Hanson Lake District - Hanson Lake, McIlvenna Bay
Zn Pb Cu Ag Au
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The Hanson Lake and McIlvenna Bay volcanic hosted massive sulphide (VHMS) zinc-lead-copper-silver-gold deposits, are located in northern Saskatchewan, in Canada, 525 km NE of Saskatoon. McIlvenna Bay is ~6 km SSE of the Hanson Lake deposit.
The regional and district scale setting and context of this group of deposits is described in the separate Flin Flon-Glennie Complex VHMS Deposits record.
Hanson Lake Deposits (#Location: 54° 40' 35"N, 102° 51' 12"W)
The small but high grade Hanson Lake underground mine operated from 1967 to 1969, producing 0.149 Mt @ 10% Zn, 5.8% Pb, 0.5% Cu, 137 g/t Ag from the known reserve of 0.257 Mt @ 11.42% Zn, 8.08% Pb, 0.61% Cu, 145 g/t Ag, 0.98 g/t Au.
The ore body is hosted by the Palaeoproterozoic 'greenstone' sequence, in the Hanson Lake block of the Flin Flon-Glennie Complex, composed of rocks dominated by juvenile island arc, felsic to intermediate metavolcanic rocks, with subordinate amounts of mafic volcanics, minor intermediate volcanics, and greywackes.
Overall the orebody is conformable occurring at the litho-stratigraphic contact between felsic volcanic rocks and soda-rhyolite porphyry rocks, although, locally, the ore is totally within the soda-rhyolite porphyry unit. Both of these units are well developed in the mine area with thickness of hundreds of metres. The soda-rhyolite predominates to the east, in the structural footwall of the deposit, and felsic volcanic rocks to the west. The overall strike direction is north-south. Both units have been metamorphosed and recrystallised to amphibolite facies.
The felsic volcanic rock unit is composed of fragmental lavas, agglomerates and tuffs (Coleman et al., 1970). In the ore deposit area, this unit is predominantly a tuff that is distinguished by very fine, even layering, and well-defined biotite and muscovite schistosity, containing large clasts of fine-grained quartz and feldspar with 10 to 30% biotite and muscovite. Accessory minerals include garnet, sericite, tremolite, hornblende, titanite and calcite. Thin (<3 m thick) bands of soda-rhyolite are common within the unit.
The soda-rhyolite porphyry unit is characterised by many sub-spherical, 2 to 5 mm diameter, colourless to blue quartz 'eyes', and white feldspar phenocrysts. Highly altered plagioclase, quartz, and a few K feldspar phenocrysts, constitute 10 to 40 vol.% of the unit. Plagioclase, which is usually untwinned, is generally altered to sericite with some calcite. The quartz phenocrysts are mostly aggregates of large quartz grains. Crudely foliated biotite, muscovite and sericite comprise <10% of the rock, and partial alteration of biotite to chlorite is common. The remainder of the groundmass is fine-grained quartz and feldspar. Tuffaceous interbands, similar to the felsic volcanic rocks, are observed. Coleman et al. (1970) suggest the soda-rhyolite porphyry is an ignimbrite flow. One narrow, 1 to 2 m thick, locally concordant, diorite band/sill, occurs in the ore zone in the southern portion of the deposit, ~10 m into the footwall.
The main orebody was a steep, westerly dipping, irregular, undulating lens of massive base metal sulphides that was 500 m long, by 120 m down dip by up to 5 m thick. Horizontally and vertically~ The ore width was very variable, both vertically and horizontally, in places changing from 3 m to <15 cm over a distance of <3 m.
The massive sulphides occur on the western margin of an up to 5 m thick envelope of silicification and shearing in its structural footwall, although locally, the ore transgresses this alteration zone to occur to its west. This alteration envelope, which is composed of sheared, chloritic and pyritic felsic rocks, has disappeared by 300 m from the extremities of the orebody.
Vertically plunging folds locally displace the ore by as much as 3 m laterally off strike. Minor open folds and refolded cylindrical isoclinal folds occur in the wallrock. There is one major and numerous minor bifurcations in the orebody, and Inclusions of wall rock over 50 cm in length are common throughout the massive sulphide lens.
The massive sulphide ore typically contains 70 to 80 vol.% sphalerite, galena, chalcopyrite, arsenopyrite, pyrite and pyrrhotite, in a gangue of large quartz-muscovite altered wallrock inclusions, quartz knots and minor amounts of carbonate, chlorite and muscovite.
