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Sturgeon Lake, Mattabi, F-Group, Lyon Lake and Creek Zone

Ontario, Canada

Main commodities: Zn Cu Ag Au Pb
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The Sturgeon Lake district exploited a number of Archaean volcanic hosted massive sulphide Zn-Cu deposits, 40 to 50 km NE of Ignace, and 200 km NW of Thunder Bay, in north-western Ontario, Canada, including Sturgeon Lake, Mattabi, F-Group, Lyon Lake and Creek Zone.   Reserves were depleted in each of these mines in 1981, 1988, 1984 and 1985, respectively.

These deposits are all within the Sturgeon Lake greenstone belt in the Wabigoon Sub-province of the Superior Province. Mineralisation is hosted within the South Sturgeon Lake assemblage, a 9 km thick, dominantly bimodal package of basalt-rhyolite volcanic rocks that appears to be built on a gneissic Archaean basement. This sequence is cut by numerous granitoid and amphibolitic intrusive bodies. The succession is divided into three depositional cycles by Morton et al. (1990) related to the formation of the 30 km diameter, 2735 Ma submarine Sturgeon Lake cauldron.

The pre-cauldron succession consists of a thick package of basaltic, massive to pillowed lava flows, scoria-tuff cone deposits and debris flows with minor interlayered rhyolite.

The overlying 4500 m thick cauldron-fill succession includes subaqueous volcaniclastic flows and at least five rhyolitic ash-flow units, plus several dacitic domes and andesitic flows with epiclastic and chemical sedimentary horizons near the top. Each of the cycles is composed of basal basaltic and upper felsic members, and is typified by pillowed and massive andesite flow units ranging to ~1000 m in thickness, intercalated with and overlain by approximately equal amounts of dacitic to rhyolitic ashflow tuffs. The felsic rocks are commonly irregularly banded or bedded and contain intercalated laterally extensive graphitic and pyritic shale members. Based on the stratigraphic distribution and thicknesses of the five ash-flow tuff units and associated debris flow deposits, it has been interpreted by Morton et al. (1990) that the Sturgeon Lake caldera represents a series of smaller nested or overlapping calderas and that each ash-flow unit is associated with a separate collapse event. Most of the cauldron succession is believed to have been emplaced in <500 m of water (Groves et al., 1987). The six Cu-Zn VHMS deposits are located within the syn-cauldron volcanic succession (Trowell, 1983).

The post-cauldron succession is dominated by mafic volcanic rocks interlayered sedimentary wacke-siltstone of volcanic provenance, including oxide and carbonate facies iron formation and massive to volcaniclastic rhyolite.

Two major granitoid bodies are evident in the district. The Bell Lake monzonite complex underlies the volcanic pile (Trowell, 1970), while the 2100 m thick, sill-like quartz diorite-granodiorite Darkwater complex was emplaced ~500 m above the base of the volcanic pile. In addition, irregularly shaped amphibolitic intrusive stocks are abundant in the area 13 km east of the Mattabi Mine.

Host rocks are composed of felsic ash flow tuffs (at Sturgeon Lake and F-Group), intermediate to felsic pyroclastics (at Mattabi) and fragmental rhyolite (at Lyon Creek and Lake Zone) all of which are associated with the 2735 Ma submarine volcanic caldera.

