite iron formation, and as dispersed siliciclastic particles within a matrix of hematite felutite. The latter occur as single dropstones or dropgrains, or as dropstone-grain accumulations of varying concentrations in laminae, pockets or bands in the iron-formation. The former siliciclastic interbeds are composed of finely laminated mudstone and siltstone with dropstones, massive mudstones and siltstones, and massive or normally graded gritty mudstone, siltstone, greywackes, gritty or pebbly greywackes, lithic arenites, diamictites, and a polimictic conglomerate. Single beds have sharp to erosional bottom contacts, commonly loadcasted or with sole structures, and sharp upper contacts. These beds range from a few cm to a maximum of about 4 m in thickness.
Stratified diamictite-shale within the iron formation comprises very thin to thin massive or graded diamictite beds alternating with thin shale laminae or beds. In the lower part of the iron-formation, there are two well demarcated thick composite siliciclastic beds. The lower comprises dropstone-free shale with interbeds of well-packed massive polimictic conglomerate, greywacke and a gritty greywacke, both of which are normally graded. The upper siliciclastic bed comprises a thick, massive bed of gritty greywacke and an upward-coarsening sequence of massive dropstone-free mudstone, overlain by stratified shale-gritty greywacke and mixtite. The iron-formation below and between these two composite siliciclastic units is relatively pure and free of siliciclastic dropstones or laminae, although it is slumped and contains disoriented jasper nodules. The upper composite siliciclastic unit is overlain by a relatively thick central iron-formation zone that contains essentially individual interbeds of predominantly graded greywackes and siltstone. These interbeds gradually increase in abundance upward, to develop into an upper zone composed of composite siliciclastic units separated by thin iron-formation beds. These upper composite siliciclastic units are composed of coarsening-upward sequences of mudstone to stratified diamictite-shale to stratified diamictite. Hematitic arenite beds are associated with this upper siliciclastic-rich unit. The iron-formation is overlain by an upward-coarsening sequence of mudstone, gritty mudstone, greywacke with well-developed Bouma cycles, stratified diamictite-shale, and coarse diamictite. The colour of the siliciclastics changes from red to grey-green in the middle of the sequence. The upward-coarsening unit is overlain by stacked upward-fining gritty greywacke and diamictite beds, each with an erosional base. A similar type of upward-coarsening siliciclastic sequence is developed at the base of the iron-formation in the lowermost part of the iron formation (Klein and Beukes, 1993). Klein and Beukes (1993) also note the presence of volcaniclastic sandstones and mudstones, containing dispersed sand-sized volcanic particles, found in close association with the iron-formation arenite beds.
Alteration - There is a marked increase in post depositional mineralogical alteration of the iron-formation downward in the sequence. This alteration starts to be mesoscopically conspicuous at depths of ~90 m, where the iron formation becomes a fine, dull grey specular hematite femicrite (Beukes, 1983) grading into a recrystallised, metallic-looking, somewhat coarser grained specularite. Red jasper mesobands change into purple- coloured cherts due to the recrystallisation of red hematite dust to fine specular hematite, whilst jasper nodules and bands tend to be replaced by carbonate. This alteration is not totally pervasive, resulting in bands or zones with varying degrees of alteration (Klein and Beukes, 1993).
Hematite within the iron-formation occurs as five texturally different varieties, as follows (after Klein and Beukes, 1993):
• Type 1 - the finest grained form, which is essentially hematite dust that occurs in jasper nodules and bands and is responsible for the red colour of the jasper.
• Type 2 - the most common variety, a very fine-grained specular hematite femicrite or a matrix of 1 x 5 µm specular hematite needles displaying a meshlike intergrowth with micro-quartz. These matrix specular hematite produce the dull metallic lustre of the laminae in the iron-formation.
• Type 3 - which is micro-nodular hematite, comprising small pseudoclasts of hematite in a cement of micro-quartz.
• Type 4 - comprises relatively coarse (~0.05 mm) specular hematite, intergrown with quartz, which forms the bright metallic laminae or streaks in the iron-formations and the metallic specular hematite nodules. It is also developed in specular hematite-rich rims, in internal concentric zones, or in the centres of chert nodules, and appears to be the result of recrystallisation of type 2 matrix hematite.
• Type 5 - is the coarsest and latest form of hematite, and consists of coarse 0.5 to ~1 mm plates of specular hematite intergrown with coarse calcite and macro-quartz. All three of these minerals are late replacements of chert nodules.
