|
Citronen |
|
|
Greenland |
| Main commodities:
Zn Pb
|
|
 |
|
|
 |
Super Porphyry Cu and Au
|
IOCG Deposits - 70 papers
|
All papers now Open Access.
Available as Full Text for direct download or on request. |
|
|
The Citronen sediment hosted zinc-lead deposit is located in northern Peary Land, northern Greenland. It lies at the head of the Citronen Fjord, a southern tributary of the larger, east-west oriented Frederick E. Hyde Fjord to the north. It is situated ~250 km NW of the Danish military support base Station
Nord and 100 km south-east of Kap Morris Jesup, the northern-most cape of Greenland (#Location: 83° 6' 9"N, 28° 15' 44"W).
Early expeditions to Peary Land were conducted in 1906-07, 1947-50 and 1953. The first recorded indication of sulphides in the area were from a sample taken by the Danish Geological Survey in the second of these, in 1950, which comprised shale with veins of quartz and calcite that was reported to contain 'centimetre-large polycrystalline pyrite balls' (Ellitsgaard-Rasmussen 1955). The next was by US Geological Survey geologists in 1960 who conducted a helicopter reconnaissance and noted gossans at Depotbugt, to the south of the Frederick E. Hyde Fjord and 20 km east of Citronen Fjord. These were noted from the air and described as having "... extreme plays of color and every appearance of mineralisation”. Gossans were also located in the vicinity of Citronen Fjord by a British Joint Services Expedition in 1969 who also sampled the Depotbugt gossans. One of these that was sampled, was reported to be hosted by a carbonate breccia-conglomerate in a thick succession of early Silurian calcareous sandstones. It was accompanied by sulphide mineralisation that was associated with intense calcite veining. Samples from these gossans were assayed, but gave no indication of significant mineralisation.
The Frederick E. Hyde Fjord region was revisited as part of a 1978-80 helicopter supported systematic regional mapping program of the area by the Geological Survey of Greenland. All strata at Citronen Fjord were assigned to the Merqujôq Formation, the basal section of the Silurian Peary Land Group turbidite succession. Carbonate breccia-conglomerates, which are interbedded with mudstones and siltstone turbidites, are conspicuous in the western part of
the area, along Citronen Fjord and to it's SW. These were interpreted as 'base of slope' debris flows, and mapped as the Citronens Fjord Member of the Merqujôq Formation. The type section contains two main intervals of conglomeratic rocks, with the lower and thickest containing a conglomerate unit that is ~80 m thick. This work was reported by Hurst and Surlyk (1982). These carbonate conglomerate units are markers in a fairly uniform sedimentary succession.
Pedersen (1982), in an unpublished PhD thesis, noted that sulphide mineralisation within limestone conglomerates south of Citronen Fjord is associated with fault splays of the NW-SE trending Trolle Land Fault Zone. The same author noted fracture-fill copper-zinc mineralisation accompanying quartz-calcite veining, the main mineral assemblage of which was stratabound pyrite, sphalerite, chalcopyrite and trace galena with an epigenetic appearance. This mineralisation is all on the western side of the Citronen Elv, the river that flows north into the head of the Citronen Fjord.
In 1992 Platinova A/S, a Greenland subsidiary of Canadian owned Platinova Resources, participated in a joint venture with Nanisivik Mines Ltd. to explore for base metals in the northwestern Greenland section of the Palaeozoic Franklinian Basin.
The Franklinian Basin extends east-west for >2500 km, from NE Greenland, westward through the Canadian Arctic Islands to the Beaufort Sea. In Greenland it rests on Proterozoic basement, and on Archaean rocks in Canada. The basin hosts the Polaris Zn-Pb-Ag deposit to the west in Canada, and Citronen to the east in Greenland, and numerous other occurrences in between. Throughout the Early Palaeozoic, the basin comprised a southern shelf and slope facies and a northern deep-water trough. The shelf succession was dominated by up to 4 km of carbonates, whilst the trough sequence was ~8 km thick and predominantly siliciclastic rocks. The transition between shelf and trough facies shifted with time, with the basin migrating south until the shelf foundered in the Silurian. In the Lower Palaeozoic, this facies boundary corresponded to the so-called Navarana Fjord Escarpment, a marked east-west palaeo-topographic feature with a relief of >1 km. Sedimentation was terminated by the Late Devonian to Early Carboniferous Ellesmerian Orogeny, characterised by east-west to NE-SW trending folds. Deformation was most intense in the north of Peary Land, the only place affected by regional metamorphism, which was only low grade. Lead-zinc mineralisation appears to occurs in both deep-water and shelf facies, with Citronen being found just north of the transition. These sequences are overlain to the SE in Greenland and to the NW in the Canadian Arctic by a late Palaeozoic to Cenozoic basin.
