The Sustut occurrence is located near the headwaters of the Sustut River, approximately 6 kilometres south of Savage Mountain and 193 kilometres northeast of Smithers (Geology, Exploration and Mining in British Columbia 1973).
The general geology comprises a sequence of northwest-striking formations that are increasingly younger to the southwest. The oldest rocks are bands and inliers of sedimentary and volcanic rocks of the Permian Asitka Group. Unconformably overlying these rocks as outliers to the east and as a broad belt to the west are rocks of the Upper Triassic Takla and Lower to Middle Jurassic Hazelton groups. This forms a thick sequence of volcanic flows and volcaniclastics with minor nonvolcanic sedimentary rocks. In the extreme west, the Upper Cretaceous to Eocene Sustut Group overlies the Takla Group unconformably. The Sustut Group is made up of nonvolcanic sedimentary rocks with minor tuffs. Stocks of diorite to granodiorite intrude the Takla Group rocks in the east. They form a northwest-trending belt, probably related to the Jurassic Omineca Intrusions. A few small stocks, apparently of the same intrusive phase, occur west of the main belt of Takla Group rocks. Minor basalt, andesite and porphyry dikes, sills and flows are found throughout the area of Takla Group rocks. Two groups of intrusive rocks are distinguished: an older group, probably Cretaceous, and a younger Tertiary group. These intrusions are probably related to the Cretaceous Bulkley and the Eocene Kastberg intrusions.
The immediate area of the Sustut deposit is underlain by three fundamental stratigraphic subdivisions of the Upper Triassic Savage Mountain Formation (Takla Group), comprising 6096 metres of rapidly deposited green and red basaltic to andesitic rocks. The ‘lower member’ is characterized by subaqueous extrusions of dark green massive flows and pillow lavas. The flows range from augite porphyry basalt through feldspar porphyry, and amygdaloidal feldspar porphyry to aphanitic basaltic andesite. Predominantly overlying, and to a minor extent interbedded with, the flows are massive breccia and bedded tuffs and breccias. The breccias consist of large blocks of the underlying and adjacent flows in a tuffaceous matrix of similar composition. Fragments are poorly sorted in the massive breccias but exhibit some grading in the bedded tuffs and breccias. Individual members are of limited lateral extent and vary greatly in thickness.
The base of the ‘middle member’ is marked by an accumulation of a thin layer of fine volcanic detritus composed of tuffaceous siltstones and sandstones. Overlying this is a major pile of volcaniclastics, which completes the lower cycle of the 'middle member'. The upper cycle is a repetition of the lower, commencing with a thin tuffaceous siltstone and wacke-sandstone unit overlain by a thick pile of volcaniclastics. The volcaniclastics are generally massive agglomerates but locally display graded bedding, crossbedding and, high in the upper sequence, mud cracks and rare ripple marks. The clast content is substantially more heterogeneous than that in the 'lower member' volcanic breccia, increasing in heterogeneity upward. Throughout the whole of the 'middle member', all detritus is apparently derived from the 'lower member'. Within both volcaniclastic sequences is a general trend upward from green to red colour.
Clasts throughout the volcaniclastic piles are poorly sorted, ranging in size from grit to blocks 4 metres in diameter. The larger clasts tend to be subrounded or rounded, whereas the smaller fractions are almost entirely angular to sub-angular.
The ‘upper member’ is a highly heterogeneous assemblage. The upper part is characteristically a sequence of argillaceous and arenaceous clastic sedimentary rocks, largely of volcanic composition and predominantly red in colour. Bedding is common and well developed. The lower part of the member is more varied, with lenses of green and red volcanic conglomerate containing clasts of both the underlying Takla assemblage and foreign chert, limestone, rhyolite and jasper. These lower units are of limited extent, marked by rapid change in thickness and composition.
The upper volcaniclastic unit of the 'middle member' is host to the copper mineralization of the Sustut deposit. It is a highly variable sequence of rocks ranging from augite porphyry basalt to andesite in composition. The sediments range from rocks composed of greater than 60 per cent clasts by volume (many of which are 0.6 metre or more in diameter), to rocks with abundant small clasts, to rocks with a few large and a few small clasts, to an arenaceous grit.
Interbedded with the grits are argillaceous tuff bands. Two colours predominate in the rocks: dark green and deep brownish red. On a large scale, green tends to predominate at the base and red tends to increase in abundance upward. Most of the sequence comprises massive, unsorted volcaniclastics. Interspersed, however, are sections displaying abundant graded bedding and crossbedding on several scales. The clasts throughout the sequence include red and green aphanitic andesites, green augite porphyry, augite-feldspar porphyry, grey bladed feldspar porphyry and red and green tuff and tuff breccia.
