The Robb Lake occurrence is located 1.75 kilometres north of Halfway River and 6.5 kilometres east of the summit of Mount Kenny, about 180 kilometres north of Mackenzie.
Regionally, a Lower and Middle Devonian platformal carbonate sequence consists of two assemblages. The lower assemblage comprises the Upper Silurian(?) and Lower Devonian Muncho-McConnell and Lower and Middle(?) Devonian Stone formations, which consist almost entirely of dolomite with various admixtures of quartz sand or sandstones and argillaceous dolomite. The upper assemblage, comprising the Middle Devonian Dunedin Formation, consists of dolomites or variably dolomitized limestones. The overall character of the lower assemblage formations suggest that these represent low energy, virtually completely dolomitized, high-salinity lagoonal, intertidal and supratidal carbonates. In contrast, the upper assemblage is of a more normal salinity, open-marine character, and appears to represent environments ranging from shallow-water lagoons of Bahama Banks type to high energy, stromatoporoid, and coral-rich carbonates characteristic of bank margins. The Lower and Middle Devonian sequence lies disconformably on Lower Silurian carbonates of the Nonda Formation and is overlain by basinal shales of the Upper Devonian–Lower Carboniferous Besa River Formation. All formations of both assemblages pass laterally into deeper-water shales and argillaceous carbonates of the basinal Besa River Formation, which also includes lateral equivalents of Upper Devonian and Mississippian platform carbonates.
In the Robb Lake occurrence area, a narrow belt of resistant Silurian and Devonian carbonate rocks form parts of thrust panels and easterly verging folds along the western margin of a topographic depression floored by recessive shales of the Besa River Formation. Lead-zinc mineralization in dolomite breccias is distributed along the western flank of a large, southeasterly plunging anticline of Muncho-McConnell and Stone formations dolomites that makes up part of a broadly folded thrust panel. This panel has moved more than 4 kilometres upward and eastward over tightly folded Muncho-McConnell and Stone carbonates and Besa River shales. The mineralized panel has, in turn, been overridden by tightly folded shales and carbonates of Ordovician and Silurian age. A facies change from platform carbonate of the Lower Silurian Nonda Formation to slope breccias and shales occurs. The Muncho-McConnell and Stone dolomites extend westward over this facies change as a rigid unit sandwiched between Besa River shales above and Silurian slope breccias and shales below. This is a unique stratigraphic setting which may have a bearing on the origin of the mineralized breccias. The actual Muncho-McConnell, Stone formations facies change in not exposed, but it is clear that the lead-zinc showings are situated very close to the depositional edge of the carbonate platform.
The Muncho-McConnell/Stone formations interval is approximately 550 metres thick. The top of the interval is directly overlain by Besa River Formation shales and argillaceous carbonates. To the east, the undivided Muncho-McConnell/Stone interval is overlain, possibly disconformably, by a 60-metre thick interval of limestones, argillaceous limestones and stromatoporoidal dolomites of the Dunedin Formation. These are in turn overlain conformably and gradationally by argillaceous carbonates and calcareous and non-calcareous shales of the Besa River Formation.
The Muncho-McConnell/Stone interval at Robb Lake contains more abundant white, sparry dolomite than the same stratigraphic interval elsewhere. White sparry dolomite fills fractures, forms the matrix of breccias, occurs as irregular patchy replacements of yellowish brown host dolomite, lines or fills open spaces, and forms peculiar and striped zebra-like anastomosing, and irregular fabrics on a scale and abundance unique to this locality. Any part of the interval may be brecciated, with mosaic and rubble breccias most abundant. Spectacular mosaic and rubble breccias are developed in the upper half of the interval.
Breccias occur throughout the undivided Muncho-McConnell/Stone interval at Robb Lake, but appear to be best developed in the middle and upper parts of this interval. Individual breccia bodies have variable dimensions; some are broadly stratigraphic whereas others are strongly crosscutting. Commonly, they comprise angular dolomite fragments with sharp boundaries set in a matrix of white, coarsely crystalline dolomite cement. Within the same breccia zone, however, angular crackle, mosaic and rubble fabrics may grade into distinct patchy breccias in which fragments have corroded(?) and sutured outlines are partly recrystallized, and are set in a fine carbonaceous matrix (bitumen is common). Every gradation exists between the two textural types. Fragment size and composition vary; 1–5 centimetres is a common range but clasts as small as a millimetre to several metres in breadth occur. Thicknesses of the breccia zones normally ranges from a few centimetres to tens of metres and may rarely exceed 45 metres. Lateral dimensions on the order of 304 metres are common.
