Geology and geochemistry of the \caprocks\" above VHMS deposits at Myra Falls Vancouver Island British Columbia"

The aim of this thesis was to characterise and understand the evolution and significance of several large 'cap zones' that overlie Myra Falls mineralised zones. The Myra Falls volcanic hosted massive sulphide (VHMS) district is located in Strathcona Provincial Park in the middle of Vancouver Island, British Columbia, Canada. The orebodies are hosted by the Devonian Sicker Group, a volcanosedimentary package which forms the basement rocks of Vancouver Island. The Cu-Pb-Zn deposits are located in two mineralised horizons, and this study concentrates on the lower ore horizon, which includes the Battle and HW deposits, located on or near the contact with the footwall Price Andesite. The orebodies are hosted by the HW horizon, a rhyolitic package, 5 to 200 m thick, of volcaniclastic and sedimentary rocks, and quartz-feldspar porphyry bodies. The HW horizon is overlain by an andesite-dacite volcano-sedimentary sequence, which is up to 450 m thick. Paleoseafloor reconstructions indicate that the Battle and HW orebodies formed in small basins along a NVV-trending ridge. Argillite deposits above the orebodies and in the South Flank area coincide with paleo-topographic lows and represent depocentres. The sequence has undergone multiple deformation events, including early extension and formation of growth faults; D i folding and development of an E-W oriented foliation; D2 shear zones; D3 aand D3b compression with steep strikeslip faults and shallow N-S dipping thrust faults; D4 extension with steep planar normal faults; and D5 compression resulting in large shallow N-S dipping thrusts and steep NE-oriented strike-slip faults. Chert is best developed above the Battle orebody, where it appears to form a thick 'cap' (3-5m thick) above the massive sulphides. Chert horizons are also located above the Ridge and Extension ore zones. However, the HW orebody is overlain by a thick argillite sequence, with only minor chert. The siliceous caprocks above the Battle deposit are replacement cherts (after mudstone), and these cherts share many sedimentological and petrological features with the adjacent unaltered argillite, including parallel laminations, interbedded turbidites, radiolarians, soft-sediment deformation and phosphatic concretions. Immobile elements indicate a closely related provenance for the argillite and chert, significantly different to the Price Andesite or rhyolitic rocks. Trace element geochemistry, and the spatial association of the cherts with the Battle orebody, indicate that the chert represents hydrothermal alteration of seafloor muds. There is a gradational transition, from white chert above the orebody, to black chert, 120m from the orebody, to unaltered argillite. Lateral metal zonation is observed in the chert, is interpreted to reflect precipitation from a cooling hydrothermal fluid, with a higher temperature Cu-Zn-Cd zone in the white chert, and a lower temperature Pb-Ag-Sb zone in the black chert. Fluid inclusions in spherical megaquartz patches in the Battle chert, indicate temperatures between 135 to 250¬¨‚àû C and salinities from 3 to 12.1 eq. wt % NaC1, which are similar to fluid inclusions in Battle orebody quartz (140 to 250¬¨‚àû C, 3 to 12.4 eq. wt % NaC1). Densities, estimated from inclusion data, range from 0.88 to 1.05 g/cm3 and are close to the density of ambient seawater at 2¬¨‚àû C and 2000m depth (1.03 g/cm3). These densities suggest that the hydrothermal fluids potentially displayed a range of behaviour types where they entered the submarine environment, from buoyant plumes to intermediate brines, which flowed laterally through porous seafloor sediments. At the time of ore formation and caprock deposition, bottom water conditions in the Battle and HW basins were moderately anoxic to euxinic, indicated by DOP values > 0.90, Fe-sulphur-carbon relations, low Fe and Mn values, and high V/Cr, V/Ni and V/(Ni+V) ratios. Barite and haematite, which are common in VHMS caprocks globally, are absent in the caprock horizon, most likely reflecting low O2 conditions. Zn, Pb, Cu, As, Ag, V, Ba and Cd are variably enriched in argillites from the Battle and HW basins, and these are attributed to a hydrothermal origin, because they are all enriched in the orebody. Argillites from the South Flank show no metal enrichment, interpreted