The Big Bull property is dominantly underlain by rocks of the Mount Eaton Block, a low metamorphic grade island arc volcanic sequence of Devono-Mississppian to Permian age contained within the Stikine Assemblage. These rocks lie east of the Llewellyn fault and are mainly located north of the Taku River and east of the Tulsequah River.
The Mount Eaton Block hosts the Tulsequah Chief and Big Bull volcanogenic massive sulphide deposits and a number of other similar occurrences and prospects.
The stratigraphy has been subdivided into three divisions. The Lower Division is dominated by Devonian to early Mississippian age bimodal volcanic units which include the Mine series felsic rocks hosting the Tulsequah Chief and Big Bull deposits. The Middle Division, Mississippian to Pennsylvanian in age, is composed dominantly of pyroxene bearing mafic breccias and agglomerates with locally extensive accumulations of mafic ash tuffs and volcanic sediments. The transition from the Middle to Upper Divisions is marked by polymictic debris flows and/or conglomerate. The Upper Division, Pennsylvanian to Permian in age, consists primarily of volcanic derived and clastic sediments with lesser mafic flows. Distinctive bioclastic rudite and intercalated chert, shales and occasional sulphidic exhalite occur near the top of the Upper Division. The Mount Eaton suite is overprinted by subgreenschist to middle greenschist facies metamorphism. Late Tertiary Sloko rhyolite and mafic dykes cut the Paleozoic units and commonly intrude along re-activated north-trending faults.
Structure in the Mount Eaton block is dominated by the north trending, eastward verging Mount Eaton anticline which plunges moderately north and dips steeply west. A number of parasitic upright to overturned folds (F1) which range from open to near isoclinal occur on the western limb of this anticline. Penetrative fabric is poorly developed except in extremely appressed folds. North to northwest trending faults are most common and generally exhibit long-lived, complex displacement histories. Displacement appears to be small on these faults except for the major Llewellyn (Chief) Fault. Younger east-west faults are less common on the property. However, based on regional mapping, these faults may have significant displacements. In particular, the Chief Cross Fault was identified as potentially offsetting the regional Llewellyn (Chief) fault in a dextral sense by as much as two kilometers.
The Volcanogenic massive sulphide mineralization at Big Bull occurs within a strongly foliated zone of intense sericite-pyrite alteration which is over-and underlain by laminated and chaotically banded dacite crystal tuffs (the Mine series). This sequence has been intruded by irregularly-shaped, aphanitic to fine-grained dark green diabase sills. The Big Bull stratigraphy has been affected by two phases of folding and sits on the eastern limb of a northwest trending synclinal structure. Several brittle faults cut the deposit area.
On the property, rocks are altered to chlorite-rich greenstones, which are generally massive and host lenses and disseminations of magnetite. Alteration of the magnetite to hematite has produced much jasper-like rock. The principal rock exposed in the mine workings is an andesitic volcanic. Immediately west of the mine is a fine-grained, southwest dipping phyllite which is cut by chloritic quartz veins which are hematite stained. North of the mine, the predominant rock type is andesite, which is heavily altered to chlorite and sericite and hosts very fine calcite stringers. The volcanic rocks are cut by a shear zone that strikes northwest and dips from vertical to 45 degrees southwest. Within the shear zone, but much narrower than it, is an altered zone which ranges from very narrow up to 60 to 90 metres in width. The intensely altered rock is composed of quartz, light colored mica, pyrite and possibly some talc. The principal orebody was about 275 metres long with a maximum width of about 8 metres and extended 90 metres below the surface. Mineralization consisted of a conformable lens of pyrite, sphalerite, chalcopyrite and galena in a gangue of barite, quartz, some calcite and altered country rock. Channel samples taken from the surface showings in 1929 assayed 2.05 grams per tonne gold, 233.1 grams per tonne silver, 2.8 per cent copper, 20.2 per cent zinc and 0.8 per cent lead over 8.2 metres, and 6.9 grams per tonne gold, 257.1 grams per tonne silver, 2.0 per cent copper, 14.4 per cent zinc and 2.8 per cent lead over 1.5 metres (Minister of Mines Annual Report 1929).
