Bedrock Geology of the Old Fort Mountain
Map Area, North-Central BC
(NTS 93M/01)

BCMEMPR Open File 1997-10 and 1997-10a

By D. MacIntyre, I. Webster and P. Desjardins

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See Bulletin 110 for more comprehensive data.

Open File 1997-10 portrays the geology and mineral occurrences of the Old Fort Mountain area (93M/1) in north central British Columbia. The geology map is at 1:50 000 scale includes a legend, geologic cross sections, and tables of sources of information and mineral occurrences. Open File 1997-10A presents till geochemical sampling results for the same area.

The oldest rocks exposed are Upper Triassic limestone and interbedded marine sediments of the Takla Group; volcanic interlayers may also be present. Lower to Middle Jurassic Hazelton Group rocks comprise eight mappable units. The three oldest are Lower Jurassic: the Basalt member; the Telkwa Formation, which consists of air fall tuffs, andesite flows and breccia, basalt flows and related epiclastic and volcaniclastic rocks; and the Nilkitkwa Formation, which consists of feldspathic siltstone, wacke, argillite and minor pebble conglomerate and limestone. Lower to Middle Jurassic units include: mainly ash flow tuff, ignimbrite, lapilli and crystal tuff; basaltic flows, breccia and tuff; volcanic sandstone, siltstone and tuff; the Saddle Hill volcanics, which are interlayered amygdaloidal and pyroxene phyric basalt flows and breccia, tuffs, tuffaceous sandstone, ash flow tuff, and rhyolite flows. The Middle Jurassic Smithers Formation contains feldspathic sandstone, siltstone, wacke and volcanic pebble conglomerate. Above the Skeena Group, the Middle to Upper Jurassic Ashman Formation consists mainly of siltstone and sandstone. The Upper Jurassic Trout Creek Formation is mainly chert pebble conglomerate, quartzose wacke, and siltstone. Lower Cretaceous Skeena Group units are: trachytic, andesitic and basaltic andesite flows that locally have coarse bladed feldspar crystals; porphyritic andesite, basalt, lapilli tuff, volcanic breccia and tuffaceous siltstone; feldspar phyric dacite and rhyodacite to rhyolite dome complexes; and the Red Rose Formation quartzose and micaceous sandstone, siltstone, shale, and chert pebble conglomerate. The Eocene Ootsa Lake Group is represented by the Newman volcanics, which are characterized by porphyritic andesite, volcanic breccia, lahar and volcanic conglomerate.

Intrusive rocks are important in the area. The Late Triassic to Early Jurassic Topley intrusions are quartz monzonite, monzonite, megacrystic granite, and rhyolite to aplite dikes. Jurassic to Cretaceous intrusions are diorite, gabbro, and basalts dikes and stocks. Cretaceous granodiorite and quartz diorite bodies occur. The Babine intrusions include: rhyolite to dacite dikes and flow dome complexes; hornblende-augite diorite; quartz diorite and granodiorite cut by porphyry dikes; and crowded porphyry dikes and plugs that are the main hosts for porphyry mineralization.

Four major tectonic events are reflected in the region. Mid to Late Jurassic folding and uplift was followed by mid Cretaceous contraction that produced northwest trending folds and northeast directed thrusts. Crustal extension in Late Eocene time produced north trending grabens and horst, and an event as young as Miocene tilted fault blocks.

The most important mineral occurrences are gold-bearing porphyry copper deposits associated with the Eocene Babine intrusions and the Late Cretaceous Bulkley intrusions. The former producing bell mine and five major prospects are known in the map area. There is also epithermal potential with silver-lead-zinc mineralization like that at the Fireweed prospect in Skeena Group rocks.

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Till Geochemistry of the Old Fort Mountain
Map Area, North-Central BC
(NTS 93M/01)

BCMEMPR Open File 1997-10a

By V.M. Levson, S.J. Cook, J. Hobday, D.H. Huntley
E.K. O'Brien, A.J. Stumpf and G.W. Weary

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See Bulletin 110 for more comprehensive data.

Open File 1997-10A describes selected results of a till geochemical sampling program conducted in the Old Fort Mountain map area (NTS 93M/1) by the British Columbia Geological Survey as part of a survey of the entire Babine copper porphyry belt. Several new exploration targets are highlighted by multi-element geochemical anomalies and most existing mineral properties in the map region are also detected by the regional till geochemical data. In addition to reflecting known mineralization, geochemical results around some mineral properties suggest that further exploration in those areas may be warranted. New prospective areas, where recorded mineral showings are not present, include multielement anomalies with greater than 95th percentile concentrations of copper, arsenic and/or antimony in five areas: northeast of Hautéte Lake, northwest of Natowite Lake, northeast of the northern end of Hatchery Arm and near the southern tip of Nakinilerak Lake. Elevated (>95th percentile) gold and molybdenum occurs northeast of Hautéte Lake. Two areas of interest with clusters of sites with elevated arsenic, lead and zinc occur southwest of Takla Lake and several kilometres southwest of Morrison Lake. Antimony and molybdenum concentrations, above the 95th percentile, also occur in till at the latter area.