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Aqqaluk |
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Alaska, USA |
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Zn Pb Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The adjacent Red Dog and Aqqaluk sediment hosted zinc-lead-silver orebodies are located in the de Long Mountains of the western Brooks Range of northwestern Alaska, some 200 km to the north of the Arctic Circle and 135 km to the north of Kotzebue (#Location: 68° 4' 8"N, 162° 49' 38"W).
Summary
Mineralisation is hosted by a Carboniferous sequence of black siliceous shale and chert. The ore is located in the second lowest of a stack of eight imbricated and folded allochthonous thrust slices. The main deposit lies within two major and one minor mineralised plates within this slice. The main Red Deg orebody overlies a tectonic mélange zone which separates it from the underlying Cretaceous rocks of the lower allochthon.
Ore is present as a stratabound accumulation of silica rock, barite and sulphides. The silica rock consists dominantly of growth zoned mosaic aggregates of quartz with accessory sulphides, within and peripheral to the main mass of sulphides. The barite facies comprises coarse and massive barite, contains accessory sulphides and silica, and is found towards the top and periphery of the deposit. The sulphide textures vary from massive, chaotic or fragmental to poorly banded. The major sulphides in decreasing order of abundance are sphalerite, pyrite, marcasite and galena. Rare disseminated chalcopyrite and pyrrhotite are found in sphalerite, while boulangerite occurs in galena. The dominant gangue constituents are quartz, barite and minor shale.
Published reserve and resource figures include:
77 Mt @ 17.1% Zn, 5% Pb, 82 g/t Ag (Preliminary Resource, 1984, Lange, et al., 1985),
68.4 Mt @ 16.6% Zn, 4.9% Pb (Red Dog Reserve, 1994, Mining Journal, Sept. 15, 1995),
76 Mt @ 13.7% Zn, 3.6% Pb (Aqqaluk Deposit, Cominco Annual Report, 1995),
85 Mt @ 18.2% Zn, 4.6% Pb. (Proved+probable reserve, 2005).
45.4 Mt @ 15.8% Zn, 4.1% Pb, 72.6 g/t Ag (Proved+probable reserve, December 31, 2013), plus,
7.5 Mt @ 25.7% Zn, 6.9% Pb, 137 g/t Ag (Measured+indicated resource, December 31, 2013), plus,
0.2 Mt @ 10.7% Zn, 3.4% Pb, 68.8 g/t Ag (Inferred resource, December 31, 2013).
Regional Setting
Click here for a regional setting image.
Geology
The De Long Mountains are characterised by eight imbricate, stacked and folded thrust sedimentary and igneous allochthons. These allochthons have an aggregate thickness of several kilometres. The ore at Red Dog and Aqqaluk is located within the second lowest of the allochthonous thrust slices, and is hosted within the Kuna Formation, a Carboniferous sequence of black siliceous shale and chert, that overlies a distinctive interbedded light grey calcarenite and dark grey calcareous shale (Moore, et al., 1986).
The allochthons were thrust northwards by an orogenic episode that commenced in the middle Jurassic and waned in the early Cretaceous. They are separated by undulatory thrust surfaces along which gouge, tectonic breccia or melange is commonly observed. The six lowest allochthons are composed of Devonian to Cretaceous clastic and chemical sediments, while the upper two contain Jurassic and older volcanic and plutonic mafic and ultra-mafic sequences. Each of the lower six allochthons is composed of distinctive facies of chert, shale, limestone and sandstone. A single allochthon may contain one or more internal repetitions or sequences, and range in thickness from hundreds to thousands of metres, although in places they are tectonically thinned or are absent (Moore, et al., 1986).
The reconstructed sequence in the Red Dog area comprises, from the base (Moore, et al., 1986):
Upper Devonian to Lower Carboniferous (Mississippian),
Noatak Sandstone, >100 m thick - which is generally composed of thin bedded, bioturbated, sandstone and siltstone, although it is not present in the Brooks Range allochthon in the immediate Red Dog area.