By 2011 and after significant work the deposit was described as follows (Assessment Report 33482). The mineralization ranges from massive, banded sulphides, to 30 to 40 per cent disseminated and stringer sulphides in a matrix of barite, sericite and silica. The mineralogy comprises pyrite, sphalerite, galena, chalcopyrite and tetrahedrite, in a matrix of barite and sericitized lithic fragments. The sulphides are recrystallized, with well-developed annealed textures that have obliterated any primary features. The sericitic fragments within the mineralized lenses may represent altered lithic fragments that were incorporated in the mineralized interval. The main Big Bull lens parallels the Bull Fault for over 1000 metre strike length, trending 140 degrees with dips at 60 degrees to the southwest. The main lens averages about 2 metres in width and has been defined by drilling to about 350 metres down dip. The mineralization is dominated by sphalerite in clots and bands, with associated galena and lesser chalcopyrite and tetrahedrite. Clots and bands of mineralization also occur discontinuously within the intense quartz sericite pyrite altered footwall. At the southern end of the Big Bull trend the mineralization becomes more complex with several 1-3 metres repetitions of the same massive sulphide mineralized stratigraphy in a single drillhole separated by 1-5 metres of relatively unaltered basalt flows. This mineralization is part of the high grade southwest or previously called “62 Zone”. Mineralization is dominantly massive to banded, disseminated and stringer medium brown sphalerite with finely disseminated galena and wisps and blebs of chalcopyrite, minor disseminated pyrite and several flecks of visible gold. Intense quartz sericite pyrite alteration is present in the footwall, but grades abruptly into mafic volcanic flows and tuffs, rather than the dacite found in the footwall to the main lens. Numerous closely spaces drillholes have attempted to resolve the complexity of this high grade area characterized by drillhole BB06062 which gave 7.08 metres grading 20.00 grams per tonne gold, 253.42 grams per tonne silver, 0.66 per cent copper, 11.59 per cent lead, and 26.63 per cent zinc (Assessment Report 33482. Evidence suggests possible continuity between the massive sulphide in adjacent drillholes with the complexity caused by interference folding and parasitic folds. Late mafic intrusions also cut the mineralization and hamper continuity interpretations. Adjacent to the mineralization is a strongly foliated sericite-quartz-pyrite assemblage, containing 5 to 20 per cent disseminated and stringer pyrite, with local base metal sulphides and tetrahedrite. The quartz-sericite-pyrite alteration appears to form a stratiform layer near the top of the felsic tuffs, but may in places be discordant to stratigraphy. Chlorite is also present with a locally strong staining effect.
The showings were discovered in 1929 by a prospecting syndicate headed by Mr. V. Manville of Juneau, Alaska, and the Big Bull group of 18 claims and 2 fractions were located at this time. In 1929, the Alaska Juneau Gold Mining Company in association with the Treadwell Yukon Mining Corllpany optioned the property along with the adjoining Potlatch and Banker groups. The following year the company holdings were increased to 54 claims by the addition of the Walker and Moose groups adjoining to the northwest, and the Big Bull extension claim adjoining to the south. During 1929-30 exploration and development work by the company on the Big Bull claims consisted of 609.5 metres of drifting, 289.5 metres of cross-cuts, and 1523.9 metres of diamond drilling. The company relinquished the option late in 1930. The property lay idle for a number of years. Leta Explorations Limited apparently held the property in the 1940s but no work was reported at this time. The Consolidated Mining & Smelting Company of Canada Limited (Cominco) optioned the Big Bull and Big Bull Extension claims from the Manville syndicate in 1946 and located 6 other claims adjoining. Exploration and development work was carried on until 1951 when Tulsequah Mines Limited was formed to combine the Big Bull and Tulsequah Chief mines into one operation. The ore from both mines was treated at the Polaris-Taku concentrator. The Big Bull mine was in production from 1951 until 1956 when operations ceased. The property was developed by an open pit, and underground on three levels from one adit.
From 1951 to 1956, inclusive, the combined production from the Tulsequah Chief (MINFILE 104K 002) and the Big Bull properties totalled 804,262 tonnes of ore. About 353,314 tonnes of ore was apparently mined from the Big Bull deposit and trucked 8 kilometres to the Polaris-Taku mine (MINFILE 104K 003) where it was mixed and processed with ore from the Tulsequah Chief mine (MINFILE 104K 002). See Tulsequah Chief for details on Big Bull production.
