Michael D'Angelo MSc thesis abstract

Thesis Title: 
Geochemistry, petrography and mineral chemistry of the Guichon Creek and Nicola batholiths, southcentral British Columbia

The Guichon Creek Batholith (GCB) is a Late Triassic calc-alkaline intrusive complex approximately 60km x 35km in size and located approximately 54km southwest of Kamloops, British Columbia, Canada. The Guichon Creek Batholith forms part of the Quesnel Terrane, comprised of stacked volcanic-arc assemblages and associated sedimentary units, which in addition to the Stikine and Cache Creek terranes, comprise the Intermontane Belt of central British Columbia. The Guichon Creek Batholith intrudes the western volcanic belt of the Upper Triassic Nicola Group and is partially overlain by volcanic and sedimentary rocks belonging to the Kamloops Group and are of Jurassic age and younger.

The Guichon Creek Batholith is host to the Highland Valley Cu-Mo porphyry system comprised of at least five known significant porphyry centers (Valley, Lornex, Highmont, Bethlehem and J.A.). It is one of two mineralized calc-alkaline batholiths that form a Late Triassic belt parallel to younger mineralized, alkaline and calc-alkaline belts to the east. Most of this belt is buried under a thin veneer of Jurassic and younger cover. Understanding the petrogenesis and composition of the Guichon Creek Batholith is important in developing exploration strategies to locate similar buried porphyry deposits along this belt.

The Guichon Creek Batholith consists of six concentrically zoned intrusive facies ranging from diorite in the core to granodiorite in the margin. Field relationships indicate that the facies young towards the centre of the batholith although observable contacts are rare in outcrop. The six intrusive facies are from the margin inward: 1) Border facies; Highland Valley facies [subdivided into the 2) Guichon sub-facies; and 3) Chataway sub-facies]; 4) Bethlehem facies; 5) Skeena facies; and 6) Bethsaida facies. 

The marginal Border facies is the most heterogeneous facies and contains numerous autoliths near the contact between Guichon Creek Batholith and the host Nicola Group basalts which are brecciated and intruded by the Border facies. Rocks belonging to the Border facies are olivine-bearing leuco-gabbros to diorites with equigranular, phaneritic textures. 

The Highland Valley facies is comprised of the Guichon and Chataway sub-facies. The composition of both sub-facies is similar, varying from quartz monzodiorite to granodiorite compositions and the Guichon sub-facies is most prominent to the northeast of the batholith whereas the Chataway sub-facies is most prominent to the southeast. The key difference between both sub-facies is the presence of conspicuous pink K-feldspar in the Guichon sub-facies and white K-feldspar in the Chataway sub-facies. Optically continuous, interstitial, sub-ophitic amphibole, K-feldspar and quartz are characteristic of all rocks belonging to the Highland Valley facies. 

The three youngest facies are all similar in composition (granodiorite) and show a progressive increase in quartz and K-feldspar, decrease in total mafic minerals (with an increase in biotite relative to hornblende) and nearly constant plagioclase contents progressing from the Bethlehem facies to the Bethsaida facies. The Bethlehem facies is characterized by ophitic amphibole phenocrysts that poikilitically enclose smaller plagioclase chadacrysts, whereas the Bethsaida facies is characterized by biotite and amoeboid quartz phenocrysts. The Skeena facies is texturally and compositionally intermediate to the Bethlehem and Bethsaida facies, lacking both amphibole and biotite phenocrysts but containing finer-grained amoeboid quartz phenocrysts than those present in the Bethsaida facies. 

Weak chlorite-epidote-sericite alteration is ubiquitous across the batholith, and even least altered samples typically contain biotite and amphibole crystals that have been affected by <5 to 60% alteration to chlorite and epidote and plagioclase that has been weakly to moderately sericitized. Alteration is most prominent in samples of the Bethsaida facies, particularly those closest to the mineralized porphyry centers. 

The Guichon Creek Batholith is a magnesian, calcic to calc-alkalic (MALI = -5 to 7.3) and metaluminous to weakly peraluminous (ASI = 0.77 to 1.28; AI = 0.02 to 0.13) batholith. Major and trace element geochemistry are consistent with fractional crystallization of the predominant minerals observed petrographically and plots of Zr vs. molar Al/Ti and Al2O3 vs. TiO2 are effective at discriminating between the Border, Highland Valley and Bethlehem-Skeena-Bethsaida facies. 

The petrographic and geochemical characteristics of the Guichon Creek Batholith in addition to cross-cutting relationships between facies and recent U-Pb ages suggest that the batholith was emplaced as at least two, but possibly three different magma pulses. 

High Sr/Y, low La/Yb, fractionated LREE and HREE relative to MREE and concave primitive-mantle-normalized multi-element diagrams indicate fractional crystallization of hornblende and clinopyroxene in a deep crustal magma reservoir. These geochemical signatures also preclude a significant role for garnet in magma genesis, either as a restite phase left behind by adakite melts of eclogite-facies subducted slabs or by assimilation and contamination by garnetiferous metamorphic rocks in the deep crust. This is consistent with Sm-Nd systematics (Nd(T) = +6.7 to +7.5) which are consistent with <2% contamination of primitive mantle melts by partial melts of subducted sediment, although 87Sr/86Sri values of 0.703367 to 0.703493 suggest minor contamination of the Border and Guichon facies by radiogenic Sr derived from Nicola Group limestones or contaminated Nicola Group basalts. 

Plots of Sr/Y, Al2O3 and V/Sc vs. SiO2 suggest that the parent magmas for the Guichon Creek Batholith were hydrous and oxidized, two criteria key for the production of porphyry deposits. Amphibole chemistry indicates temperature, pressure and ƒO2 conditions of crystallization were 712 ± 23.5 to 846 ± 23.5°C, 2.5 ± 0.3 to 0.9 ± 0.1 kbar (equivalent to 7.4 to 2.6km depth) and ΔNNO = 0.03 to 1.81, respectively. Pressure estimates are consistent with gentle tilting to the northeast, and imply that the Valley deposit may have formed at pressures where a single-phase supercritical fluid would have been stable, possibly leading to mineralization styles and an alteration footprint that are atypical of porphyry environments.