Little evidence exists of intrusive activity in the vicinity of the Sustut deposit. Intrusive rocks are restricted to two suites of dikes. Narrow, erratic lenses, rarely longer than 30 metres and wider than 1.5 metres, are apparently randomly oriented; they are composed of subvolcanic andesite to dolerite.
At least two recognizable directions of folding are evident on the Sustut property. Both are broad, open concentric folds with resultant domes and basins. Faulting along north-northwest and northeast trends is of major proportions. The Two Lake Creek fault is one of the most significant structural features in the area and has a vertical displacement of more than 1000 metres. It is identified as part of the Omineca fault zone, a northern extension of the Pinchi fault system.
Regional metamorphism throughout the Takla Group rocks is of the zeolite facies. Metamorphic grade increases northeast from laumontite subfacies in the 'upper member' through the prehnite-pumpellyite subfacies in the 'middle and lower members'. Green rocks of the 'middle member' and to a lesser extent of the 'lower and upper members' are characterized by an unusual abundance of chlorite and epidote. The greatest concentrations of epidote are found in the upper volcaniclastic unit of the 'middle member'. Here, epidote, chlorite, quartz and calcite are common in fracture and open-space fillings and amygdules replacing minerals. Finer grained sequences are present, composed of 50 per cent epidote. The copper mineralization in this unit is not specifically associated with the greatest epidote concentrations but is within the broad epidote envelope.
The Sustut deposit consists of a sulphide-rich sheet-like zone up to 76 metres thick in the upper volcaniclastic unit of the middle member. Downdip, the zone becomes increasingly irregular and generally becomes steeper more rapidly than the bedding. The zone is composed essentially of hematite, pyrite, chalcocite, bornite, chalcopyrite and native copper in decreasing order of abundance. All occur as very fine grains disseminated through both matrix and clasts of the volcaniclastics. Increased mineral concentrations occur in the finer grained tuff and tuffaceous matrix fractions. Hematite is ubiquitous throughout the zone; pyrite tends to form an incomplete envelope around the cupriferous lenses.
The northern part of the mineralized zone is characterized by a uniform, closely stratabound, continuous zone of copper mineralization. It is 15 to 24 metres thick and has little pyrite associated. Copper minerals are essentially chalcocite and lesser native copper. Separating the north from the south part of the zone is a deep cirque following a fracture zone.
In the eastern part of the south zone, copper mineralization is continuous and up to 45 metres thick. Chalcocite and bornite are the dominant minerals, with sporadic fringes above and below showing a gradation through chalcopyrite to pyrite. Downdip, the copper mineralization breaks up into a series of erratic lenses separated laterally and vertically by pyrite. Chalcocite is still the dominant copper mineral. Farther downdip, as the lenses become steeper and apparently transgressive to bedding, the mineralogy changes gradually to bornite and chalcopyrite. In the updip portion of the south part of the zone, a clear vertical zoning sequence from pyrite fringes through chalcopyrite to bornite and a chalcocite core is apparent.
The mineralized zone transgresses coarse- and fine-grained rocks with no discontinuity. Pyrite has not been observed in proximity to native copper or chalcocite. Malachite development is observed on cliff faces. Lensing veins of massive bornite, chalcocite and native copper, up to 15 centimetres wide, are found in some epidote-, quartz- and calcite-filled fractures. They are most abundant in the southwest on fractures trending 110 to 150 degrees. The mineralized zone covers an area 457 by 701 metres in the north and 548 by 853 metres in the south.
The Sustut deposit is approximately concordant with the strata over 30 to 61 metres of the 'middle member' succession. It lies below the zone of transition from subaqueous to subaerial deposition in the top unit. Concentration in this specific zone is believed due to upward leaching of copper during low grade metamorphic and metasomatic reaction. Precipitation occurred in the zone of hydration characterized by prehnite-pumpellyite below the oxidized subaerial rocks.
The Sustut deposit has recently been classified as a basaltic copper deposit (Church, 1992). The deposit formed from the metamorphism of copper-rich basaltic rocks. Mineralizing solutions deposited minerals in openings (permeable sediments, breccias, amygdules and fractures) when reducing environments were encountered.
