Andrew Jedemann MSc thesis abstract

Thesis Title: 
Tectonomagmatic evolution and green rock mineral chemistry of Jurassic Bonanza arc rocks at Pemberton Hills, Vancouver Island, British Columbia, Canada

The Pemberton Hills study site is situated in northern Vancouver Island, British Columbia, and forms part of a northwest trending belt of porphyry deposits and prospects within the Wrangellia Terrane. The study site is underlain by late Triassic to middle Jurassic Bonanza Group volcanic rocks, with porphyry-related alteration and mineralization thought to be related to the emplacement of the Jurassic Island Plutonic Suite (181 – 141 Ma).

Whole-rock geochemistry, high precision geochronology combined with trace element and Hf isotope analysis of zircons from various intrusive units has led to an improved understanding of the magmatic evolution of plutonic rocks at Pemberton Hills. The oldest unit is a granodiorite dated at 172.34 ± 0.11 Ma. Following this is a quartz diorite with an age of 172.01 ± 0.17 Ma, which overlaps with a tonalite dyke sample dated at 171.77 ± 0.13 Ma. A second quartz diorite sample yielded the youngest reported age at 171.44 ± 0.25 Ma. These results imply that felsic and intermediate plutonism were broadly synchronous at Pemberton Hills.

Zircon trace element geochemistry reveals contrasting conditions in the felsic and intermediate plutonic magmas, despite broadly similar emplacement ages. The granodiorite and tonalite dyke samples are characterized by higher, overlapping Eu/Eu* (~0.2-0.3) and lower, overlapping Dy/Yb (~0.15-0.2) ratios, whereas the quartz diorite samples show low and increasing Eu/Eu* ratios (~0.1-0.17) as well as high and decreasing Dy/Yb ratios (0.25-0.45) over time. These results suggest that conditions remained stable throughout the emplacement history of the felsic plutonic magmas, but varied during the emplacement of the intermediate magmas.  Zircon Hf isotope compositions of the granodiorite (+11.4), tonalite dyke (+11.2), and one of the quartz diorite samples (+9.5) suggest these rocks were derived from a juvenile mantle source that experienced minimal crustal contamination. Together, zircon chemistry and Hf isotope compositions suggest that the granodiorite and tonalite dyke magmas were the most fertile, meaning that felsic plutonic rocks have the most potential to be associated with a larger and more mineralized porphyry system at Pemberton Hills.

Field mapping combined with detailed petrography have shown propylitic alteration assemblages to be present throughout most of the porphyry environment at Pemberton Hills.  Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analyses of epidote, chlorite, and pyrite have identified that major and trace element variations in these minerals vary systematically across the study site, and can be used to discriminate multiple porphyry-related alteration events at Pemberton Hills more effectively than whole-rock geochemistry.

Distal pathfinder elements displayed the most variation in epidote. Elements such as As and Sb in epidote were highest in a cluster of samples from the center of study site to the northeast of the lithocap. A small group of samples proximal to the southwestern margin of the quartz diorite pluton also displayed high As in epidote. Chlorite displayed clear systematic spatial distribution patterns, illustrated by increasing Ca and Sr concentrations in chlorite from samples moving towards the east from the southeast margin of the quartz diorite pluton. Fertility assessment plots of Sb vs. As in epidote and Zn vs. Mn in chlorite were used to discriminate spatially distinct groups of samples interpreted to relate to discrete porphyry alteration events at Pemberton Hills.

Pyrite mineral chemistry combined with pyrite element maps illustrate a complex paragenetic history at Pemberton Hills. Pyrite in samples located around the southeastern margin of the quartz diorite pluton are enriched in elements such as Au, As, Sb, Te, and Ag, whereas pyrite from samples overprinted by phyllic alteration assemblages are enriched in Mn, V, and Zn, consistent with abundant silicate micro-inclusions in these pyrites. Pyrite in samples located along the surficial margin of the lithocap are enriched in Re, likely reflecting the overprinting of fluids derived from the lithocap.

Trace element mineral chemistry of epidote, chlorite, and pyrite have proven to be effective at vectoring towards potential porphyry targets in an area with limited geologic context, and as a useful method to discriminate samples from different porphyry alteration events. The results of this study demonstrate the application of mineral chemistry as a tool for exploration in greenfield porphyry environments.