Ultramafic Intrusions, Detailed Geology and Geobarometry of the Jurassic Bonanza Arc in the Port Renfrew Region, Southern Vancouver Island (NTS 092C) D. Canil, School of Earth and Ocean Sciences, University of Victoria, Victoria, BC; [email protected] D. Canil (2010): Ultra mafic intrusions, detailed geology and geobarometry of the Ju ras sic Bo nanza Arc in the Port Renfrew region, southern Van cou ver Is land; in Geoscience BC Sum mary of Ac tiv i ties 2009, Geoscience BC, Re port 2010-1, p. 141–148. Introduction Survey Methods Sev eral iso lated bod ies of ultram afic rock were discov ered Bedrock geol ogy was mapped at 1:25 000 scale on Terrai n in the Port Renfrew area of south ern Vancou ver Island over Resource Inven tory Mapping (TRIM) map sheets and com - the past nine years by G. Pearson. A field mapping study piled to 1:50 000 scale (Figure 1). Sam ples for thin-sec tion jointly funded by Geoscience BC and Em eralds Fields Re- pe trog ra phy, geo chem is try, fire as say and geo chron ol ogy sources was con ducted to as cer tain the ar eal ex tent of the were taken (Fig ure 2). De tailed ex am ina tions of se lect ultra mafic bod ies and to de ter mine their re la tion ship with ultram afic bodies could not be com piled onto the 1:50 000 other rocks of southern Vancou ver Island. The objec ti ve scale ge ol ogy maps but are de scribed in de tail in re lated pa - was to use this and other petro log i cal infor m ati on from the pers (Larocque and Canil, 2007, 2009). surround ing area to assess the econom ic signif i canc e of the ultra mafic bod ies as a po ten tial target for Ni-Cu or plat i - Regional Geology Overview num-group el em ent (PGE) de posit s, and the overall struc - tural thickness and set ting of the Juras sic Bonanza Arc. Pre vi ous geo log ical maps in the area were mapped at This re port sum ma rizes re sults of the geo log i cal field work, 1:100 000 scale and com piled to 1:250 000 scale (Mul ler, includ ing detai led mapping, sampling, geobarometry and 1983). Most of the area is un derlai n by rocks of Wrangellia. geochemical assays of the ultramafic rocks. The De vo nian Sicker Group forms the base ment to Wrangellia on Vancou ver Island, and consis ts of mafic and Field Area and Access fel sic vol ca nic and volcaniclastic rocks, over lain by The field area on southern Vancou ver Island (NTS 092B, C, epiclastic and car bon ate sed im en tary rocks of the Permian F, G) is bordered by the San Juan River to the south, Buttle Lake Group. Uncon form ably overly ing the Buttle Cowichan Lake to the north, Nitinat Lake to the west and Lake Group are the Tri as sic Karmutsen bas alts, in turn the Fleet River to the east (Fig ure 1). The area of in ter est is overlai n by the Quatsino Form ati on, a thin (<75 m) se- in and surround ing the NTS 092C map area. The physi og - quence of micritic limestone . The Parsons Bay Form ati on, raphy of the area is one of gener all y steep forest ed slopes, a 35 m thick se quence of thinly bedded ar gil laceous divided by streams cut ting through bed rock and allu vium to mudstone, limestone , siltstone and sandstone , conform ably cre ate con sid er able re lief. Open val leys are pres ent along overli es the Quatsino Formation (Nixon and Orr, 2007). the San Juan River and to a lesser degree along the Gordon The Port Renfrew region is under lai n mostly by rocks of the River. Juras sic Bonanza Arc, which intrude s, as well as uncon - The area is ac cessed by a net work of logging rods in vari ous formably over lies, the older units of Wrangellia. Clastic states of pres er va tion. Over all, bed rock ex po sures are best sedi m entar y rocks of the Oligocene Carmanah Group un- along acti ve logging roads and in ele vate d areas that have conform ably overli e all older units along the western coast been re cently logged. Most ar eas were ac cessed by four- of the is land. Qua ter nary gla cial till and al lu vium covers wheel drive truck, but a few rem ote moun tain