Hope Bay - Madrid, Doris, Boston
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The Hope Bay deposits of Madrid, Doris and Boston are located close to the Arctic coast of Melville Sound in the territory of Nunavut of northern Canada, approximately 745 km north-east of Yellowknife (NWT) and 160 km south-west of Ikaluktutiak (Cambridge Bay). (#Location: 68° 2'N, 106° 35'W).
The deposits lie within the Hope Bay volcanic belt, and Archaean greenstone belt in the north-east part of the Slave Structural Province, approximately 50 km west of the Thelon Fault which marks the western margion of the Thelon Tectonic Zone of the Churchill Province. This greenstone belt is isolated from the rest of the Archaean Slave Province by the Proterozoic cover of the Kilohigok Basin
The Hope Bay greenstone belt is exposed over a distance of around 80 km north-south and a width of 7 to 15 km. The stratigraphic trend parallels the elongation of the greenstone belt and comprises mafic and intermediate to felsic volcanic rocks, with lesser sediments and ultramafic units. Mafic volcanics dominate the eastern portion of the belt, while the western segment is more varied and include felsic volcanics and siliciclastic sediments. The felsic volcanics have been dated at 2716 to 2663 Ma. Archaean intrusives are minor and include ultrmafic, gabbro and quartz-feldspar porphyry dykes and sills. These are all cut by 723±3 Ma Neoproterozoic dykes and sills of dolerite. However at least one larger body may represent the cumulate phase of a differentiated mafic magma.
To the east and west the Hope Bay greenstone belt is bounded by felsic intrusions, including granodiorites dated at 2672 Ma to the east which separate it from the narrow Elu Inlet greenstone belt further east. To the west, the plutonic intrusives contain foiliated mafic fragments dated at 2608 Ma. The south-eastern margin of the greenstone belt is occupied by a heterogenous gneiss terrane with titanite age of 2598 Ma and U-Pb age of 26495±2.7 Ma.
The main deposits of the Hope Bay greenstone belt are localised in the eastern half of the belt which is occupied by a bimodal sequence of mafic and felsic volcanic rocks that are folded aboiut a large scale antiform whose axis parallels the elongation of the greenstone belt. The mafic volcanic sequence can be divided into Mg and Fe tholeiites, both of which are characterised by pillows, are amygdaloidal and are composed of variolitic and non-variolitic units. Several felsic successions are recognised that vary from quartz-feldspathic porphyritic intrusives, volcaniclastics and tuffs, with sedimentary (conglomerates, argillites and arenites) bands and massive, coherent flows or sills.
The interior of the greenstone belt is lower greenschist facies and the margins amphibolite facies metamorphism. The major antiform in the eastern section of the Hope Bay greenstone belt is a tight, isoclinal structure the axis of which variably plunges shallowly north and south along its length. The greenstone package has been deformed during multiple events and is cut by major north-south trending shear zones that appear to have a significant control on the localisation of mineralisation, particularly at dilational flexures where these cut the antiformal axis.
Three main gold deposits have been defined within the belt, as follows:
Doris - which occurs towards the northern end of the greenstone belt and consists of a steeply dipping, >3 km long quartz vein system in folded and metamorphosed pillow basalts. It is localised at an inferred inflexion in the regional Hope Bay shear zone. On its northern end, the veins are folded to produce a high-grade anticlinal 'Hinge Zone' lying close to surface (Doris North). Within the same vein system, some 1.2 km to the south, two structures intersect to creates another high-grade zone (Doris Central). A 500 m long interval if mineralisation, known as the Doris Connector zone is situated approximately 100m north of Doris Central. The alteration assemblage includes carbonate, paragonite, pyrite and sericite. Gold is found at the contacts between the quartz vein and wall rocks, associated with dark-coloured tourmaline-pyrite septa or ribbons. At Doris North, the gold occurs as continuous white quartz veins, hosted within a conformable succession of mafic volcanic and gabbroic rocks which have been folded into a tight, shallow, north plunging antiform. The quartz veins display semiregular septa of sericite and tourmaline, with associated Fe-dolomite, sericite, paragonite and pyrite alteration assemblage. Three main, generally north-south trending vein systems have been defined, the West Valley Wall, Central and Lakeshore veins. The well developed, thick Hinge Zone is where the narrower Central and Lakeshore veins coalesce and then terminate to form a thick hinge to an apparent, shallow, north-plunging isoclinal fold.
