Geology of the Robb Lake Pb-Zn Deposit, Northeastern B.C.
(NTS 94B/13)

BCMEMPR Open File 2000-03

by JoAnne Nelson, Willem Zantvoort and Suzanne Paradis

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Open File 2000-03 describes the geology of the Robb Lake lead-zinc deposit (94B/13) in northeastern British Columbia. The coloured map at 1:20 000 scale includes a legend, cross section, a schematic presentation of the textural and mineralogical zonation of the deposit, photographs, a regional geologic setting map, and a summary of the property geology.

The Robb Lake Mississippi Valley-type Pb-Zn deposit (MINFILE 094B 005) is hosted by platform carbonate rocks in the Rocky Mountains of northeastern British Columbia (56°56N, 123°43W; 94B/13). The deposit, hosted by Silurian-Devonian dolostone of the Muncho-McConnell Formation, consists of a series of interconnected bedding-parallel and crosscutting breccia bodies with sparry dolomite, sphalerite, galena, pyrite, quartz, calcite, and pyrobitumen in their matrix. It is the most promising and the best-developed carbonate-hosted lead-zinc occurrence in the northern Rockies, with significant showings over an 8 square km area.

Lead-zinc mineralization was discovered near Robb Lake in 1971 by Arrow-Interamerican Corp., Barrier Reef Resources Ltd., and Ecstall Mining Ltd. Drilling between 1972 and 1975 led to the incomplete delineation of three deposits, the "lower zone", the East Webb ridge zone, and the West Webb ridge zone (at depth south of the Webb showing). Drilling was continued by Texasgulf in the summers of 1980 and 1981. The geological resource at Robb Lake is quoted as 6.5 million tonnes at 7.11 % combined lead and zinc (2.4 metre mining width, 5% cutoff grade; Consolidated Barrier Reef Resources, Rights Offering Circular, November 29, 1984). Core is stored at an airstrip at the confluence of Mississippi Creek and the Halfway River.

Most of the lead-zinc mineralization at Robb Lake occurs within the Muncho-McConnell Formation, although a few occurrences lie within a thin overlying sequence attributed to the Stone-Dunedin formations. The deposit is located next to the tectonically telescoped shelf-slope facies boundary. The strata that host Robb Lake lie in the immediate footwall of a major thrust fault, which has deep water, early Paleozoic strata in its hanging wall.

The showings are in and near the valley of Mississippi Creek. Mineralization occurs as breccias, veins and vein stockworks, with breccia mineralization the most important. The breccias form interconnected, bedding-parallel and/or crosscutting bodies, which are partly to wholly mineralized. The favorable mineralized sections form a stratabound zone occupying the upper 200 metres of the lower unit of the Muncho-McConnell Formation and the lower 130 metres of the upper unit. The base of the cliff-forming unit exerted a strong control on the location of the main stratiform breccia bodies, as is illustrated on the cross-section of the valley of Mississippi Creek. The south slope of Tennessee Mountain is more or less a dip slope, and the showings along the creek - the Lower, Canyon, and Cascade zones - are stratigraphically equivalent to those on the south side of Tennessee Mountain - the Upper, Waterfall, Camp, and North Face zones.

The Robb Lake deposit has unexplored stratabound potential in the zone bounded above by unit Silurian-Devonian Muncho-McConnell Formation cliff forming dolostone unit. Breakthrough zones of cross-cutting mineralization may be associated with zones of more intense mineralization, such as Webb Ridge and the as yet unexplored zone around Tennessee Mtn., Monocline and Bear on the south side of Tennessee Mountain. The area of the Talis zone is also of interest because of the steep faults outlined there.

Last updated June 05, 2007