Diagnostic Features of the Rocks and Minerals Peripheral to the Highland Valley Copper District, British Columbia, Canada: Implications for the genesis of porphyry Cu systems and their footprints

• Author / Creator

  Byrne, Kevin

• The Highland Valley Copper (HVC) porphyry Cu (±Mo) district is Canada’s largest and longest operating mine. Globally, exploration expenditure has been increasing, yet very few new resources have been discovered, despite the anticipated Cu demand for the future. Hosted in the Triassic Guichon Creek batholith (GCB) in the Cordillera of British Columbia, the HVC district is an excellent site to study the distal expression of porphyry Cu systems, integrate disparate data sets, and develop new exploration tools and models to aid the discovery of porphyry Cu deposits for the next generation. This study aimed to determine the diagnostic features of the rocks and minerals peripheral to the prolific HVC porphyry Cu centers– the footprint. Specifically, how, to what degree, and to what distance are the rocks outside of the porphyry centers affected by hydrothermal processes? Can vein mineral chemistry be used to vector to Cu mineralization and identify the presence of a porphyry system? Was there an external fluid entrained in the magmatic-hydrothermal systems in the porphyry Cu centers, and what was its origin? Whole-rock lithogeochemistry, mineral geochemistry, and a combination of stable (H, O, C and B) and radiogenic (Sr) isotope techniques are used to address these questions. The HVC district is hosted in calc-alkalic plutonic rocks of the GCB. Well-developed, feldspar-destructive, white-mica–chlorite alteration is indicated by (2Ca+Na+K) /Al values 0.75 wt. % calcite and have measured 13Ctotal values > -10 ‰. Up to 6 km away from Cu mineralization, propylitic, sodic-calcic, and a few least-altered rocks have 13Ctotal values between -11 and -7 ‰ that contrast with lower whole-rock 13Ctotal values (-25 and -21 ‰) in more distal propylitic alteration and fresh rocks. Calcite is interpreted to have formed via CO2 degassing from a late-stage magmatic volatile phase that contained ~1.6 mol. % CO2 in H2O.Estimates of δ18Ofluid and δDfluid values derived from epidote and tourmaline in sodic-calcic domains range from 3.0 to 8.2 ‰ and -40.6 to -3.9 ‰, respectively, which contrasts with the dominantly magmatic fluid values from Cu-mineralized potassic and white-mica–chlorite alteration facies. Tourmaline δ11Bfluid values range from 4.2 to 18.3 ‰. The highest δ11Bfluid values occur in albite-altered rocks and indicate a fluid source rich in 11B, such as seawater or seawater-derived formation water in country rocks. Sodic-calcic alteration is interpreted to be caused by the inflow and heating of seawater-derived fluid along regional-scale fracture-sets at low water–rock ratio (~0.01–0.1), and locally mixed with magmatic fluids in and adjacent to the porphyry centers. Epidote veins from the fertile HVC porphyry district can be discriminated from the epidote occurring in non-prospective geological environments by a combination of compositional features: large positive Eu anomaly, low sum HREE concentration, moderate negative REEN slope, and principally by analyses of >30 ppm As and Ge, and >10 ppm Sb. A coherent domain of epidote veins with a Ge anomaly (>30 ppm) forms a halo around the porphyry centers for ~ 5 km. Epidote veins with the highest As and Sb values (~ >20 ppm) occur within ~4 km of the porphyry Cu centers. The elements Mn (>6000 ppm) and Zn (> 25 ppm) are most enriched in epidote between 1.5 and 4 km of the porphyry centers, whereas Pb enrichment (~>120 ppm) occurs between ~ 4–6 km away. Prehnite veinlets are enriched in Mn (>2500 ppm) between 3 and 6 km away from the porphyry centers, and enriched in Pb (~>130 ppm) within a window of ~ 5.5–8 km. The pathfinder metals in epidote are interpreted to be primarily sourced from outflowing H2S-depleted, cooling, magmatic volatile phase emanating from the porphyry Cu centers or from the fringes of the subjacent cupola.Collectively, these results demonstrate methodologies and approaches that will be useful in elucidating hydrothermal footprints in porphyry Cu systems globally.

• Subjects / Keywords

  • Magmatic-hydrothermal
  • Porphyry
  • Footprint
  • Lithogeochemistry
  • Quesnel
  • Isotopes
  • Alteration
  • Na-Ca
  • Highland Valley Copper
  • Propylitic
  • Cu-Mo
  • Sodic-calcic

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-g57q-zm25

• License

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