Sulphide mineralization at Robb Lake is largely confined to dolomite breccias and comprises predominant pale to medium brown sphalerite with lesser galena and minor pyrite. The sulphides occur as rims around dolomite fragments and as large crystals and crystal aggregates with the sparry filling. Pyrite, where present, occurs as thin, fine grained fragment coatings. The dolomite breccia zones are in places traversed by essentially vertical, north striking fractures of little or no displacement; some of the better grade material appears to occur near these fractures where they intersect the breccia zones.
In 2005, a representative chip sample (19753) from a mineralized (galena, sphalerite) crystallized dolomite mosaic breccia assayed 7.4 per cent lead, 1.6 per cent zinc and 1.2 grams per tonne silver (Assessment Report 28060).
Origin of the breccias at Robb Lake is undetermined. An alternative mechanism to a solution collapse origin is hydraulic fracturing: the fracture of rocks through application of excess fluid (hydrostatic) pressure. In simplest terms, hydrostatic pressure has the effect of reducing rock strength to the point of brittle failure without any increase in applied stress. At Robb Lake, hydraulic fracturing may be a result of high pore pressures generated by dewatering of the fine clastic facies that envelope the host dolomites (Property File Canadian Journal of Earth Sciences, 1978).
In 1972, lead-zinc mineralization was discovered in the Robb Lake area by Barrier Reef Resources Limited. This discovery was known as the Barrier Prospect; on which 29 drillholes totalling 4473 metres were completed. Measured geological reserves over a minimum mining width of 2.4 metres were 6,449,481 tonnes grading 7.11 per cent combined lead-zinc at a 5 per cent cutoff grade; or 10,794,490 tonnes grading 6.6 per cent combined lead-zinc at a 3.5 per cent cutoff grade; or 20,137,620 tonnes grading 5 per cent combined lead-zinc at a 2 per cent cutoff grade (Consolidated Barrier Reef Resources Ltd., Rights Offering Circular, November 29, 1984). The reserves are combined from three separate deposits: the Lower Showing, East Webb and West Webb.
Recent interpretation (Bill McMillan, 1998) suggests mineralization appears to be hydrothermal and stratabound. NATMAP crews are investigating.
A summary in Fieldwork 1998 (pages 89–101) suggests that the Robb Lake deposit is possibly a Mississippi Valley-type deposit. They note that there is a textural and mineralogical zonation in the Robb Lake system. Bedding parallel, rock matrix breccia with adjacent crackle and mosaic breccia in the upper 200 metres of the lower unit of Siluro-Devonian Muncho-McConnell Formation, is particularly rich in sphalerite; whereas, separated by 70 metres or so, in the lower 130 metres of the upper Siluro-Devonian Muncho-McConnell Formation, in crosscutting, crackle breccias and adjacent dolomite veins and quartz stockworks, galena seems to predominate.
In 1971, lead-zinc mineralization was discovered in the Robb Lake area and a protracted staking rush ensued. In the fall of 1971 three companies who had independently staked adjoining claim groups in the Robb Lake area formed a Joint Venture consisting of three member companies: Arrow Inter-America Corp., Barrier Reef Resources Ltd., and Texasgulf Canada Ltd. (Texasgulf Inc., 1974). From 1972-75, annual exploration was conducted on the property resulting in significant diamond drilling, geochemical sampling, and geological mapping. The property lay dormant from 1976 until 1980, when a favourable market and political conditions allowed exploration activities to begin anew and lasted for two years. Between1972 and 1982, several major drill programs drilled a total of 119 holes (24,182 metres). In 1992, limited reclamation was completed. In 1993 over 300 claims were dropped and in 1995 further reclamation was conducted. Exploration expenditures by Falconbridge through 1999 total in excess of C$2,000,000 (personal communication, Falconbridge, 1999). In 1999, Doublestar Resources Ltd. planned to acquire the property from Falconbridge Limited.
In 2005, Doublestar Resources Ltd. conducted prospecting and a geological mapping program. The exploration program consisted of locating and sampling the known showings, prospecting between these showings, and a general study of the structure and lithology. This latter aspect was designed toward determining if the property was permissive of hosting economic grade/tonnage quantities of mineralization. The consequences of the program were the collection and analyses of 32 rock chip and float samples (six of which were analyzed by petrography).