Multiple deformation episodes at Myra Falls volcanic-hosted massive sulfide camp, central Vancouver Island, British Columbia, Canada

Abstract

A detailed deformation history for central Vancouver Island was determined at Myra Falls volcanic-hosted massive sulfide camp with early ductile deformation overprinted by several distinct episodes of brittle deformation. Brittle structures were subdivided into separate groups based on their morphology, geometry, kinematics, and crosscutting relations. The central location of this study provides a link between previous deformation studies in northem and southem Vancouver Island. Late Paleozoic northeast-southwest compression (D I ) produced open upright folds with variably developed north-northwest-striking axial planar cleavage zones (SI) and subhorizontal stretching lineations (LI) subparallel to F I fold axes. Renewed northeast-southwest compression during the collision of Wrangellia and North America produced a second foliation (S2) in localized shear zones, slightly oblique to the dominant SI foliation. These twO events are recorded throughout Vancouver Island wherever the basement is exposed. Mid-Cretaceous northeast southwest compression during D 3 produced early Steep conjugate strike-slip faults (D 3a ), overprinted by northeast-and southwest-dipping thrust faults and bedding-parallel shears (D 3b ). D 3 structures have been previously recognized in northem Vancouver Island but not as far south as Myra Falls. North-south extension (D 4 ) produced east, north, and east-southeast-striking normal faults . These faults consistently crosscut earlier D I -D 3 structures and reactivate steep D 3a faults. Normal faulting is correlated with the development of the Upper Cretaceous Nanaimo Basin, but no faults of this age have previous been reported from onshore studies. The youngest structures at Myra Falls are large northwest striking, northeast-dipping tbrust faults and steep west-to west-northwest-striking sinistral strike-slip faults formed during the D s event. These faults are gouge-rich, wavy anastomosing structures, with cleaved wall-rock zones up to several metres wide. The D s faults are correlated with Eocene deformation caused by the accretion of the Pacific Rim and Crescent Terranes along the southwestern margin of Vancouver Island. Myra Falls is the northemmost location to have been reported, at which the structures formed as part of the Cowichan fold and tbrust belt.

Precious Metal Enrichment at the Myra Falls VMS Deposit, British Columbia, Canada

“…aqueous phase, carbonic liquid and vapour phases. Both populations were identified in an earlier study [10], while a later study [8] only reported P1. Our observations for the P2 FIs concur with the earlier study of Barrett and Sherlock [10] Femtosecond-laser excitation has proven useful in nucleating meta-stable vapour bubbles in some fluid inclusions [12], but was deemed beyond the scope of this study as the ice melting temperatures could be measured in the absence of the vapour phase in some of our larger fluid 246 inclusions, and was consistent with fluid inclusions within the same FIAs, that were able to 247 nucleate vapour bubbles.…”

“…The Sicker Group is overlain unconformably by the Permian Buttle Lake Group limestones and younger rocks. Rocks of the Wrangellia terrane are generally metamorphosed to greenschist facies, with some amphibolitefacies contact-metamorphic zones near the Jurassic Island intrusive rocks [8].…”

“…First, pre-Permian event D1 caused northwest-trending folding [7]. Early to Mid-Jurassic [7] folding event D2 resulted in localized shear zones and reactivated F1 folds [8]. Steep, conjugate strike-slip faults and bedding-parallel thrust faults and shears resulted from event D3 [7,8], normal faults from event D4, and large thrust faults and steep, west-to west-northwest-striking sinistral strike-slip faults from event D5 [8].…”

“…3). Paragenesis for the deposit is complicated by numerous volcanic depositional events and by numerous deformational and subsequent alteration events [8]. However, the gold-bearing veinlets are very late in the deposit paragenesis and cross cut all of the typical sub-and seafloor sulphide mineralization associated with VMS deposits [1] including barite/silica cap rocks [8,10].…”

“…Paragenesis for the deposit is complicated by numerous volcanic depositional events and by numerous deformational and subsequent alteration events [8]. However, the gold-bearing veinlets are very late in the deposit paragenesis and cross cut all of the typical sub-and seafloor sulphide mineralization associated with VMS deposits [1] including barite/silica cap rocks [8,10]. Gold grains, ranging up to 4 mm in size, are commonly visible in the veinlets, and contemporaneous with quartz, barite, sphalerite, galena, bornite, tennantite, chalcopyrite, pyrite and rarely stromeyerite, covellite, chalcocite, acanthite, and native silver.…”