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Miramar Con

Northwest Territories, Canada

Main commodities: Au
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The Miramar Con gold deposit is located on the western shore of Yellowknife Bay on Great Slave Lake, in the Northwest Territorries of Canada. It lies within the 50 km long Yellowknife Greenstone Belt, near the southwestern margin of the Archaean Slave craton in the Northwest Territorries of Canada.

The Slave craton is a preserved fragment of a larger land mass, composed of a basement of Mesoarchaean gneisses, overlain by a Neoarchaean supracrustal assemblage, the 2.8 to 2.6 Ga Yellowknife Supergroup. The latter sequence was deposited over a protracted period and can be subdivided into a number of chronologically and tectonically distinct phases. These include: (i) ~2.73 to 2.70 Ga rifting and mafic volcanism (the greenstone rocks - massive, pillowed and variolitic mafic to intermediate tholeiitic volcanic rocks, with minor volcniclastic and sedimentary rocks); (ii) ~2.69 and 2.67 bimodal arc volcanism; (iii) ~2.66 Ga arc-rifting and turbidite deposition; and (iv) ~2.63 Ga arc-plutonism- volcanism and turbidite deposition (dominated by older volcanic rock detritus with a lesser from Mesoarchaean basement rocks) in a back-arc basin. These supracrustal units were deformed, exhumed and unconformably overlain by late orogenic, <2.6 Ga, molasses-type conglomerates, that were deposited on an incised palaeosurface, and subsequently deformed along first order, crustal-scale fault zones. Widespread Neoarchaean plutons, forming a batholithic complex (the Western Plutonc Complex) extending over a 500 km interval along the western part of the craton, were emplaced in pulses at ~2.70, 2.67, 2.635 to 2.620, and 2.610 to 2.602 Ga, with a final bloom of granitoids at 2.60 to 2.58 Ga which range from two-mica S-type, to hornblendebiotite I-type granites.

Multiple episodes of metamorphism and deformation are evident in the supracrustal and older granitic rocks. In the immediate Yellowknife area, metamorphic grade is generally greenschist facies, with local amphibolite hornfels related to intrusion of 2.63 to 2.62 Ga Defeat Suite plutons and ~2.592 Ga S-type plutons.

In the Yellowknife area, gold mineralisation is principally hosted by second order quartz-carbonate-bearing, north-south striking, west-dipping shear zones that crosscut the ~2.70 Ga mafic volcanic-dominated rocks of the Yellowknife Greenstone Belt, although numerous prospects and past-producing mines are found as quartz lodes within 2.66 Ga greywacke-mudstone turbidites to the east of the greenstone belt. There are also numerous structurally hosted gold prospects associated with banded iron formation that is interbedded with the <2.63 Ga greywacke-mudstone turbidites. The turbidite-hosted mineralisation is spatially related to ~2.592 Ga S-type Prosperous plutons and associated pegmatites and contact metamorphic aureoles. In contrast, the strictly greenstone-hosted deposits lack a spatial relationship with the S-type plutons, although the metal budgets (e.g., As and Sb) and radiogenic isotopic signatures of the ore does indicate some sedimentary or plutonic contribution. Gold mineralisation in these deposits is localised near the ~2.63 Ga amphibolite-greenschist isograd, although the mineralisation postdates this plutonic-metamorphic event (Ootes et al., 2011).

The Miramar/NERCO Con deposit is a shear hosted lode deposit is located within the amphibolite-facies aureole of the Western Plutonic Complex that overprints the Yellowknife Greenstone Belt. The greenstone belt commences with the Kam Group, comprising an upright (northeast striking, 75 to 80° SE dipping) monoclinal basaltic sequence of flows and intercalated pillowed and pillow breccias, cut by compositionally identical sills. Some flows incorporate sills of fining upwards massive, coarsely gabbroic textured rocks. Some pillowed flows are variolitic. To the southeast, basalts, gabbros and variolitic pillowed flows are increasingly intercalated with semi-continuous, weakly to well laminated tuffaceous chert and more granular sandy units, some of which are reasonably well mineralised with local 10 to 15% pyrite beds. The main lower basaltic sequence is overlain to the southeast by pyroclastic and volcaniclastic rocks, which include lapilli and crystal tuffs, that grades into sedimentary rocks, mainly composed of tuffaceous units with alternating siltstone and argillaceous bands. All of these lithologies are intruded by sills of gabbro up to 350 m thick. The Kam Group sequence is juxtaposed across the Kam Fault with conglomerates of the younger Jackson Lake Formation, which include well-sorted sdandstone to granite-cobble to boulder-conglomerates. The entire sequence is intruded by swarms of north-south trending fine- to medium-grained matadiorite and gabbro dykes that pre-date the Western Plutonic Complex, and finally by northwest trending late dolerite dykes. The Western Plutonic Complex is represented locally as a series of granitic plugs and dykes, and a number of related bodies of breccia that extend easterly from the batholithic Western Granodiorite to the west, to the West Bay Fault which is 2 km to the east.

