Minto
GEOLOGY
Geological Setting
Property Geology and Lithological Description
Deposit Types
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.
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
Property Geology and Lithological Description
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.
Copper sulphide mineralization is found only in rocks that have a structurally imposed fabric, ranging from a weak foliation through to strongly developed gneissic banding. The primary hypogene sulphide mineralization consists of chalcopyrite, bornite, euhedral chalcocite, and minor pyrite. 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 and textural evidence suggests they postdate the major ductile deformation event. In addition, semi-massive to massive mineralization is also observed; this style of mineralization tends to obliterate the foliation altogether and is interpreted as the coalescing of multiple stringers. However, some massive bornite has been observed as a cement in narrow breccia zones, that are interpreted as late brittle deformation in fold noses. 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. Variable amounts of magnetite mineralization are ubiquitously associated with all Sulphide mineral occurrences. However, there is now relationship between the grade of copper, gold or silver mineralization and the amount of magnetite present.
Supergene mineralization occurs where hypogene sulphide zones come near-surface (up-dip extensions), locally beneath an unconformity defined by conglomerate that is mostly locally derived from eroded and sometimes partially weathered and decomposed granodiorite. The primary supergene mineralization consists of chalcocite and trace amounts of malachite and/or azurite. The conglomerate has been dated as late Createcous Age and the supergene alteration is interpreted to be of similar age and thus brought about by the position of a Cretaceous Age position of the water table.
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 granodioritic 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 a previously unrecognized deposit type.