An anomalous occurrence of ultramafic rocks associated with high Ni, Cu, Cr and PGE anomalies in soils and streams is documented near the western edge of Wrangellia terrane, at its southern contact with the Jura-Cretaceous Pacific Rim terrane on southwestern Vancouver Island (NTS092C). Rocks of this portion of Wrangellia belong to the Paleozoic- Jurassic West Coast Crystalline Complex, Jurassic Island Plutonic Suite and Triassic Karmutsen metabasalts. The area has only been mapped at 1:100,000 scale by Mueller (1977). Specific goals of this project are to undertake geological mapping at 1:10,000 scale of an area surrounding the anomalous ultramafic rocks near Port Renfrew, and to follow-up with geochemical and petrological study of the ultramafic rocks explaing processes for their occurrence. Stream sediment sampling of key streams draining the region of anomalous ultramafic rocks will be used to identify possible anomalies and host minerals for Ni, Cu or PGE, and to determine the nature and/or extent of Ni, Cu and PGE mineralization. Geochronological information on pertinent rock units will put the area in a regional and stratigraphic context with that known for other parts of Wrangellia. Research at this detailed scale will be brought into the broader context to provide a better geological database outlining the process, age, tectonic setting and possible metallogenic significance for PGE, Ni, and Cu mineralization within the Wrangellia terrane on Vancouver Island.

Posters and Presentations

2007 : Ultramafic Occurrences in the Bonanza Arc, near Port Renfrew, Southwestern Vancouver Island
- Mineral Exploration Roundup Poster (pdf, 2MB)

Technical Articles

2010: "Ultramafic intrusions, detailed geology and geobarometry of the Jurassic Bonanza Arc in the Port Renfrew region, southern Vancouver Island (NTS 092C)"
- Summary of Activities 2009, Report 2010-1 p.141-148 (pdf, 3.6 MB)

2007: "Ultramafic Rock Occurrences in the Jurassic Bonanza Arc near Port Renfrew, Southern Vancouver Island"
- Geological Fieldwork 2006, Paper 2007-1 p.319-324 (pdf, 2.81MB)

Final Deliverables

M.Sc. Thesis - J.P. Laroque

The role of amphibole in the evolution of arc magmas and crust: the case from the Jurassic Bonanza arc section, Vancouver Island, Canada - University of Victoria, 2008 - Available digitally through University of Victoria (pdf, 12.6MB)

Abstract: Exposed on Vancouver Island, British Columbia, the Jurassic Bonanza arc is believed to represent the southerly continuation of the Talkeetna arc. Small bodies of mafic and ultramafic cumulates within deeper plutonic levels of the arc constrain the fractionation pathways leading from high-MgO basalt to andesite-dacite compositions. The removal of amphibole from the most primitive non-cumulate compositions controls the compositions of mafic plutons and volcanics until the onset of plagioclase crystallization. This removal is accomplished by the intercumulus crystallization of large amphibole oikocrysts in primitive olivine hornblendite cumulates. Experimental hornblende compositions that crystallize from high-MgO basalts similar to primitive basalts from the Bonanza arc show a good correlation between octahedral Al in hornblende and pressure, and provide a means of estimating crystallization pressures during differentiation of primitive arc basalt. Application of an empirical barometer derived from experimental amphibole data (P = Al(6)/0.056 -- 0.143; r² = 0.923) to natural hornblendes from this study suggests that crystallization of primitive basalts took place at 470-880 MPa. Two-pyroxene thermometry gives a result of 1058 +/- 91 ºC for the only olivine hornblendite sample with both pyroxenes. Lever rule calculations require the removal of 30-45 % hornblende from the most primitive basalt compositions to generate basaltic andesite, and a further 48% crystallization of hornblende gabbro to generate dacitic compositions. Hornblende removal is more efficient at generating intermediate compositions than anhydrous gabbroic fractionating assemblages, which require up to 70% crystallization to reach basaltic andesite from similar starting compositions. There are no magmatic analogues to bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic cumulates will push the bulk arc composition to high-Mg# andesite. Garnet removal appears to be a key factor in producing bulk continental crust.