Summary:
Deposit Types
Gold mineralization on the Madsen property is localized along major structural trends, similar to other deposits found throughout the Red Lake district and ranging in scale from localized showings to past-producing or currently producing mines. Based on extensive work, Pure Gold made advances in the geological understanding of these deposits and developed a new interpretation that deviates from those of past workers (e.g., Dubé et al., 2000). As of the effective date of this report, the deposits are now classified as Archean orogenic gold deposits (Groves et al., 1998), though they have been intensely modified by deformation and metamorphism following gold deposition.
Madsen Mine Style Gold Mineralization
Controls on mineralization at the Madsen Mine are consistent with a typical orogenic gold system. Many deposit-scale features such as control by lithological / structural contacts and association with felsic dykes are typical in these systems. Smaller-scale features have been used to support the interpretation that the Madsen Mine deposits are an unusual or end-member type of orogenic gold system. For example, Dubé et al. (2000) conclude that the Madsen deposit is a disseminated, stratabound deposit that shares similarities with mafic-hosted gold-skarns and also with highertemperature Australian deposits. Recent work, however, indicates that, apart from its early timing of emplacement prior to the dominant regional deformation and metamorphism, Madsen Mine shares many characteristics with typical orogenic gold deposits, including the Red Lake Mine deposit.
All significant gold mineralization on the mine property is demonstrably early relative to the most significant, penetrative deformation (D2) and metamorphic events. Quartz veins at 8 Zone, Wedge and Russet are boudinaged, recrystallized, folded and overprinted by the penetrative S2 foliation. Mineralized bodies of the Austin, South Austin and McVeigh Zones are locally folded and transposed into S2. In addition to this intense deformation overprint, the mineralized veins and alteration have been subject to the relatively high temperatures of amphibolite facies metamorphism, which led to extensive recrystallization and growth of the skarn-like mineral assemblage of diopside-amphibole-quartz-biotite. By contrast, more typical deposits of the orogenic gold deposit class are characterized by lower temperature greenschist facies mineral assemblages. It is this intense deformation combined with the amphibolite grade metamorphism that obscured the primary features of the Madsen Mine style gold mineralization and led to suppositions of a syngenetic, or other atypical origins for the deposit. However, numerous structural and petrological features identified by Pure Gold contradict such interpretations and support an orogenic gold deposit classification.
At the property-scale, the mineralized zones follow planar corridors defined by patterns of gold mineralization, alteration and high strain that are continuous on a kilometre-scale (e.g., Austin/ South Austin Zones are planar and continuous over an area of at least 600 by 2,000 m). These planar structures transect their host Balmer stratigraphy and the Balmer / Confederation Assemblage unconformity at a low angle, which refutes the possibility of a syngenetic or stratabound origin. This corridor is also host to discontinuous, coplanar lenses of quartz porphyritic felsic dykes, which predate mineralization but preferentially host significant rinds of mineralization along their contacts – a further indication of an epigenetic origin and structural control for the mineralization.
At the deposit scale, primary local control controls on the localization of gold mineralization include competence contrasts between different lithologies, in particular felsic dykes and ultramafic sills, but also iron formations, which is a common feature of structurally-controlled hydrothermal deposits of all types. These competence contrasts manifest deposit scale controls on the shape and plunge of the deposits at their intersections with mineralizing structures. Reversals in the plunge direction of mineralized shoots are thereby linked to the primary orientations of vein emplacement relative to their host stratigraphy - originally moderately-dipping structures host shoots that tend to plunge east (e.g., Austin and South Austin), whereas originally steep-dipping and more northerly-trending structures tend to plunge southwest (e.g. McVeigh and Wedge). Also, the presence of shallowdipping veins within the original vein structure, as evidenced by the shallower dip of enveloping surfaces around tightly-folded veins, in conjunction with moderately- and/or steeply-dipping structures, is another feature common in structurally-controlled gold vein systems.
Petrologically, the skarn-like alteration assemblage of the mine style mineralization is an unusual feature in gold vein systems but is now recognized here as a metasomatic overprint on the original quartz-carbonate (+sericite?) vein and alteration mineral assemblage. Numerous examples of preserved quartz-carbonate veins and vein remnants with biotite-rich reaction rims inside massive diopside-amphibole-altered rock have been found, clearly indicating the primary alteration assemblage to be quartz-carbonate rich, as is more typical of orogenic gold deposits. The skarn minerals (diopside, amphibole, biotite and garnet) therefore represent the recrystallized equivalents of the original alteration assemblage, grown at higher temperatures following emplacement.
Planar, Quartz-Sulphide Vein-hosted
Gold The second style of gold mineralization found on the mine property is hosted by discrete, planar quartz veins with little associated alteration. Setting and orientation of these veins is variable – multiple examples occur proximal to iron formation, but they are also present in both Balmer-age mafic and Confederation-age felsic host rocks. The orientation of these veins is often – though not universally – roughly parallel to the axial plane of D2 folds. In cases where the veins do not display this orientation, they typically track sub-parallel to a stratigraphic feature such as an iron formation. The key features which distinguish these veins from the earlier veins described above are their lack of associated alteration and co-incident quartz porphyry or non-quartz vein material (e.g., ankerite/diopside). The timing of this vein style is not well constrained, but if the #1 Vein is included in the set, a post-Confederation age is required. The localized association with D2 axial planes suggests that they were emplaced synchronous with the D2 deformation event. Examples of this style include the Dev Northwest, Roberts and #1 Vein exploration targets.
Quartz-Tourmaline Vein-hosted Gold
The third style of gold mineralization is associated with quartz-tourmaline veins. These veins show evidence for emplacement under a brittle tectonic setting and typically occur as en echelon vein arrays. As such, these veins have short strike lengths and vein orientations are somewhat chaotic, displaying only a rough alignment of orientations within swarms along large (km-scale) trends.
Despite being generally narrow (cm to 10’s of cm scale) the veins can host high gold grades making swarms of sufficient density an attractive exploration target. Where they are present in the same location as pre D2 ankerite-quartz veins (e.g., Redaurum deposit), they show a clear cross-cutting relationship with those earlier veins. They do not show evidence of deformation during D2, suggesting that they post-date that tectonic event. These veins are part of the same system that forms the historical Buffalo deposit, which has been interpreted to be associated with emplacement of the Dome Stock. The primary example of this style on the mine property is the Treasure Box showing.