GEOLOGY
Geological Setting
Property Geology and Lithological Description
Structure
Deposit Types
Mineralization
Geological Setting
The Minto Project is located in the Carmacks Copper Belt along the eastern margin of the Yukon-Tanana Composite Terrane, which is comprised of several metamorphic assemblages and batholiths (Figure 1). The north-northwest trending Copper Belt is host to intrusion-related Cu-Au mineralization. The Yukon-Tanana Composite Terrane is the easternmost and largest of the pericratonic terranes accreted to the Paleozoic northwestern margin of North America (e.g., Colpron et al., 2005). It is regarded to be the product of a continental arc and back-arc system, preserving meta-igneous and metasedimentary rocks of Permian age on top of a pre-Late Devonian metasedimentary basement (e.g., Piercey et al. 2002).
Figure 1: Yukon Geology from Yukon Geologic Survey "Maps Yukon" website (www.geology.gov.yk.ca)
The Minto Property and surrounding area are underlain by plutonic rocks of the Minto Pluton (Early Mesozoic Age) (Figure 2) of the Granite Mountain Batholith that have intruded into the Yukon-Tanana Composite Terrane. They vary in composition from quartz diorite and granodiorite to quartz monzonite. The batholith is unconformably overlain by clastic sedimentary rocks thought to be the Tantalus Formation and andesitic to basaltic volcanic rocks of the Carmacks Group. Both are assigned a Late Cretaceous age. Immediately flanking the Minto Pluton, to the east, is a package of undated mafic volcanic rocks, outcropping on the shores of the Yukon River. The structural relationship between the batholith and the undated mafic volcanics is poorly understood because the contact zone is not exposed.
Figure 2: Regional Geology
Geobarometry and geothermometry data (Tafti and Mortensen, 2004) suggests that the Minto Pluton was emplaced at a depth of at least 9 km, while the presence of euhedral to subhedral epidote, interpreted by Tafti and Mortensen as magmatic in origin, suggests a deeper emplacement depth in the order of 18-20 km.
Property Geology and Lithological Description
Much of the geological understanding of the rock around the Minto deposits is based on observations from diamond drill core and extrapolation from regional observations. The reason for this is poor outcrop exposure, and due to the moderate to deep weathering and oxidation of the limited exposed outcrop. The terrain was not glaciated during the last ice age event.
The Minto Main deposit is now exposed in open pit mining. The Area 2 and Area 118 deposits are considered continuous, and reported as one deposit known as Area 2 / Area 118, located immediately south of Minto Main. The Ridgetop deposit is located approximately 300 m south of the Area 2 / Area 118 deposit. The most recently discovered copper deposit to be reported is the Minto North deposit located approximately 700 m north of the Main Minto deposit. In addition to these deposits which all have NI43-101 compliant mineral resources and mineral reserves there are several significant mineral prospects. The most recent of these discoveries is "Minto East" which was discovered in late 2009. These deposits and prospects define a general north-northwest trend informally called the Priority Exploration Corridor or PEC.
Hypogene copper sulphide mineralization at Minto is hosted within the Minto pluton, which intrudes near the boundary between the Stikinia and Yukon-Tanana terranes, however since the contact is not exposed it is unclear if the pluton stitches the two terrains. The Minto pluton is predominantly of granodiorite composition. Hood et al. (2008) distinguish three varieties of the intrusive rocks in the pluton. The first variety is a megacrystic K-feldspar granodiorite. It gradually ranges in mineralogy to quartz diorite and rarely to quartz monzonite or granite, typically maintaining a massive igneous texture. An exception occurs locally where weakly to strongly foliated granodiorite is seen in distinct sub-parallel zones several metres to tens of metres thick. A second variety of igneous rock is quartzofeldspathic gneiss with centimeter-thick compositional layering and folded by centimetre to decimetre-scale disharmonic, gentle to isoclinal folds (Hood et al., 2008). The third variety of intrusive rock is biotite-rich gneiss. Capstone geologists consider all units to be similar in origin and are variously deformed equivalents of the same intrusion.
