Geology and genesis of the polymetallic Ridge Zone West VHMS deposit, Myra Falls, Vancouver Island, British Columbia, Canada
Creators: name: Chong, Albert Gordon
Publisher: name: University Of Tasmania
Year: 2023

description: 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 (&gt;4.0 g/t) and Ag (&gt; 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 &gt; 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.