The area is underlain by carbonate and clastic rocks of the Cambrian-Ordovician Kechika Group, Ordovician to Devonian Road River Group and the Lower and Middle Ordovician Skoki Formation. The Kechika Group comprises cleaved, phyllitic, calcareous siltstone and shale, silty limestone, banded limestone and sandstone. The upper contact of the Kechika Group with the overlying Skoki Formation is gradational and conformable. The Skoki Formation consists of three distinctly coloured units. A middle, light grey dolomite is bounded by dark grey, more massive dolomite units. The middle unit contains a conspicuous dark green weathering volcanic unit consisting of basaltic flows and pyroclastics. The rest of the section consists of medium bedded sandy dolomite and dolomite. The Skoki Formation has a definite western limit beyond which it changes facies to black shales, cherts and siltstones of the lower part of the Road River Group. The upper contact is conformable, comprising a gradation from dolomite, silty dolomite and dolomitic siltstone to brown weathering laminated siltstone of the Road River Group.
The Road River Group comprises an Ordovician graptolitic shale-quartzite unit, a Silurian breccia unit and carbonaceous unit, a Silurian-Devonian brown siltstone unit, and a Devonian dolomitic quartz sandstone unit. In the Aley property area, the Road River Group is represented by the Ordovician graptolitic shale-quartzite unit.
This miogeoclinal succession, of the continental margin of ancient North America, was deposited near the shelf/off-shelf boundary and intruded by the Mississippian Aley Carbonatite Complex prior to the main Late Jurassic-Early Cretaceous orogenic event (formation of the northern Rocky Mountains). The youngest unit affected by the intrusion is the Skoki Formation volcanic sequence. Two potassium-argon dates have been obtained from mica separates from the Aley complex, 339 plus/minus 12 million years and 349 plus/minus 12 million years (Open File 1987-17).
The carbonatite complex is oval in outline with a diameter of 3 to 3.5 kilometres, occupying an area of approximately 7 square kilometres. The body is cylindrical in the third dimension with a near vertical axis and has probably been only slightly tilted from its original orientation. It consists of a rauhaugite (dolomitic carbonatite) core zone surrounded by an older, outer ring of amphibolite. Some sovite (calcitic carbonatite) and rare-earth carbonate sweats occur in the rauhaugite core. A contact aureole of recrystallized carbonate rocks surrounds the amphibolite margin. Rare-earth carbonate-rich ferrocarbonatite dikes intrude the contact aureole. Ultramafic lamprophyre dikes and a diatreme breccia pipe intrude altered and fresh carbonates outside the complex.
The Aley complex and its contact aureole are part of an imbricate thrust sheet of the northern Rocky Mountains, bounded to the west by a high-angle thrust fault juxtaposing Cambrian rocks of the contact aureole against unmetamorphosed Silurian rocks. The Silurian rocks form part of the tectonically thinned eastern limb of a tight anticline with a Cambrian core to the west. This structural element is dissected by faults striking at high angles to the Rocky Mountain trend. Along the eastern side of the complex a tectonically thinned, reversed stratigraphic section, with a set of subparallel lower angle thrust faults, is thrust onto an imbricated sheet containing Silurian rocks (to the east of the area mapped). Parts of the carbonatite complex may be faulted out above and below the exposed level. The fault zones along the eastern and western side of the Aley complex are mapped as two branches of the Burden thrust (Fieldwork, 1986).
Approximately 50 mineral species are identified in the Aley carbonatite complex (Fieldwork 1986, page 285). Niobium-rich phases of economic interest include fersmite, pyrochlore and columbite. The abundance of apatite also provides a possible phosphate inventory.
The rauhaugite core zone of the Aley complex is approximately 2 kilometres in diameter. It comprises more than 50 per cent of the exposed complex and consists of dolomite (80 to 95 per cent) and apatite (5 to 15 per cent) with minor amounts of phlogopite, pyrite, magnetite and zircon. It is generally a massive and homogeneous unit, weathering buff to brownish. Different degrees of deformation and alteration resulted in a variety of textures. Apatite occurs as prismatic crystals or disk-like flattened aggregates oriented parallel to the planar fabric. Fersmite forms fibrous to fine-grained aggregates replacing euhedral pyrochlore. Primary fersmite is rare. Columbite is present as a replacement of fersmite. Alteration of this core zone includes extensive chloritization and minor silicification of narrow fracture zones with relatively abundant fersmite and/or pyrochlore. Metallic black, granular aggregates are widespread and consist of chlorite-rutile mixtures or dolomite with thin niobium rutile lamellae grown along the rhombohedral cleavage.
Generally, mineralization is characterized by a preferred orientation of mineral crystals and grains, swirl-textures and compositional banding. Economic minerals are also associated with massive pods and bands of accessory minerals (magnetite). It appears that the niobium phases are zoned within the carbonatite complex, namely: 1) pyrochlore occurs at the top and outer margins of the carbonatite; 2) columbite occurs lower and at the central core of the carbonatite; and 3) fersmite occurs at the transition between pyrochlore and columbite.
Sovite zones (dikes?) occur locally near the margin of the rauhaugite core zone and in the surrounding amphibolite zone. It typically displays a strong parallel fabric marked by a cleavage and mineral layering. The sovites exhibit a more varied mineralogy than the rauhaugites. Calcite with or without dolomite dominates and also includes apatite, biotite, magnetite, sodic amphibole (richterite), pyrochlore and fersmite. Accessory minerals include zircon and rare baddeleyite associated with zirkelite. Small amounts of chlorite and secondary quartz may form in zones of higher strain.