Lower to Middle Carboniferous (Lower to Upper Mississippian & Middle or Lower Pennsylvanian),
Kuna Formation, which has been subdivided into,
Kivalina Unit, >122 m thick - alternating laminated, black to dark grey, calcareous shale and light grey calcarenite. Calcarenite-shale couplets 4 to 10 cm thick are present as are medium to thick bedded calcarenite or shale dominated zones. The unit is overall distinctively rhythmically banded and in places contains dark chert or flaggy crinoidal biosparite. This unit is the stratigraphic footwall to the Red Dog mineralisation.
Ikalukrok Unit, 45 to 212 m thick at Red Dog, averaging 120 m - which hosts the bulk of the ore at Red Dog. The majority of the unit is composed of laminated, poorly to well indurated, sooty black shales, characterised by a very high silica content. Two, or possibly three zones of turbiditic lithic quartz rich sandstone and sandy limestone occur in the upper half of the unit.
The uppermost part of the unit comprises black chert, from a few metres to >24 m in thickness. The top of this chert is marked by a single undulatory chert bed with pronounced mammillary structures which are sometimes diapiric. This bed, which has been traced over a distance of 246 km, marks the contact with the overlying Siksikpuk Formation.
Lower to Upper Carboniferous (Pennsylvanian), to Lower Triassic,
Siksikpuk Formation, 76 m thick at Red Dog, averages 68 m - the basal section of the formation is composed of a distinctive orange-weathering, thin to medium bedded, grey chert and lesser grey to white shale. It often contains disseminated to nodular or bedded pyrite and/or marcasite. Barite is also commonly present. This chert is a regional feature, that has been traced for up to 190 km from Red Dog, and averages 11 m in thickness. At Red Dog it grades into a thick zone of barite that caps the main deposit.
The middle part of the formation consists of 35 m of laminated to thin bedded, commonly silicified, greenish grey and maroon shale, with subordinate chert. Euhedral barite is widely distribute, and in some cases barite beds are present.
The upper part of the unit comprises 9 to 20 m of thin to medium bedded grey and maroon chert with shale partings.
Lower Triassic to Middle Jurassic,
Otuk Formation, >46 m thick - is composed of four sub-divisions, namely:
* Black shale, a 20 m thickness of laminated, thin bedded, black shale, to thin bedded micrite and dolomite, with subordinate chert. It locally contains pyrite and barite;
* Chert, 21 m of thin bedded grey ribbon chert with shale partings;
* A carbonate unit, composed of 4 to 8 m of planar, thin bedded, buff weathering, grey chert and cherty carbonate; and
* Black and grey, thin bedded shale which is generally <8 m thick.
Upper Jurassic to Lower Cretaceous,
Ipewik Formation, >20 m thick at Red Dog, averages 6 m elsewhere - maroon and grey shale, coquinoid limestone, siltstone and clean quartz sandstone.
Disconformity to Unconformity
Lower Cretaceous,
Okpikruak Formation, >365 m thick - grey to black mudstone, medium to fine grained greywacke and polymictic conglomerate. Olistostromes of exotic lithologies are found near the base.
On a deposit scale, the structure mimics the regional setting, with thrust slices which have been subjected to at least two directions of compression. The Main deposit lies within two major and one minor mineralised plate all within the main Brooks Range Allochthon. These plates exhibit various degrees of movement with respect to each other. In ascending order they are the (Moore, et al., 1986):
1). Footwall waste plate which largely comprises the tectonic melange separating the ore from the underlying un-mineralised plate. The melange is a major glide zone and includes diverse lithologies of the Kuna, Siksikpuk, Otuk and Okpikruak Formation lithologies;
2). Lower ore plate - mainly exposed to the north of Red Dog Creek, and dipping below the Upper Ore Plate to the south. The plate thins to zero over the sole of the thrust to the south-west, and thickens to 168 m on its northern extremity. Much of the plate is composed of sulphide veined, silicified, Ikalukrok Member shale, while a thick sulphide bearing barite cap is preserved at its northern-most extremity;
3). Lower waste plate - consists of Okpikruak Formation, minor Otuk Formation chert, and abundant Siksikpuk Formation chert, shale and sulphide poor barite. The Lower Waste Plate is generally conformable on the Lower Ore Plate, although local movement between the plates and imbrication is observed;
4). Upper ore plate - which lies to the south of red Dog Creek and contains the majority of the ore at Red Dog;
5). Upper waste - commences with the bleached shale of the orange cherty unit at the base of the Siksikpuk Formation, overlain by the middle and upper Siksikpuk Formation, and the middle Otuk and the Okpikruak Formation. The basal contact with the Upper Ore Plate is locally a thrust fault, but is generally a normal, stratigraphic relationship; and
6). Remnant mineralised plate - which comprises a flat lying sheet of Kivalina unit limestone and shale, Ikalukrok unit siliceous shale and sulphide bearing barite rock. Erosion has removed and significant sulphide deposit that may have been present.
