An integrated study of epigenetic gold mineralization, Duder Lake Area, Northeastern Newfoundland

Churchill, Rodney A. (1994) An integrated study of epigenetic gold mineralization, Duder Lake Area, Northeastern Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Duder Lake area is located within the eastern Dunnage Zone of the Newfoundland Appalachians, being part of the volcano-sedimentary Exploits Subzone. The study area is underlain by a mixed succession of late Ordovician to middle Silurian sedimentary and volcanic rocks which are intruded by numerous small gabbroic dykes and sills of Silurian to Devonian age. These sills and dykes were emplaced late in the deformational history of the area, but exhibit features suggesting that emplacement coincided with movement along a major fault system termed the Dog Bay Line. All units preserve three phases of deformation as well as a low- to mid-greenschist facies metamorphic mineral assemblage. -- Recent exploration activity in this area, and elsewhere in the eastern Dunnage Zone, has indicated that the gabbros and graphitic sedimentary rocks host gold-bearing pyrite and arsenopyrite mineralization. The sulphide mineralization in most instances is structurally controlled following low angle Riedel shears which appear to be late structures related to the Dog Bay Line fault system. Fluid/rock and CO₂/H₂O ratios, as determined from alteration mineral assemblages, diminish away from the mineralized shear zones producing a distinctive alteration zonation that is characterized by sulphidation, carbonatization and silicification. The hydrothermal alteration overprints the regional metamorphic assemblages. -- Detailed geochemical, isotopic and microthermometric studies suggest that the fluids responsible for the auriferous mineralization were derived from metamorphic devolatilization reactions in both Gander Zone basement rocks and the overlying allochthonous cover rocks of the Dunnage Zone during a Silurian orogenic event. The fluids, having sampled lithologically complex and diverse country rocks, were enriched in CO₂, H₂O, Au, As and Sb and were focused into large, late structures transecting the crust. -- Geochemical data suggest that the Au was transported as bisulphide complexes that were destabilized due to changes in redox conditions in the carrier fluid from reducing to relatively more oxidizing. The physico-chemical controls acting at the time of mineralization were such that Au precipitated simultaneously with As and S, resulting in its incorporation within arsenopyrite. As a result, gold at the Duder Lake occurrences is not free and is best described as invisible. -- Similarities between the Duder Lake gold occurrences and others elsewhere in the Dunnage Zone with similar ages and geological relationships suggest that the processes inherent in the formation of the Duder Lake occurrences may have been of a regional extent. As such the Duder Lake genetic model may be applicable to these other occurrences as well.

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