Copper sulphide mineralization is found only in the rocks that have a structurally imposed fabric, ranging from a weak foliation through to strongly developed gneissic banding. For this reason all core logging by the past and present operators separates the foliated to gneissic textured granodiorite as a distinctly discernable unit. It is generally believed by Capstone geologists that this foliated granodiorite is just variably strained equivalents of the two primary granodiorite textures and not a separate lithology. While this interpretation, based upon detailed observations from logging of tens of kilometers of drill core is highly likely, it has not been conclusively proven. Tafti & Mortensen (2004) noted that the relatively massive plutonic rocks have similar mineral and chemical composition as the foliated rocks. Research in collaboration with the Mineral Deposits Research Unit of the University of British Columbia is on-going.
The contact relationship between the foliated deformation zones and the massive phases of granodiorite is generally very sharp. These contacts do not exhibit chilled margins and are considered by Capstone geologists to be structural in nature, separating the variably strained equivalents of the same rock type. Tafti and Mortensen (2004) had interpreted the sharp contacts to be zones of deformed rock within the unfoliated rock (i.e. rafts or roof pendants). More recent deep drilling and deep penetrating geophysics suggest this is not the case, but again has not been conclusively proven. Supergene mineralization occurs proximal to near-surface extension of the primary mineralization and beneath an unconformity defined by conglomerate that is mostly locally derived from eroded and sometimes partially weathered and decomposed granodiorite. The conglomerate has been dated as Cretaceous Age (unpublished date pers. com. Dr. Maurice Colpron - Yukon Geological Survey), and is now recognized in outcrop in a borrow pit exposure located west of the airstrip as well as in numerous recent drill holes. Observations of foliated and even copper mineralized cobbles in drilling indicate that "Minto-Type" mineralization was exposed, eroded and reincorporated in sedimentary deposits by the Cretaceous Age.
Conglomerate and volcanic flows have been logged in drill core by past operators. New drilling has confirmed a widespread presence of conglomerate, but not the volcanic flows. The latter cannot be confirmed by the authors as the drill core from historic campaigns was largely destroyed in forest fires and no new drilling has intersected such rocks. However, undated volcanic rocks are mapped by Hood, near the southwest margin of the property, south of a fault that is inferred from geophysics to separate them from the Jurassic Age intrusive rocks.
Other rock types, albeit volumetrically insignificant include dykes of simple quartz-feldspar pegmatite, aplite, and an aphanitic textured intermediate composition rock. Bodies of all of these units are relatively thin and rarely exceed one metre core intersections. These dykes are relatively late, and observed contact relationships suggest they generally postdate the peak ductile deformation event; however some pegmatite and aplite bodies observed in a rock cut located north of the mill complex are openly folded. It is unclear if this folding is contemporaneous with foliation development in the deformed rocks or post-dates the foliation development. Observations from drill core and open cut benches in the mine show examples where the foliation and the pegmatitic/aplitic intrusions are both folded, as well as examples where the intrusions are not folded, suggesting two populations of minor dykes.
Structure
There are examples of both ductile and brittle phases of deformation affecting the Minto deposits. As noted above copper-sulphide mineralization is strongly associated with foliated granodiorite. This foliation is defined by the alignment of biotite in areas of weak to moderate strain and by the segregation of the minerals into mafic and felsic bands. The mafic areas are comprised of millimetric bands of predominantly biotite with lesser amounts of magnetite and accessory amounts of hornblende and garnet. The felsic bands tend to be thicker, forming layers of a centimeter or so in thickness that are comprised of predominantly plagioclase feldspar, sometimes with orthoclase over growths and lesser amounts of quartz. The banded rock is interpreted to represent areas of higher strain, and these rocks have a distinct gneissic texture. Individual deformation zones form sub-horizontal horizons within the more massive plutonic rocks of the region and can be traced laterally for more than 1000 m in the drill core. They are often stacked in parallel sequences analogous to stacks of pancakes. The regular, sub-horizontal nature of the deformation zones allows a high degree of predictability when planning diamond drilling campaigns. Contrary to some previous reports, the foliated zones do not appear to inter-finger with the more massive rocks. Rather, observations from pit benches within the mine show that blocks of unfoliated granodiorite are sometimes incorporated within the thicker deformation zones that surround them, more in the manner of horses within the deformation zones.
