The paleozoic Sicker Group of Vancouver Island, British Columbia

The Sicker Group encompasses the entire sequence· of Paleozoic volcanic and sedimentary rocks of Vancouver Island. It is exposed below its Mesozoic cover in three structural culminations: The Buttle Lake Uplift, the Cowichan-Horne Lake Uplift and the Nanoose Uplift. In th is paper a new lower Ni tinat Formation and a new middle Myra Formation with overlying informally named Sediment- Sill Unit are introduced. The overlying Buttle Lake Formation forms the upper part of the group. The Nitinat Formation, only exposed in the Cowichan-Horne Lake Uplift, is composed of dark, basic agglomeratic, locally pillowed lava, breccia and luff with distinctive large phenocrysts of uralitized pyroxene. The Myra Formation contains interbedded massive and well bedded cherty tuff and commonly variegated maroon-and-green breccia in its lower part and alternating thinly laminated to thick bedded and massive black argillite and light coloured rhyodacite tuff and breccia. Porphy ritic, partly in trusive rhyodacite, named Tyee Quartz Fe ldspar Porphyry, is an importan t component of the Myra Formation in the southeastern part of the Cowichan-Horne Lake Uplift. ln the Buttle Lake Uplift quartz porphyry, rhyodacite and (?) rhyolite and associated brecc ia are the ore-bearing zones carrying the polymetallic (Cu-Zn-Pb-Cd- Ag- Au) sulphides being mined by Western Mines Ltd. The Salt spring Stock of south Saltspring lsland and of Maple Mountain on Vancouver Island is a small intrusive body of sheared and altered granodiorite that appears to be r elated to the quartz-feldspar prophyry of the Myra Formation. Zircon U/ Pb age determinations of quartz porphyry and rhyodacite yield a least-squares-fitted chord on the concordia plot with upper intercept of earliest Silurian 440 Ma age and determinations of the granodiorite yield an upper intercept of Early Devonian 386 Ma age. Alternatively, combined ages of porphyry, rhyodacite and granodiorite yield a chord with upper intercept of Early Devonian 410 Ma if a lower intercept of 180 Ma is used, similar to the metamorphic age of 180 Ma, obtained with K-Ar dating of sericite from the seri citic quartz prophyry sampl e. The Sediment-Sill Unit is composed of thinly bedded silicified argillite and greywacke, interleaved with thick sills of, commonly plagiophyric, diabase that may be related to the basalt of the younger, Triassic Karmutsen Formation. The Buttle Lake Formation is composed of crinoidal and calcarenitic limestone with chert lenses, interbedded with varying proportions of siltstone and argillite. It carries Middle Pennsylvanian fusulinids together with brachiopods and bryozoans that have been variously determined as indicating Pennsylvanian or Permian age. The structure of the Sicker Group shows commonly southwesterly verging, large- and small-scale asymmetric, overturned, and isoclinal folds. In steep and overturned limbs of folds the rocks are commonly highly sheared and metamorphosed to chlorite-actinolite and chlorite-sericite schist. Polymetallic ore deposits of the exhalite type in the Myra Formation are being mined in I.he Buttle Lake Uplift by Western Mines Ltd. and were former ly mined in the Mount Sicker area of the Cowichan-Horne Lake Uplift. Similar deposits may yet be found.