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Manitouwadge - Willroy, Willecho, Nama Creek, Geco

Ontario, Canada

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The Manitouwadge volcanic hosted massive sulphide (VHMS) zinc-copper-silver deposits, which include Geco, Willroy, Willecho and Nama Creek are located approximately 250 km ENE of Thunder Bay, north of Lake Superior in south-western Ontario, Canada (#Location: 49° 9' 19"N, 85° 47' 21"W)

Geco is the eastern-most of the main Manitouwadge deposits, on the eastern limb of the Manitouwadge Synform.   The Willroy group of bodies are from 1 to 2 km to the west, while Nama Creek is just over 4 km WNW of Geco, east of the axis of the Manitouwadge Synform. The Willecho group of ore lenses are on the western limb of the synform, close to it's axis, approximately 3.6 km WNW of Nama Creek.

These deposits lie within the Manitouwadge (also known as the Schreiber-Hemlo) Greenstone Belt, a highly deformed remnant of upper amphibolite facies supracrustal rocks in the volcano-plutonic Wawa subprovince of the Archaean Superior Province, immediately south of the major tectonic boundary with the metasedimentary-migmatitic Quetico subprovince. It comprises a single 2720 Ma mafic to felsic volcanic succession that includes a large synvolcanic trondhjemite body within the Manitouwadge synform. Two additional zones of mafic rocks, known as the Dead Lake suite, are also found within the Manitouwadge synform. These comprise a package of interleaved, foliated, gabbro, diorite and layered mafic to intermediate rocks of probable supracrustal origin, as well as magnetite-garnet-rich rocks. The greenstone belt is enclosed by foliated multiphase dioritic to granitic rocks of the Black Pic batholith, while the related Loken Lake pluton is outlined by the Dead Lake suite in the core of the Manitouwadge synform.

The felsic volcanic rocks of the Manitouwadge Greenstone Belt succession are intercalated with banded iron formation and the associated volcanogenic massive sulphide deposits of the Manitouwadge district, of which Geco is the largest. The volcanic rocks are stratigraphically overlain by greywackes which lie in an early fold (Agam Lake syncline) along the southern limb of the Manitouwadge synform.

Five stages of deformation are recognised in the district. D1 and D2 are regionally developed isoclinal folding. D3 is a regionally developed Z-style folding, while D4 and D5 are localised shearing episodes. A pervasive compositional layering, migmatitic veining in tonalite bodies, meta-sedimentary and meta-volcanic strata, and a schistosity in less metamorphosed rocks related to D1 and D2. D4 deformation is localised along the southern limb of the Manitouwadge synform, centred on the ore zone where highly altered micaceous rocks which have focussed strain and disrupted the orebodies.

The Manitouwadge orebodies are located within a highly deformed section of the sequence near the boundary between the Quetico and Abitibi-Wawa belts to the north and south respectively. The regional sequence in the vicinity of the transition between the belts comprises, from north to south: i). the Quetico paragneiss; separated by a band of hornblende-biotite gneiss from ii). a granitised zone of interbanded felsic paragneisses and mafic orthogneisses; iii). the Manitouwadge Mine Series; and iv). a granitised zone of felsic orthogneisses and paragneisses.   All of these units have been folded to form the easterly plunging Manitouwadge Synform and less well developed parallel structures. All of the known orebodies are within the Manitouwadge Synform, in which the Mine Series has been overturned.

Within the Manitouwadge Synform the Mine Series comprises, from north to south:

i). the Granite Gneiss, 0 to 500 m thick, of medium to coarse grained mafic to intermediate rocks containing biotite-hornblende gneiss, biotite-sillimanite gneiss and biotite-anthophyllite (gedrite)-cordierite gneiss, all of which may be garnetiferous;
ii). the ore-hosting Sericite Schist Group, which is 0 to 200 m thick and contains all of the stratabound massive sulphide mineralisation. The sulphide mineralisation at Manitouwadge is generally sandwiched between the 'lower' mafic volcanic unit, represented by the lower sections of the structurally overlying Granite Gneiss and the quartzite unit of the Lower Grey Gneiss as described below, which contains abundant, conspicuous iron-formation. The metamorphic rocks of the Sericite Schist Group immediately enclosing, and interleaved with the orebody exhibit an overall mineralogy including quartz-primary chlorite muscovite-biotite-almandine-plagioclase-cordierite-anthophyllite-hedenbergitic pyroxene-staurolite-tourmaline-gahnite-sillimanite, the latter being very abundant in some sections;
iii). the Grey Gneiss Group, which is 0 to 1500 m thick and commences a lower grey to buff coloured, well banded quartzite interbanded with fine- to coarse-grained quartz-feldspar-biotite gneiss, with intercalated silicate facies iron formation on the flanks of the ore stratigraphy. The iron formations are characterised by an assemblage of chlorite, garnet, hornblende, pyrrhotite and magnetite. Oxide facies iron formation overlie the ore zone in the mine area, with a low alumina. Both iron formations contain low grade Cu and Zn, and grade laterally outwards into garnetiferous quartz-feldspar-biotite gneiss; and
iv). the Hornblende Schist Group.

