Gold mineralization and its position in the geological evolution of the Beardmore - Tashota area

CIM Bulletin, Vol. 80, No. 900, 1987
DEREKE. McBRIDE
Abstract Exploration work in the Beardmore - Tashota area has led to a reinterpretation of relationships between gold mineralization and the geological evolution of the area. The host volcanic rocks form two parallel volcanic belts connected by a much smaller cross-cutting band, which trends northeast from Beardmore to Tashota. This connecting band consists of mafic and felsic volcanic, plus clastic sedimentary rocks in a sequence conformable with the contact of the Onaman Lake Batholith to the east. A metamorphic aureole or incorporated xenoliths are lacking along this major contact and the only intrusive relationship observed is chilling between mafic dikes and a cold host granite in proximity to it. Granite dikes are totally lacking in the volcanic succession, or the overlying sedimentary sequence. A major part of this sequence consists of a polymict basal conglomerate with granite boulders and volcanic clasts in a finegrained chloritic matrix. The nearest source for these boulders is the Onaman Lake batholith.
Gold mineralization in significant economic quantities (+ 50 000 ounces) occurs at the top or above the mafic volcanic succession as quartz bands. In the Northern Empire Mine, the band is conformable, folded, boudinaged and the mineral zoning shows a relationship to variations in the host rock, not structural overprinting. In general, where the host mafic tuff is medium-grained, the quartz band is thicker, up to a metre wide, and carries economic grade; where the tuff is finer-grained and bedded, the band is very thin and low grade.
Stratigraphic control and metal zoning shows that the mineralization formed as part of the development of the volcano-sedimentary succession. The other major deposits in the area (Leitch and Sturgeon River) have a similar structural history suggesting that they are also syngenetic in terms of the stratigraphic development. Regionally, these deposits were emplaced prior to any significant tectonic deformation but at the end of the mafic volcanic event.
The mafic activity was related to a period of extension which caused the volcanic troughs to develop along centres of rifting. The hiatus at the end of this event permitted extensive hydro-thermal systems to form and deposit siliceous exhalite bands within sedimentary or terminal mafic volcanic environments in promixity to major thermal sources. Subsequently, the Kenoran Orogeny deformed the entire succession into upright folds with a sub-vertical schistosity. Evidence shows that the deposits have remained intact since deposition.
Keywords: Gold mineralization, Geological evolution, Mineralization, Beardmore - Tashota area, Economic geology, Exploration.
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