The similarity of chemistry and texture of both the deformed and the massive granodiorites suggest the deformation zones are structural in origin and not stratigraphic, but this still needs to be confirmed by more rigorous science. Several of these foliated units can be traced in drill holes over long distances at similar elevations and are good marker horizons locally; they are used to guide exploration drilling and 3D geological modeling.
However the absence of chill margins or absorption rims at contacts, combined with the great depth of emplacement (Tafti and Mortensen, 2004) likely preclude them from being remnant rafts or roof pendants of metasedimentary or metavolcanic strata, as some workers have postulated. No sedimentary or volcanic features have been observed in these foliated and mineralized rocks. A structural origin remains the best explanation.
Therefore it is postulated that the foliated granodiorite represent healed, shallowly dipping shear zones within the Minto Pluton, and may have formed when the rocks passed through the brittle/ductile transformation zone in the earth's crust in transition from a deep emplacement environment to eventual exhumation of the batholith. They may represent thrust faults related to regional crustal thickening of the Yukon-Tanana Terrain when the batholith was being exhumed.
Internally to the enveloping surfaces the foliation exhibits highly variable orientations within individual deformation zones including frequently observed small-scale folds. The foliation is often observed to be at a high angle to contacts with the more massive textured rock units but occasionally it is also observed to align parallel to the contacts locally making a sense of shear unclear. Observations by Hood et al. (2008) along a transect in the Area 2 deposit suggest that foliation orientations within deformed horizons have a geometry of tight to isoclinal folding with a wavelength on the order of about 30 m. The observed trend of folds within this area is approximately north-northwest, parallel to regional structural trends (Tempelman-Kluit, 1984). The ore--bearing zones are also occasionally openly folded on a scale of several hundred metres. Based upon horizon modeling for resource estimation by Capstone geologists at Ridgetop and Area 2 the folds have wavelength of about 280 m. The folds appear to be gentle in amplitude with approximately north-south axial traces. Simple shear strain of the foliated zones is also noted adjacent to some late cross-cutting fault zones.
Late brittle fracturing and faulting is noted throughout the property area. Some of these faults are significant from an economic standpoint. The Minto Creek fault (MC Fault) bisects the Minto Main deposit, dividing it into north and south areas and is modeled as dipping steeply north-northeast with an apparent left lateral reverse displacement. The northern block moved up and to the west relative to the southern block. Both the vertical and horizontal displacements are evident by offsets in the main zone mineralization, but appear to be minimal. A lack of marker horizons in the plutonic rocks, however, makes it difficult to determine the absolute magnitude of the movement (Figure 3).
Figure 3: North- South Cross Section through Minto Deposit showing DEF Fault and MC Fault
The DEF fault defines the northern end of the Minto Main deposit. It strikes more or less east-west and dips north-northwest and cuts off the main zone mineralization, as shown in Figure 6.3. The vertical orientation of most of the drilling is less than optimal to intersect steep to vertical faults. It may share a similar sense of movement to the MC fault, but a significant amount of displacement is inferred. Determining the magnitude of this displacement could lead to locating an extension of the main zone mineralization on the north side of the DEF fault. This late block faulting is noted throughout the Minto Pluton and in some instances a rotational component is noted as well. Tafti & Mortensen (2004) found the Cretaceous Age Tantalus Formation rotated up to 60 degrees from horizontal in areas located south of the Minto deposit.
A zone of pervasive fracturing on the west side of the Main deposit limits ore grades in this direction. Limited historical drilling west of this structure did intersect some weak copper mineralization, although foliated horizons do not line up across this fracture zone. It is presumed to be one of the north-south faults that are part of the late brittle conjugate set.
While the limits to Minto Main mineralization on the north and west sides are structural in nature, the southern limit is an erosion channel cutting below the elevation of the mineralization and thereby removing it. This zone of deeper erosion is a paleo-channel that is interpreted to follow another roughly east-west striking fault. Only on the east side does mineralization appear to fade out and have no obvious structural limit.
