"Rock of the Month # 299, posted for May 2026" ---

The Robb Lake deposit

in British Columbia is an example of the carbonate-hosted Mississippi-Valley type (MVT) class of lead-zinc deposits. It is a stratabound deposit with epigenetic sphalerite and galena, hosted exclusively within local brecciated portions of the succession (Manns, 1982). Robb Lake is near the headwaters of the Halfway River in the Rocky Mountains of northeast B.C. Three companies staked the initial claims in late 1971. Mapping and core logging in 1972-1975 indicated that the showings occur in a presumed lower to middle Devonian carbonate sequence >800 m thick.

A 1980-1981 drill program undertaken by Texasgulf (Graham and Bending, 1981) affirmed the major extent of mineralization. A hole in the Cascade sector included a near-surface intersection of 14.18% Pb + Zn over 3 m. Breccias at Webb Ridge are either tectonic (fault-related) or solution-induced, excluding slumps and intraformational conglomerates, recognized as syndepositional features. The breccias are as follows: 1) crackle breccia, "systems of fractures and anastomosing veinlets without evidence for significant settling, dilation, or rotation of individual fragments"; 2) mosaic breccia, a fabric with significant settling and collapse but without much rotation, such that the original fabric "can be visually reassembled"; 3) rubble breccias, due to more complete collapse and rotation of fragments; 4) pseudobreccia, formed by solution and infilling without actual collapse; 5) zebra (zebroid) fabrics, characterized by 2-10-mm-wide bands of dolostone separated by bands of dolomite, or less often sulphides or quartz, and vugs; 6) porphyrotopic textures, like pseudobreccia but with smaller vugs, showing contrast between fine host dolomite and coarser vug-lining dolomite; and 7) tectonic breccias, usually evident by fault geometry, and distinct mineralogy (cemented by quartz, but not sulphides nor dolomite).

Beales and Hardy (1980) noted that dolomites are the "favored host rocks" for low-temperature Pb-Zn ores, perhaps because of their origins in evaporitic settings, with local sulphur sources, and the reduction of sulphate in the presence of organic matter. Macqueen and Thompson (1978) studied fluid inclusions, and the maximum temperature in the host carbonates was inferred to be 200-230C. MVT deposits in northeast B.C. occur east of a suite of Devonian shale-hosted sedex deposits, while further to the east, minor sulphides accompany coarse secondary dolomite in petroleum reservoirs of the Western Canada sedimentary basin, in Alberta. The age and tectonic setting of the MVT deposits is poorly known: they have been ascribed to Devonian-Carboniferous or Cretaceous (Laramide) orogenic events At Robb Lake, the most significant known MVT deposit in the region, a late Devonian age, 362±9 Ma, is indicated for the Pb-Zn mineralization, favouring the Devonian- Mississippian model (Nelson et al., 2002). The Robb Lake deposit is hosted by platform carbonates of the Silurian-Devonian Muncho and McConnell formations. Robb Lake consists of some 19 stratabound Pb Zn showings hosted by interconnected breccia bodies, both transgressive and stratabound, in a sequence as much as 200 m thick. Some barren breccias are known (Paradis et al., 1999; see also Paradis et al., 2007; Paradis and Nelson; 2007).

REFERENCES

Beales,FW and Hardy,JL (1980) Criteria for the recognition of diverse dolomite types with an emphasis on studies of host rocks for Mississippi Valley-type ore deposits. SEPM Spec.Publ. 28, 197-213.

Graham,RAF and Bending,DA (1981) Robb Lake Project, report of 1981 drilling and review of complete 1980-81 program, NTS 94B. Texasgulf Inc., Vancouver, report + map folder, December.

Macqueen,RW and Thompson,RI (1978) Carbonate-hosted lead-zinc occurrences in northeastern British Columbia with emphasis on the Robb Lake deposit. CJES 15, 1737-1762.

Manns,FT (1982) Stratigraphic Aspects of the Silurian-Devonian Sequence Hosting Zinc and Lead Mineralization near Robb Lake, Northeastern British Columbia. PhD Thesis, University of Toronto, 252pp.

Nelson,J, Paradis,S, Christensen,J and Gabites,J (2002) Canadian Cordilleran Mississippi-Valley-type deposits: a case for Devonian-Mississippian back-arc hydrothermal origin. Econ.Geol. 97, 1013-1036.

Paradis,S and Nelson,JL (2007) Metallogeny of the Robb Lake carbonate-hosted zinc-lead district, northeastern British Columbia. In `Mineral Deposits of Canada' (Goodfellow,WD editor), GAC MDD Spec.Publ. 5, 1061pp., 633-654.

Paradis,S, Hannigan,P and Dewing,K (2007) Mississippi Valley-type lead-zinc deposits. In `Mineral Deposits of Canada' (Goodfellow,WD editor), GAC MDD Spec.Publ. 5, 1061pp., 185-203.

Paradis,S, Nelson,JL and Zantvoort,W (1999) A new look at the Robb Lake carbonate-hosted lead-zinc deposit, northeastern British Columbia. GSC Current Research 1999-A, 61-70.