Platinova A/S geologists and partners began regional exploration in the western half of the northern Greenland section of the Franklinian Basin in 1992, focussed on deep-water clastic sedimentary rocks known as the Amundsen Land Group, the stratigraphic equivalent of the widespread Early Cambrian to Early Silurian Hazen Formation of neighbouring Arctic Canada. This group occurs as a narrow east-west striking belt over a distance of ~300 kilometres (Higgins et al., 1992), and hosts numerous, occurrences of zinc and lead sulphides in various settings, as identified during helicopter reconnaissance. To the east In Peary Land, exposures of equivalent sedimentary rocks are largely confined to land bordering the Frederick E. Hyde Fjord, occurring as discontinuous outcrops over a strike length of ~125 km. Work in this latter area commenced with skidoo and sledge ground/ice based access in May 1993, focussed on inspecting gossans and sulphide occurrences reported in the Citronen Fjord area that had been observed in the 1969 and 1979 expeditions, as described above. 'Massive' sulphides of the Citronen deposit were encountered in outcrop on the first day of exploration, in what is now known as the Discovery Zone. This zone is on the eastern side of the Citronen Fjord and Elv, within sight across the river from the gossans encountered in earlier expeditions. The discovery was so promising that a full-scale exploration programme that included drilling was executed during the same summer, continuing for five years, into 1997. Work during that period included geological mapping, gravity and electromagnetic surveys, and the drilling of 143 exploration holes for a cumulative total of 32.4 km of core. In 1998, an order of magnitude study of the project was undertaken by Kvaerner Metals. However, in the same year, a downturn in the zinc price caused this project to be placed on hold. Then, in 2002, Platinova A/S entered into administration and relinquished all interests in the Citronen deposit. In 2005, Globe Star Mining Corp. acquired title to the property, and then transferred the title to a holding company Bedford (No.3) Limited. In 2007, the Australian company Ironbark Gold acquired 100% of the Citronen Zinc Project from Bedford (No.3), and then in 2009 changed the company name to Ironbark Zinc Limited. Ironbark undertook further proving and related activities to bring the deposit to production. Their shareholders included the global zinc smelter group Nyrstar and Glencore International AG. By December 2020, after a 9 year long permitting process, Ironbark had obtained the necessary Exploitation licence and Closure Permit required to commence production. In 2022 the company signed an MOU with a Norwegian Arctic operations specialist contractor for the mine construction, operational mining and logistics. However, in December 2024, Ironbark announced that it had initiated a strategic review of its Citronen asset, and that fresh eyes and capital were required to further advance the project. Following on from this review, Ironbark Zinc entered into a binding agreement to divest 100% of its Citronen Zn-Pb Project through the sale of its wholly owned subsidiary, Ironbark A/S, to Almeera Ventures Limited based in Dubai, for a consideration of AUD 1.4 million (Ironbark Zinc Release to the ASX, December 2024).
Regional Setting
The Citronen deposit is located near the eastern preserved extremity of the Franklinian Basin, just north of the facies boundary between the southern carbonate platform and shelf, and the northern deep-water trough that is characterised by dominantly fine clastic sedimentation. This facies change is abrupt, and was established in the late Neoproterozoic to early Cambrian, during the initial subsidence of the basin, remaining relatively stable until the Early Silurian. As detailed above, the shelf and trough settings are separated by the Navarana Fjord Escarpment in the basement, which has a relief differential of >1 km and was apparently sufficiently steep, such that no sediments were deposited on the slope between the shelf and the trough. The mudstones, siltstones, fine-grained sandstones and carbonate debris flows that were deposited in the trough regionally through the Ordovician and Lower Silurian have been assigned to three groups - the Vølvedal and Amundsen Land groups and the lowest part of the Peary Land Group (Friderichsen et al., 1982; Higgins et al., 1992).
Renewed deposition of sandstones and turbidites in the trough commenced in the middle-Early Silurian (late Llandoverian) as a result ot the onset of the Caledonian Orogeny to the east, and by the late-Early Silurian (Wenlockian), had buried the Navarana Fjord Escarpment and filled the trough. This sediment load caused the outer margin of the platform to subside and then fail, and the carbonates were transgressed by shales of the Wulff Land Formation.
Deformation within the basin took place in the Devonian to Carboniferous, producing the North Greenland Fold Belt, a band of tectonism and metamorphism that now occupies the northern, Arctic Ocean facing coast of Greenland. This belt more-or-less coincides with the deep-water trough. Thus, the host rocks to the main sulphide mineralisation at Citronen are folded, deep-water, argillaceous Ordovician rocks with interbedded carbonate debris flows, derived from the nearby carbonate platform.
Within the Citronen deposit area, the Silurian Peary Land Group comprises Merqujôq Formation sandstone turbidites and calcareous siltstones, with the interbedded carbonate debris flows designated the Citronens Fjord Member.
Stratigraphy of the Citronen Deposit area
The stratigraphy of the Citronen Fjord area may be summarised as follows (after Kragh et al., 1997 and van der Stijl and Mosher, 1998):
CAMBRIAN
Buen Formation, which is locally subdivided into,
- Unit 1, Green Siltstone - Fine grained, thick-bedded to massive, greenish grey siltstone, with a variable thickness that ranges from at least 300 m in the north, near the Frederick E. Hyde Fjord, thinning to <100 m along the southern Citronen Fjord. The base of the unit is not exposed, so the thicknesses are not definite. The age is also uncertain, as no fossils have been encountered.
- Unit 2, Cigar Debris Flow - the oldest of the mapped debris flows, a pale weathering unit composed of elongate clasts of carbonate
and calcareous mudstone set in a siliceous matrix. Individual clasts appear to be 10 to several tens of cm long and 1 to 5 mm in diameter. The flow has a thickness that ranges up to 15 m, and locally it has a basal, metre thick layer of massive dolostone. Where absent, the boundary between units 1 and 3 is difficult to pinpoint. Both are un-fossiliferrous.
