On 28 October 2024, Gold Fields Limited is announced completion of the acquisition of Osisko Mining following receipt of all regulatory approvals and support from shareholders for the transaction at the Osisko shareholder meeting on 17 October 2024.
The transaction consolidates Gold Fields’ 100% ownership of the Windfall Project and the extensive surrounding exploration camps in Québec, Canada, which were previously jointly and equally owned by Gold Fields and Osisko.
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Summary:
The Windfall deposit is classified as a pre-Temiskaming intrusion-associated gold deposit due to: 1) a temporal and spatial association of gold with felsic calc-alkaline QFP intrusions; and 2) the main gold event (i.e., vein- and replacement-type mineralization) being interpreted to pre-date known regional scale deformation.
Mineralization consists of a network of quartz-carbonate-pyrite-tourmaline veins and an associated silica-sericite-pyrite alteration assemblage. The mineralization and alteration have strike lengths of >2 km that show, as of yet, no recognized vertical zoning. Gold mineralization is only locally spatially associated with calc-alkaline QFP dikes but shows no genetic association with them. The QFP intrusions were emplaced mainly as a product of tectonism and deformation and act only as competent host rocks that concentrate deformation and gold-bearing hydrothermal fluids.
Mineralization
Gold mineralization in the Windfall deposit is observed in two main settings and include: 1) vein-type mineralization; and 2) replacement type mineralization.
Vein-type mineralization consists of grey to translucent coloured quartz veins that contain subordinate amounts of ankerite, tourmaline, pyrite and commonly visible gold. The veins have sharp contact margins that are straight or folded. Texturally these veins are massive, but locally can form laminated textures characteristic of fault-fill veins (Robert and Poulsen, 2001). The veins vary in thickness from 0.1 m to 1 m and are generally associated with the highest gold grades ranging on average from 20 g/t to >100 g/t. In the veins, sulphide content ranges from 1% to 80% and is dominated by pyrite with minor concentrations (<1% total sulphide) of chalcopyrite, sphalerite, arsenopyrite, galena, pyrrhotite, tennantite and other Bi-Te minerals, as identified by internal petrographic and microanalytical analyses. This mineralization style is commonly observed to occur in felsic volcanic dominated domains of the deposit (i.e., Caribou and Lynx).
Replacement-type mineralization occurs at the margins of vein-type mineralization or in high strain zones that lack the development of quartz veins. This mineralization style consists of pyrite replacement zones and stockworks associated with a strong pervasive silica-sericite ankerite ± tourmaline alteration of the host rock. The gangue and precious-metals are identical to those mentioned above in the vein-type mineralization. The gold is associated with disseminated pyrite, which varies from 1 to 80% over mineralized intervals. This mineralization style is commonly observed to occur in the mafic volcanic dominated domains of the deposit (i.e., Main zone).
Visible gold mineralization is commonly observed in the Windfall deposit. In drill core, the gold ranges from millimetre-sized nuggets to locally centimetre sized patches commonly associated with post-vein formation fractures containing cloudy white quartz-carbonate. The late overprint of visible gold suggests late-stage remobilization.
Other than the auriferous vein-type and replacement-type mineralization noted above, less significant vein-types include: 1) early gold barren carbonate-quartz veins with colloform textures (pre-ore); 2) gold-barren sheeted blue quartz veins (pre-ore); 3) laminated-quartz carbonate- tourmaline veins and tourmaline breccias (post-ore); 4) carbonate-quartz stockworks and breccias cross-cutting mineralized zones and remobilizing gold (post-ore); and 5) late white quartz veins with coarse pyrite and remobilized gold (post-ore).
At the Windfall deposit, the high-grade gold mineralization is contained within narrow deformation zones that cross-cut the synvolcanic rocks and syn-deformation QFP intrusions and are locally spatially associated with the contacts of the latter. Mineralization consists of vein-type quartz- carbonate-pyrite-tourmaline-gold veins, or replacement-type pyrite-rich corridors that are zoned from a high-grade inner gold-silica > silica-carbonate-tourmaline mineral assemblage to an outward low-grade gold-sericite > sericite-carbonate-tourmaline assemblage, which in turn transitions to a background of gold-barren chlorite-carbonate > sericite.
The mineralization is currently known for a lateral extent of 3,000 m and a confident vertical extent of approximately 1,600 m. It is separated into four sectors: the Lynx zone (Lynx Main, Lynx HW, Lynx SW, Triple Lynx and Lynx 4), the Main zone (Zone 27, Caribou 1, Caribou 2, Caribou Extension, Bobcat, Mallard, Windfall North, F-Zones), the Underdog zone, and the Triple 8 zone. Current drilling is testing the extensions of many of these zones, mainly in the Lynx zone. All zones generally trend east-northeast and plunge roughly 35° to 40°.
The Lynx zone consists of five gold mineralized zones located in the east-northeast portion of the deposit. Most of the Lynx mineralization zones form an extensive anastomosed network of quartz-rich and pyrite-rich veins hosted within strongly silicified felsic volcanic rocks or gabbros. This system is located on the southern limb of an open fold plunging at 40º towards the east-northeast along the Bank fault-shear zone.
The Main zone consists of five gold mineralized zones located in the central portion of the deposit. The gold mineralization is constrained along east-northeast oriented contacts of narrow subvertical granodioritic dikes within tilted volcanic rocks. Most mineralized envelopes in the Main zone are associated with pyritic stringers occurring near contacts between volcanic rocks and younger intrusive rocks. Generally, the gold mineralization is hosted in a mafic dominant domain (i.e., basalt and andesite) with lesser syn-deformation QFP intrusions and mafic intrusions.
The Underdog zone is located in the southwestern portion of the deposit and is separated from the Main zone by the post-mineral Red Dog QFP intrusion. The gold mineralization is hosted in a syn-deformation QFP dominant domain (i.e., I2P, I1P QFP dikes) with minor mafic and felsic volcanic rock. The mineralization in the Underdog zone is composed of disseminated to semi-massive pyrite intervals associated with strong silica and sericite alteration, generally following main intrusive contacts and/or deformation zones. The top of this deeper mineral zone starts at around 600 m depth and continues to depths of roughly 1,600 m where it is still open at depth and down-plunge.
The F-Zones are located in the northern portion of the deposit. Gold mineralization in the F-17, F-11 and F-51 zones differs from that of the Main and Lynx zones. The F-Zones trend to the northeast, subparallel to the Main zone, but dip steeply to the north. F-17 and F-51 are aligned along the same trend but separated by approximately 800 m. The zones are interpreted to be associated to the Northern fault, and the mineralization is typical of shear-hosted replacement-type mineralization. Mineralization continuity between the two zones cannot be established from the current drilling data. F-11 lies in a similar structural context but is located 500 m to the northwest.