| Summary: | 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. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
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