At Mattabi the host sequence is structurally simple comprising of a homoclinal, north-facing and north-dipping (65 to 75°) sequence of volcanic rocks, The host sequence comprises the Footwall Rhyolite Agglomerate, principally composed of 2000 to 4500 m of lapilli tuff, with a 60 m thick footwall rhyolite 250 m below the ore zone. The 'agglomerate' is overlain by the ~60 to 120 m thick Mine Rhyolite, with the irregular, but broadly tabular, 650 m long, massive sulphide mineralisation within its upper half. The ore is underlain by a cherty/silicified zone. The ore is overlain by the 0 to 100 m thick Hanging wall Rhyolite and the 500 to 600 m thick Hanging wall Andesite. A north-trending cylindrical dyke-like intrusive mass of rhyodacite cuts the Footwall Rhyolite Agglomerate, Mine Rhyolite and ore zone, and assumes a semi-concordant sill-like attitude in the Hanging wall Rhyolite. To the west, a massive fine-grained mafic dyke cuts the footwall and hanging wall rhyolite units, but not the hanging wall andesite, and may be a feeder dyke for the latter unit (Franklin et al., 1975).
    All of the rocks in the mine area are sheared, with the shearing fabric subconcordant with, but deviating from the strike by as much as 25°' Pillows and lapilli fragments are essentially undeformed. Minor folds are absent, with rare minor faulting that has horizontal offsets of<5 m (Franklin et al., 1975).
    The Mattabi orebody is a conformable, stratabound, zoned accumulation of massive to semi-massive sulphides in a volcanic or volcaniclastic host sequence, and may be considered as two closely stacked ore zones separated by a cherty tuffaceous rhyolite unit. The ore is zoned, with a footwall stringer zone, lower chalcopyrite-sphalerite interval, in turn overlain by banded pyrite-sphalerite ore in each zone. The mineralogy of the ore includes pyrite, sphalerite, chalcopyrite, galena, tetrahedrite-tennantite, arsenopyrite, minor pyrrhotite, and rare boulangerite and bournonite (Franklin et al., 1975).
    The altered silicate mineral assemblage observed in the footwall to the ore in approximate order of abundance are quartz (locally up to 95% of the rock), sericite (locally up to 80% of the rock), carbonate (locally up to 60% of the rock), chloritoid, chlorite, andalusite, garnet, kyanite and biotite, with pyrite, pyrrhotite, chalcopyrite, sphalerite, magnetite and gahnite, occurring as crosscutting veins and as massive pervasive replacement (Franklin et al., 1975).

The F-Group deposit is hosted within the High Level Lake pyroclastic flow deposits, which represent the second pyroclastic event/cycle associated with the caldera complex, and can be subdivided into two separate units: i). a lower quartz crystal-rich rhyolite (average SiO2 = 74%) and ii). an upper pumice-rich dacite (average SiO2 = 67%). The lower quartz crystal ash-flow tuff forms lenses (up to 1000 m in strike length by 100 m thick) within deposits of mesobreccia and a 100 to 300 m thick unit which overlies the mesobreccia. The F-Group massive sulphide deposit occurs close to the top of the quartz crystal pyroclastic flow unit (Hudak, 1989; Morton et al. 1991).
    The Mattabi deposit is hosted by the third and most voluminous eruptive event/cycle within the caldera, which formed deposits that exceed 800 m in thickness, and can be traced across the caldera complex for more than 15 km. Late (post-Mattabi) caldera volcanism is represented by a complex succession of rocks which range from quartz and quartz-plagioclase-bearing pyroclastic flow deposits through plagioclase-porphyritic lava flows and domes to sedimentary sequences that range from 250 to 1000 m in thick, with the domes, lava flows and dome-related sediments comprising the bulk of the stratigraphy. The pyroclastic flow deposits form two distinct units, each of which may represent a major pyroclastic eruption. The second of these pyroclastic units is host to the Sturgeon Lake Mine, Lyon Lake and Creek Zone ore deposits, which are similar in character to the Mattabi deposit described above (Morton et al. 1991).

The larger of the orebodies contained the following:

    Sturgeon Lake - 2.51 Mt @ 9.17% Zn, 1.21% Pb, 2.55% Cu, 160 g/t Ag, 0.6 g/t Au.
    Mattabi     - 13.8 Mt @ 8.28% Zn, 0.85% Pb, 0.74% Cu, 105 g/t Ag, 0.2 g/t Au.
    F-Group     - 0.42 Mt @ 9.51% Zn, 0.58% Pb, 0.64% Cu, 60 g/t Ag.
    Lyon Lake & Creek Zone - 3.49 Mt @ 8.67% Zn, 0.99% Pb, 1.26% Cu, 140 g/t Ag, 0.3 g/t Au.

The most recent source geological information used to prepare this summary was dated: 2000.     Record last updated: 20/10/2012
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:
Galley A, Van Breemen O, Franklin J  2000 - The relationship between intrusion-hosted Cu-Mo mineralization and the VMS deposits of the Archean Sturgeon Lake mining camp, northwestern Ontario: in    Econ. Geol.   v95 pp 1543-1550
Holk G J, Taylor B E and Galley A G,  2008 - Oxygen isotope mapping of the Archean Sturgeon Lake caldera complex and VMS-related hydrothermal system, Northwestern Ontario, Canada: in    Mineralium Deposita   v.43 pp. 623-640
Morton R L, Walker J S, Hudak G J  1991 - The early development of an Archean submarine caldera complex with emphasis on the Mattabi ash-flow tuff and its relationship to the Mattabi massive Sulfide deposit: in    Econ. Geol.   v86 pp 1002-1011


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