- Unit 3, Black Siltstone - black, locally phyllitic, monotonous carbonaceous, planar-laminated and thin-bedded siltstone that commonly contains disseminated pyrite. It is interbedded with black and dark grey cherts to the NE. The siltstones include a poorly exposed layer of Early Cambrian trilobites. It has a maximum thickness of 100 m, but thins markedly where the upper contact is with the overlying Silurian Units 9 and 10 and units 4 to 8 are absent. Where this occurs, the contact is manifested as a variably rusty-weathering carbonate 'conglomerate' with a matrix that is locally siliceous. Individual clasts are commonly elongated and cigar-shaped, and in many respects simulates the Cigar Conglomerates of Unit 2. Where present, it represents the absence of an appreciable thickness of Cambro-Ordovician strata, including the whole Vølvedal and Amundsen Land Groups, and is interpreted to represent a regional low-angle thrust.
The Buen Formation (Jepsen 1971) is defined as a siliciclastic shelf facies sequence that grades over the outer shelf and margin interval into the monotonous, >2 km thick, slope and trough sequence of turbiditic sandstones, siltstones and mudstones that is the Polkorridoren Group (Higgins et al., 1991).
ORDOVICIAN
Amundsen Land Group, which was deposited in a sediment-starved trough a few kilometres basinward of the ~1 km high, east-west trending Navarana Fjord escarpment that formed the outer margin of a wide carbonate platform. It is composed of dark, recessive, argillaceous strata, punctuated by intermittent pale weathering, resistant, re-sedimented carbonate debris flows, and is subdivided into,
- Unit 4, Lower Mudstone, dominated by dark grey interbedded dolomitic siltstone and graphitic mudstone, with common intercalated arenite and minor disseminated pyrite. It is similar to Unit 6, although lenses and 'eyes' of chert formed by the in situ replacement of siltstone and mudstone are common. It is only encountered in drilling which has not found the base of the unit.
- Unit 5, Lower Debris Flow, a clast-supported carbonate debris unit that is strongly dolomitised and occasionally silicified. Minor pyrite is found in the matrix, particularly in the upper sections. The clast size distribution indicates sorting. It is not exposed and based on drill data, the thickness of the unit is estimated to average ~45 m, with drill intersections of between 2 and 65 m.
- Unit 6, Middle Mudstone, a dark unit of rhythmic layered black graphitic, dolomitic and/or calcitic mudstone and dark grey siltstone, with variously calcareous arenite, minor carbonate debris flows and fine- to medium-grained massive pyrite beds. It is characterised by soft-sediment deformation and
dehydration textures and varies from ~15 to 85, averaging 50 m in thickness. The upper section of the unit is commonly brecciated by depositional erosion related to the overlying Unit 7 debris flow. It is the host rock of 'Level 3' sulphide mineralisation.
- Unit 7, Middle Debris Flow, a variably matrix- and clast-supported calcareous and dolomitic limestone conglomerate, which, particularly in the uppermost sections, exhibits sorting of clasts. Clasts locally have a preferred orientation with an imbricate structure. These clasts are generally fossiliferous, containing crinoids, reef corals and occasionally brachiopods, and are set in an argillaceous matrix. The conglomerate is locally altered by a penetrative dolomitisation with associated gradational replacement of both the matrix and clasts by pyrite, and occasionally sphalerite and minor galena. The lower part of the debris flow generally contains numerous angular clasts of finely laminated sulphides within a carbonate debris matrix, interpreted to have been ripped up from the immediately underlying 'Level 3' sulphides hosted within Unit 6. The thickness of this unit varies between a minimum of 2 and a maximum of 104 m, averaging ~50 m, and is typically inversely proportional to the amount of underlying massive sulphides in Unit 6. The average thickness encountered in drilling is 50 m.
The Middle Debris Flow is laterally variable. To the south, along Citronen Elv, west of the Discovery Zone and along the western margin of the Citronen Fjord, it is a largely monomictic, strongly pyritic conglomerate that is 'rusty weathering'. However, further north, towards the Frederick E. Hyde Fjord, there is an abrupt change into a strongly pyritic, polymictic debris flow containing abundant, large (‘house-size’) boulders of dolostone with common intercalations and enclosures of siliceous arenite, black mudstones and massive, fine-grained pyrite. Further to the north, on the shores of the Frederick E. Hyde Fjord and west of the Citronen Fjord, in the West Gossan area, drilling indicate abundant and extensive slump and tectonic breccias, resulting in a stratigraphic-tectonic pattern not seen elsewhere in the Citronen Fjord area. These rock occur in the vicinity of the long lived regional Harder Fjord Fault Zone, which may have influenced this increased disclocation and deformation.
- Unit 8, Footwall Shale, so-named because it formed the footwall and host to the outcropping massive sulphides of the original 'Discovery Zone' 'Level 1' sulphide mineralisation. However, it is also host to the 'Level 2' sulphide mineralisation. It is composed of black graphitic mudstone with dark grey, weakly calcareous siltstone, and commonly contains intercalations of calcarenite and thin carbonate debris flows. Scattered, disseminated pyrite occur throughout. Soft sediment deformation structures are evident, but are not characteristic. It thickens rapidly to the NE, accompanied by an increase in the number of intercalated limestone debris flows. It's thickness varies from between 45 and ~100, averaging 55 m.
