Old Fort copper-molybdenum occurrence, Old Fort Mountain, Omineca Mining Division, British Columbia, Canadai

Regional Level Types
Old Fort copper-molybdenum occurrence	Prospect
Old Fort Mountain	Mountain
Omineca Mining Division	Mining Division
British Columbia	Province
Canada	Country

The Old Fort copper-molybdenum prospect is located on the east flank of Old Fort Mountain, near Babine Lake, about 11 kilometres north-northeast of Smithers Landing and 61 kilometres north-east of Smithers, British Columbia, in the Omineca Mining Division.
There is an extended description of the property on the British Columbia “Minfile” site, current to 2020, to which interested readers are referred. Relevant portions pertaining to geology are quoted below:
“The area is underlain by [a] calc-alkaline stock of the Eocene Babine Plutonic Suite. The stock is approximately 760 by 1130 metres in size, trending north east, and intrudes hornfelsed argillaceous siltstones of the Jurassic Hazelton Group. Four phases have been mapped and include the main quartz diorite phase, a smaller interior quartz monzonite phase, a quartz poor diorite and a feldspar porphyry phase. Copper and molybdenum mineralization appear to be primarily associated with the feldspar porphyry phase. Most of the known copper and molybdenum mineralization lies to the west and north of the quartz monzonite body. Andesitic tuffs and breccias, also of the Jurassic Hazelton Group, outcrop nearby. Potassium/Argon dating of a mineralized sample of biotite feldspar porphyry yielded an age of 49 million years.”
“Locally, chalcopyrite with minor bornite and molybdenite mineralization occur as fracture fillings and disseminations in both quartz diorite and porphyry dikes adjacent to the western margin of the inner quartz monzonite body. Alteration minerals include silica, potassium feldspar, sericite, biotite, calcite, actinolite, magnetite and malachite. Pyrite and pyrrhotite are widely disseminated in all of the intrusive rocks as well as in the hornfelsed sedimentary rocks. Minor sphalerite is also reported in thin veins cutting altered sandstone.”
Giles Peatfield comments:
This is an inconsequential occurrence from an economic point of view, but it has features common to those found in the various occurrences, deposits and mines in the Babine Porphyry Belt.
One interesting feature of the occurrence is that it appears to lie near the centre of what has been interpreted (MacIntyre, 2001) as a 20 kilometre-wide caldera structure which he called the Old Fort Mtn. Caldera. His description was “An arcuate belt of resistant, rhyolite domes previously dated as mid-Cretaceous (MacIntyre and Villeneuve, in press [2001]) and interpreted to be a ring dike structure related to development of a submarine caldera crop out north of Old Fort Mountain in the Babine Lake area . . . . These rhyolites are emplaced into mafic lapilli tuffs typical of the [Cretaceous] Rocky Ridge Formation. The volcanic succession is overlain and underlain by marine and non-marine clastic sedimentary rocks of the [Cretaceous] Skeena Group. Near the rhyolite domes, the sedimentary strata contain numerous angular clasts of rhyolite suggesting explosive volcanism accompanied emplacement of the domes . . . .” Note that the host strata for the Old Fort copper-molybdenum occurrence are mostly Jurassic Hazelton strata, and the host intrusions for the mineralization are dated as Eocene, thus bracketing apparent age of the formation of the caldera. The location of the occurrence within the caldera may be fortuitous, but it is interesting.
Carter (1981) published two K-Ar dates. The first, 49.0±2 Ma, was on biotite from a biotite-feldspar porphyry body in the area of the principal showing. The second, 52.0±2 Ma, was on biotite from a similar rock in a sill intruding argillaceous sedimentary rocks about 2.5 kilometres south of the main mineralized stock.
Giles Peatfield comments on the minerals reported:
The following comments, derived from several reports, give some details of the various minerals reported from the Old Fort area.
Amphibole group: Bysouth (1966) reported that, in a trench on the North Zone, “The minerals magnetite, potash feldspar[,] actinolite, and to a lesser extent, pyrite, quartz and hematite, are common, but not invariable, associates of the copper and molybdenum mineralization.” Gaard (1980) mapped Trench 3 in the North Zone, and wrote that “Alteration includes silicification, k-spar, sericitization, secondary biotite, calcite, actinolite and magnetite.” Bysouth (1966) reported hornblende in quartz diorite. Carter (1967) found, by examining a thin section, that what he called a hornblende diorite was “An unaltered rock consisting essentially of fresh zoned plagioclase and green hornblende exhibiting only minor biotite alteration which is probably deuteric rather than hydrothermal.”
Apatite: Carter (1967) reported apatite in thin section, as an accessory mineral in quartz diorite.
