TELLURIDE ASSEMBLAGES IN A REDUCED INTRUSION-RELATED GOLD (RIRG) DEPOSIT, CLY GROUP PROSPECT, SOUTHEASTERN BRITISH COLUMBIA, CANADA

The BiTe Knoll (CLY Group) exploration target, Nelson District, B.C., comprises a set of veins and skarns located near the contact to the mid-Cretaceous Bunker Hill sill, an outlier of the Wallack Creek granite. The latter belongs to the Bayonne magmatic belt marking the boundary between Lower Jurassic rocks of the Quesnel Terrane to the NW and Paleozoic sedimentary rocks of the Kootenay Terrane. Ore minerals occur in fracture-controlled clots in m-plus wide outcropping quartz veins. The mineralization has all criteria of a Reduced Intrusion-Related Gold deposit, e.g., as in the Tintina Belt, Alaska-Yukon, including a marked Au-Bi-Te correlation. Veins in three different sets occur over a 1.5 km N-S extent on the CLY property; the economic potential as an open pittable deposit is high.

The Bi-mineral assemblage occurs as droplets consisting of native Bi, Bi-(sulfo)tellurides and bismuthinite. The (sulfo)tellurides include joséite-B (Bi₄Te₂S), joséite-A (Bi₄S₂Te), hedleyite (Bi₇Te₃), unnamed Bi₂Te, ingodite [Bi(Te,S)] and ikunolite (Bi₄S₃). We note the presence of coarse symplectites of joséite-A and -B, and exsolutions of joséite-B in Bi₂Te. Such morphologies, phase associations and the presence of mineral pairs representing eutectics at 266ºC in the systems Bi-Te or Bi-S (e.g., Bi+Bi₇Te₃ and Bi+Bi₂S₃, respectively) suggest crystallisation from Bi-Te-S melts. Native gold occurs as inclusions within such patches, or forms rims at ingodite-Bi₂Te boundaries. Gold is also present at concentrations of 0.02-55 ppm within Bi-minerals. However, the eutectics on the Bi-rich side of the system Au-Bi-Te include maldonite instead of native gold as observed at CLY. Neither maldonite nor its breakdown products (Au+Bi symplectites) are observed within the blebs. Moreover, the native gold rim between ingodite and Bi₂Te is considered to result from subsequent Au release from tellurides. A parallel can be drawn to the transition reaction: liquid+BiTe=Bi₅Te₃+Au at 374ºC in the system Au-Bi-Te. Remobilization of Au is also suggested by the irregular Au signals in the LA-ICPMS Bi-telluride analyses, indicating the presence of <µm inclusions. Even if deposited as Bi-rich melts exsolved from a magmatically-derived fluid at >266ºC, the associations were overprinted and recrystallized during a subsequent (orogenic?) event.