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Ministry of Energy Mines and Responsible for Core Review

Geology and Industrial Minerals in the Gang Ranch Area
(NTS 092O/8W,9W)

BCMEMPR Open File 1989-27

by K. Green

View Open File (PDF, 1.91 MB)


Open File 1989-27 presents the geology and industrial mineral occurrences in the Gang Ranch Area (92O/8, 9) in southern British Columbia. It is part of an ongoing investigation of industrial minerals in Tertiary basins, and was funded by the Canada/British Columbia Mineral Development Agreement. It comprises a 1:50 000-scale geological map of the area of Tertiary volcanic and sedimentary rocks west of the Fraser River fault system between Gaspard Creek and Lower Cabin Creek.

Accompanying notes briefly describe the Tertiary stratigraphy, eight zeolite, three bentonite and two perlite occurrences. Data on Mercury, Calcium, Sodium and Potassium cation exchange capacity and radiometric dating are provided.

This open file presents current data from an ongoing investigation of industrial minerals in Tertiary basins funded by the Canada/British Columbia Mineral Development Agreement.

In the vicinity of Gang Ranch, approximately 80 to110 kilometres at right-lateral, strike-slip movement on the Fraser fault has juxtaposed a western assemblage of Lower Cretaceous and Eocene volcanic, volcaniclastic and sedimentary rocks against Pennsylvanian to Triassic marine sediments and mafic volcanic rocks of the Cache Creek complex. This package of Lower Cretaceous and Eocene strata is bounded to the southwest by the Hungry Valley Thrust that places Jurassic to Cretaceous marine clastics of the Jackass Mountain Group on top of Eocene volcanic rocks.

East of the Fraser fault, siliceous volcanic tuffs, black and green ribbon chert and sheared, siliceous black argillite comprise the major part of the Cache Creek complex. In the southeast corner of the map area, a 150-metre-thick lens of light grey, recrystallized limestone is interbedded with black argillite and chert. Along the northern side at Gaspard Creek, mafic volcanic flows and gabbro underlie green chert and argillite.

West of the Fraser fault, approximately 700 metres of maroon, green-brown, and grey, crystal-phyric to aphanitic andesite flows and local hornblende porphyry flows of the Lower Cretaceous Spences Bridge Group are the oldest rocks exposed. A subgreenschist grade metamorphic assemblage of pumpellyite, zeolite, calcite and chlorite fills fractures and amygdules throughout the Spences Bridge Group volcanic package. This alteration assemblage provides a field criterion for distinguishing between Cretaceous and overlying Eocene volcanic flows.

Approximately 2000 metres of Eocene volcanic and volcaniclastic rocks overlie the Spences Bridge Group volcanic rocks on a gently west-dipping surface. The base of the Eocene package consists of red, yellow-beige and brown, andesitic to dacitic volcaniclastics and interlayered volcanic flows. In the western portion of the map-area, these volcanic rocks are overlain by a thick accumulation of pink, grey and white rhyolite flows, quartz-rich welded crystal tuffs and lithic tuffs. Towards the east, the coarse-grained, rhyolite tuff grades laterally into layers of white and green rhyolite crystal and ash tuffs that are interlayered with andesite, dacite and basalt volcaniclastics and flows. In the southwestern corner of the map sheet a grey-green feldspar porphyry flow overlies rhyolite lithic and crystal tuff. A sequence of well-bedded sediments containing layers of bentonitic ash and thin coal seams overlies the volcanic strata at a number of localities immediately west of the Fraser fault.

Flat-lying, erosional remnants of Miocene to Pleistocene plateau basalts and minor Miocene fluviatile sediments, in places overlain by a cream-coloured, rhyolitic ash layer, occur throughout the map area. In places they directly overlie the trace of the Fraser fault.

Volcanic rocks of the Spences Bridge Group are exposed in the footwall of a major northwest-trending normal fault, informally referred to as the Empire Valley fault. The Empire Valley fault appears to merge with the Fraser fault, but most likely postdates dextral, transcurrent motion. East-dipping reverse faults and northwest-trending upright folds developed in Eocene sediments and volcaniclastics west of the Fraser fault. Latest movement on the Fraser fault was dip-slip.

Eocene rhyolite crystal and ash tuffs are extensively altered to zeolite and bentonitic clay. Montmorillonite, illite and heulandite group zeolites have been identified. Perlite occurs at two locations in the southern part of the map-area. One of these is the Frenier perlite deposit. Fragments of diatomite were found in Miocene rhyolite ash west of the Gang Ranch.


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