to reflect a lack of hydrothermal activity in this area. ˜í¬•34S values in the unaltered Battle basin argillite range from ‚ÄövÑvÆ35.2 to ‚ÄövÑvÆ10.3 ‚ÄövÑ‚àû, which are much lighter than ˜í¬•34S values in the black chert ( ‚ÄövÑvÆ18.4 to ‚ÄövÑvÆ5.3 ‚ÄövÑ‚àû), and the Battle orebody (-1.1 to +4.1 ‚ÄövÑ‚àû). The distinct shift from light ˜í¬•34S values in the unaltered argillite, to heavier ˜í¬•34S values in the black chert provides evidence that hydrothermal fluids, originally with Battle-like signatures, progressively leached regional argillites and so obtained intermediate ˜í¬•34S compositions. A model for the formation of the Battle orebody and siliceous caprocks consists of 5 stages, including: 1) Basin formation and the onset of rhyolitic volcanism, followed by a period of tectonic and/or volcanic quiescence, with the deposition of several metres of laminated muds, silts, and minor interbedded turbidites in the Battle basin. Restricted circulation within the basin resulted in a stratified water column with low to fluctuating O2 bottom water conditions. 2) The onset of hydrothermal activity during deposition of the seafloor muds resulted in the deposition of rutile, apatite and minor sulphides in primary pore space, principally by diffuse flow of hydrothermal fluids through the sediments. Buoyant venting also occurred, producing fine layers of sulphide-rich mud. In places, these layers were entrained into overlying sandstone turbidites. 3) Subsequent low to moderate temperature, silica-bearing fluids flowed laterally through porous seafloor muds to form the Battle chert. Silicification took place early in the depositional/diagenetic history of the muds, to account for the incorporation of chert into overlying mass flows. Ubiquitous bedding-parallel micro-stylolites indicate that silicification occurred prior to compaction. Low metal contents in the chert reflect relatively low fluid temperatures (<250¬¨‚àû C) and likely prior deposition of sulphides in the underlying coarse-grained rhyolitic volcaniclastic layer. Low flow velocities combined with relatively high salinities and moderate temperatures may have enhanced subsurface lateral fluid flow, rather than buoyant venting. Lateral fluid flow is consistent with the sheet-like form of the Battle orebody and caprocks, replacement textures, and lateral metal zonation in the chert. Plugging of the porous seafloor sediments by silica precipitation changed the hydrothermal flow regime from diffuse to more focussed flow, leading to enhanced lateral flow of subsequent ore-bearing fluids beneath the silica cap; the basal contact of the chert is crosscut by massive sulphides. 4) Formation of the siliceous caprocks was followed by renewed extension and volcanism, with the rapid deposition of rhyolitic mass flow units, interlayered with rhyolitic sandstones and volcaniclastic rocks. Finely bedded jasper clasts are common in the overlying mass flow units and most likely originated from Fe-Si deposits forming on the margins of Battle basin, above the anoxic-oxic boundary. The end of rhyolitic volcanism was marked by the intrusion of a massive quartz-feldspar porphyry body at the top of the rhyolitic sequence. 5) Ore deposition continued throughout the deposition of the HW Horizon rhyolitic volcaniclastic rocks and intrusion of the quartz-feldspar porphyry. The barite-rich Gap lens formed by upflow of a more focussed hydrothermal fluid, along the main growth structure at the northern edge of the basin. The upper ore lenses formed by diffuse fluid flow through the rhyolitic pile and ponding beneath the massive porphyry. A more oxygenated environment developed during deposition of upper zone sulphides, indicated by ubiquitous barite in upper ore lenses. A similar structural and depositional environment is proposed for the HW orebody. However, the HW orebody formed prior to deposition of the caprock horizon. Massive sulphides were deposited within the coarse-grained rhyolitic detritus on the basin floor and the top surface of the HW orebody is eroded in places, with ore clasts common in overlying mass flows. Elsewhere, there is a sharp depositional contact between argillite and the orebody. This study indicates that the formation of siliceous caprocks required the interaction of hydrothermal fluids with a pre-existing carapace of fine-grained sediment.

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