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Geology and genesis of the polymetallic Ridge Zone West VHMS deposit, Myra Falls, Vancouver Island, British Columbia, Canada

Chong, Albert Gordon 2005 , 'Geology and genesis of the polymetallic Ridge Zone West VHMS deposit, Myra Falls, Vancouver Island, British Columbia, Canada', Coursework Master thesis, University of Tasmania.

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The polymetallic Zn-Pb-Cu-Ba-Au-Ag-rich Ridge Zone West volcanic-hosted
massive sulphide (VHMS) deposit is one of twelve known deposits in the Myra Falls
property located on Vancouver Island, British Columbia, Canada. The current pre-mining
geological resource estimate for the Ridge Zone West is approximately 980,000 tonnes
grading 6.8% Zn, 0.8% Pb, 0.9% Cu, 2.0 g/t Au, and 72 g/t Ag. The aims of this thesis are
to (1) derive a genetic model for the Ridge Zone West VI-1MS deposit by characterizing the
geological setting and mineralization, and (2) evaluate the exploration potential of the Ridge
Zone West area.
At Myra Falls, the Ridge Zone West and other VHMS deposits are located near the
base of the Devonian Sicker Group, Wrangellia Terrane, of the Canadian Cordillera. The
Ridge Zone West geological setting comprises from stratigraphic footwall to hangingwall:
(1) the basement Price Formation basaltic andesite sequence of flows, flow breccias, and
tuffaceous sediments which is conformably overlain by the Basal Conglomerate, a cobble
conglomerate deposited prior to the onset of the hydrothermal system; (2) the Myra
Formation H-W Horizon, a sequence of predominantly felsic, coarse to fine-grained
volcano-sedimentary rocks intercalated with stacked lenses of Zn-rich mineralization; and
(3) the post-mineralization Myra Formation Hangingwall Mafic flow-sill complex that is
overlain or laterally equivalent to the intermediate to mafic Lower Mixed Volcaniclastic
Unit consisting of graded turbidite deposits, sills, and breccias.
Rifting formed a series of restricted basins in the Myra Formation in the present
Myra Falls VHMS property. A high-energy mass flow deposited the Basal Conglomerate
into the basin prior to or during the early stages of the hydrothermal mineralizing event.
The Ridge Zone West is located in a subordinate paleo-depression. Intermittent infill of the
depression by volcanic activity is inferred from intercalated fine and coarse-grained
volcano-sedimentary deposits, angular siliceous mudstone clasts, and sulphide clasts.
Following cessation of the hydrothermal system, the Hangingwall Mafic Complex
(HWMC) was emplaced by intrusive and possible extrusive events excavating previously
deposited, wet unconsolidated volcano-sedimentary deposits and sulphides. The Lower
Mixed Volcaniclastic Unit was deposited predominantly after the HWMC. Three post-mineralization phases of deformation are interpreted for the Ridge Zone West area and are explained in the context of the Myra Falls structural model proposed by
Jones (2001). The phases are: (1) DI, a weak WNW-ESE (mine grid) striking foliation
related to a Paleozoic folding event; (2) Mesozoic D3a strike-slip faults, and infrequent D3b
shallow dipping striking thrust faults; and (3) Mesozoic D4 normal faults. The D4 normal
faults are the most frequently occurring faults in the study area.
The footwall Price Formation and Basal Conglomerate, and hangingwall rocks in
the H-W Horizon are hydrothermally altered to the same mineral assemblage sericitequartz+/-chlorite.
Alteration intensity is weak to moderate as original textures are
preserved. The footwall alteration is pervasive throughout the Basal Conglomerate,
generally within the top 30 m of the Price Formation, and is observed to extend at greater
depths near two faults. Hangingwall alteration of mudstones is interpreted as silicification
that occur below, lateral to, and above the Zn-rich mineralization.
The Ridge Zone West VHMS deposit comprises stacked lenses of disseminated to
massive mineralization associated with fine and coarse-grained volcano-sedimentary rocks.
The main deposit minerals are sphalerite, pyrite, chalcopyrite, galena, tetrahedritetennantite,
and barite. Four phases of mineralization are identified in the Ridge Zone West
deposit: (1) Lower lens mineralization formed on the paleo-seafloor within fine-grained
sediments during a period of quiescence and comprises disseminated and banded
mineralization with pyrite framboids; (2) Main lens mineralization, the most economically
significant mineralization, formed by Zn-rich fluids infiltrating and replacing permeable,
dominantly coarse volcaniclastic deposits in a shallow sub-seafloor environment. Sulphide
textures include anastomosing veins progressing up to massive sulphide with relic lithic
clasts; (3) Upper lens mineralization deposited on the paleo-seafloor during a secondary
period of quiescence and comprises sulphide bands with graded grains and soft-sediment
deformation textures; and (4) Upper lens mineralization consisting of late pyritechalcopyrite+/-sphalerite
veins crosscutting siliceous mudstone breccias.
Based on its overall metal content, the Ridge Zone West is classified as a Zn-Pb-Cu
deposit with an average Cu ratio [100*Cu / (Cu+Zn)] of 11 and an average Zn ratio [100*Zn
/ (Zn+Pb)] of 90. Element associations from assay data are Zn-Pb-Ag, Au-Ag+/-Ba, and
Zn-Cu. Above average Au (>4.0 g/t) and Ag (> 150 g/t) values occur in the Lower lens and
Main lens. A focused up-flow zone of hydrothermal discharge has not been identified as
metal zoning patterns for Fe and the Cu ratio are broadly dispersed and unfocussed. Exploration potential in the Ridge Zone West area includes: (1) eastward towards
the Gopher lens of the Battle deposit where drill hole intersections with Zn-rich
mineralization and siliceous mudstone occur; (2) below the Ridge Zone West within the
footwall Basal Conglomerate; (3) north of the main study area where at least 12 drill hole
intersections are up to 7.6 m in length and have Zn contents > 5%; and (4) westward for at
least 200m to -250E on the mine grid, and potentially up to approximately 590 m westward
to -640E where Zn-rich intersections occur in favourable host rocks similar to the Ridge
Zone West.

Item Type: Thesis - Coursework Master
Authors/Creators:Chong, Albert Gordon
Keywords: Geology, Geochemistry
Copyright Holders: The Author
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Copyright 2005 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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Additional Information:

Thesis (MSc(ExGeo))--University of Tasmania, 2005. Includes bibliographical references

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