Types 1 to 3 hematite are commonly associated, particularly on the margins of many chert nodules. The type 1 hematite dust occurs in jasper laminae that are somewhat less compacted than those in which type 2 matrix specular hematite is developed, and it is probable that the former represents the earliest form of hematite, as preserved by early chert cementation. In the more compacted laminae, type 2 matrix specular hematite occurs as a fine intergrowth with quartz. The micro-quartz associated with matrix specular hematite is devoid of hematite dust, suggesting that a separation of quartz and hematite has taken place. Type 3 appears to be slightly later than types 1 and 2 and is developed where quartz has replaced matrix specular hematite or where jasper has recrystallised. This process results in complete separation of quartz and hematite, with the hematite becoming concentrated as small nodules in clear grey microquartz. This separation results in the pseudogranular texture in chert nodules or bands (Klein and Beukes, 1993).
Five texturally different varieties of carbonate have been recognised, all representing late-stage replacement features, as follows (after Klein and Beukes, 1993):
• Type 1 - is a coarse euhedral to subhedral clear sparite intergrown with specular hematite needles, as described above.
• Type 2 - is composed of a sparite mosaic, replacing either jasper, with a yellow to red colour due to hematite dust inclusions, or quartz, in which case it has a grey colour.
• Type 3 - is similar to type 2, except that it comprises microsparite that replaces chert or jasper.
• Type 4 - occurs as euhedral sparite crystals that commonly have a poikilotopic zoned texture, with matrix or micronodular specular hematite
• Type 5 - is coarse clear sparite filling cracks, in some cases septarian cracks in nodules.
Electron microprobe analysis showed that all five types of carbonate are essentially pure calcium carbonate. Sandy lenses or stringers in the iron-formation may contain sand-sized dropstones, whilst some of these same laminae contain reworked iron formation fragments.
Structural and sedimentary control
There is a considerable variation in the occurrence, thickness and facies distribution of the Rapitan Group in northwestern Canada and eastern Alaska, including the thickness and geographic extent of the iron formation, both within and external to the Snake River and Redstone basins. These variations appear to be predominantly depositional and not deformational or erosional (Baldwin, 2014). The absence of the Rapitan Group over the 135 km interval between the two sub-basins appears to record a lack of deposition, rather than later erosion (Eisbacher 1981), whilst within each of these basins, variations in the thickness and distribution of the Rapitan Group and the iron formation are significant, as indicated by the descriptions above. Internal variations are relatively minor in the smaller Snake River basin, but are more pronounced in the exposed section of the more extensive Redstone basin (Baldwin, 2014).
The margins of each of the basins, as well as changes in the thickness of key formations (e.g., the Sayunei Formation) and presence or absence of iron formation), correspond to the locations of 9 (or more) mapped and inferred major, crustal-scale fault systems, most of which trend NNE, roughly perpendicular to the axis of the rift basins. Inferred syndepositional faults are based on overall thickness changes in both the Coates Lake and Rapitan groups (Eisbacher, 1981, 1985), but also on unusual thickness and facies patterns of units within the underlying Katherine and Little Dal groups of the Mackenzie Mountains Supergroup (Turner and Long, 2008). Baldwin (2014) compared the areal distribution of the iron formation and the interpreted locations of many of these faults and concluded that these structures may have exerted dominant control on the distribution, and even thickness, of the iron formation and were active prior to deposition of the Rapitan Group (Turner and Long, 2008), and some, if not all, were active throughout Rapitan deposition (Baldwin, 2014).
These structural controls result in the Rapitan iron formation being highly variable in both thickness and character over relatively short distances. Major changes in iron formation thickness are also reflected by the changing character of the underlying Sayunei Formation, which is very fine-grained in areas where iron formation is <30 m thick, but includes significant thicknesses of coarse clastic lithologies, either sandstone or sandy diamictite, where iron formation is thicker. The thicker iron formation sections commonly contain several distinct chemostratigraphic zones, as defined by the REE+Y signatures, that are dominated by either chemical or siliciclastic components. Zones of chemical deposition in the thickest iron formation sections are accompanied by several cycles of shallowing-upward and subsequent deepening of the basin, and coincide with sedimentary evidence for cyclic basin infill and subsequent subsidence. The geochemical signatures of iron formation units that were consistently isolated from siliciclastic sedimentation remained purely hydrogenous over their entire thickness (Baldwin, 2014). Baldwin (2014) concluded that these lines of evidence collectively indicate that the thickest and most iron-rich iron formation intervals were deposited in the deepest and most tectonically active sub-basins. These observations, in conjunction with the presence of extensive soft-sediment deformation in thick iron formation intervals, suggests active subsidence in a relatively young rift basin. Baldwin (2014) suggest this interpretation is consistent with the absence of the Coates Lake Group in areas with thick iron formation in much of the Snake River Basin, whereas, in more evolved basins, such as most of the Redstone Basin, the iron formation is much thinner. The Snake River Basin has a fairly steep western fault margin and a gradual, shelf-like eastern section, producing a small, deep basin, which allowed for the deposition of coarse clastic rocks in the deep basin prior to iron formation deposition, but the limitation of siliciclastic input later, possibly by a covering anchored ice sheet, allowing for deposition of thick iron formation (Baldwin, 2014). This also suggests the hydrogenous iron formation facies are deposited in proximity to active growth faults.