The occurrence of the key components of the ore is as follows:
• Sphalerite constitutes 70 to 80 vol.% of the sulphides in the ore body, most commonly with a grain size range of 0.02 to 0.3 mm.
• Galena comprises 4 to 15 vol.% of the sulphides, occurring predominantly as: i). grains of between 0.04 and 0.3 mm; ii). grains along sphalerite grain boundaries that are less than 0.04 mm long; and lesser iii). anomalously large grains with dimension 0.5 to 2.5 mm.
• Sphalerite constitutes 70 to 80 vol.% of the sulphides in the ore body, most commonly with a grain size range of 0.02 to 0.3 mm.
• Chalcopyrite which typically ranges between zero and 3 vol.% of the massive sulphides, although, in local areas of high silicate content, it may reach 20 vol.%. The gain size is normally <0.1 mm, although aggregates up to 1 mm across are common. Individual chalcopyrite grains are mainly found along sphalerite grain boundaries although large sphalerite grains contain structurally oriented exsolution lamellae of chalcopyrite along twin boundaries. Minute chalcopyrite grains <<0.05 mm occur along the margins of rounded sphalerite inclusions in quartz and in brittle fractures in pyrite. The concentration of chalcopyrite on opposing sides of silicate grains suggests that chalcopyrite was deposited as pressure shadows.
• Arsenopyrite typically forms 3 to 7 vol.% of the massive sulphides, although locally it can constitute up to 50%. It occurs as 0.01 to 1.3 mm subhedral to euhedral grains, although aggregates of small grains may attain a maximum diameter of 1.5 cm. Growth features are common within euhedral arsenopyrite grains, which may be the only mineral in its primary form.
• Pyrite constitutes <4 vol.% of the massive sulphides, although massive and disseminated pyrite also occurs throughout the alteration envelope. Pyrite in the massive ore differs texturally from wall-rock pyrite in that it contains many inclusions while grains from the wall-rock contain very few. In the massive sulphides, it occurs as irregular anhedral grains with dimensions ranging from 0.1 to 3.0 mm, with almost ubiquitous Inclusions of silicate minerals, sphalerite, arsenopyrite, galena and chalcopyrite, whilst marcasite and chalcopyrite occur along some pyrite fractures. Fractured pyrite grains are common and some smaller grains appear to be the result of granulation of larger grains. No growth features are evident. Pyrite in the wall rock has a similar grain size to that in the massive ore, although in the immediate hanging wall and footwall, it has been brecciated, while faint concentric growth features have been observed on a few small wallrock pyrite grains.
• Pyrrhotite generally comprises <3 vol.% of the sulphides, although it locally ranges from zero to 15%, occurring as distinct grains interstitial to sphalerite
• Gangue within the massive ore comprises a near-constant 20 to 30 vol.%, with local variations attributed to the frequency of wall-rock inclusions. The gangue within massive sulphides consists of up to 4 cm long quartz knots, 2 to 25 cm long muscovite and quartz-muscovite-sericite knots, some carbonate and minor individual mica grains. Sericite and K feldspar are common in the wallrock inclusions and the biotite content ranges from zero to 10 vol.%. The schistosity in some inclusions near the margin of the ore lens has the same orientation as that of the wall rock although most have been mechanically rotated. Foliation in the smaller wall rock inclusions is typically bent along the margins of the inclusion or highly contorted throughout. No quartz veins have been observed in the ore body although local quartz-galena-rich areas occur throughout the ore. Carbonate minerals (calcite and minor siderite) are common interstitial gangue minerals and occasionally as thin veinlets. Textural evidence of mechanical deformation of the gangue to the sulphide matrix is evident.
The alteration envelope is marked on the footwall side by a decrease in the number of phenocrysts (mainly plagioclase), an increase in the amount of muscovite, quartz and sericite, and an increase in shearing as the massive sulphide is approached. On the hanging wall, alteration is marked by an increase in muscovite, which usually occurs in segregated bands, and an increased amount of shearing, from country rock to ore. Quartz-muscovite schist makes up most of the alteration which is generally confined to the footwall of the ore body, accompanied by considerable shearing and silicification. Sericite is present throughout the altered wall rock, although locally in the ore zone, it attains 20 vol.%. The biotite content of the alteration zone and of the unaltered country rocks is the same, i.e., <:4%. Chloritisation is absent from the alteration halo, except as a minor replacement of biotite, which is a regional feature unrelated to ore. Disseminated pyrite and a traces of chalcopyrite occur throughout the alteration, whilst narrow bands of highly contorted and brecciated massive pyrite <30 cm wide occur on the immediate hanging wall and footwall of the ore lens.