This Unit 8 is very similar to the calcareous siltstone of Unit 10, as described below, except that sedimentary features like ripple marks and cross-laminations are absent.
SILURIAN
Peary Land Group and Merqujôq Formation, which forms the uppermost sequence of the Lower Palaeozoic in the immediate
Citronen Fjord area, where it is only overlain by Quaternary and Recent cover. It is divided into:
- Unit 9, Hangingwall Debris Flow, which immediately overlies the original 'Discovery Zone' 'Level 1' massive sulphides, and is a clast-supported limestone conglomerate. The clasts, which are sub-angular to sub-rounded, appear to range from 1 to 30 cm across, are variably sorted by size, particularly in the upper sections, and are occasionally aligned in a preferred orientation. The carbonate clasts commonly contain crinoids, reef corals, and a Llandoverian (Lower Silurian) coral-crinoid-brachiopod fauna (Bjerreskov and Poulsen, 1973). These conglomerates generally have a brown to rusty appearance in outcrop, after pyrite. Pyrite is a common constituent of the groundmass, locally forming the entire matrix, with occasional traces of sphalerite and galena. The thickness of the unit varies from a few to 90, averaging ~55 m. In general, it has a sharp lower contact with the argillaceous rocks of Unit 8.
The Hangingwall Debris Flow, which is ~80 m thick in the type section, is the lower and thicker of the two debris flows that together constitute the Citronens Fjord Member of Hurst and Surlyk (1982). The upper of the pair is Unit 11, as described below.
- Unit 10, Calcareous Siltstone, generally a somewhat monotonous, 500 to 450 m thick sequence composed of dark grey, thin-bedded calcareous siltstone and muddy carbonate turbidites. These sometimes exhibit a discrete banding of darker and paler silt beds, and commonly contain well-preserved cross-lamination. Graptolites, which are locally abundant, indicate a Late Llandovery age. Scattered pyrite is common, although no massive sulphides have been observed within the unit, which is virtually indistinguishable from Unit 8, the Footwall shale. Unit 10 forms a discrete stratigraphic interval separating the Units 9 and 11 debris flows, although calcareous siltstones indistinguishable from it, interdigitate with the turbidites of the overlying, and uppermost Unit 12 debris flow.
- Unit 11, Upper Debris Flow, a clast-supported limestone conglomerate in which clast size varies from centimetres to a couple of metres. The clasts are, in general, poorly sorted, with local vague preferred orientations. The matrix is argillaceous, but becomes increasingly sandy to the north, where the unit thins and gradually grades into a matrix-supported conglomerate. As in the other debris flows, the clasts are fossiliferous most commonly containing crinoids and reef corals. This debris flow is characterised by sharp upper and lower boundaries, and ranges up to 90 m in thickness, averaging ~20 m.
- Unit 12, Sandstone Turbidite, the uppermost exposed unit in the immediate Citronen Deposit area. It comprises a reddish brown weathering fine-grained turbiditic sandstone, with frequent intercalations of carbonate and quartz-chert debris flows, as well as abundant calcareous siltstone. Graded bedding, cross-bedding, load casts, flute marks and other sedimentary structures are common. These sediments interfinger with calcareous siltstones of Unit 10 where unit 11 is absent or poorly developed. Although, the upper limit of the preserved sequence of Unit 12 is either the current erosional surface or the unconformity with the Cenozoic cover sequence, it's estimated thickness is ~700 m, although it may be thicker to the south.
This sequence of Cambrian to Early Silurian rocks, were deposited near the shelf/slope to trough transition, marked by the interpreted Navarana Fjord Escarpment. The deposit lies within the trough sequence fine siliciclastics sedimentary rocks that are periodically intercalated with fossiliferous debris flow carbonate breccia-conglomerates derived from the shelf facies to the south. The shelf facies are separated from the trough sequence by the narrow, steeply north-dipping slope of the fault governing the >1 km, north-down fault that produced the Navarana Fjord Escarpment. Periodic seismic activity on this fault is interpreted to have triggered the debris flows.
The information for this stratigraphic section was drawn from van der Stijl and Mosher (1998) and Kragh, Jensen and Fougt (1997).
Mineralisation
Mineralisation Style - The main mineralisation at Citronen comprises stratabound massive and banded pyrite containing variable sphalerite, minor galena and trace silver, barium and copper. The sulphides are generally fine- to medium-grained, and range from weakly-banded to laminated, while others are massive and lack any planar features. There is a further variation in the textures, to the intermittent presence of dendritic-textured pyrite forming a net-like texture (e.g., Kragh et al., 1997). Where best developed, this dendritic texture forms a fern-like pattern of open spaces lined with pyrite framboids that are locally overgrown by euhedral pyrite crystals. The open space between the sulphides, which comprises up to 50% of the rock by volume, is filled by calcite or dolomite spar. This pattern can be seen to overprint or replace the banding textures, although in most cases the process has advanced such that no primary features remain.
Zinc and lead grades are generally <3% and <1% respectively within the massive and dendritic-textured pyrite. Within these, sphalerite and minor galena appear to have been deposited into open spaces and fractures as irregular, medium- to coarse-grained concentrations.
