Northwest Territories, Canada

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The Lupin gold deposit is located on the shores of Contwoyto Lake, ~400 km NNE of Yellowknife in the Northwest Territories of northern Canada, and lies ~80 km south of the Arctic Circle (#Location: 65° 48' N, 111° 15'W).

The deposit is hosted by an Archaean Banded Iron Formation within a metaturbidite sequence of the Contwoyto Formation, which has been mapped as part of the Yellowknife Supergroup of supracrustal metasedimentary and metavolcanic rocks that are part of the Slave Slave Archaean Province in northern Canada (Tremblay, 1976; Bostock, 1980; King et al., 1989).

The Lupin deposit is contained within of supracrustal rocks (Henderson, 1970). In the Contwoyto Lake area, the Yellowknife Supergroup is subdivided into four formations (Bostock, 1980) as follows:
Contwoyto Formation, composed of interbedded greywacke and mudstone, containing lenses and horizons of iron-formation, and a locally significant volcanic component. This suite is interpreted to represent turbiditic deposition in a submarine fan (Bostock, 1980; Lhotka and Nesbitt,1988). Iron-formation appears to be preferentially deposited within either greywacke or mudstone-dominated sequences.
Point Lake Formation, composed of mafic flows and tuffs to felsic volcanic rocks and minor sedimentary units ( Bostock, 1980) and is a highly variable sequence.
Keskarrah Formation, which is only found in the Point Lake area and comprises clast- and matrix-supported conglomerate and lithic sandstone. Conglomerate clasts include granitic and mafic volcanics, in addition to minor gneissic rocks (Henderson and Easton,1977).
Itchen Formation, which is very similar to the Contwoyto Formation, but is typically coarser grained, arenaceous, thicker bedded, and devoid of iron-formation (Bostock, 1980; King et al., 1988).

Detailed regional structural studies (King et al., 1989; Relf, 1989) indicate that the region has been affected by three deformation events. The first is characterised by tight to isoclinal folds with steep axial planes and subvertical plunges. Fold wavelengths vary from a few cm to 100 m. The second event produced tight to isoclinal folds with wavelengths of only a few metres and typically steep plunges and near-vertical axial planes. During D3, folding created crenulations to folds, with wavelengths that are commonly less than 1 m, but which may be up to 50 m.

Six episodes of plutonism have been recognised (King et al., 1989). The first three are gabbroic to granodioritic in composition and are coeval with volcanism. The next two events were coeval with D2 and show a wider compositional variation from gabbroic to granitic. The youngest of the plutons are granitic to syenogranitic and locally pegmatitic and are post D2. The Contwoyto batholith, lying to the north and west of Lupin, is thought to have been emplaced as part of this final event.

The Contwoyto Lake area can be divided into zones separated by the biotite, cordierite, and sillimanite isograds (Relf, 1989). Peak metamorphism coincided with, or slightly post-dated D2 deformation (Lhotka,1988).

All ore zones at Lupin are confined to a continuous amphibolitic iron-formation within the Contwoyto Formation, which has been mapped at surface over a strike length in excess of 3 km and a down dip extent of 1.2 km. The main ore-hosting unit is dominated by sulphide-rich amphibolitic iron-formation with minor interbeds of phyllite and quartzite.

The main ore unit at Lupin is underlain by massive quartzitic metagreywacke with pelitic beds and lenses. Phyllites commonly overlie the iron-formation and grade upward into metagreywacke. A garnet-amphibolite-chlorite-biotite schist, up to a few metres thick is often found at the upper contact of the iron-formation with the phyllites. The garnet content decreases away from the iron-formation in these areas. Discontinuous sulphide-rich or sulphide-poor amphibolite horizons up to 2 m thick with lateral extents of up to several tens of metres occur sporadically in pelitic zones in the overlying and underlying metasedimentary rocks away from the main amphibolite.

The structure that hosts the Lupin orebody resulted from at least three phases of deformation. Stratigraphic younging directions, radiate outward from the centre of the structure indicating a modified dome. The first two phases of folding are responsible for the major folds outlined by the banded iron-formation. The F1 foliation strikes NE, associated with isoclinal folds. The F2 axial planar foliation that strikes northerly, parallels bedding and forms the main schistosity in the mine area. The south fold nose of the west zone, which formed during D2, plunges steeply south and has been defined by drilling from surface to the 250 m level. A set of parasitic F2 folds, with amplitudes of a few metres, are locally strongly developed within the amphibolitic iron-formation horizon forming. The axes of these parasitic folds splay as the F2 fold noses broaden with depth. Axial plane cleavage and faulting associated with this folding controlled quartz vein emplacement and the related sulphide and gold distribution in the iron-formation. The west zone fault, is a graphitic slickensided shear zone in the footwall of the west zone which it parallels. Several north-easterly trending, near-vertical faults cut the centre zone amphibolite and west zone fault and postdate D3. No displacement of >20 m have been observed on the west zone fault and no displacements of more than 2 to 3 m have been observed on the later faults.