This summary is largely drawn from "Kennedy, D.S., 1971 - A Textural and Chemical Analysis of the Hanson Lake Ore Deposit, Saskatchewan; Master of Science Thesis, Department of Geological Sciences, University of Saskatchewan, Canada, 134p."
McIlvenna Bay Deposits (#Location: 54° 37' 44"N, 102° 48' 41"W)
The McIlvenna Bay deposit was discovered in 1988 by drilling a geophysical anomaly from a helicopter borne INPUT survey which delineated a 1.2 km long, ESE striking anomaly.
Like Hanson Lake, it is hosted within the Hanson Lake Block of the Flin Flon-Glennie Complex. This block is bounded to the east by the Sturgeon-Weir Shear Zone and to the west by the Tabbernor Fault Zone, and extends an unknown distance to the south beneath a nearly flat-lying cover of Ordovician sandstones of the Winnipeg Formation, and dolomites of the Red River Formation. In the immediate Hanson Lake area, the exposed Palaeoproterozoic rocks are dominated by juvenile island arc and felsic to intermediate metavolcanic rocks, with subordinate mafic volcanic and minor intermediate volcanic rocks, as well as greywackes. Oxide facies iron formations are also represented. Various felsic intrusions cut the sequence, some of which are interpreted to be subvolcanic intrusions. Abundant diorite and gabbro plugs and dykes also intrude the sequence, as well as minor ultramafic intrusions. The supracrustal rocks generally dip moderately to steeply west, east and north-east. South of Hanson Lake, at McIlvenna Bay, the Palaeoproterozoic sequence is poorly exposed, and only locally projects to sub-surface through the sedimentary cover.
At least two distinct deformation events are recognised at Hanson Lake, both with northerly trending fold axes. Peak regional metamorphism is interpreted to have reached upper amphibolite facies, as seen in rocks away from the deposit, and as evidenced by relict cordierite, anthophyllite, garnet and andalusite that are commonly observed in the retrograde greenschist metamorphic facies alteration package rocks enveloping the McIlvenna Bay deposit. U-Pb age dating of a quartz-feldspar porphyry (a possible subvolcanic intrusion) which intruded the supracrustal sequence yielded a date of 1888±12 Ma.
The volcanic and sedimentary sequence hosting the McIlvenna Bay deposit has been divided into six formations over a 2 km strike length, as follows, from the base:
• McIlvenna Bay Formation - >200 m of variably altered felsic volcanic, volcaniclastic and/or volcanic-derived sedimentary rocks of rhyolitic composition. The massive and semi-massive sulphide deposits are hosted by this formation.
• Cap Tuffite Formation - a 35 to 55 m thick sequence of finely banded to laminated, intercalated, felsic volcanic and cherty metasedimentary rocks, intruded by sills and dykes of the Davies Gabbro. An east to west zonation is observed, from chert-dominated in the east, to rhyolite-dominated in the west.
• Koziol Iron Formation - a 0.1 to 25 m thick oxide-facies iron formation which stratigraphically overlies the Cap Tuffite, and is interpreted to represent an extensive, continuous exhalative horizon, traceable over several kilometres in drill core and by geophysics, and acts as a stratigraphic marker horizon.
• Rusk Formation - an ~100 m thick package of massive and calcite-altered mafic volcanic rocks, most likely massive flows, that overlies the Koziol Formation.
• HW-A Formation - another exhalative horizon, which ranges from 1 cm to 5 m thick and shows a transition from west to east, from oxide-facies iron formation, to massive sphalerite-pyrite. The iron formation is overlain by a 1 to 10 m thick unit of massive mafic volcanic rocks, whilst the 10 to 75 cm thick sphalerite-pyrite layer is overlain by a 5 to 15 m thick massive, grey, felsic volcanic unit.
• Upper Sequence - a >400 m thick package of highly variable, bimodal volcanic rocks, including aphanitic, grey, felsic volcanics and fine-grained mafic volcanic rocks. Some of the mafic rocks may be gabbroic intrusions, whilst ~5% of the unit comprises greywackes with at least two additional oxide-facies iron formation horizons.