Most contacts between ore and younger rocks is believed to be thrusts, although the amount of movement is variable (Moore, et al., 1986).
Mineralisation
The main Red Dog orebody extends for 1600 m in a north-west direction, and varies in width from 150 to 975 m. The high grade portions are up to 135 m in thickness (Moore, et al., 1986).
The orebody is a stratabound accumulation of: i). silica rock; ii). barite; and iii). sulphides, contained within the Ikalukrok Unit of the Kuna Formation. On the margins of the orebody the Ikalukrok Unit is 60 to 100 m thick, while in the centre of the deposit only the basal 10 to 40 m is present, the remainder being occupied by the sulphide orebody (Moore, et al., 1986).
The silica rock consists dominantly of growth zoned mosaic aggregates of quartz with accessory sulphides, within and peripheral to the main mass of sulphides. The barite facies comprise coarse and massive barite, contain accessory sulphides and silica, and is found towards the top and periphery of the deposit (Moore, et al., 1986).
The sulphide rocks at Red Dog are subdivided into massive sulphides with >70% sulphide, semi-massive with 40 to 70%, and sulphide bearing containing 1 to 40%, the latter usually being accompanied by barite. The sulphide textures vary from massive, chaotic or fragmental to poorly banded. Coarse angular breccias are found throughout the deposit, but are spectacularly developed near the base of the deposit where fragments of silicified shale, crustiform banded sphalerite and a variety of other materials, including barite and sulphides, are found in a fine grained sulphide matrix. These breccias are apparently not a post ore tectonic feature. Fine grained fragmental textures are also common in the massive sulphides (Moore, et al., 1986).
The major sulphides in decreasing order of abundance are sphalerite, pyrite, marcasite and galena. Rare disseminated chalcopyrite and pyrrhotite are found in sphalerite, while boulangerite occurs in galena. Silica is the dominant gangue mineral, occurring as mosaics of strongly growth zoned quartz grains, or as irregular cherty masses. In addition the gangue also includes barite and minor shale. The sulphides are generally fine grained, although coarser grained veins are also observed. Late stage activity is characterised by thin pyrobitumen-calcite veins (Moore, et al., 1986).
The shales which host the ore have been silicified, while carbonate, which is characteristic of the host outside of the ore area, is absent in the vicinity of Red Dog. The deposit is capped by silica and sulphide poor barite of the overlying Siksikpuk Formation (Moore, et al., 1986). Figure 241 and Figure 242 illustrate the geological setting of the Red Dog ore deposit.
The upper ore plate has a central thick, metal-rich zone, while the lower plate thickens and becomes more metal rich to the north and south-east. The thickest accumulations of barite fringe the thickest sulphide developments. Irrespective of the thickness, the metal zonation shows abundant Pb to the south-east in both plates. Fe has a central zone of depletion relative to Zn and Pb, while an Fe rich halo is evident in both ore plates. This Fe halo is a reflection of the relative abundance of pyrite in the adjacent shales. There is a weak upward increase in Pb and a relative upward decrease in Fe. Most drill holes show a systematic upward decrease in the Fe:Zn+Pb+Fe ratio, while the Zn:Zn+Pb ratio decreases less markedly. Vertical zonation is independent of changes in the total Zn+Pb+Fe content (Moore, et al., 1986).
The most recent source geological information used to prepare this decription was dated: 2001.
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.
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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.
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