The banded and laminated sulphides, which are characteristically composed of fine grained, planar-laminated and thinly-banded sulphides, contain the highest grades of zinc and lead. Individual layers range from 1 mm to 1 m in thickness, but most commonly are up to a few tens of centimetres, with any >30 cm being rare. Textural variations are common within individual sulphide beds. Well preserved sedimentary structures such as graded bedding, cross-lamination and dewatering features have been identified within the laminated sulphides (Kragh 1997; Kragh et al., 1997). Some beds are characterised by laminated framboidal pyrite, whilst others show recrystallisation and replacement textures and spheroidal/colloform intergrowth of two or more of carbonate, pyrite, galena, sphalerite and silica. These spheres/framboids are either massive or have complex radiating textures, and are often overgrown by diagenetic or metasomatic subhedral pyrite (Kragh,1997). With a few exceptions, mudstones intercalated with these sulphide beds have sharp, but often irregular contacts. Adjacent sulphides contain clay and silt as impurities, whilst the mudstone commonly carries up to 20% disseminated pyrite.
In general, the primary lamination in the upper parts of the sulphide bearing units is disturbed by replacement or recrystallisation, particularly just below debris flow breccia-conglomerates. The more massive sulphide accumulations grade laterally into laminated framboidal pyrite with interstitial sphalerite and carbonate. In places, the massive sulphides are cut by quartz-calcite veins that include partly replaced or recrystallised wall rock fragments. These fragments can be either angular or 'deformed', and are typically overgrown by sphalerite or pyrite. Debris flow conglomerates lying directly above sulphide accumulations are often mineralised with pyrite, sphalerite and galena occurring in the matrix and as rims on the clasts (Kragh, 1997).
The colour of the massive pyrite varies in proportion with the amount of zinc present. When the zinc content is in the 15 to 20 vol.% range, the host massive pyrite has a brownish hue, and where >20 vol.%, the pyrite is pinkish to reddish. The highest zinc content of a pyrite bed noted to date is 35%. However, where the sphalerite content of the pyrite is <10 vol.%, it is visually difficult to detect. Sphalerite commonly forms laminae within the massive pyrite, but rarely forms thicker beds. It normally occurs interstitial to pyrite framboids (Kragh 1997). Other than within the veins, no examples have been found of sphalerite that has been deposited without accompanying pyrite, but pyrite without sphalerite is common, with many beds of pyrite containing <1% Zn.
Pyrite occurs as microscopic spheres in all textures, i.e., massive, bedded/laminated and dendritic (Kragh 1997), whilst sphalerite and galena occur both as inclusions and, more commonly, as overgrowths on the pyrite spheres.
Late stage mineralisation is commonly seen in both drill core and exposure, particularly in units 7 and 9, the Middle and Hangingwall debris flows, as well as the massive sulphides. A significant example is the so-called Zone XX to the immediate north of the Discovery Zone, which is a 20 to 30 m wide x 30 m thick corridor that overprints the Middle and/or Hangingwall debris flows, and was formed on the northern side of a tectonic lineament. This lineament is partly exposed at surface, but can be traced along trend in drill core for a strike length of >1 km, subparallel to the strike of the Trolle Land Fault Zone. This late stage mineralisation is irregular and discontinuous, and is closely related to the penetrative dolomitisation and minor silicification seen in the debris flows. It resulted in replacement textures and associated fading and local bleaching of the clast-supported fabric and the subsequent accentuation of the original carbonate clasts by colloform linings of pyrite. This initial stage of replacement progresses gradually, forming an increasingly massive pyrite-sulphide with dendritic textures and floating 'clasts' of recrystallised carbonate. Solution vugs with or without secondary fillings of sparry dolomite and/or bitumen are commonly developed. Veins and veinlets of quartz/carbonate are numerous and occasionally develop into a stockwork-like pattern. Abundant light brown to green-grey sphalerite and minor galena are restricted to particular well-defined intervals in the highly pyritic carbonate rocks of the Middle and Hangingwall debris flows that constitute units 7 and 9.
NOTE: The description by van der Stijl and Mosher (1998) of this Late Stage mineralisation within the units 7 and 9 debris flows is very similar to the same authors' description of the dendritic sulphide mineralisation of shales and siltstones that host the main massive to semi-massive sulphide mineralisation in units 6 and 8.
Distribution of Mineralisation - The zinc-lead sulphide deposits of the Citronen Fjord comprises at least five major, massive sulphide accumulations that form a 10 km long x ~2 km wide, NW-SE trending corridor that is approximately parallel to the Late Palaeozoic to Mesozoic Trolle Land fault zone (Håkansson and Pedersen, 1982; Birkelund and Håkansson, 1983). Whilst there is no evidence of pre-Carboniferous activity on this fault, it has been considered to possibly represent a reactivated Early Palaeozoic structure (Kragh et al., 1997). The individual sulphide accumulations are up to 1500 m long x 600 m wide, with a maximum thickness of ~25 m. Each is laterally separated by intervals that vary from a few hundred metres to 4 km. The central parts of each accumulation comprises sulphides, dominantly pyrite, with lesser sphalerite and galena, and the carbonates calcite, dolomite and ankerite. Minor chalcopyrite and quartz are also present. The sulphide content of these accumulations varies from ~25 to almost 100%. These sulphides are confined to beds that are normally 30 to 60 cm thick, but locally expand to as much as 2 m, with irregular mudstone interbeds from a few mm to 10 cm thick.