tops with stream valleys. over grown logging roads could be ac cessed only by helicopter. Jurassic Bonanza Arc In the field area, rocks of the Bo nanza Arc are sep a rated from the Ju ras sic-Cre taceous Pa cific Rim Terrane to the south by the San Juan fault and from the Sicker Group to the north by the Cowichan fault. The Juras sic -aged rocks of the Keywords: ultra mafic, arc, Ju rassic, nickel, Bo nanza, geo bar- ometry Bonanza Arc in Wrangellia include , from base to top, the This publi ca tion is also avail able, free of charge, as col our dig i tal Westcoast Crys tal line Com plex (WCC), the Is land files in Adobe Ac ro bat® PDF for mat from the Geoscience BC Plutonic suite and the Bonanza Group volcanics. These website: http://www.geosciencebc.com/s/DataReleases.asp. rocks have under gone ze o lite- to lo cally greenschist-fa cies Geoscience BC Re port 2010-1 141 Fig ure 1. Geol ogy of the field area and loca tion of the field area (in set map), south ern Van cou ver Is land, Brit ish Colum bia. The cross-sec tion (lower left) is along the line of A-A’. The region was mapped at a scale of 1:20 000 and com piled to 1:50 000 scale. Also shown for ref erence are the town of Port Renfrew, Nitinat Lake, Cowichan Lake and the trace of the ma jor struc tures (San Juan fault and Cowichan up lift). Fig ure 2. Areas tra versed with all sam ple lo ca tions (red in verted tri an gles), the main line for estry roads (red lines) and ultra mafic out crop lo ca tions (black cir cles), south ern Van cou ver Is land, Brit ish Co lum bia. The box in the cen tre shows the lo ca tion of the Pearson claim block. 142 Geoscience BC Sum mary of Ac tiv i ties 2009 meta mor phism, but orig i nal ig ne ous lithological names are meta morphosed na ture of these carbon ate rocks, they are used in their description. here sug gested to repre sent fragm ents and/or faulted slices of carbon ate rocks be longing to the Perm ian Buttle Lake Westcoast Crystalline Complex Form ati on or the older Sicker Group, which are now screens or septa in the intrusive rocks of the WCC. The Westcoast Crys tal line Complex (WCC) rep re sents the deep est pre served struc tural level of the Bo nanza Arc and is Ultramafic Rocks dom inantl y quartz diorite and gabbro with vary ing amounts of hornblende, bi o tite, orthopyroxene and The larg est con cen tra tions of ultra mafic rocks are pres ent clinopyroxene with ac ces sory mag ne tite, il men ite, py rite along the Granit e Creek mainli ne and at the top of Fairy and pyrrhotite. Weakly con cordant diabase bodies are mountai n (Figure 2). Other bodies are recog nize d west of found local ly in the WCC in the field area southwest of the the Griersen main line in the Twin Lakes area. De tails of Gordon River. Grain sizes vary from fine grained to pegma- their geochem istry , petrol ogy and ori gin can be found else- toid and are het er o ge neous over a scale of metres. where (Larocque, 2008; Larocque and Canil, 2009) Disti nct units of light grey im pure marble 3–10 m thick oc- Ultra mafic rocks oc cur as dis crete bod ies within the WCC cur in the diorite. Some mar ble units can be traced for a few diorite, ranging in size from a metre to several tens of kilo metres along an ori en ta tion par al lel to re gional strike. metres. Al though their borders are often obscure d by over - Mar ble out crops found in the east ern part of the field area bur den, there is some lat eral con ti nu ity or con cen tra tion of are more ir reg u lar in out crop pat tern. Mi nor mag ne tite-rich the ultra mafic bodies over distanc es of up to 1 km. Contac t skarn bod ies, with vari ably de vel oped di op side-gar net and re la tion ships be tween the ultra mafic bod ies and the WCC mag ne tite-chal co py rite-py rite as sem blages, are found at diorite are quite variable . Smaller bodies , which tend to be the contac t of diorite or diabase with the marble. Due to the more ol ivine rich, have either abrupt, undeformed contac ts Fig ure 3.