Petrographic, geochemical, and fluid inclusion evidence for the origin of siliceous cap rocks above volcanic-hosted massive sulfide deposits at Myra Falls, Vancouver Island, British Columbia, Canada

Massive sulfides at the Myra Falls volcanic-hosted massive sulfide (VHMS) camp, Vancouver Island, British, Columbia, Canada, are overlain by white chert, black chert, argillite, and siltstone. White chert is best developed above the Battle orebody, where it forms a siliceous caprock (3-5 in thick) above the massive sulfides. There is a gradational lateral change from white chert above massive sulfides to black chert and unaltered argillite, 100 to 150 in south of the Battle orebody. Chert horizons are also located above the Ridge and Extension ore zones, but only minor chert lies above the HW orebody, which is instead overlain by a thick argillite sequence. The chert and argillite share similar sedimentologic and petrologic features, including abundant parallel laminations, interbedded turbidites,radiolarian-rich layers, soft-sediment deformation, scours, flame structures, and small phosphatic concretions. These features indicate that white and black chert formed as a replacement of mudstone rather than as exhalative or biogenic deposits. Silicification occurred early in the depositional history of the fine-grained sediments and was contemporaneous with some ore formation. Early syndepositional features are still visible in the chert, with primary pore spaces such as radiolarian tests filled by silica, rutile, apatite, and minor sulfides displaying open-space crystal growth. The presence of minor ore-clast breccias above the orebody indicates that at least parts of the Battle orebody were exposed on the sea floor. Metal zoning is observed in the cap-rock horizon above the Battle orebody, with higher Cu, Zn, and Cd contentrations in chert directly above massive sulfides, higher Pb, Sb, and Ag contentrations in black chert at the edge of the siliceous cap rocks and lower metal concentrations in the distal argillite. Primary fluid inclusions in spherical quartz patches in chert above the Battle orebody indicate that hydrothermal fluids passing through sediment were between 135 to 250°C and had salinities ranging from 3.0 to 12.1 wt percent NaCl equiv. These data are similar to those for fluid inclusions measured in quartz interstitial to sulfides in the underlying Battle orebody, which have temperatures of homogenization ranging from 140° to 250°C and salinities from 3.0 to 12.4 wt percent NaCl equiv. Fluid inclusions in the Battle orebody display a slight increase in salinity and homogenization temperature with depth, which may reflect the overprinting of earlier high temperature stages by cooler fluids as the hydrothermal system waned or varying degrees of mixing between hydrothermal fluids and seawater. A minimum depth for deposition of the cap rocks (>200 m) is estimated, based on sedimentologic features such as fine parallel laminations and interbedded sandstone turbidites, which indicate deposition below storm wave base. Greater water depths (1,000-1,500 m) are suggested by the lack of evidence of boiling in fluid inclusions. Low O 2 concentrations in the bottom water of the Battle basin are suggested by the absence of bioturbation, lack of fossils of benthic fauna, degree of pyritization values >0.90, elevated Zn, Pb, Cu, Cd, As, Sb, Ag, Ba, and V, low Fe and Mn, and V/(V + Ni) > 0.8 in the unaltered argillite. Paleosea-floor reconstructions indicate that the Battle and HW orebodies formed in small basins along a northwest-trending ridge. The fine-grained sediments were deposited in depocenters within paleotopographic lows. Hydrothermal fluid densities, estimated from fluid inclusions at Myra Falls, range from 0.88 to 1.05 g/cm 3 and are higher than for many other VHMS deposits. However, they are close to the density of seawater at 2° C and a 2,000-m depth (1.028 g/cm 3). Replacement textures in the siliceous cap rocks above the Battle deposit, the sheetlike morphology of the siliceous cap rocks, and lateral metal zonation indicates that diffuse lateral flow of hydrothermal fluids through the porous sea-floor sediments was more important than the buoyant rise of hydrothermal fluids into the water column. However, buoyant venting during formation of the Battle deposit is indicated by positive Eu anomalies and elevated metal values in argillite of the Battle basin, reflecting the wide dispersal of plume particulates. The range of fluid densities indicate that the hydrothermal fluids emerging onto the sea floor and flowing laterally through porous sea-floor sediments would have varied from buoyant to neutrally buoyant, to negatively buoyant. © 2006 Society of Economic Geologists, Inc.