The northwestern extremity of this corridor is occupied by the uneconomic, pyrite dominated sulphides of the West Gossan or West Showing Zone to be discussed below. This zone abuts the southern shore of the Frederick E. Hyde Fjord, ~2 km west of the Citronen Fjord.
To the SE, there are three main accumulations which are, from SE to NW, the Discovery, Beach and Esrum zones. The Discovery and Beach zones are semi-continuous, whilst the Esrum Zone is offset from the northern extremity of the Beach Zone by 1.5 to 2 km to the WSW. Each of these zones host mineralisation at three vertical levels that have been traced by geophysics and drilling, with fault offsets, thinning and 'pinchouts', over a strike length of >5 km, a width of up to 500 m, and a gross strike of ~145°.
In recent announcements by Ironbark Zinc (2024), a fourth, Sulphide Level 4, is mentioned, particularly within prospects along the eastern margin of Citronen Fjord and the Western Gossan, but not in the main Discovery, Beach and Esrum zones. However, little information has been encountered on its geology, mineralogy and stratigraphic position.
Within the Discovery, Beach and Esrum zones, the three mineralised levels recognised by van der Stijl and Mosher (1998) may be summarised as follows, from the lowest, upwards:
Sulphide Level 3, which is hosted within Unit 6, the Middle Mudstone, between the underlying Lower Debris Flow and overlying Middle Debris Flow. This level contains the greatest tonnage of sulphides, yielding intersections of >10 m cumulative thickness that persist continuously for almost 3 km in a strike direction of 140 to 180° in the Beach and Discovery zones, and for at least 1.5 km, striking 160 to 180° in the Esrum Zone. It occurs as a series of laterally separated or adjoined accumulations of massive and dendritic sulphide, the thickest of which is 57 m. However, this thick sulphide core has only very low Zn and Pb grades. Outward from these thick centres of the main sulphide accumulations, the dendritic texture becomes less conspicuous with an increase of fine-grained and laminated, more zinc-rich sulphide bands with intercalated mudstone and calcarenite. In contrast to the sulphides in Unit 8 (see below), Unit 6 is also mineralised between the different main lateral sulphide accumulation, and can therefore be traced continuously from SE of the Discovery Zone to the north-western end of the Esrum Zone. There is evidence that Unit 7, the Middle Debris Flow has, in part, eroded the underlying Level 3 sulphides during deposition.
Sulphide Levels 1 and 2, both of which are hosted within Unit 8, the Footwall Shale, but at different vertical positions with respect to Unit 9, the Hangingwall Debris Flow. Locally, within the Discovery Zone, the Hangingwall Debris Flow immediately overlies Sulphide Level 1, although elsewhere this upper sulphide sheet is separated from the debris flow by a variable thickness of siltstone. Similarly, whilst the separate levels 1 and 2 may be readily distinguished in some areas, elsewhere, most of the unit is composed of massive sulphides and the two may not be differentiated.
In the Discovery Zone, Sulphide Level 2 is only found locally and is not well developed.
In the Beach Zone, Sulphide Level 2 is defined as occurring immediately above, or separated by a thin mudstone unit, from Unit 7, the Middle Debris Flow, which, in turn, separates it from the underlying Sulphide Level 3. In this Beach Zone, Level 2 does not outcrop, but forms a concealed 2 km x up to 500 m, north-south striking belt, dipping at 5 to 8°N. It is characterised by a series of massive to dendritic-textured sulphide accumulations that are up to 20 m thick, surrounded by thin, 1 to 5 m thick aprons of banded sulphides. The highest combined zinc/lead grades occur in a ‘corridor’ of mainly fine-grained, bedded sulphides, that is ~1 km long, and some hundreds of metres wide, that runs along the eastern rim of this Level 2 belt. This sulphide sheet characteristically consists of two prime layers, which in most cases, are separated by a less mineralised siltstone up to 10 m thick. In the southern and western part of this corridor, the highest Zn/Pb values occur in the upper layer, whilst in the north-eastern part, the best combined grades come from the lower layer. Detailed exploration drilling at 100 m intervals within this Level 2 sulphide corridor has indicated a resource for a high grade core of 7 Mt @ 9% zinc, 1% Pb at a cut-off grade of 6% Zn over 2 m.
Within the Esrum Zone, Level 2 sulphides occur as a concealed 1 km x 250 m belt that is sub-parallel to, and offset from that in the Beach Zone. However, massive sulphides are laterally absent from the intervening Unit 8, Footwall Shale, between the Beach and Esrum zones.
Sulphide Level 1 is the uppermost and most southerly development of the sulphide sheets, and is the exposed massive sulphide of the Discovery Zone, covering an area of ~1200 x 300 m. It is also occurs as minor accumulations of pyrite laminae further to the north-west in the Beach and Esrum zones.
