Geology and genesis of the Battle Zone VHMS deposits, Myra Falls district, British Columbia, Canada
Creators: name: Sinclair, BJ
Publisher: name: University Of Tasmania
Year: 2023

description: The Myra Falls volcanic hosted massive sulfide (VHMS) district, is located on Vancouver Island, British Colombia, Canada. The district has a pre-mining reserve of 40Mt @ 6.2% Zn, 1.8% Cu, 47.5 g/t Ag and 2.1 g/t Au. The Battle Mine is the youngest in a series of mines, which have kept the district in continuous production since 1966. The Battle mine was discovered in May 1991 through mine site exploration. The Battle Mine contains a pre-mining reserve of 7Mt @ 12.5% Zn, 1.8% Cu, 0.7% Pb, 53.2g/tAg and 1.4g/t Au and is made up of several lenses: Battle, Gopher, South Trough, Upper Zone and Gap. These lenses lie at two stratigraphic levels and represent two distinct styles of VHMS mineralisation. Host rocks to the Battle Mine lenses are part of the Devonian Sicker Group, which forms the base of the Wrangellia Terrane of the Canadian Cordillera. Footwall to the Battle, Gopher and South Trough lenses is the Price Formation. The Price Formation is a &gt;300m thick sequence of basaltic andesite flows, flow breccias and associated volcaniclastic deposits. The Battle, Gopher, and South Trough lenses are overlain by up to 30m of fine-grained \chert\" and then by 30-70m of rhyolitic mass flow deposits with minor intercalated non-volcanic sediments. The rhyolitic mass flow deposits are overlain by 10 to 60m of massive to autoclastic quartz-feldspar-phyric (QFP) rhyolite. The Upper Zone lenses lie at the top of the rhyolitic mass flow deposits along the contact with the QFP while the Gap lens is hosted within the QFP. The entire sequence is overlain by basaltic-andesite to andesite lavas and related breccias of the Hanging Wall Andesite. The mine sequence has been subjected to at least two stages of prehnite-pumpellyite to lower greenschist facies metamorphism and at least five stages of post-depositional deformation. Several stages of pre- to syn-mineralisation faulting are also interpreted. Throughout the host sequence there is a strong sub-vertical cleavage with stretching lineation trending NW to NNW. The cleavage is axial planar to folds and the stretching lineation parallels the fold axis. This cleavage is interpreted as prePermian and marks the first recognisable post-mineralisation structure in the mine area and is named D2. D3A structures include north striking dextral west striking sinistral faults and high angle reverse faults with NW strike and are associated with quartz carbonate veins. D3B structures include a variety of south dipping thrust faults range from flat lying to thrusts to high angle structures. Both D3 events show brittle ductile textures (discrete faults surrounded by narrow zones of cleavage development). D3A structures are interpreted as Mid to Late Jurassic in age while D3B structures probably correlate with the a Cretaceous event of England and Galon (1991). The fourth major structural event (D5) is an Eocene thrusting which forms broad zones of anastomosing gouge and brecciation and bounds the ore lenses in NW to NNW orientation. A variety of gougy faults showing normal movement exists through out the Battle Mine and are correlated with D4 and D6 events. Alteration in the Battle Zone is divisible into several types depending on precursor rocks (Price Formation andesite or HW Rhyolite) and mineralogy and style of alteration. The Price Formation directly underlying the Battle Gopher and South Trough lenses is intensely altered by texturally destructive sericite-pyrite ¬¨¬± quartz alteration. Zones of intense Mg-chlorite - pyrite alteration also occur. Approximately 20m below the base of the massive sulfides alteration changes into less intense sericite-chlorite-quartz alteration relic textures (e.g. pillow margins) are visible. The HW Rhyolite is also altered by texturally destructive quartz-sericite alteration which is heterogeneous creating pseudo-fiamme. Dolomite alteration occurs in the Battle Zone HW Rhyolite away from massive sulfide mineralisation. Alteration in the Price Formation in the footwall to Battle Gopher and South Trough lenses is characterised by large gains in S and K for all alteration types and gains Mg and Mn in Mg-chlorite alteration. These gains are due to addition of muscovite and sulfides to the altered rocks and the formation of chlorite in Mg-chlorite alteration. Calcium Na Sr Ni and Cr are depleted in all footwall alteration types while Mg and Mn are lost in the intense sericite-pyrite ¬¨¬±quartz alteration. Hangingwall alteration is characterised by the addition of S Cu Ba Zn Ni and As associated with the growth of barite and sulfide minerals. The silicified samples also gain Si due