For detail of the Geco deposit, see the separate Geco, Manitouwadge record.

At Willroy the stratigraphic footwall is represented by garnetiferous and amphibole rich, altered and deformed mafic meta-volcanic and minor meta-sedimentary rocks of the Granite Gneiss Group. These rocks have been enriched in alumina and magnesia to produce an assemblage that includes well foliated anthophyllite (gedrite), biotite, garnet, sillimanite and cordierite with local sericite. The surface of the foliation exhibits L2 rodding, plunging at 40 to 50° east, paralleling the elongation of ore.

The stratigraphic hangingwall is occupied by a 'laminated' biotite-quartzite with minor garnet of the Grey Gneiss Group, similar to that found at Geco. The Sericite Schist Group of the ore zone immediately adjacent to the quartzite comprises a leucocratic quartz-sericite schist with an intense schistosity and rusty colouration at surface, grading stratigraphically downwards into a schist composed of quartz and sericite with sillimanite clots which grades into the massive sulphide mineralisation. Four rod like massive sulphide bodies spread over an interval of nearly 1 km make up the deposit. The ore zone and schist host terminate at a sharp contact with the stratigraphically underlying granite gneiss of the structural hangingwall.

At Nama Creek, as at Willroy and Geco, the stratigraphic footwall to ore is represented by garnet-anthophyllite altered mafic meta-volcanic rocks of the Granite Gneiss Group. Both the massive sulphides and the sericite schist hosts have been intruded by a suite of NE trending pegmatite sheets. The massive sulphides occur in a lenticular schist and are coarser than at the other deposits of the district, possibly as a result of the pegmatite intrusion.

At Willecho, three elongate, shallowly plunging lenses of massive sulphide, from 1 to >3 m in thickness are distributed over an interval of around 1 km, parallel to the L2 lineation. The ore zone is hosted by iron formation and by garnet- and sillimanite-bearing sericite schist. The coarse grained, often zinc-rich, ore zones contain pyrite, pyrrhotite and sphalerite. They exhibit strong deformation, but appear in general to parallel the structurally overlying contact of the Granite Gneiss Group which has been deformed by mylonitised zones paralleling the regional foliation.

The total size of each of the deposits is as follows (after Franklin, 1996):

      Geco - 58.4 Mt @ 3.45% Zn, 1.86% Cu, 0.15% Pb, 50 g/t Ag
      Willroy - 3.95 Mt @ 2.84% Zn, 1.64% Cu, 27.8 g/t Ag
      Nama Creek - 0.18 Mt @ 4.16% Zn, 0.83% Cu, 0.02% Pb, 35.7 g/t Ag
      Willroy - 1.96 Mt @ 4.43% Zn, 0.50% Cu, 0.18% Pb, 67.9 g/t Ag.

The ores averaged 0.2 g/t Au overall.

The most recent source geological information used to prepare this summary was dated: 1990.    
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.


Geco

  References & Additional Information
   Selected References:
Petersen E U  1986 - Tin in volcanogenic massive Sulfide deposits: an example from the Geco Mine, Manitouwadge district, Ontario, Canada: in    Econ. Geol.   v81 pp 323-342
Schandl E S, Gorton M P, Wasteneys H A  1995 - Rare Earth element geochemistry of the metamorphosed volcanogenic massive Sulfide deposits of the Manitouwadge Mining Camp, Superior Province, Canada: a potential exploration tool?: in    Econ. Geol.   v90 pp 1217-1236
Zaleski E, Peterson V L  1995 - Depositional setting and deformation of massive Sulfide deposits, Iron-formation, and associated alteration in the Manitouwadge Greenstone Belt, Superior Province, Ontario: in    Econ. Geol.   v 90 pp 2244-2261


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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