Geological setting of the Hedley gold skarn camp with specific reference to the French mine, south-central British Columbia
Creators: name: Dawson, Garnet Linn
Publisher: name: University of British Columbia
Year: 2009

description: The Hedley gold skarn camp in south-central British Columbia is the second largest gold producer in the province. From the period 1902 to 1955 about 51 million grams (1.6 million ounces) of gold were produced from four gold bearing skarn deposits; over 95% of this came from the Nickel Plate and Mascot mines that mined underground a large skarn deposit centered on Nickel Plate Mountain. The Nickel Plate deposit was reopened in 1987 as a large tonnage, low grade open pit deposit; from 1987 to the end of 1993, 18.6 million grams (0.54 million ounces) of gold were recovered from 7 236 430 tonnes milled. The French mine produced 1.36 million grams of gold from 69 508 tonnes of ore during the periods 1950 to 1955, 1957 to 1961, and in 1983. The camp is underlain predominantly by the sedimentary facies of the Late Triassic Nicola Group that unconformably overlies more deformed oceanic rocks of the middle to late Paleozoic Apex Mountain complex. The sedimentary facies of the Nicola Group is subdivided into four sedimentary and one volcaniclastic formation that were deposited in a north trending, westward deepening, fault controlled basin along the eastern margin of the main Nicola arc in Quesnellia. They are represented by: (i) siltstones and thick limestones as the shallow water Hedley formation, (ii) siltstones and thin limestones as the intermediate depth Chuchuwayha formation, and (iii) argillite and rare limestone as the deeper water Stemwinder formation. Collapse of this basin is marked by deposition of the Copperfield breccia, which separates the Hedley, Chuchuwayha and Stemwinder formations from the overlying volcaniclastics of the Whistle formation. The Copperfield breccia is a limestone breccia that represents a catastrophic massive gravity slide deposit derived from uplifted and faulted reef material with a provenance to the east. The overlying Whistle formation forms an extensive unit that grades from thinly laminated tuffaceous siltstones at its base to massive alkalic and subalkalic intermediate to mafic ash. Three periods of calcalkaline intrusive activity and associated mineralization recognized are: (i) Late Triassic Hedley intrusions (gold skarn), (ii) Early Jurassic Bromley batholith - Mount Riordan stock (minor W-Cu and industrial garnet skarns), and (iii) Middle Jurassic Cahill Creek pluton (minor W-Mo porphyry and skarn), Lookout Ridge pluton and rhyolite porphyry dykes. Hedley intrusions form a texturally diverse suite of intermediate to mafic calcalkaline dykes, sills and stocks that are spatially and temporally associated with gold skarn mineralization. Features such as wavy sill contacts, destruction of sedimentary structures, peperite, quench textures, sedimentary dykes, and preliminary U-Pb zircon dates suggest that they preferentially intruded unconsolidated to poorly consolidated siltstone between beds of lithilled limestone in the Hedley formation. Implications of this synsedimentaiy sill interpretation are: (i) contemporaneous sedimentation and intrusive volcanism, (ii) an extensional setting, and (iii) a shallow depth of intrusion and associated skarn formation. In addition, this interpretation helps to explain and integrate the lithologic, stratigraphic and structural controls to gold skarn mineralization in the camp. At the French mine gold-arsenopyrite-telluride mineralization is associated with skarn zones on both sides of limestone - biotitic aphyric intrusion contacts. The skarn displays consistent mineralogical zoning from (i) biotitic aphyric intrusion, (ii) orthoclase (endoskarn), (iii) Mg-rich clinopyroxene (endoskarn), (iv) Fe-rich clinopyroxene (endoskarn and exoskarn spanning the aphyric - limestone contact) and (vi) Fe-rich garnet (exoskarn). Quartz, calcite, vesuvianite, scapolite, arsenopyrite, Cu and Fe suiphides, tellurides and associated gold occur in microfractures and vugs between the iron-rich garnet ± pyroxene skarn. This mineralogical zoning is complicated locally by crosscutting hydrothermal overprinting. Late stage, retrograde alteration is limited, and is represented by minor replacement of garnet and clinopyroxene by chlorite ± actinolite ± titanite ± epidote.