The Gisby group of claims is located south of the confluence of the Nahatlatch and Fraser rivers, approximately 14 kilometres north-northwest of Boston Bar. The main showings are at the south abutment of the railway bridge across the Nahatlatch River, on both sides of the railway, 180 metres upstream from the railway and another 45 metres upstream where the talc is exposed in a 4.5-metre bank and is highly sheared.
Before 1920, the property was worked for silica and shipments were made to Vancouver. An adit was driven on the Gisby claim (Lot 1078), apparently in search of gold, which intersected a talc body. About 91 tonnes of talc were extracted from the workings up to 1923, at an average rate of slightly greater than 20 dollars per tonne. Other adits reportedly occur on the Salmon River claim (Lot 1077), but recent visits to the area found no such workings. They were probably covered during road construction (which has resulted in some new exposures).
The best, and most abundant, talc is found on the Laura and Salmon River claims (Lots 1080 and 1077) immediately south of the Nahatlatch River.
The host rocks are mixed metasediments of the Permian to Middle Jurassic Bridge River Complex (Group). Here, lithologies include thinly bedded slaty argillite with minor quartzite interlayers, greywacke, calcareous bands and chlorite-carbonate schists. The sediments are highly sheared, contorted and quartz veined. Pods of ultramafic rock, mainly serpentinite, are common in the area.
The Gisby adit was driven perpendicular to the enclosing slates, which dip nearly vertically. At 45 metres, a 1.5 to 2.4-metre wide talc bed was exposed, bordered by 15 metres of talcose slate. More magnesia-rich talcose rock with much quartz and calcite veining occurs on either side of the talc bed.
A talc bed is exposed southeast of the tunnel, adjacent to the largest of two northwest trending diorite bodies. The talc in this area contains a large percentage of carbonate. The host is sheared talc-carbonate schist mixed with pods of quartzite and argillite. In the tunnel however, the talc is reportedly much purer. It is light to dark olive green and translucent. Occasional quartz impurities are present as thin layers, but the talc grinds to a soft powder with no discernable grit. The talc breaks into irregular pieces with slickensided surfaces. An analysis of this material was made by the Mines Branch in 1926 with the following results (in per cent) (Geological Survey of Canada, Economic Geology Series 2):
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Silica 59.88
Ferrous oxide 4.54
Ferric oxide nil
Alumina 1.18
Lime 0.10
Magnesia 29.51
CO2 0.02
H2O > 105 C 4.73
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The talc showings along the Nahatlatch River have an aggregate length of approximately 150 metres. The talc is light green to pearly grey, massive to lightly laminated, with granular quartz and minor sulphide impurities. All the showings contain abundant carbonate, most of which is iron-rich magnesite. One bed, described as the "uppermost" showing, is distinctive in that it contains bright green chrome mica (fuchsite) and may be part of a listwanite assemblage (Geological Survey of Canada, Economic Geology Series 2).