• Davies Gabbro - which appears to be a series of sills that are up to 100 m thick, intruding along the bedding planes of the Cap Tuffite Formation, extending westward from east of the deposit, toward the centre of the sulphide body, where it narrows into a number of thin dykes. It plunges along an axis parallel to the sulphide body and appears to exert some sort of control over the limits of mineralisation along the bottom plunge line of the deposit.
The stratigraphy at the McIlvenna Bay deposit strikes at 95 to 115° and dips at 65 to 70°N, although locally it is almost vertical. The deposit is concordant with the host rocks and plunges at ~45° to the NW. The host sequence varies from massive to strongly foliated, with the intensity of foliation being a reflection of the competency of individual units and the degree of alteration.
The deposit comprises structurally modified, stratabound lenses of copper-zinc sulphides, with associated silver and gold, overlying a zone of stockwork mineralisation. It includes three distinct styles of mineralisation and comprises five different zones. The three different styles of mineralisation are i). massive sulphides, ii). semi-massive sulphides and iii). copper stockwork.
The five zones identified to date are
• Lens 2 Massive Sulphide Zone - is the largest known, and most significant massive sulphide zone, comprising a zinc-silver-rich lens with a 115° trending strike length of 400 m to 550 m, and average dip of 68°N. It ranges from 0.40 to 16.75 m in true thickness, averaging 5.55 m, and plunges ~45° to the north. The zone has been defined from the base of the cover sequence, at a vertical depth of 35 m, down to 1230 m where it remains open down plunge.
• Upper West Semi-massive Sulphide Zone - is a relatively copper-gold-enriched zone, occurring as a continuous, elongate strip/sheet, lying parallel to, and along the top of the plunge line of the Lens 2 Massive Sulphide. It has a strike length of 150 to 300 m, and has been delineated over the full vertical extent of Lens 2, also remaining open down plunge. It varies from 2.80 to 10.60 m true thickness, averaging 4.81 m.
• Lens 3 Massive Sulphide Zone - is a discontinuous, and generally very thin zinc-rich massive to semi-massive sulphide lens, located 10 to 30 m above the Lens 2 and Upper West Zones. It has a strike length of 100 to 350 m and plunges parallel to the underlying mineralised zones, with a true thickness of from 0.18 to 6.65 m, averaging 2.39 m.
• Lens 4 Semi-massive Zone - is a zinc-rich zone, located ~40 to 50 m below the Upper West and Lens 2 zones.
• The Copper Stockwork Zone - which underlies, and is in contact with the Upper West Zone and the western half of the Lens 2 Massive Sulphide Zone. This copper-gold-rich zone is wedge-shaped, with the blunt edge running parallel to the plunge of the Upper West Zone. It is terminated near the central axis of the Lens 2 Massive Sulphide, being thickest below the Upper West Zone. Stockwork mineralisation in this area is hosted in chlorite-altered rock. To the east, immediately underlying the Lens 2 Massive Sulphide Zone, the stockwork mineralisation is hosted in fine fracture networks in silicified and sericitised rock, containing multiple narrow stringers between the vertical depths of 35 and 1230 m. The best grades of this zone are developed over a strike length of 300 to 600 m and ranges from 0.57 to 48.30 m true thickness, averaging 8.08 m. The copper stockwork zone remains open to the west and down plunge below the 930 m level.
This description is drawn from "Rennie, D.W., 2011 - Technical Report on the Mcilvenna Bay Project, Saskatchewan, Canada"; an NI 43-101 Technical Report prepared for Foran Mining Corporation by Roscoe Postle Associates Inc.
Reserve + production figures for these deposits are as follows:
• Hanson Lake - 0.257 Mt @ 11.42% Zn, 8.08% Pb, 0.61% Cu, 0.98 g/t Au, 145 g/t Ag (total pre-mining reserve, Kennedy, D.S., 1971)
0.147 Mt @ 0.51% Cu, 9.99% Zn, 1.09 g/t Au, 137.14 g/t Ag, (Production, Gibson et al., 2011)
• McIlvenna Bay - 13.900 Mt @ 1.28% Cu, 2.67% Zn, 0.49 g/t Au, 17 g/t Ag, (Indicated resource, January 2013, Foran Mining, 2014)
11.311 Mt @ 1.32% Cu, 2.97% Zn, 0.43 g/t Au, 17 g/t Ag, (Inferred resource, January 2013, Foran Mining, 2014)
The most recent source geological information used to prepare this summary was dated: 2011.
Record last updated: 20/11/2014
This description is a summary from published sources, the chief of which are listed below.
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