An amphibolitic margin, approximately 1 kilometre in width, encircles and complexly interfingers with the rauhaugite core. The marginal zone includes massive and breccia phases. No distinct pattern to the spatial distribution of the two phases is evident. Carbonatite dikes cut both members. The massive phase is a medium to coarse-grained, dark green rock consisting primarily of sodic amphibole (arfvedsonite), quartz, albite and aegirine. It is more extensively developed than the breccia phase and resembles fenites associated with some other carbonatite complexes in British Columbia (Open File 1987-17). However, Mader (Fieldwork 1986), has recognized microsyenitic textures in the massive amphibolite, and suggests that it is a primary igneous phase with a metasomatic (fenitic) overprint. The breccia phase contains subrounded clasts of dominantly orthoquartzite, with some siltstone, albitite, and syenite fragments in a matrix that is similar to the massive phase. The clast-to- matrix ratio is highly variable and clast-supported breccias are locally developed. The subrounded nature of clasts give this unit the appearance of a conglomerate. The massive and breccia phases locally grade into one another.
Sedimentary rocks adjacent to the Aley carbonatite complex have been altered for a distance of approximately 500 metres beyond the amphibolite margin. This alteration is characterized by a colour change from grey to buff which is indicative of a limestone to dolomite transition. The altered rocks can look superficially similar to material from the rauhaugite core zone. Apatite, pyrite and magnetite are developed in the alteration zone. White mica and potassium feldspar are the only common metamorphic minerals observed and the degree of alteration decreases outward from the complex. Silicification and growth of richterite amphibole is observed with 10 to 40 centimetres of the contact. Trace element abundances (niobium, rare-earth elements, thorium, fluorine) and radioactivity can be correlated with the degree of alteration, also decreasing outward.
Dikes or "sweats" enriched in rare-earth elements (REE) occur throughout the complex but are most common in the outer alteration halo. The dikes weather dark reddish brown, are generally intruded parallel to bedding, and average 0.5 to 1.5 metres in thickness. Their primary component is dolomite. Accessory minerals include purple fluorite, pyrite, barite, secondary quartz, bastnaesite and other rare-earth carbonate minerals. Burbankite, cordylite and huanghoite are probably primary igneous rare-earth carbonates whereas the hydrous carbonates and various calcium-strontium-barium carbonates are part of the alteration assemblage (see Aley Dykes, 094B 028).
Preliminary exploration has identified eight niobium-bearing zones within the rauhaugite core zone. These are the Ridge, Bear, Bear Extension, East, Saddle, Saddle West, Goat and Central zones. The potential exists for open pittable bodies of mineralization grading 0.66 to 0.75 per cent niobium (Assessment Report 16484).
The average grade of the carbonatite is approximately 2.18 per cent phosphorus representing approximately 10 per cent apatite. If the entire carbonatite core contains this amount of apatite then the deposit may have a resource potential exceeding 15 billion tonnes at this grade (Fieldwork 1987, page 407).
The Aley deposit was discovered in 1980 by Cominco. From 1983 to 1986, approximately 3000 metres of diamond drilling was completed in 20 holes, outlining near-surface mineralization in the range of 20–30 million tonnes. Assays for 18 of Cominco's 20 holes had intersections of greater than 8 metres in length and averaged 0.524 per cent niobium.
In 2007, Taseko Mines Ltd. completed an exploration program on the Aley deposit to update Cominco's work and will continue exploration in 2008 to develop a NI 43-101 compliant resource estimate for the deposit.
In 2010, Taseko Mines Ltd. completed an exploration program comprised of geological mapping and 23 diamond drillholes totally 4460 metres. Highlights include drillhole 2010-023, which returned 141.7 metres grading 0.573 per cent niobium (Assessment Report 32798).
In 2011, Taseko Mines Ltd. also released a NI 43-101 compliant inferred resource estimate of 158 892 tonnes grading 0.3 per cent niobium with a 0.14 per cent cut-off grade (Press Release, Taseko Mines Ltd., September 12, 2011). Taseko Mines Ltd. also conducted a program of helicopter-supported exploration drilling, rock characterization and subsequent 3D geological modeling. Highlights of the 43-hole, 17 600.07-metre drill program include drillhole 2011-095, which returned 225.1 metres grading 0.357 per cent niobium (Assessment Report 33237).
In early 2012, Taseko Mines Ltd. upgraded the inferred resource estimate to a measured and indicated resource estimate. This showed measured resources of 112 651 tonnes grading 0.286 per cent niobium with a 0.14 per cent cut-off grade, and indicated resources of 173 169 tonnes grading 0.244 per cent niobium with a 0.14 per cent cut-off grade (Press Release, Taseko Mines Ltd., March 28, 2012).
Taseko Mines Ltd. released an updated Technical Report on Mineral Reserves for the Aley project on October 30, 2014. Reserves were calculated at 0.30 per cent Nb2O5 cut-off and resources were calculated at 0.20 per cent Nb2O5 cut-off.
Category Amount (tonnes) Nb2O5 (%)
Proven 44,272,000 0.52
Probable 39,543,000 0.48
Probable 83,815,000 0.50
Measured 112,651,000 0.41
Indicated 173,169,000 0.35
Indicated 285,820,000 0.37
Inferred 144,216,000 0.32