The boundary between the Area 2 and Area 118 deposits is an intermediate NE dipping fault. The displacement of the mineralization is significant. At least two parallel structures displace mineralized domains in Area 118. The shear sense on this structure has not been analyzed in detail, but attempts to correlate ore zones across the main boundary fault are complicated by the difficulty in finding a specific characteristic to unambiguously identify the zones. The easiest zone to identify (based on mineralization and texture) is the "N" zone and it has up to 66 m of vertical throw across the boundary fault. Other zones show changes in thickness and orientation, suggesting the presence of pure strain and block rotation. A similar NW striking fault zone appears to be present that defines the northeastern boundary of the Ridgetop deposit, and defines the outcrop of Cretaceous conglomerates. The dip of this structure is unknown.
All mineralized horizons exhibit locally pervasive fracturing (typically chloritic or hematitic), which are interpreted to postdate the main copper-sulphide mineralization event. This late structural/hydrothermal event may have potential economic significance, as coarse-grained visible gold has been logged on chloritic and epidote-lined fractures.
Deposit Types
All of the known deposits on the Minto property have the same style of mineralization and are all considered to be similar deposit types. In all cases, mineralization is associated with primary copper sulphide mineralization (except for supergene enriched zones noted in Ridgetop and Area 118) restricted to sub-parallel foliated horizons within a grandioritic pluton.
There are no deposits closely analogous to the Minto deposit on a world-wide basis because there is no consensus as to the origin of the Minto deposit. At various times since its discovery the Minto deposit has been described as an example of Porphyry Copper, Volcanogenic Massive Sulphide (VMS), Redbed Copper, Magnetite Skarn (see discussion by Pearson and Clark, 1979) and Iron Oxide Copper Gold "IOCG"(Minto Explorations Ltd., 2003).
Based on the preceding paragraph it is reasonable to say that the origin of the Minto deposit is enigmatic. Various workers appear to have ascribed different interpretations for the most part based on their empirical observations, the background of the observer, and/or the popular models of the day. The abundance of the high Cu/S mineral bornite in a moderately oxidized magmatic system along with the obvious magnetite association suggests that Minto belongs to one of two recognized deposit types: Magnetite Skarn or Iron Oxide Copper Gold. The lack of a typical calc-silicate skarn mineral assemblage seems to preclude the skarn deposit type, thus appears to leave the IOCG model or alternatively to a previously unrecognized deposit type.
The host rocks to the Minto deposits were emplaced in a deep batholitic setting (exceeding 9 km deep to perhaps as much as 18-20 km deep), which is not considered to be the typical porphyry environment. The host is a moderately oxidized magma (Tafti and Mortensen, 2004) with widespread iron oxide (predominantly magnetite with lesser hematite) mineralization. At least some of the hematite is supergene in origin, but it is unclear if some hematite is also primary. There are very strong structural controls on ore mineral emplacement and there is no apparent genetic link to a specific phase of intrusion. Typical porphyry-type alteration zoning such as widespread propylitization, argillization, barren silicic core, or large barren pyritic halo is not recognized. Stockwork style, fracture or vein mineralization is also not present.
Examples of IOCG mineralization with some similar characteristics and setting to Minto include Copperstone in Arizona, Caldelaria in Chile and Ernest Henry in Australia (Williams et al., 2005). From a genetic and structural prospective, albeit not size wise, the Sossego Deposit in Brazil may be a reasonable analog. While an IOCG origin for the Minto Deposit cannot be unequivocally demonstrated, the authors are of the opinion that this style of deposit provides the most consistent model for our current level of understanding.
Mineralization
The Minto deposits have essentially no surface exposure with the exception of minimal exposure in historical trenches of the shallow oxidized zones associated with the Ridgetop deposit and more recently now exposure in mine benches in the Main deposit open pit. Observations for the deposits are based on hand-specimen and petrographic studies of drill core. The primary hypogene sulphide mineralization consists of chalcopyrite, bornite, euhedral chalcocite, and minor pyrite. Metallurgical testing also indicates the presence of covellite, although this sulphide species has never been logged macroscopically. Texturally, sulphide minerals predominantly occur as disseminations and foliaform stringers along foliation planes in the deformed granodiorite (i.e. sulphide stringers tend to follow the foliation planes). Sulphide mineral content, however, tends to increase where this foliation is disrupted by intense folding. In addition, semi-massive to massive mineralization is also observed; this style of mineralization tends to obliterate the foliation altogether. Silver telluride (hessite) is observed in polished samples but has not been logged macroscopically. Native gold and electrum have both been reported as inclusions within bornite and accounts for the high gold recoveries in test concentrates. Occasionally, coarse free gold is observed associated with chloritic or epidote lined fractures that cross-cut the sulphide mineralization. The free gold may be due to secondary enrichment during a later hydrothermal process overprinting the main copper sulphide-gold event. Sulphide mineralization is always accompanied by variable amounts of magnetite mineralization.
