The Butte zone of the Galore Creek deposit is located at the headwaters of Galore Creek, a northerly flowing tributary of the Scud River, some 85 kilometres south-southwest of Telegraph Creek.
At least 12 alkalic porphyry copper-gold deposits are known to occur within the Galore Creek syenite complex. This complex comprises a series of Late Triassic to Early Jurassic orthoclase-porphyry syenitic bodies which have intruded coeval Upper Triassic Stuhini Group volcanic rocks and related sediments. Faults which offset and segment the intrusive rocks and a sub-horizontal fracture cleavage are the two main structural elements in the syenite complex. The complex is roughly 5 by 2.5 kilometres in area.
The deposits are hosted primarily by highly altered potassium-enriched volcanic rocks and pipe-like breccias adjacent to syenite dikes and stocks. Typically, the deposits are manto-shaped and have a north to northeast trend related to the syenite contacts and zones of structural weakness.
The syenite complex is made up of four intrusive phases that are most closely associated with the copper deposits. Six other phases are recognized but are peripheral to the Central zone deposit. The copper-bearing rocks near the syenite intrusion are extensively metasomatized, recrystallized and locally brecciated. These may include pyroclastic and intrusive breccia, trachyte, phonolite, lithic tuff, crystal tuff, pyroxene basalt, pyroxene andesite and minor sediments. These rocks have been converted to skarns and fenitic porphyroids so that original rock types are unclear. The term "hornfels" was frequently applied to these meta-volcanic rocks in the early stages of exploration.
Alteration and mineralization are contemporaneous and spatially overlap. The hydrothermal system was extensive and the resultant alteration led to the formation of large gossans. Potassic alteration consisting of potassium feldspar, titanium biotite and magnetite have converted the syenites and volcanic rocks to pink, white and orange rocks composed mostly of orthoclase. Alteration of pyroxene, hornblende and biotite to assemblages of chlorite and calcite plus/minus albite and epidote characterizes the propylitic zone, best developed in the syenitic rocks. Calc-silicate alteration consisting of abundant garnet, diopside, epidote, albite and anhydrite is an unusual feature of the complex. Garnet replaces up to 50 per cent of the metavolcanic rocks and infills breccias near the northern end of the Central zone breccia pipe but is generally absent from the other deposits. However unusual this metasomatic overprint is, the distribution of sulphides, precious metal and magnetite is considered consistent with the expected zoning pattern for alkalic porphyry deposits.
The Butte deposit outcrops on the western margin of the complex. Barr (CIM Bulletin, July 1966) offers the following brief description of the deposit: "The deposit appears to be localized by infolding of the altered Mesozoic rocks at the contact of porphyritic units of the Complex. A prominent west-dipping major fault structure bounds the western part of the deposit, and the northern portion of the syenite porphyry. The principal alteration products are biotite and potash feldspar. Mineralization occurs principally in disseminated form, and includes bornite, chalcopyrite and chalcocite, with secondary cuprite, covellite and copper carbonates."
Drilling on the main Galore Creek property during 2007 totalled 4,547 metres in 17 drillholes. The drilling was distributed among many areas, including the Southwest zone, the Central Zone, the Lower Butte Zone and in some reconnaissance areas. Initially discovered in 2005, work at Lower Butte was a follow-up to the minimal drilling on the zone done in prior years and was focused to help expand the extent of the deposit.
In 2011, the Junction zone was described in a technical report (43-101) by AMEC Americas Limited on behalf of NovaGold (Galore Creek Project by AMEC, July 17, 2011) as follows:
"The Butte deposit crops out on the west edge of the syenite complex, 2 kilometres west of the Central Zone. It is localized along a west-dipping faulted contact between altered volcanic rocks and syenite intrusions.
The South Butte deposit crops out on a nunatak in the West Fork Glacier, 4 km south of the Central Zone. North-trending dykes and mineralized shear zones cut the altered host volcanics, but most of the chalcopyrite and pyrite mineralization is fracture-controlled.
Copper mineralization occurs as disseminated, fine-grained bornite, subordinate chalcopyrite and minor chalcocite in three northeast oriented bands. Mineralization is sometimes difficult to see due to its fine-grained nature in a mottled, dark, pseudoleucite-bearing tuff host. This peculiar, altered, pseudoleucite-bearing tuff is the preferred host to bornite mineralization at Butte."
Refer to the Central Zone deposit (104G 090) for further details of the Galore Creek deposits. Further details of a common work history and 2011 reserves and resources are given. AMEC reports five zones as being modelled toward the 2011 Mineral Resource Estimate: the Central Zone (including the Bountiful deposit), Southwest Zone, Junction/North Junction Zone, Southwest Zone, and West Fork Zone.