The metamorphic grade of the host sequence varies from hornblende-amphibolite along the margin of the Western Granodiorite batholith, to greenschist facies to the east. Metamorphic isograds cut across the volcanic sequence, paralleling both the batholith contact and the mineralised shear zones. Gold mineralisation is restricted to north-striking, west-dipping shear zones, where hangingwall amphibolite-facies assemblages retrogress to chlorite-carbonite-sericite-sulphide bearing schists which host multiple generations of quartz-ankerite veins. Auriferous quartz-lodes occur within the brittle-ductile Con, Negus-Rycom and Campbell shear zones that cross-cut lithological boundaries. The gold-producing sections of the Con and Campbell shears correspond to th intervals where these shears intersect an east-west-trending thermal corridor defined by a granitic plugs and related breccias that can be traced to the batholith contact to the west.

The Campbell Shear extends over an interval of >10 km, strikes at 5° and dips at ~50° W near surface, shallowing with depth, before steepening again to ~70°. The shear zone comprises several anastomosing bands of strongly-foliated, chlorite-carbonate and sericite-chlorite-carbonate schists with an aggregate thickness of >175 m. Ore is not present over the entire width, but occurs in structuraly controlled, brecciated, contorted and boudinaged, quartz-ankerite-sericite lenses up to 150 m long, by 1 to 25 m in thickness, separated by barren chlorite schist. There are two related multiple vein-sets in shears, sub-vertical (parallel to foliation/shear zones) and sub-horizontal in extensional fractures usually cutting the steep set. The mineralised veins contain a wide variety of metallic minerals, including arsenopyrite, pyrite, stibnite, chalcopyrite, sphalerite, galena, scheelite, various sulphosalts and native gold.

The Con Shear occurs as two merged, 2 to 10 m wide strands of sheared rock that defines a 35 m wide zone striking at 30° and dipping at 65°NW. Retrogression of the hangingwall volcanic rocks produced an assemblage of carbonate-epidote-quartz-chlorite-plagioclase-leucoxene.

The Rycon-Negus Shears cross-over between the Con and Campbell shears, striking at 165° and dipping at 55 to 67°W, ranging up to 3 m wide, averaging around 1.5 m. Enclosed quartz veins occur at shear flexures and junctions and comprise a number of narrow quartz and quartz-carbonate filled brecci zones, with ore occurring where these contain pyrite, arsenopyrite, stibnite, chalcopyrite, sphalerite, galens, native gold and various sulphosalts.

Con Mine entered production in 1938 and ceased operations in 2003. It produced around 175 t Au (more than 5 Moz) of gold from 12.2 Mt of ore processed at an average grade of ~17 g/t Au. Production in 1994 was 3.7 t Au.

The most recent source geological information used to prepare this summary was dated: 1997.    
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
   Selected References:
Elevatorski E A  1988 - World Gold, Mines-Deposits-Discoveries (Extract): in    Minobras Mining Services    pp 99-102
McDonald D W, Duke N A, Hauser R L  1993 - Geology setting of the NERCO Con Mine and the relationship of gold mineralisation to metamorphism, Yellowknife, NWT: in    Exploration & Mining Geology, CIM   v2 pp 139-154
Ootes L, Morelli R M, Creaser R A, Lentz D R, Falck H and Davis W J,  2011 - The Timing of Yellowknife Gold Mineralization: A Temporal Relationship with Crustal Anatexis? : in    Econ. Geol.   v.106 pp. 713-720


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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