Upper Devonian to Mississippian Earn Group rocks are preserved in a series of synformal fold keels and thrust plates that form sinuous, semi-continuous northwest trending belts. This package overlies and is overthrust by strata belonging to the Cambrian- Ordovician Kechika Group and Ordovician-Silurian Road River Group. The Devono-Mississippian section can be split into four main subdivisions. The Lower to Middle Devonian limestones and shales are characterized by massive, grey, fossiliferous limestone (Kwadacha and Pesika reefs), limestone debris flows and chert breccias that interfinger laterally with graptolitic shales, cherts, and distal calcareous turbidites (Paul River Formation).
The Akie Formation (Earn Group) comprises rusty brown and grey-weathering shale, silty shale and siliceous shale which unconformably overlie the Lower to Middle Devonian strata. Some shales which have been mapped as Akie Formation may be facies equivalents of the Lower to Middle Devonian units, or basal shales of the Gunsteel Formation (Earn Group). An unconformity between the top of the Silurian and the base of the Upper Devonian is indicated in drill core by the conglomeratic, reworked top of Silurian siltstone of the Road River Group. The duration and regional nature of the unconformity are poorly understood. The problem is complicated by depositional thickness and facies change in Lower to Middle Devonian strata and lack of paleontologic control.
The Gunsteel Formation overlies the Akie Formation and consists of silvery-grey weathering, black, siliceous, carbonaceous shale and chert. The Gunsteel Formation is host for known barite-sulphide mineralization and most of the known stratiform barite deposits in the region.
The Warneford Formation (Earn Group) comprises Upper Devonian to Lower Mississippian submarine fans of chert and shale conglomerates in the west, interbedded to the east with silty, distinctly laminated shales which have thin dolomitic siltstone interbeds. The Warneford Formation is interbedded with and overlies the Gunsteel Formation. It locally contains nodular and bedded barite that may be stratigraphically equivalent to the barite in the Gunsteel Formation.
The Cirque occurrence is underlain by strata of the Earn Group (Warneford, Gunsteel, Akie formations). The Gunsteel Formation is the primary host of the Cirque deposit and is characterized by black, carbonaceous, locally pyritic, siliceous shale and ribbon-bedded porcellanite approximately 140 metres thick. The porcellanite members occur at the upper and lower contacts and are 10 to 20 metres thick. The Akie Formation consists of soft, grey, laminated shales and in the vicinity of the Cirque deposit is interbedded with it. It contains minor calcareous siltstone and intraformational siltstone breccia beds, some of which are also found within the ore deposit. The Warneford Formation, which consists of coarse sandstone and chert pebble conglomerate interbedded with grey to black shale, is not prevalent.
The Cirque deposit is a plunging, elongate, east tapering, lensoid body 1000 metres long, 300 metres wide and 60 metres thick. It dips 30 to 45 degrees to the southwest and plunges 30 degrees to the south. In plan view the deposit extends north-south, with the up plunge end truncated by the present day erosion surface. The orebody lies along the southwest dipping limb of a northwest trending anticline and is cut by normal faults.
The deposit consists of three facies; a barite-rich facies fringed by pyritic and laminar banded pyritic facies. The baritic facies consists of fine to medium-grained barite and less than 40 per cent sulphides, with the sulphides occurring as 1 to 5 millimetre thick laminations of pyrite, sphalerite, and to a smaller degree galena. By comparison, the pyritic facies, which grades into the baritic facies, contains from 40 to 100 per cent sulphides comprised of pyrite, sphalerite and galena, in a gangue of barite with minor amounts of quartz and carbonate. The laminar banded pyritic facies consists of 0.1 to 20-centimetre thick beds of fine framboidal pyrite in siliceous shale. The sphalerite, galena and barite occur as sparsely disseminated grains in these layers.
Overall, the pyritic facies dominates in the northern part of the deposit and the baritic facies in the southern part, with the laminar banded pyritic facies occurring to the east and above the baritic and pyritic facies. The ore grade of the laminar banded facies is low, constituting only a minor part of the total lead-zinc-silver reserve.
Diluted mineable reserves at Cirque are reported to total 22,084,000 tonnes grading 9.4 per cent zinc, 2.8 per cent lead and 60 grams per tonne silver (Application for Mine Development (Stage 1 Report), Cirque Project, Curragh Resources Inc. January 1991).
The Cirque deposit formed within a fault-bounded sub-basin or trough which had restricted seawater circulation. This hypothesis is based on the assumption that the anomalous thickening of the Gunsteel Formation is a primary depositional feature. Interbeds of coarse, poorly sorted conglomerate or breccia within the southern part of the massive barite horizon suggests the inferred bounding blocks were tectonically active during the main pulse of mineralization. The thickness and lensoidal shape of the barite-rich horizon and general lack of pelitic interbeds is consistent with Sato's (1972) model of accumulation of dense metalliferous brines in a seafloor depression and subsequent rapid crystallization. The reported occurrence of ammonites within the barite further supports a syngenetic origin for the deposit. A period of alternating pelitic sedimentation and syngenetic to early diagenetic crystallization of pyrite followed the main episode of barite precipitation and was apparently restricted to the same basin of deposition. It is suggested that the source fluids for both types of mineralization emanated from rift zones bounding this basin (Fieldwork 1979).
