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