The West Gossan or West Showing Zone, which is 4 to 5 km NW of the Esrum Zone. Drilling from this zone reported by van der Stijl and Mosher (1998) indicates sulphide mineralisation occurs in both the carbonate breccia-conglomerates and the intervening argillaceous and siliciclastic strata. It has also encountered an extensive occurrence of extremely dense, massive pyrite at a stratigraphic level that is interpreted to be comparable to Level 3 of the Beach and Esrum Zones. However, whilst the massive sulphides contain slightly enhanced lead, zinc grades are very low to almost zero, although the overlying arenites and siltstones do carry some disseminated zinc-rich sulphides. However, more recently (Ironbark Zinc, Sept. 2024) drilling has intersected 2.0 m @ 4.5% Zn+Pb and 1.2 m @ 4.77% Zn+Pb. Stijl and Mosher (1998) had reported that three drill holes in conjunction with a pronounced gravity anomaly have been interpreted to indicate the largely barren sulphide of the Western Gossans is probably in the order of 30 Mt. The textures and composition of the massive sulphide and the host rock lithologies in this area are markedly different from those in the Beach, Esrum and Discovery zones, i.e., the sulphides are dense, contain no dendrites, contain low zinc and weakly enhanced lead grades, whilst the host lithologies, are characterised by slumping, tectonic brecciation, abundant quartz-arenites and polymict, matrix-supported debris flows with generally rounded, lenticular clasts. These suggest a more active tectonic regime during and after deposition and mineralisation.
Sulphur Isotope Chemistry - The δ34S values of sulphides from the Citronen ore deposit range from +7 to +35 ‰ (CDT), with the bulk being within the range from +10 to +25 ‰. The three principal sulphide styles are reflected in the sulphur isotope ratios. The very heavy sulphur (δ34S from +30 to +35‰) of the laminated sulphides that are characterised by sedimentary structures, including diagenetic overgrowths by colloform pyrite, is considered to be inherited from bacteriogenically reduced sea water sulphate. At the other end of the spectrum, the light δ34S isotope values of from +5 to +20 ‰ characterise the vein sulphides, which vary in composition from sulphide- and carbonate-rich types to nearly pure quartz with scattered aggregates of sphalerite or galena. The intermediate δ34S values of the net-like dendritic intergrowth between carbonate and sulphide and the associated massive and semi-massive sulphides (i.e., minor 30 to 35 ‰ and majority 15 to 25 ‰) were interpreted by van der Stijl and Mosher (1998) to reflect overprinting of hydrothermally derived and seawater-derived sulphur (Fougt, 1997).
Notes on the Origin of the Deposit
According to van der Stijl and Mosher (1998), and (Kragh et al., 1997), as detailed above, there is evidence that:
• pyrite in the Citronel Fjord area grew within the host sediment or at the water/sediment transition zone, before consolidation of the host shales/mudstones. The resultant lithofacies totalled up to 20% disseminated pyrite with preserved sedimentary structures;
• these barren to weakly zinc-lead mineralised pyrite and resultant gossans are widespread over hundreds of km within the carbonaceous muds/shales trough facies of the southern Franklinian Basin in northern Greenland, whilst those hosting potentially economic zinc grades are very restricted;
• where present, sphalerite has clearly been deposited contemporaneously with the immediately enclosing pyrite;
• sphalerite is always accompanied by pyrite, although much of the pyrite contains no anomalous sphalerite;
• the intraformational debris flows generally contain numerous angular clasts of finely laminated sulphides within a carbonate debris matrix, interpreted to have been 'ripped-up' from the immediately underlying bedded sulphides;
• elsewhere, debris flow conglomerates lying directly above mineralised stratabound sulphide accumulations are often mineralised with pyrite, sphalerite and galena which occurs in the matrix, and as rims on the clasts, suggesting post-deposition mineralisation;
• the sedimentary textures seen in the massive and banded sulphides are overprinted or replaced by a dendritic textured sulphides with open spaces lined with pyrite framboids that are locally overgrown by euhedral pyrite crystals. The open spaces, which represent up to 50% of the rock by volume, are filled by calcite or dolomite spar;
• in places, some debris flows are overprinted by irregular replacement textures and bleaching, closely related to penetrative dolomitisation and minor silicification, progressing to massive pyrite with dendritic textures and floating 'clasts' of recrystallised carbonate;
• laminated sulphides on the margins of the deposit comprise very heavy sulphur (δ34S from +30 to + 35‰), considered to be inherited from bacteriogenically reduced sea water sulphate;
• dendritic replacement, grading to massive- to vein-sulphides have progressively lighter δ34S values suggested to reflect hydrothermal overprinting of the laminated sulphides;
• zinc and lead grades are generally <3% and <1% respectively within the massive and dendritic-textured pyrite, with sphalerite and minor galena in open spaces;
• the more massive sulphide accumulations grade laterally outward into the laminated framboidal pyrite with interstitial sphalerite and carbonate;
• in places, the more massive sulphides are cut by quartz-calcite veins that include partly replaced or recrystallised wall rock fragments, that are typically overgrown by sphalerite or pyrite;
• the banded and laminated sulphides, which are characteristically composed of fine-grained, planar-laminated and thinly-banded sulphides, contain the highest grades of zinc and lead.
These observations could be interpreted to imply that a number of generations of sulphide were generated and overprinted as follows:
• Early diagenetic pyrite was deposited regionally with, or soon after the host carbonaceous mudstones, but before consolidation, and is characterised by heavy sulphur;
• these early pyritic beds were overprinted by a structurally controlled, post-depositional, hydrothermal, reduced, iron- and sulphur-rich fluid which produced dendritic pyrite and eventually massive pyrite, by both replacement of the diagenetic pyrite, and the addition of new hydrothermal pyrite;
• these, structurally controlled hydrothermal fluids evolved to become zinc-rich, with minor lead. With continued inflow along controlling structures, they produced an outer and upper sphalerite rich zone overprinting the earlier, less massive sulphides, accompanied by carbonate-silica infill/replacement and veining within the massive sulphides.