Baldwin (2014) interpreted data on the REE+Y and the redox-sensitive elements Mo and U from the Rapitan iron formations, to show the Rapitan iron formation was deposited in a partially restricted basin from biogenically reduced iron under variable redox conditions. He also concluded that elemental Re and Mo isotopes implied that, although oxic and ferruginous conditions predominated during deposition of the iron formation, a transition towards a sulphidic water column locally terminated deposition. The young, deep rift basins in which the iron formation was deposited, had undergone marine incursion, and were intermittently sealed by an ice shelf, allowing for the generation of an anoxic, iron-rich water column. The absence of any Eu anomaly and the heavy Mo isotopic signature indicate that the open ocean was fully oxygenated at the time of Rapitan iron formation deposition.
The Rapitan Group was deposited during the distributed global mafic magmatic and rifting event that accompanied the early stages of break-up of the Rodinia supercontinent. It is also coincident with the global Sturtian glaciogenic event and is broadly coeval with a variety of magnetite and/or hematite bearing iron formations and/or iron-rich clastic sedimentary rocks on most continents, e.g., (after Yeo, 1984), the
• Braemar and Holowilena iron formations, magnetite and lesser hematite bearing diamictite in South Australia in the Umberatana Group that
includes the Sturtian type section, situated within the Adelaide Fold Belt/Rift Basin;
• banded hematitic quartzites of the Macaúbas Group, in the Araçuaí Basin, SE of the São Francisco Craton in eastern Brazil;
• banded iron formation of the Espitito Santo Subgroup of the Jaibaras Group in the Caririan Fold Belt of Ceara, Brazil;
• the associated nodular manganese beds and hematite-jaspilite units of the Jacadigo Group in the Corumba-Urucum district of western
Brazil and eastern Bolivia;
• magnetite or hematite rich siliceous rocks, locally interbedded with manganese-rich beds, in the Chuos Formation of the Damara
Supergroup in Namibia;
• similar iron formation within the possibly correlatable Numees Formation of the Gariep Belt in SW Namibia and NW South Africa;
• jasper-hematite iron formation of the upper Bissokpabe Group of the Dahomeyide Belt in Togo, West Africa;
• the hematite and minor magnetite in the Chestnut Hill Iron Formation in the western New Jersey Highlands of the USA;
• hematite-jaspilite iron formations of the Kingston Peak Formation in eastern California, USA;
• the Upper Tindir Group extension of the Rapitan Group in eastern Alaska;
• hematite, magnetite and quartz with minor ankerite in the Red Sea coast of Egpyt, on the Arabian-Nubian Shield;
• iron formation of the Fulu Formation, South China.
The Crest deposit is located within the Snake River Basin, hosted by the Rapitan Group in a section of conglomerate, mudstone, shale and sandstone that is 2000 m or more in thickness. The iron formation is relatively fresh and unaltered, and crops out in three structural blocks separated by northwest-trending faults. The Crest iron deposit lies in the westernmost of these fault blocks (Dustan, 2008).
The iron deposit comprises unaltered layers of hematite and jasper oxide facies, with interspersed beds, lenses, and 1 to 5 cm thick nodules of dolomite and ankeritic carbonate. The iron layers have a cumulative thickness of 85 to 105 m and are distributed through 120 m of stratigraphic section (Dustan, 2008).
The average composition of the iron formation within the deposit varies from 40 to 50% Fe (averaging 43%); 0.02 to 0.8% Mn (averaging 0.25%); 0.2 to 0.7% P2O5; 0.02 to 0.08% S; 0.02 to 0.11% TiO2 and varying amounts of SiO2 (~30%), Al2O3 (~1.4%) and CaO (~3%). The hematite and silica are believed to have been carried in solution by fumarolic waters and precipitated in grabens on the sea floor. Phosphorus, the main impurity, occur as finely disseminated apatite (Dustan, 2008).
A feasibility study between 1963 and 1964 estimated a resource of 3.2 Gt @ 43% Fe, 26.6% SiO2, 0.34% P2O5. The resource could be mined from open pits with a stripping ratio of 1:1. An additional 3.6 Gt of iron ore formation were estimated in the vicinity of the potential open pits (Dustan, 2008).
Other, smaller deposits in the Rapitan belt of rocks. The total iron resource in the Snake River Basin was estimated at 18 Gt. Beneficiation studies showed that the fine-grained iron formation of the Snake River Basin can be beneficiated by selective agglomeration methods. Material containing 54.6% Fe and 0.39% P2O5 was treated to provide concentrate containing 65.9% Fe, less than 0.02% P2O5 and 5.3% SiO2, with 85 percent of the iron being recovered in the concentrate (Dustan, 2008).
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2015)