Koocanusa: Copper-Bearing Lithologies
Koocanusa: Introducing the Geology
Copper mineralization has been observed at a variety of locations across the property, but are most dominant in the Creston and Sheppard Formation rocks. Minerals are found along bedding planes, fracture fill or as as disseminations throughout the rocks, appearing as black-brown speckles.
Pathfinders in the field include growths of garnets in a coarse sandstone, and the presence of iron or copper oxide dendrites along fracture surfaces. Exploration activities over the past few years at Koocanusa have led to the discovery of several “fertile” horizons across the project area.
Creston Formation
The Creston Formation is a thick series of siltstones and argillites which underlie the western side of the Koocanusa project. This unit runs continuously south into the United States (where it is called the Ravalli Group) and hosts the copper mineralization at the Hecla-owned mines (Troy, Spar Lake, Montanore).
Mineralization consists of copper-bearing minerals such as chalcopyrite, chalcocite and bornite, but also hosts other elements such as galena (lead sulphide) and various silver sulphosalts.
Sheppard Formation
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Phillips Formation
The Phillips formation is a relatively minor bed at Koocanusa and has only been observed in the southwestern part of the property.
It comprises of a purple to red, fine-grained and micaceous sandstone with occasional argillite layers nearer the top of the formation.
Volcanic ash from the equivalent formation in the United States (the contiguous Bonner Formation) has a Uranium-Lead isotopic age of 1,401 million years (+/- 6 million years).
Gateway Formation
The Gateway Formation overlies the Sheppard Formation. It is comprised of fine-grained, light grey-green siltstone and sandstone with minor dolomitic limestone. The base of the Gateway Formation is marked by the first occurrence of salt casts, mud cracks, and rip-up clast beds within siliceous fine-grained sediments that overlie the massive dolomitized limestone beds that mark the top of the Sheppard Formation. Sedimentary facies of the Gateway Formation fine upwards from predominantly fine-grained sandstone at the formation base to fine-grained siltstone and argillite at the formation top.
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Vein Styles
A wide range of veins occur across the property, from thin quartz-carbonate stockworks to half-meter-wide quartz / barite / magnetite veins.
The majority of the large vein systems have been observed in the Nicol Creek Basalts where the brittle rocks allowed for the formation of wider fractures. In the overlying Sheppard Formation, these veins quickly become more diffuse as fluids pass into porous sandstones or the reactive dolomites. The theory is that these veins are “synsedimentary”, i.e. they are forming at the same time as the sediments are deposited and so these rocks do not have a brittle fracture.
The veins observed in outcrop contain extensive mineralization and oxidized “ghosts” of crystals, sometimes replaced by later infill. Studies are underway to determine the full mineralogy, genesis and potential of these veins.
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Nicol Creek Formation
The Nicol Creek Formation is comprised of amygdaloidal and phenocrystic basalt flows, shallow marine volcaniclastic to siliciclastic sediment, and minor tuff. Zircon from a rhyolitic tuff within the contiguous Purcell Lava in the USA has a U-Pb age of 1443 +/- 7 Ma.