Interest in the Tulsequah Chief property was rekindled in the early 1970s with the recognition that the deposits are volcanogenic rather than structurally controlled replacements. Cominco resumed exploration in 1987, mainly at Tulsequah Chief, with only limited work at the Big Bull deposit. In 1987, Cominco conducted 1:1000 surface mapping at Big Bull (Assessment Report 16983) and 1:10 000 in the surrounding area (Assessment Report 17054). In 1988, Cominco continued geological mapping on the Big Bull crown grants (Assessment Report 18428.) In 1992, Cambria Geological Ltd. undertook a detailed surface mapping program at Big Bull and, in 1993, Redfern Resources Ltd. initiated a detailed compilation and exploration program. During 1993 to 1994, Redfern drilled 9084 metres in 27 holes, successfully demonstrating that massive sulphide mineralization continued below the old workings, with several holes intersecting ore grade material over widths up to 6 metres.
Drilling at the Big Bull property in 1994 successfully demonstrated that massive sulphide mineralization remains open outside the limits of the historic workings and the 1993 drilling. Four of the 15 holes drilled in 1994 intersected ore grade material (greater than $45 NSR) over mineable widths (greater than 3 metres), and three other holes intersected ore grade over widths between 1 and 3 metres (Assessment Report 24188).
The 2006 exploration drill program was the first since 1994. Redcorp Ventures Ltd. completed 15,312 metres of drilling in 37 holes and a new zone (60-62) was discovered southwest of the Main zone. This new lens of mineralization has a true width of 5 metres grading 20 grams per tonne gold, 253.42 grams per tonne silver, 0.66 per cent copper, 11.59 per cent lead, and 26.63 per cent zinc. This is a top priority for follow-up drilling in 2007. Drilling along strike north of Big Bull obtained narrow intercepts of low grade mineralization.
In 2007, twenty core holes at Big Bull focused on the exceptionally high grade 60-62 zone discovered in 2006 totaling 7371.55 metres. A total of 12,484 m was drilled at the Big Bull and Tulsequah Chief where a further 15 core holes were completed. A geotechnical assessment of the plant site and the tailings impoundment, a detailed topographic survey (LIDAR) and several environmental programs were also completed. A new mining support plan for development is based on an air cushion barge that will be towed by an amphibious tug and operate year-round on the Taku River. The shipment of equipment and supplies during construction and operation, and the shipment of concentrate would all be done using this system, via Juneau Alaska. . The first NI 43-101 resources for the Big Bull deposit were estimated by Wardrop. An indicated resource of 211,000 tonnes grading 3.043 grams per tonne gold, 162 grams per tonne silver, 0.40, per cent copper 1.25 per cent lead and 3.33 per cent zinc; an inferred resource of 669,000 tonnes grading 4.139 grams per tonne gold, 195 grams per tonne silver, 0.35 per cent copper, 2.59 per cent lead and 5.97 zinc (Assessment Report 33482).
In 2008, Redfern conducted substantial infrastructure and preparatory activities on the property including: constructing 1030 metre airstrip and Satellite 30 man camp at Shazah Creek; completed 19km exploration road from the Barge Landing (kilometre 0) to the Shazah Air Strip (kilometre 19) via the Tulsequah Chief mine (kilometre 14). Technical work associated with the construction included geological mapping along the exploration road, and geotechnical studies on the Shazah fan; also surface and underground acid rock drainage studies at Tulsequah Chief.
Redfern Resources Ltd. wound down construction activities in early 2009 and was placed into receivership in May, 2009 by its creditors, who were unwilling to work cooperatively with the company while it restructured its debt.
Chieftain Metals acquired the Tulsequah Chief and Big Bull Properties lien free from the receivers in October 2010, and engaged SRK Consultants Canada Ltd. to complete a 43-101 technical report and resource estimation. This is the current Big Bull resource estimate. Indicated resources are estimated at 231,000 tonnes grading 2.9 grams per tonne gold, 152 grams per tonne silver, 0.38 per cent copper 1.20 per cent lead and 3.22 per cent zinc; 728,000 tonnes grading 3.9 grams per tonne gold,185 grams per tonne silver, 0.34 per cent copper 2.42 per cent lead and 5.61 per cent zinc (Assessment Report 33482).
In 2011, Chieftain Metal Inc drilled 8,527.38 metres in 22 NQ surface diamond-drill holes. No significant new resources or extensions were identified during the 2011 drilling program but it was reported to have successfully completed its goals of increasing the confidence of the drilling information at Big Bull.
See Tulsequah Chief (MINFILE 104K 002) for related details.