Physical volcanology and hydrothermal alteration of the Rainy River Gold Project, northwest Ontario.

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Physical volcanology and hydrothermal alteration of the Rainy River Gold Project, northwest Ontario.

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The Rainy River Gold Project (RRGP) is located 75km northwest of Fort Frances, Ontario, within the Rainy River Greenstone Belt. This advanced stage exploration project has a NI43-101 compliant gold resource of 6.6 Moz indicated and inferred and 14.7 Moz indicated and inferred silver (from Rainy River Resources Press Release, February 2011) represented by low grade (<2g/t), low-moderate grade (2-10g/t), and high grade (>10g/t) gold mineralization. The nature of the gold mineralization in this deposit has been the subject of controversy, and several competing models have been proposed to explain its genesis. Initial exploration in 1967 suggested that the deposit was a shear zone-hosted resource. However, recently completed exploration drilling has now defined large, diffuse zones of gold mineralization in dacitic volcanic and volcaniclastic rocks, suggesting, in part, a syn-genetic genesis for the gold mineralization. While previous studies have examined structural regimes and timing of gold mineralization, this research focuses on the physical volcanology and hydrothermal alteration associated with the deposit. Field mapping is difficult due to a paucity of outcrop, and geological correlations are complicated by polyphase deformation, hydrothermal alteration, and both regional, and locally contact, metamorphism. This study included comprehensive fieldwork involving mapping of all available outcrops and compiling them at a 1:25,000 scale and exploration drill core logging along three sections totaling ~9000 m of core. Fieldwork, supplemented with petrographic studies of 210 thin sections and lithogeochemical investigations comprising 69 samples, has enabled distinction of stratigraphy, volcanic facies, and hydrothermal alteration assemblages, and enabled processes associated with hydrothermal metasomatism to be evaluated. Drill core is locally intensely altered and deformed, resulting in many of the units having false pyroclastic textures (Allen, 1988). Despite this, strata associated with the RRGP contain some well-preserved primary textures. These primary textures indicate that the volcanic facies in the deposit include coherent dacitic flows and associated synvolcanic intrusions with autoclastic breccias, hyaloclastites, peperites, and syn- to postdepositional resedimented volcaniclastic deposits. The coherent dacitic flows are massive, range in thickness up to 150 m, and in lateral extent for 2500 meters. Coherent dacite flows grade into a heterogeneous facies, characterized by pods and lobes of coherent dacite, enveloped by autoclastic breccia and hyaloclastite. Flows are interspersed with strongly altered volcaniclastic sediments that are locally punctuated by peperites. Volcanic facies reconstruction indicates the presence of lobe-hyaloclastite dome/flow complex fed, and locally intruded by, synvolcanic dacite hypabyssal intrusions. An apparent feeding fissure is centered to the west of the main area of mineralization. Hydrothermal alteration is widespread throughout the deposit and is marked by silicification, chloritization, sericitization, and carbonitization (as well as minor epidote and local biotite alteration). Quartz, sericite and chlorite are ubiquitous in the deposit. The isocon method developed by Grant (1986) and the box plot method developed by Large et al. (2001) were utilized to quantify the chemical alteration resulting from the hydrothermal alteration in RRGP rocks. Alteration assemblages are dominantly stratabound, and their distribution is related to original rock permeability, with flow tops, autoclastic breccias, and volcaniclastic sediments being most strongly altered. Shearzones also preserve stronger alteration intensities. Gold mineralization appears to have initially occurred within a synvolcanic, lowsulfidation (Simmons et al., 2005) epithermal system. Elevated gold values are strongly correlated with highly permeable units and increased alteration intensity, suggesting enhanced mineralization in areas that experienced higher water: rock ratios. Postvolcanic remobilization of the gold appears to have occurred, as the highest gold values in the deposit are spatially related to shear-zones and associated quartz-carbonate-epidote veins.


University of Minnesota M.S. thesis. July 2011. Major: Geological sciences. Advisor:Dr. Ron Morton. 1 computer file (PDF); vii, 154 pages, appendices I-II.

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Wartman, Jakob Michael. (2011). Physical volcanology and hydrothermal alteration of the Rainy River Gold Project, northwest Ontario.. Retrieved from the University Digital Conservancy,

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