Bulletin 79: The Geology and Mineralization of the Coquihalla Gold Belt
and Hozameen Fault System, Southwestern British Columbia
Table of Contents
Bulletin 79 and the accompanying maps describe the geology, gold deposits, mineral occurrences and exploration criteria adjacent to the Hozameen fault system and Coquihalla serpentine belt in southwestern British Columbia. The map area covers 170 square kilometres within parts of the Hope (92R/6), Spuzzum (92H/l I) and Boston Bar (92H/l4) map sheets.
The northerly trending Hozameen fault is associated with both the Coquihalla serpentine and Coquihalla gold belts, and separates two distinct crustal units. East of the fault are greenstones of the Early Triassic (?) Spider Peak Formation, which forms a basement for the Jurassic to Middle Eocene turbidite and successor basin deposits of the Methow-Pasayten trough. The Jurassic Ladner Group, the oldest sedimentary rocks in the trough, unconformably overlies the Spider Peak Formation and contains a locally developed basal coarse clastic unit that hosts many of the mineral occurrences in the Coquihalla gold belt, including the Carolin deposit. Recently discovered Late Jurassic Buchia fossils in the Ladner Group succession suggest the group extends beyond its formerly accepted age of Early to Middle Jurassic. West of the Hozameen fault, the Permian to Jurassic Hozameen Group is interpreted as a highly deformed, dismembered ophiolite suite comprising ultramafic rocks of the Petch Creek serpentine belt at the base, stratigraphically overlain in turn by volcanic greenstone and chert units. Volcanic greenstones on either side of the Hozameen fault are geochemically distinguishable. Those in the Spider Peak Formation represent sodic, ocean-floor, subalkaline basalts probably formed in a spreading-ridge environment, while the volcanic rocks in the Hozameen Group include both arc tholeiites and oceanic island-seamount subalkaline basalts.
Farther west, the major, northerly trending and easterly dipping Petch Creek fault separates the Hozameen Group from the Custer-Skagit gneiss. This fault is associated with the lowest recognized stratigraphic section of the Hozameen Group the Petch Creek serpentine belt, and is believed to be a northern extension of the Ross Lake fault in Washington State.
The evolution of, and relationships between, the Hozameen Group, the Spider Peak Formation and the Methow-Pasayten trough are still controversial. One possibility is that they were all deposited within a single long-lived basin that evolved from a multi-rifted, marine back-arc basin into a narrow, nonmarine trough. Alternatively, it can be argued that the contrasting geochemical signatures of the volcanics in the Spider Peak Formation and the Hozameen Group suggest they were not closely associated prior to Middle Jurassic time.
The relationship between the Hozameen Group and the Custer-Skagit gneiss to the west is also uncertain, primarily because of transcurrent and vertical movements along the Petch Creek fault. Closure of the Methow-Pasayten trough during Cretaceous to Middle Eocene time resulted in easterly overthrusting of the Hozameen Group onto the Methow-Pasayten strata along a westerly dipping thrust plane that was a precursor to the Hozameen fault. Up thrust ultramafic material from beneath the Spider Peak Formation now forms the Coquihalla serpentine belt. Both the thrust and ultramafic belt were later tilted into their present subvertical attitudes. Together the Coquihalla serpentine belt and Spider Peak Formation represent steeply inclined or overturned basement to the Methow-Pasayten trough.
The Petch Creek and Coquihalla serpentine belts are dissimilar and unrelated. The Petch Creek belt mainly comprises olivine-bearing serpentinite and is not associated with gold or listwanites. The much wider and extensive Coquihalla serpentine belt is an ophiolite that is spatially associated with gold mineralization; it includes early serpentinite, later gabbroic intrusions and minor tectonic slices of listwanite. Textures suggest both serpentine belts were derived from ultramafic cumulates.
A felsic dyke swarm, possibly related to the Middle Eocene Needle Peak pluton, intrudes the rocks of the Methow-Pasayten trough, but is absent in the Hozameen Group. It is postulated that this is due to later offset by regional dextral strike-slip movement along the Hozameen fault.
The Coquihalla gold belt shows similarities in its geological setting, mineralogy and alteration assemblages to the Bridge River camp of British Columbia and the Mother Lode district of California. It comprises five past-producing mines (Carolin, Emancipation, Aurum, Pipestem and the Ward mine) as well as at least 25 minor gold occurrences. Total production from the belt was 1473 kilograms of gold from just over 800 000 tonnes of ore mined, although over 90 per cent of this came from the Carolin mine. Three forms of epigenetic mesothermal gold-bearing mineralization are recognized:
(I) Fracture-related quartz-carbonate vein systems that host the majority of the minor occurrences as well as the mineralization at the Emancipation and Pipestem mines.
(2) Sulphide-rich albite-quartz-carbonate-gold mineralization within folded sedimentary rocks in the basal portion of the Ladner Group. The Carolin deposit is representative of this type.
(3) Native gold hosted in talcose shears adjacent to the faulted eastern margin of the Coquihalla serpentine belt. The only documented example of this type is the Aurum mine.
The source and age of the gold mineralization in the belt is unknown. The Hozameen fault probably played an important role as a conduit for ore-forming fluids; most of the occurrences are hosted by the Ladner Group and lie close to the Hozameen fault. However some gold mineralization is hosted by the Spider Peak Formation or is associated with a suite of small sodic felsic intrusions of uncertain age that cut the Ladner Group. Significant spatial relationships exist between gold mineralization, the Hozameen fault, ultramafic rocks of the Coquihalla serpentine belt and the basal Ladner Group unconformity. Over 99 per cent of the gold production from the belt took place to the east and within 200 metres of the Hozameen fault, the serpentinite and the unconformity.
Limited isotopic and fluid inclusion data suggest the ore-forming fluids in the Coquihalla gold belt were highly evolved meteoric waters that were greatly enriched in 18O during deep circulation. Mineralization is believed to have occurred at temperatures of 350 to 3800C at pressures of 750 to 1250 bars, and at depths of 1 to 3 kilometres (Nesbitt et al., 1986).
There is potential for the discovery of more auriferous mineralization in the Coquihalla gold belt, particularly deposits of the Carolin type. Other mineral potential in the district includes volcanogenic massive sulphide mineralization in the Hozameen Group and base metal and gold-bearing systems associated with the intrusion of the Needle Peak pluton. The reported occurrence of placer platinum in Sowaqua Creek raises intriguing possibilities that the Coquihalla serpentine belt represents an exploration target for platinum-group elements.
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