The mineralized veins of the Grey Rock occurrence are hosted by the Mississippian to Jurassic Bridge River Complex (Group) metasediments-greywacke, hornfels, minor conglomerates, recrystallized chert breccia and silicified limestone and volcanics. The metasediments are complexly intruded by dykes of granodiorite, aplite, granite, quartz diorite and quartz latite; extensions of the Cretaceous to Tertiary Bendor batholith are found approximately 300 metres to the south. Quartz filled parallel fissures transect both metasediments and dyke rocks. The mineralized veins are found mainly in the metagreywacke. The quartz infillings in the dykes are generally barren.

There are three main veins and up to six in a parallel system, striking northeast and dipping 50 to 65 degrees southeast. The vein-fissures vary in width from several centimetres to 2 metres, and have numerous faulted minor offshoots. The main (#1) vein is continuous downdip for at least 123 metres, averaging 1 metre in width and is offset 35 metres by fractures. The mineralization occurs in lenticular masses and is constant throughout the length of the vein. Stibnite occurs as disseminations and streaks in the quartz gangue and as massive layers on the vein walls. Smaller amounts of pyrite, grey copper with associated silver, sphalerite, galena, arsenopyrite, tetrahedrite and fuchsite are found in the main #1 vein; #2 and #3 veins contain only discontinuous lenses of high grade stibnite.

Proven ore reserves are 17,780 tonnes of 4.0 per cent antimony, 2.4 per cent lead, and 342.8 grams per tonne silver. Combined with probable and possible reserves, totals are 70,488 tonnes of 3 per cent antimony, 2.1 per cent lead and 342.8 grams per tonne silver. Assay results are in grams per tonne: 0.34 gold, 40.1 silver, 8.0 per cent antimony, 0.15 per cent arsenic and trace iron (Assessment Report 837). Assays for #1 vein are reported as 1557 grams per tonne silver, 3.9 per cent lead and 10.7 per cent antimony over 1.1 metres by 30.5 metres strike length (Minister of Mines Annual Report 1954). In 1951, 3765 kilograms of antimony were recovered from 7.3 tonnes of sorted ore. There are two adits (6500 feet and 6800 feet) with "several hundred feet" of drifting on #1 vein.

Earlier prospects (Commerce, Stewart, B & M, Birthday) may have been later incorporated into Gray Rock mine; all are located near the head of Truax and Fergusson creeks (listed under National Mineral Inventory No. 92J15 Sb7).