Boston - located towards the southern end of the greenstone belt where it is associated with a flexure in the Hope Bay shear zone. Gold occurs in association with sulphides, predominantly pyrite, that form in clots within veins and as a halo in the wall rock adjacent to the veins. The best developments of gold are in zones of extensive hydrothermal alteration within a large iron-rich carbonate altered shear system. Three auriferous zones are developed sub-parallel to the shear, each of which consists of several en echelon, higher-grade lenseswith significant vertical continuity.
Madrid comprises a group of open pit deposits, Naartok, Perrin, Rand and Suluk and is approximately 8 km SSW of Doris where it occurs within a package of interbedded basalt, komatiitic volcanic flows, gabbros and black argillites. Mineralisation in the Madrid area is associated with a NW-SE trending, 8 km cumulative strike length zone of contorted quartz-mica-Fe carbonate schist with disseminated pyrite known as the 'Deformation Zone'. Straw yellow coloured mica forms semiregular, mm scale laminations that define the schistose foliation within Mg and Fe tholeiites and quartz-feldspathic rocks. Visible gold is rare and gold grades appear to coincide with an abundance of fine-grained pyrite and intense iron carbonate alteration. The Naartok zone is characterised by a west trending, steeply north-dipping zone of disseminated, stockwork and breccia-style, gold-pyrite mineralisation associated with dolomite-sericite-silica-albite alteration within mafic volcanic rocks and lies structurally above the west trending 'Deformation Zone'. At the Suluk zone, in the southeast of Madrid group, gold is associated with brecciated, silicified and sulphidised mafic and ultramafic volcanic rocks with intercalated carbonaceous and/or graphitic argillite. The better gold values are associated with more intense developments (>5%) of fine-grained disseminated pyrite within the quartz-carbonate-sericite altered horizons. Three separate, steeply west-dipping zones of mineralisation with nearly vertical to slightly northerly plunge have been outlined. The Rand Spur zone lies about 700 m north of Suluk. Mineralisation appears similar to that of Suluk, comprising silicification and carbonate alteration with 1 to 10% pyrite commonly occurring along the contact between sediments and mafic volcanics accompanying intervals of moderate brecciation, and silicification, alteration and minor quartz veining.
Resource and reserve figures are as follows (Mirimar Mining Corp., 2006):
Measured + indicated resources
Boston - 1.387 Mt @ 15.4 g/t Au
Doris - 0.763 Mt @ 23.9 g/t Au
Madrid - 4.703 Mt @ 5.5 g/t Au
Total - 6.853 Mt @ 9.6 g/t Au
Inferred resources (in addition to measured & indicated)
Boston - 2.431 Mt @ 9.5 g/t Au
Doris - 1.634 Mt @ 14.5 g/t Au
Madrid - 29.594 Mt @ 4.2 g/t Au
Total - 33.659 Mt @ 5.1 g/t Au.
The most recent source geological information used to prepare this summary was dated: 2006.
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 to this deposit in the PGC Literature Collection:
Sherlock R L, Shannon A, Hebel M, Lindsay D, Madsen J, Sandeman H, Hrabi B, Mortensen J K, Tosdal R M and Friedman R, 2012 - Volcanic Stratigraphy, Geochronology, and Gold Deposits of the Archean Hope Bay Greenstone Belt, Nunavut, Canada : in Econ. Geol. v.107 pp. 991-1042|
Stemler J U, Richards J P and Muehlenbachs K, 2006 - A Fluid Inclusion and Stable Isotopic Investigation of the Boston Lode-Gold Deposit, Hope Bay Volcanic Belt, Nunavut : in Exploration & Mining Geology, CIM v15 pp 101-121|
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