The Minto Main deposit exhibits crude zoning from west to east. The bornite zone is dominant in the west while a thicker, lower grade chalcopyrite zone is dominant on the east side of the deposit. The bornite zone is defined by the metallic mineral assemblage magnetite-chalcopyrite-bornite. Bornite mineralization is conspicuous, but chalcopyrite is the dominant sulphide species. Stringers and massive lenses of chalcopyrite with various quantities of bornite are typical. Massive mineralization occurs locally over intervals exceeding 0.5 m in thickness and semi-massive mineralization over several metres in thickness may occur. In these sulphide rich areas, textures often resemble those seen in magmatic sulphide zones with sulphide mineralization interstitial to the rock forming silicate minerals. The higher grade portion of the Minto Main deposit roughly corresponds to the bornite zone. Local concentrations of bornite up to 8% are seen. The precious metal grades are elevated in the bornite zone (very fine gold and electrum occur as inclusions in bornite) and occurrences of coarse grained native gold are noted almost exclusively in bornite-rich material. The chalcopyrite zone is characterized by the metallic mineral assemblage of chalcopyrite-pyrite +/- very minor bornite and magnetite.
Empirical observations indicate the highest concentrations of bornite are associated with coarse grained, disseminated and stringer-style magnetite mineralization, up to 20% by volume locally. The stringers of magnetite are often folded or boudinaged, suggesting that at least some of the magnetite mineralization predates peak ductile deformation.
Sulphide mineralization on the other hand, shows both evidence and absence of ductile deformation locally and is interpreted to have formed contemporaneous with, or late in the ductile deformation history.
The Minto North Deposit also exhibits a zoning from west to east. High-grade bornite-dominant mineralization is observed in the west with lower grade chalcopyrite-dominant mineralization in the east. The bornite zone is defined by the metallic mineral assemblage bornite-magnetite-chalcopyrite. Bornite mineralization occurs as strong disseminations and foliaform stringers locally > 10% to occasional semi-massive to massive lenses up to 2 m in thickness. Chalcopyrite concentrations are typically within the 1 to 2% range. Precious metal grades are elevated in the bornite zone, and visible gold has been observed on several occasions.
Mineralization at Area 2 / Area 118 is distinct in that mineralization is predominantly disseminated (+ occasional foliaform stringers) and that semi-massive to massive sulphide mineralization is absent; as a whole, the mineralization is more homogenous and consistent as compared to Minto Main and Minto North. The primary mineral assemblage at Area 2 / Area 118 includes chalcopyrite-bornite-magnetite with minor amounts of pyrite; and a crude zoning is present in that the higher grade northern half of the deposit shows increased bornite concentrations up to 8% locally.
Mineralization at Ridgetop is subdivided into the near surface oxidized upper zones (chalcocite is the dominant sulphide) and the more typical primary sulphide mineralization of the lower zones. The lower zones are defined by a mineral assemblage of chalcopyrite-magnetite with minor amounts of pyrite. Chalcopyrite is the dominant sulphide in the lower zones, and bornite is only observed in minor amounts. Texturally, chalcopyrite occurs as disseminations and foliaform stringers, and is rarely observed as semi-massive to massive veins. Magnetite is coarse grained, disseminated, stringer-style, and can occur in bands up to 0.3 m in thickness, up to 20% volume locally.
These empirical observations of bornite/chalcopyrite relative abundances are supported by a copper and gold grade trend in mineral resources discovered to date within the PEC where the Ridgetop deposit sits at the lower grade chalcopyrite dominated southern end and Minto North sits at the much higher grade bornite dominated northern end of the currently defined trend.