Bornite: Carter (1967) reported minor amounts of bornite with chalcopyrite and magnetite disseminated in quartz diorite and in hornblende-biotite-feldspar porphyry dykes. Bornite was also mentioned by Bysouth (1966) and Belik et al. (1974).
Calcite: Belik et al. (1974) described “carbonate” (probably calcite) in drill logs. Gaard (1980) mentioned calcite in quartz monzonite, and Serack (1982) reported calcite veining in siltstone.
Chalcopyrite: This is the primary mineral of economic interest at the occurrence, mentioned by all workers. Most commonly, it occurs on dry fractures in intrusive rocks. Bysouth (1966) wrote that in the North Zone, “Chalcopyrite occurs as disseminations within fault breccia and as minute veinlets filling joints and irregular fractures.” He also mentioned that “Most of the chalcopyrite mineralisation in the west zone occurs as disseminated replacements of mafics.”
Chlorite group: Carter (1967) described local chloritization of mafic minerals in quartz diorite. Belik et al. (1974) mentioned several instances in drill core logs of chlorite in intrusive rocks, but gave no details.
Epidote: Carter (1967) reported epidote in thin section, as an accessory mineral in quartz diorite. Gaard (1980) noted that epidote was a common alteration product in hornfels. Serack (1982) described it as occurring in andesite.
Feldspar group: As noted above in the comment for Amphibole group, Bysouth (1966) reported “potash feldspar” in the trench on the North Zone. Carter (1981) reported, in both the quartz diorite and quartz monzonite, plagioclase feldspar (oligoclase-andesine) and wrote that “The quartz monzonite is distinguished by a slightly coarser equigranular to seriate texture, and a lighter grey colour with pinkish cast due to the presence of ragged, poikilitic K-feldspar.”
Graphite: Belik et al. (1974) reported graphite as common in hornfels and sedimentary units.
Hematite: Bysouth (1966) reported hematite as common in a trench, but did not specify the rock type. Gaard (1980) identified this as “Trench 3” in the so-called North Zone, excavated in quartz monzonite.
Limonite: Carter (1967) reported limonite; Belik et al. (1974) reported “fracture staining”. Surface weathering limonite is no doubt common.
Magnetite: Bysouth (1966) noted magnetite in association with sulfides. Carter (1967) wrote that “Chalcopyrite occurs as disseminations in both rock types [porphyry dykes and quartz diorite] and in north-trending fractures and irregular 1-inch zones rich in mafic minerals and magnetite in quartz diorites.”
Malachite: Belik et al. (1974) noted, in the drill log for drill hole 74-6, traces of malachite at 30-40 feet in the hole, probably after chalcopyrite.
Mica group: Biotite is common, as a constituent of hornfels and in various intrusive units. Carter (1973) wrote that, within a biotite-feldspar porphyry, “Two forms of biotite are present - 1.) 2 mm. plates and books of fresh brown biotite 2.) Secondary biotite altering from hornblende.” Carter (1967) described local sericitization of feldspar in quartz diorite.
Molybdenite: This has been reported by most workers, as the second most important mineral of economic interest. Bysouth (1966) wrote that “Molybdenite occurs chiefly as Scattered grains on fracture planes.” Carter (1967) noted molybdenite in intrusive rocks, as disseminations and on fracture planes.
Pyrite: Pyrite is common, in most cases disseminated but also on fracture surfaces, reported by all workers.
Pyrrhotite: Bysouth (1966) described pyrrhotite in thin section examination of hornfels. Carter (1967) noted that “Disseminated pyrite and pyrrhotite with resultant limonite stain is a common feature of the hornfelsed rocks.”
Quartz: Quartz in common here, generally as a constituent of the intrusive rocks or as “silicification” of various units. Interestingly, it is not generally noted in the form of veins or veinlets with sulfides; these are noted as being on fractures rather than in veins.
Siderite?: Belik (1974) reported several examples of a brown mineral in hornfels, associated with sulfides, that he thought might be siderite, but gave no definitive data.
Sphalerite: Serack (1982) found small amounts of sphalerite in hornfels and altered sandstone.
Talc?: Belik et al. (1974) noted, in drill logs, numerous occurrences of talc on slip surfaces. This identification should be regarded as tentative.

Giles Peatfield comments on the rock types reported:
The rock types listed above are described in the numerous papers on the Old Fort occurrence and the immediate surrounding area. For more details, refer especially to Bysouth (1966) and Carter (1967).

Giles Peatfield
BASc. (Geological Engineering) University of British Columbia 1966.
PhD Queen's University at Kingston 1978.
Worked for Texas Gulf Sulphur / Texasgulf Inc. / Kidd Creek Mines - 1966 to 1985.
Vancouver based consultant 1985 to retirement in 2016

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Link to British Columbia Minfile:	093M 004

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