Summary:
The updated Mineral Resource Estimate for the Ikkari gold deposit, the Pahtavaara gold mine and the Heinä Central gold-copper deposit.
All three deposits set out here are considered to be orogenic-style with gold mineralisation associated with low sulphidation alteration.
Pahtavaara
The Pahtavaara gold deposit can be described as an orogenic, metamorphic, hydrothermal gold deposit. The Pahtavaara deposit has shown the deposit to lie within a mineralised envelope up to 500 m wide and up to 1.5 km longand that the deposit remains demonstrably open at depth and along strike. In the 1994 Feasibility Study the deposit was described as occurring in a gold-bearing alteration zone covering 100 m x 600 m, dipping 80° to 85° NNW (Davy, 1994).
Pahtavaara is hosted by ultramafic rocks (komatiites to high magnesian basalts). Gold mineralisation is structurally controlled and associated with low sulphidation, polyphase quartz-carbonate veining, multiple deformation phases varying from brittle to ductile and back to brittle and intense alteration, both prograde and retrograde.
Two phases of gold mineralisation have been observed; early fine grained and later, more coarse-grained. Both are ‘free’ gold, as the deposit does not exhibit refractory metallurgical characteristics.
Ikkari
The mineralization at Ikkari is considered to be orogenic-style with gold mineralization associated with low sulphidation alteration.
Mineralization
The Ikkari deposit can be described as an orogenic, hydrothermal gold deposit. Modeling of the mineralization, using over 111,000 m of drilling available, shows the deposit to lie within a mineralized envelope of up to 900 m long, 350 m wide and 750 m deep and that the deposit remains demonstrably open at depth and along strike, at depth.
Overall, the mineralization trends at approximately 065° strike and has a strong sub-vertical control. However, within the mineralized halo different grade zones have varying morphology and plunges on a local scale and these are explored later.
Mineralization at Ikkari occurs in several styles, but in all cases, gold distribution is correlated to the abundance of disseminated pyrite and intensity of veining, which are in turn considered to be principally controlled by lithological contacts, fold geometry and brittle fracturing.
The style of mineralization is principally controlled by the host lithology with significant controls on mineralization localization including:
- Brittle-fracture regime in intensely albite-altered felsic sediments that controls veinlets of gold associated with fine-grained pyrite and magnetite. Given that this style of mineralization is limited to the albite-altered sediments it is most prevalent in the north-western portion of Ikkari where the felsic sediments form a large block. It also occurs in larger felsic intercalations within the komatiite domain.
- Lithological contacts; notably intensely chlorite-sericite-siderite-magnetite-pyrite-(+fuchsite)-altered sediments with felsic sediments, quartzite and conglomerate, and siltstones.
- Complex and concentrated short-wavelength (metre-scale) parasitic folding of narrow felsic and siltstone sedimentary intercalations within intensely chlorite-sericite-magnetite-altered ultramafic rocks that appears to further focus fluid flow and pyrite deposition, particularly at fold hinges. Intense, irregular carbonate-quartz veining is frequently developed in these zones and mineralized higher in grade.
- Within and at the margins of hematite-carbonate hydrothermal breccias, that have a sub-vertical expression and overprint folding and cross-cut lithological contacts. Where these breccias host intense disseminated pyrite, bonanza gold grades are commonly seen.
Ikkari is unusual among orogenic gold deposits in the width of mineralization when compared to the strike. In typical orogenic gold systems, the strike of mineralization is an order of magnitude greater than the width, however, at Ikkari the strike length of the mineralization is only two to three times the width and this is be attributed to multiple, stacked mineralized zones perpendicular to the strike. These stacked zones are interpreted to arise from the structural interleaving of diverse lithologies pre-mineralization in D1, with no evidence to support post mineralization thickening.
Although vein arrays and stockwork zones are considered to be linked to the main gold phase, there is little consistent relationship between vein density, vein volume, and gold grade. This is attributed to much of the siderite veining, now transposed into the foliation, being relatively early, likely a product of the iron metasomatism 'ground preparation' event along with chlorite and magnetite, that may have been synchronous with the initial structural interleaving of sedimentary and komatiitic units.
There appears to be a closer relationship between gold content, pyrite and late-stage iron-oxides. Magnetite- bearing veins and breccias typically contain elevated gold grades, with associated disseminated pyrite, and where late haematite is (also) present, particularly in coarse breccias comprising haematite-carbonate (+ pyrite) in the matrix, very high grades (>10 g/t Au) are observed. These iron-rich fluids clearly post-date the main deformation event and inject at zones of weakness, particularly lithological contacts and early breccias. Late-stage hematite dominated hydrothermal breccias with a vertical control occur throughout mineralized zones 1 to 3 as described above but are by far the most extensive in zone 2 that hosts the strongest grades in the deposit.
Heinä Central
The deposit is a grassroots discovery, located under more than 10 m of transported glacial till cover and locally up to 40 m of glacial till.
Gold-bearing sulphide mineralisation at Heinä Central does not correspond well to any typical universally recognised deposit type, although deposits of its type are not uncommon within greenstone belts.
The deposit to have a mineralised footprint of up to 250 m (NE-SW) by 200 m (NW-SE) extending to a maximum of 240 m below surface. The deposit remains demonstrably open at depth and along strike.
Mineralisation is best described as two converging trends of mineralisation which both dip at approximately 40° to the north and northwest. The converging trends can be traced for 185 m and 175 m respectively and both have been traced 185 m down-dip from the bedrock surface, which is the extent of current drilling.
Mineralisation at Heinä Central occurs as a sulphide-rich fracture-fill and cement to a brecciated quartz-albite-carbonate vein and also as veinlets in immediately adjacent albite-altered sediments. Sulphide content ranges from a few percent up to greater than 90% in massive sulphides. Gold and copper mineralisation are however poorly correlated with the sulphide mineralisation which is dominated by pyrrhotite and to a lesser extent pyrite and chalcopyrite.
Gold is correlated with chalcopyrite mineralisation and in hand specimen chalcopyrite content is frequently a good indicator of gold mineralisation. However, the correlation is not absolute and a minor separation between the highest grade gold and copper mineralisation is observed. In addition, some gold occurs completely independently of the pyrrhotite-chalcopyrite sulphide assemblage and is more closely related to narrow sericite-silica alteration zones within predominantly sedimentary units, but this is a relatively minor contribution to the overall gold distribution.
Mineralisation remains open at depth and along strike and further drilling will help resolve apparent strike variations, folds or off-sets and possible later structural controls on gold mineralisation.