In 2013 the deposit was described as consisting of the North Cirque and South Cirque (Exploration in BC 2013, page 34). The North Cirque deposit is a moderately southwest dipping, east-tapering lensoid stratiform mineralized body that is 1000 by 300 metres and up to 60 metres thick. The northeastern margin is exposed at surface. Mineralization from highest to lowest grade is comprised of three major facies: pyritic, baritic, and laminar banded pyrite. The pyrite facies is dominant in the north part of the deposit and the baritic facies in the south. Previous work used metal grade distribution, metal ratios, and deposit thickness to suggest a feeder zone in the north. However, in re-examining the structural setting and deposit characteristics, and looking for expansions, the company remains open to the possibility of an alternate feeder zone. The South Cirque deposit, a kilometre to the southeast, is a partially-delineated apparent tabular mineralized body with similar features to North Cirque, but does not outcrop at surface.
Historic drilling has delineated a non-compliant indicated resource of the North Cirque orebody of over 38.5 million tonnes averaging 8 per cent zinc, 2.2 per cent lead, and 47.2 grams per tonne silver, and an inferred resource of the South Cirque orebody of 15.5 million tonnes averaging 6.9 per cent zinc, 1.4 per cent lead and 32 grams per tonne silver (Assessment Report 34274).
A joint venture regional exploration program of geochemistry and geological mapping by Cyprus Anvil Mining Corporation (50 per cent) and Hudson's Bay Oil and Gas Company Limited (50 per cent) led to the staking of the Cirque 1-14 claims (approximately 155 units) in July 1977; the discovery was a visual find in the course of geochemical surveys. Electro- magnetic and geochemical soil and silt surveys (1,950 samples) in 1978 outlined a coincident lead-zinc soil anomaly over 2 km in length. Diamond drilling in 1978-79 totalled 8 848 metres in 30 holes. Further drilling in 1980 increased drill indicated reserves to 30 million tonnes grading 2.2 per cent lead, 7.8 per cent zinc, 51.43 grams per tonne silver, with an additional geologic reserve of 6.3 tonnes (Hudson's Bay Oil and Gas, 1980 Annual Report). Drilling in 1981 confirmed a 40 million tonne deposit (Financial Post, Nov. 14, 1981). An 80 kilometre access road from Williston Lake was completed in 1981.
In 1981, Hudson's Bay purchased a 100 per cent interest in Cyprus Anvil Mining Corporation. Dome Petroleum Limited during 1981-82 purchased a 100 per cent interest in Hudson's Bay and simultaneously sold about 34 per cent of this interest to associates Dome Canada Limited, Trans Canada Pipelines Limited and Dow Chemical Canada Inc, in effect making Hudson's Bay Oil and Gas Company Limited an indirectly wholly owned subsidiary of Dome Petroleum Limited. Surface drilling on the deposit was completed in 1982 but plans for underground exploration were postponed indefinitely.
In 1985, Curragh Resources Inc. purchased the deposit. Trans Canada Pipeline was a minority partner. In 1989, Asturia de Zinc, S.A. purchased 15 per cent interest. A decline was driven in 1989 and bulk testing was started.
In December 1992, Curragh Inc was issued a Mine Development Certificate for its Stronsay (Cirque) lead-zinc-silver project. At a daily milling rate of 3500 tonnes the company estimated that the project would produce about 250,000 tonnes of zinc and lead sulphide concentrates yearly.
In 1994, Curragh Resources went into receivership and the Cirque property was acquired by Cirque Operating Corporation (25 per cent Teck Corp., 25 per cent Cominco Ltd., and 50 per cent Korea Zinc Company Ltd.).
In 1995 a Prefeasibility study was completed for an open-pit mine operation with a 7.3:1 stripping ratio and reserves of 18.5 million tonnes at 8.1 per cent zinc and 2.2 per cent lead (Exploration and Mining in BC 2013). A feasibility study in 1995 indicated that better market conditions would be required to mine the orebodies. Prior to 1995, 360 diamond drill holes (74,262 metres had been completed on the property, including 27 holes targeting the North Cirque orebody, 32 holes targeting the South Cirque orebody, and 11 exploratory holes north of the main orebodies.
In 2001, when Teck Corp. and Cominco Ltd. merged, a 50 per cent share in the property was obtained on behalf of Teck Cominco Ltd. (later Teck Resources Ltd.). Interest in the property was renewed in 2009, during which a community consultation and site visit (and reclamation) took place, and extensive compilation and digitization of the historic data commenced and continued into early 2013.
In 2013, Teck Resources Ltd returned to their Cirque property, a joint venture with smelting company Korea Zinc Co Ltd and conducted on behalf of Cirque Operating Corp. The 2013 program was the first year of a multi-year exploration program in the area, and re-establishing the Cirque camp demanded much of the first half of the season. In 2013, a geochemical soil sampling program was conducted during the 2013 field season in order to classify known Cirque-style mineralization for the purpose of continuing property-scale exploration of untested areas.
In 2014, drilling occurred on the Cirque. Details are pending publication.