The Sustut occurrence was first discovered by Gunnar Thomason during a 1971 reconnaissance exploration program that included prospecting, mapping and sampling. Falconbridge Limited acquired the property in 1972. Exploration that year consisted of 26 AQ-size diamond-drill holes totalling 2534 metres, two XRPS-size packsack-drill holes totalling 20.4 metres, geological mapping, aerial photogrammetry and preliminary metallurgical testing. Drill indicated resources based on diamond drilling in 1972 were 54,426,000 tonnes grading less than 1.25 per cent copper (Northern Miner - April 12, 1973). In 1973, exploration continued with 61 AQ-size diamond-drill holes totalling 7050 metres, mapping, mineral inventory estimation, pit design, minable resource calculation, an airborne magnetic-electromagnetic survey of the Sustut Valley, ground magnetic-electromagnetic anomaly truthing, petrographic studies and the collection and analysis of more than 3000 stream-sediment samples, approximately 350 of which were collected from streams surrounding the Sustut massif. In 1974, Falconbridge completed 39 AQ-size infill drillholes totalling 4375 metres, 13 AQ-size reconnaissance drillholes totalling 3236 metres, an in-house feasibility study, metallurgical testing, a legal claim survey, a mineral inventory estimation and trace element geochemistry on deep drillholes Numbers 88 and 110. Exploration was suspended after 1974 due to unfavourable political and economic conditions. In 1975, Falconbridge completed the feasibility studies and geochemical statistical analysis that had begun the previous year.
In 1997, Cross Lake Minerals Limited optioned the property and commissioned a mineral resource evaluation, an in situ and minable reserves calculation, a valuation report and an environmental considerations–due diligence review. In 1999, International Skyline Gold Corporation completed an in-house geological model and block model for the purpose of identifying additional high-grade reserves. Doublestar Resources Limited was granted an interest in the property in 2000. Exploration that year consisted of 22 BQTK-size infill diamond-drill holes totalling 2104.7 metres and the collection and analysis of 23 stream- sediment samples. A geological resource outlined for the Southeast zone estimated 5,937,000 tonnes of 1.87 per cent copper and 6.11 grams per tonne silver at a cutoff grade of 0.7 per cent (Assessment Report 27141). Doublestar Resources continued exploration in 2002 with infill and definition drilling in the Southeast zone. A total of 27 drillholes were completed, totalling 2290.4 metres. Five of the holes were for geotechnical purposes; the remaining 22 consisted of 17 BGM-size and seven NQ-size diamond-drill holes. The five geotechnical holes consisted of four short (less than 80 metre) NQ3-size oriented drillholes and one 396.3 metre deep BQ-size diamond-drill hole. Twenty-two of the drillholes were point-load tested and all but the five geotechnical drillholes were sampled. That year, Doublestar Resources entered into a partnership with Northgate Exploration Limited, Procon Mining and Tunneling Limited with the intention of bringing the proposed Sustut mine into production.
The infill drilling completed in 2002 was intended to provide a National Instrument (NI) 43-101 compliant resource estimate; however, the resource estimate was not able to be classified as a reserve in accordance with report standards. The feasibility study released in September 2003 found that, based on the 2003 metal price forecasts and capital and operating cost estimates, the project would not have been profitable and therefore did not qualify as a NI 43-101 compliant reserve (Pincock, Allen & Holt, 2003).
In early 2004, Doublestar Resources intended to significantly alter the mine access and ore delivery plans and had been advised by the British Columbia Environmental Assessment Office to withhold further permitting activities until the mine plans were finalized. In 2004, Northgate Minerals Corporation was granted the option to purchase 100 per cent interest in the project, with the intention of completing a prefeasibility study in 2005.
In March 2008, Northgate terminated the purchase option agreement and returned the property title to Selkirk Metals Corporation (merged with Doublestar Resources Limited in 2007). Ownership of the claims officially transferred to Selkirk Metals Corporation in 2009 and, as of May 2012, the company still held the project claims.
Resource figures released in 2003, shown in the table below, are based on the 22 drillholes completed in 2002 and are not to NI 43-101 standards.
---------------------------------------------------------------------- SOUTHEAST AND SOUTHWEST ZONES AT 0.65 per cent COPPER CUTOFF GRADE Grade Product Tonnage Cu Cu Category 000s per cent tonnes Southeast Measured 3 859 1.811 69 900 Indicated 1 638 1.780 29 200 Inferred 132 1.685 2 200 ----- ----- ------- Subtotal 5 629 1.799 101 300 ----- ----- ------- Southwest Measured 1 358 1.316 17 900 Indicated 1 385 1.223 16 900 Inferred 189 1.171 2 200 ----- ----- ------- Subtotal 2 932 1.263 37 000 ----- ----- ------- Total 8 561 1.615 138 300 ===== ===== ======= ---------------------------------------------------------------------- |
(Press Release, Doublestar Resources Ltd., February 3, 2003).