to the formation of quartz while sericite-altered samples gain K during muscovite growth. Dolomite-altered samples gain Sand metals along with Mg Ca and Mn resulting in the formation of dolomite. However in all alteration types only the absolute changes in Si significantly affect the change in mass of the rock during alteration. The Battle Gopher and South Trough lenses are composed of sphalerite pyrite chalcopyrite galena tennantite and accessory barite rutile and telluride minerals. These lenses show classic VHMS mineralogical zonation from a sphalerite-galena-tennantite-barite-rich top downward through massive sphalerite-pyrite-chalcopyrite massive pyrite-chalcopyrite and massive pyrite zones towards the base of the lenses. In contrast the Upper Zone and Gap lenses are composed of sphalerite pyrite bornite chalcopyrite galena tennantite and barite with accessory colusite renierite anilite and electrum. Metamorphism has resulted in the obliteration of most of the primary VHMS ore textures with the exception of some textures preserved in pyrite. Zonation of metals correlates with the observed mineralogical ionation within each lens and between lenses. The South Trough lens which is the stratigraphically lowest lens in the Battle mine shows the highest Fe and lowest Pb and Ba. In contrast the Gap lens which is the stratigraphically highest lens has the greatest Au-Ag-Ba-As-Pb contents. Within each lens sphalerite-galena-tennantiterich zones are marked by enrichment in Pb-Cu-Ba-Au-Ag-As and Hg. Underlying is a zone of Zn-Cd enrichment followed by a zone of Cu-Fe-Ni+Au toward the base of the lenses. Manganese Mo and P are elevated in sediments overlying the Battle Gopher and South Trough lenses while footwall alteration below the Battle Gopher and South Trough lenses is enriched in Se and P. The Gap lens shows a similar zonation in Ba-Pb-As-Zn-Cd and Fe from hangingwall to footwall although Ba-Pb-As zone is more voluminous due to the enrichment in these elements compared to the underlying lenses. However metal zoning reveals a strong zone of Cu-Ag and Mo cross cutting the typical VHMS zonation within the Gap lens. This zone of Cu-Ag-Mo corresponds to the distribution of bornite-anilite. Although metamorphism of the Gap lens has destroyed evidence of primary paragenetic relationships textural relationships of bornite and chalcopyrite suggest bornite occurred as a replacement of chalcopyrite. The most likely factor in driving this reaction would be an increase in the fO2 of the fluid and/or a decrease in temperature. Detailed mineralogical examination of the Upper Zone and Gap ores has also revealed the presence of stromeyerite (Cu-Ag-S) chalcopyrite and electrum. This Cu-Ag-Au rich assemblage cross cuts all other sulfide minerals and forms vermicular intergrowths with bornite and anilite. Delicate textures of this assemblage suggest a late (possibly prograde) metamorphic age for this assemblage the Cu Ag and Au being derived from recrystallisation of bornite anilite and gold during metamorphism. The Battle Gopher and South Trough lenses were deposited in the Devonian on top of the Price Formation on the seafloor with minimal ( &lt;10m) of sediment cover until swamped by a massive influx of rhyolitic mass flows. The hydrothermal fluid associated with the deposition of these lenses is interpreted to have had temperatures between 250-350¬¨‚àûC weak acidity (ph = 4-5) and a salinity in the range of 3-8wt%. Sulfur isotope data Indicate a reduced seawater sulfate source for the sulfides and a direct seawater source for the sulfates. The swamping of the depositional environment with volcaniclastics did not stop the hydrothermal system. Hydrothermal fluids continued to percolate up through the rhyolitic volcaniclastics until they encountered the QFP. The QFP provided a barrier to the ascending fluids which resulted in the subseafloor deposition of sulfide mineralisation as represented by the Upper Zone and Gap lenses. During percolation of the hydrothermal fluid up through the rhyolitic volcanicaniclastics the fluid cooled and became slightly more oxidised. Toward the end of the hydrothermal system the fO2 of the hydrothermal fluid increased and the temperature decreased resulting in the formation of bornite from previously formed chalcopyrite. After the hydrothermal system ceased the hangingwall volcaniclastics and associated massive sulfide lenses were covered by the Hanging Wall andesite. The entire sequence was then metamorphosed and deformed. Metamorphism caused the recrystallisation and annealing of all sulfide minerals except pyrite. Metamorphism also resulted in remobilisation of Cu Ag and Au within the Upper Zone and Gap lenses with these elements deposited in crosscutting veins during the waning stages of metamorphism."