Reserves and Resources
Published, JORC Compliant Ore Reserves and Mineral Resources include:
Ore Reserves (Ironbark Zinc Strategy Update to the ASX, 5 September 2024)
Proved Reserve - 24.6 Mt @ 5.1% Zn Equiv., 4.6% Zn, 0.5% Pb;
Probable Reserve - 24.2 Mt @ 5.1% Zn Equiv., 5.0% Zn, 0.4% Pb;
TOTAL RESERVE - 48.8 Mt @ 5.1% Zn Equiv., 4.8% Zn, 0.5% Pb.
These have been split into:
Discovery Open Pit
Proved Reserve - 5.5 Mt @ 3.5% Zn Equiv., 3.2% Zn, 0.6% Pb;
Probable Reserve - 1.4 Mt @ 2.5% Zn Equiv., 2.3% Zn, 0.4% Pb;
Beach Underground
Proved Reserve - 19.0 Mt @ 5.5% Zn Equiv., 5.2% Zn, 0.5% Pb;
Probable Reserve - 7.0 Mt @ 5.8% Zn Equiv., 5.7% Zn, 0.5% Pb;
Esrum Underground
Probable Reserve - 15.8 Mt @ 5.1% Zn Equiv., 4.8% Zn, 0.4% Pb.
Mineral Resources (Ironbark Zinc Strategy Update to the ASX, 5 September 2024)
Measured Resource - 34.286 Mt @ 4.36% Zn, 0.51% Pb;
Indicated Resource - 28.368 Mt @ 5.30% Zn, 0.46% Pb;
Inferred Resource - 22.047 Mt @ 4.55% Zn, 0.42% Pb;
TOTAL RESOURCE - 84.702 Mt @ 4.72% Zn, 0.47% Pb.
These have been split into:
Open Pit Total Mineral Resources - 17.231 Mt @ 2.8% Zn, 0.4% Pb;
Underground Total Mineral Resources - 67.472 Mt @ 5.2% Zn, 0.5% Pb;
NOTE: Resources are inclusive of Reserves.
This large, low grade resource is variously reported to contain a higher grade core of:
Within Sulphide Level 2 with a cut-off grade of 6% Zn over a 2 m interval (van der Stijl and Mosher, 1998)
Indicated Resource - 7 Mt @ 9% Zn, 1% Pb
SEG Newsletter 80 January 2010:
Mineral Resource - 22.6 Mt @ 8.2% Zn+Pb.
In addition to this Ore Reserve and Mineral Resource, Ironbark Zinc (Strategy Update to the ASX, 5 September 2024) has estimated a JORC Compliant Exploration Target of between 40 and 90 Mt @ 4.6 to 6.5% at a number of prospects, namely the West Gossans; East (on the northern eastern margin of Citronen Fjord); Beach South (an extension of the Beach Zone); Esrum Extended; Discovery SE and Discovery North/Trilobite Valley (between East and Discovery along the eastern margin of Citronen Fjord). The best intersection in Discovery North has been 6.6 m @ 8.0% Zn+ Pb, including 3.45 m @ 11.2% Zn+Pb and in Discovery SE, 1.78 m @ 14.3% Zn+Pb.
The information in this summary has almost entirely been derived from van der Stijl and Mosher (1998), and (Kragh et al., 1997), as cited below, with a minor input from Schlatter and Kolb (2023), as well as Reserves and Resources from Ironbark Zinc releases in 2024.
NOTE: The satellite imagery available for this part of Greenland appears to be very low density - you should zoom out when viewing the image below for a larger scale view of the location of the deposit.
The most recent source geological information used to prepare this decription was dated: 2023.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Citronen
|
|
|
|
Kragh, K., Jensen, S.M. and Fougt, H., 1997 - Ore geological studies of the Citronen Fjord zinc deposit, North Greenland: project Resources of the sedimentary basins of North and East Greenland: in Geology Greenland Survey Bull.176, pp. 44-49,
|
Peel, J.S. and Sonderholm, M., 1991 - Sedimentary basins of North Greenland: in Gronlands Geologiske Undersogelse Bull,160. 186p.
|
Schlatter, D.M. and Kolb, J., 2023 - The giant Citronen Zn-Pb deposit in North Greenland (83.08605° N) and preliminary geological data from the High Arctic Zn-Pb belt: in Resources of the oceanic crust Seminar at the Karlsruher Institut fur Technologie (KIT), 27th-28th March, 2023, Proceedings, 4p.
|
Thomassen, B., 2007 - The lead and zinc potential of the Franklinian Basin in North Greenland: in Bureau of Minerals and Petroleum Exploration and Mining in Greenland, Greenland Mineral Resources, Fact Sheet No. 15, 2p.
|
van der Stijl, F.W. and Mosher, G.Z., 1998 - The Citronen Fjord massive sulphide deposit, Peary Land, North Greenland: discovery, stratigraphy, mineralization and structural setting: in Geology Greenland Survey. Bull. 179, 40 p.
|
|
Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
|
Top | Search Again | PGC Home | Terms & Conditions
|
|