Garnet amphibolite occurs as thin horizons or lenses <1 m thick and up to 30 m long, within or at the margins of the iron-formation. It contain 20% garnet on average, but may be as high as 90%. Grunerite is the dominant amphibole (up to 80%). Quartz and chlorite compositions are variable.

Well-laminated, sulphide-poor iron-formation dominates in the east zone and the south nose of the centre zone below the 650 m level, but is present throughout the deposit. Locally it grades into and interfingers with the sulphide rich iron formation. Bedding is defined by alternating grunerite (50 to 60%), metachert and hornblende layers, and occasional thin laminae enriched with pyrrhotite.

The sulphide-rich iron-formation is a well-bedded unit in which intense folding and deformation are obvious. Pyrrhotite, arsenopyrite, loellingite and minor pyrite occur in varying concentrations, as disseminations in amphibolitic layers and as massive bands interbedded with layers of amphibole and metachert. Pyrrhotite comprises ~20% of the rock and may be up to 80% over thicknesses of 2 to 3 m. Sulphides comprise 10 to 30% of the rock.

The gold at Lupin is finely disseminated within pyrrhotite, arsenides, and silicates. Arsenopyrite occurs as large prismatic crystals up to 2 cm in diameter close to quartz veins which cut the iron-formation. Arsenopyrite replaces amphibole, quartz, pyrrhotite and chlorite. Metacrysts commonly have loellingite cores and much of the gold associated with the arsenides occurs at the loellingite-arsenopyrite boundaries. Pyrrhotite replaces and is intergrown with, hornblende, grunerite, other silicates, and arsenides in amphibolitic iron-formation. Both pyrrhotite and arsenides occasionally show minor deformation, implying some straining of the iron-formation subsequent to crystallisation of sulphides. The amphiboles are both hornblende (35%) and grunerite. Grunerite comprises 3 to 30% of the unit, occurring as layers intercalated with hornblende and chert layers. Quartz id occasionally up to 60% of the rock, but averages 35%, occurring as granoblastic matrix quartz and in the cherts. Chlorite may comprise up to 15% of the rock and is oriented parallel to S2. Pyroxene is locally present. Accessory minerals include calcite, epidote, graphite, ilmenite and scheelite.

The gold-bearing portions of the host iron-formations are characterised by the presence of multiple quartz veins and the pervasive replacement of grunerite by hornblende. In heavily mineralised sections of the deposit, sulphides appear to be laterally continuous. However, in parts of the deposit where mineralisation is less extensive, sulphide banding is seen to be discontinuous and clearly associated with quartz veins. Gold is directly and intimately associated with the sulphide. Just as the sulphide concentration is generally most intense immediately adjacent to quartz veins and decreases with distance from veins, so does the gold content. Although most of quartz veins are internally only weakly mineralised, a few contain some sulphide and gold.

The quartz veins contain 0.03 to 1.00 ppm Au, while a zoned sequence of hydrothermal alteration in the iron formation envelopes the quartz veins. Immediately adjacent to the veins, there is a calc-silicate assemblage of hedenbergite + quartz ± epidote ± scheelite ± grossular, also with 0.03 to 1.00 ppm Au. The next zone is iron-sulphide rich, with pyrrhotite-pyrite + hornblende + quartz ± hedenbergite ± epidote ± actinolite. This zone carries 5 to 30 ppm Au. Further from the veins the iron formation is un-mineralised (<0.03 ppm Au), lacks sulphide and comprises grunerite + quartz± magnetite. In the transition zone, hornblende replaces grunerite, and iron sulphides replace amphibole and magnetite. The alteration sequence is developed on a scale of millimetres to metres and in areas of dense veining there is no un-mineralised iron formation between adjacent veins.

Production between start-up in 1982 and 1993 was 6.66 Mt @ 10.63 g/t Au.

In 1993 proven and probable reserves were 3.0 Mt @ 9.61 g/t Au, with a further 2.7 Mt @ 8.56 g/t Au in the possible category. Production in 1995 was 5.6 t Au.

The mine opened in 1982 and closed in February 2005 and was subsequently put on care and maintenance. During this period the deposit produced 104.5 t of gold at an average grade of approximately 9.3 grams per tonne, from 11.24 Mt of ore. The current inferred mineral resources are between 1.110 and 1.027 Mt grading between 11.32 and 10.73 g/t Au, containing 12.5 to 10.7 t of gold (Elgin Mining website).

The most recent source geological information used to prepare this summary was dated: 1994.     Record last updated: 4/11/2014
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.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Bullis H R, Hureau R A, Penner B D  1994 - Distribution of Gold and Sulfides at Lupin, Northwest Territories: in    Econ. Geol.   v 89 pp 1217-1127
Elevatorski E A  1988 - World Gold, Mines-Deposits-Discoveries (Extract): in    Minobras Mining Services    pp 99-102
Lhotka P G, Nesbitt B E  1989 - Geology of un-mineralised and gold-bearing iron formation, Contwoyto Lake - Point Lake region, Northwest Territories, Canada: in    Can. J. Earth Sci.   v26 pp 46-64

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