The Keystone mine is located on the west side of the Coldwater River, approximately 6 kilometres north of the Coquihalla Lakes. Base and precious metal mineralization were originally discovered at this locality in the early 1900's and underground development had taken place by 1936. The only production from the mine occurred in 1955, when 81 tonnes of ore were shipped for processing (Minister of Mines Annual Report 1955, page A48).
The geology of the upper Coldwater River area is characterized by Late Triassic Nicola Group metavolcanic and metasedimentary rocks derived from the emplacement of plutonic rocks assigned to the Late Jurassic and Early Cretaceous Eagle Plutonic Complex to the west. Along the Coldwater River, the Nicola Group comprises amphibolite, foliated diorite, mylonite and chlorite schist with minor marble in contact with gneissic granodiorite. A dioritic stock of Early Tertiary age has intruded these plutonic rocks west of this contact.
The Keystone mine is situated near the centre of the Early Tertiary quartz diorite (Keystone) stock, which has been estimated to be approximately 1300 by 2200 metres. The southern half of the stock is brecciated, as is the older granodiorite adjacent to it. The brecciation has been attributed to the subsequent emplacement of small stocks and dykes of porphyritic rhyolite. This event was apparently accompanied by pervasive alteration and the introduction of metallic mineralization.
In the area of the mine, rhodochrosite, sphalerite and hematite with galena and minor chalcopyrite and magnetite occur with or without quartz(-carbonate) as veins, veinlets and stringers in shears and brecciated zones. Erratically distributed gold and silver values reportedly occur with the quartz.
The dominant structure hosting this mineralization is a steeply dipping, north-northeast striking shear zone expressed, on the surface, by conspicuous rock alteration and manganese staining. This shear extends from the Stonewall adit (092HNW034)/Julie zone (092HNW023) on Mine Creek northeast past the Keystone mine, and attains widths in excess of 150 metres (diamond-drill hole 80-W2).
Three "vein zones" are known to be hosted along the structure. The No. 1 Vein zone, developed from two levels at the Keystone Mine, comprises quartz, calcite and rhodochrosite with pyrite, sphalerite, galena and rare tetrahedrite. It averages from 5 to 10 centimetres wide, but pinches and swells from a one-centimetre wide pyrite-gouge clay zone to a 30-centimetre wide massive pyrite-quartz vein with minor base metals. It also splits and branches where exposed on the lower level of the mine. A total strike length of approximately 275 metres has been explored, both in underground workings and drill holes.
Detailed sampling of the vein has produced erratic precious metal values. Silver values have ranged from better than 35 to over 754 grams per tonne. Gold values have been generally low. The best assay from the upper level has been one of 29.5 grams per tonne gold and 576.0 grams per tonne silver across a "very narrow" hangingwall vein (Assessment Report 19139). Values up to 5.07 grams per tonne gold and 275.7 grams per tonne silver have been obtained from samples from the upper level.
Two mineralized intersections averaging one metre in width were encountered in drill holes northeast of and below the mine workings, but precious metal values were very low. A second vein, which assayed 23.25 grams per tonne gold and 41.14 grams per ton silver across a 3.05-metre intercept, was discovered at depth further to the northeast (Assessment Report 19139). Veining intersected in a follow-up hole, however, did not contain significant precious metal values.