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Geco, Manitouwadge

Ontario, Canada

Main commodities: Zn Cu Ag
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The Geco volcanic hosted massive sulphide (VHMS) zinc-copper-silver deposit is located near Manitouwadge, 250 km ENE of Thunder Bay in south-western Ontario, Canada (#Location: 49° 09' 15"N, 85° 47' 40"W).

It lies 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 Geco orebody is located within a highly deformed section of the sequence near the boundary between the Quetico and Abitibi-Wawa belts. 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. It commences at the structural base with a 0 to 5 m thick, strongly deformed "quartz-pebble conglomerate" containing local copper mineralisation. This is overlain by 0 to 20 m of quartz-biotite-anthophyllite hornfels, then a band of sericite schist (with variable biotite, sillimanite and quartz), followed by the 0 to 100 m massive sulphide zone. This is then succeeded by the 0 to 70 m thick disseminated copper zone, further sericite schist and an upper 0 to 5 m thick zinc-zone composed of sericite schist with disseminated sphalerite, pyrite and minor galena marking the structurally upper limit of the Sericite Schist Group; 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.

The sulphide mineralisation at Manitouwadge is sandwiched between the lower mafic volcanic unit, represented by the upper sections of the Granite Gneiss and the overlying quartzite unit of the Lower Grey Gneiss as described above, 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.

The mineralisation at Geco occurs in three distinct types, namely: i). a core of coarse-grained massive pyrite-pyrrhotite-chalcopyrite-sphalerite, which varied from a few cm to over 50 m in thickness, surrounded by ii). disseminated pyrite-sphalerite, and structurally overlain by iii). disseminated chalcopyrite-pyrrhotite-pyrite within a siliceous sericite schist, formed adjacent to the massive sulphide mass, and grading laterally into silicate facies iron formation outside of the sericitic alteration halo. Structurally higher, the disseminated zone grades into a light coloured schist which contains less biotite and sillimanite

The orebodies trend easterly, dip steeply to vertical and plunges at around 35° to the east, parallel to the L2 lineation. The thickest part of the ore zone is spatially associated with, and localised within an easterly plunging Z-style asymmetric fold, the Geco Drag Fold.

Production from the orebody from 1957 to 1990 totalled of 48 Mt @ 3.8% Zn, 1.9% Cu, 52 g/t Ag. Remaining reserves in 1990 were 11.3 Mt @ 2.94% Zn, 1.62% Cu, 35 g/t Ag. The ore contained an average of 0.3% Pb as galena (Williams, et al., 1990).

Franklin (1996) quote a total deposit resource of 58.4 Mt @ 3.45% Zn, 1.86% Cu, 0.15% Pb, 50 g/t Ag for Geco.

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:
Araujo S M, Scott S D  1996 - Oxygen isotope composition of alteration zones of highly metamorphosed volcanogenic massive sulfide deposits: Geco, Canada, and Palmeiropolis, Brazil: in    Econ. Geol.   v91 pp 697-712
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
Williams H R, Breaks F W, Schnieders B R, Smyk M C, Charlton S G, Lockwood H C  1990 - Field guide to the Manitouwadge area: in   8th IAGOD Symposium, Field Trip Guidebook, Mineral Deposits in the western Superior Province, Ontario, Field Trip 9 Geol. Surv. Canada   Open File 2164 pp 7-25
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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