Gnat Pass porphyry copper deposit, Lower Gnat Lake, Liard Mining Division, British Columbia, Canadai

Regional Level Types
Gnat Pass porphyry copper deposit	Deposit
Lower Gnat Lake	- not defined -
Liard Mining Division	Division
British Columbia	Province
Canada	Country

The Gnat Pass porphyry copper deposit (also known as the Gnat Lake deposit) is located at Lower Gnat Lake, on British Columbia Highway 37 (the Stewart-Cassiar Highway), about 22.5 kilometres south-east of Dease Lake, 91 kilometres east-northeast of Telegraph Creek, and 61.5 kilometres north of the operating Red Chris porphyry copper-gold mine. Refer also to Minfile Number 104I 029 (MOSS, GALAXIE).
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 predominantly covered by overburden and largely devoid of outcrop. Regional mapping indicates that the area is underlain by rocks of the Upper Triassic Stuhini Group consisting of andesite and basalt flows, tuffs and breccias with some sediments intruded by small stocks and sills of porphyritic andesite and basalt. The property is adjacent to the contact of the Upper Triassic Cake Hill pluton, part of the Jurassic-Triassic Hotailuh batholith. The pluton consists mainly of hornblende quartz monzonite, granodiorite and rare hornblende diorite.
Locally, the rocks consist of dark green hornblende porphyritic andesite, fine grained andesitic greenstone, volcanic breccia and tuff. Basaltic rocks and basaltic lithic tuffs are evident in drill core. These volcanic rocks are intruded by an irregular mass of fine grained, broken, feldspar porphyry rock that is highly variable in texture. Much of this rock is a leucocratic reddish stained, fine-grained felsite or alaskite that could be in part highly altered versions of the volcanic rocks. Quartz monzonite is reported south of the main areas of mineralization.
Near the two main mineralized areas known as the Hill zone and Creek zone, all the rocks exhibit considerable alteration. Carbonate is widespread throughout and occurs as veinlets. Sericite is distributed as patches. Both the volcanic rocks and feldspar porphyry have been bleached, locally silicified and have widespread iron oxide staining and hematite on many irregular fractures. Chlorite and dense black tourmaline veins occur on fractures in the volcanic rocks. In places, fine grained potassium feldspar occurs in the volcanic rocks. All the rocks locally exhibit cataclastic breccia textures with evidence of deformation.
Two distinctive types of mineralization are present in the Gnat Pass (Hill and Creek zones) occurrence. One is characterized by chalcopyrite-magnetite-hematite with minor bornite as veinlets, fracture fillings and lesser disseminations in Stuhini Group volcanic rocks. Veinlets rarely exceed 1.3 centimetres in width and are often accompanied by chlorite. Local (but rare) replacement of volcanic rocks has produced small, higher grade lenses measuring up to 0.6 by 0.6 by 0.3 metre in size which copper grades commonly exceed 1 per cent. Rare flecks of molybdenite and locally abundant specular hematite are associated with the replacement-style mineralization.
A second type of mineralization is characterized by chalcopyrite-tourmaline-carbonate breccia zones which occur proximal to or within irregular, eastward-dipping feldspar porphyry and quartz feldspar porphyry intrusions. Brecciation varies from fine mosaic structures to coarse fragments to 5 centimetres across cemented in a fine-grained matrix. Chalcopyrite occurs as fine disseminations in the matrix, often accompanied by chlorite. More rarely, narrow chalcopyrite veinlets cross-cut quartz feldspar porphyry lithologies. Little magnetite is present but fine-grained pyrite was noted locally.”
Giles Peatfield comments:
There is an extensive published literature regarding radiometric dates for intrusive rocks both within the deposit and in the general area of the property. Detailed references to these data are beyond the scope of this review. The best summaries are in Anderson and Bevier (1992) and van Straaten et al. (2012). Readers interested in following up on this topic could refer to: Souther (1971); Wanless et al. (1971); Stevens et al. (1982a, 1982b); and Anderson (1983). van Straaten et al. (2012) reported a preliminary date of ca. 217 Ma for their “Gnat Pass intrusive”, which agrees well with the various dates derived, by various methods, for similar rocks of the Cake Hill plutonic suite, as reported by the Geological Survey of Canada. This does not, of course, provide an age for the copper mineralization. The Cake Hill intrusion is cut by the later (ca. 160-180 Ma) Three Sisters pluton. Although this later intrusive body does not apparently crop out in the immediate area of the Gnat Pass deposit, it seems possible that it might have had an influence on the copper mineralization.
Bowen (2013) reported that “In a Canadian Stock Exchange Listing Statement dated November 7, 1972, Lytton Minerals Ltd. reported “indicated reserves” for the Gnat Deposit of 30,387,850 tonnes grading 0.389% Cu, including 20% dilution by wallrock grading 0.15% Cu. To the author’s knowledge, no technical report supporting this historical resource estimate exists.” This resource estimate, quoted by Minfile, would not be compliant with National Instrument (NI) 43-101 standards; there do not appear to be any more recent estimates. The deposit as presently known is relatively small and of low grade. It is one of a number of similar occurrences and deposits in the general region, the largest and most important being the presently operating Red Chris porphyry copper-gold mine (Minfile No. 104H 005), 61.5 kilometres to the south.
The Gnat Pass porphyry copper deposit is included in the USGS compilation by Singer et al. (2008). The references quoted in this report for the deposit are Lefebure and Höy (1996) and the Minfile inventory page for 2005. There is some confusion here in that the mineral list given in Singer et al. (2008) is incomplete and incorrect as it includes scheelite. Nowhere in the Lefebure and Höy (1996) paper, or in Minfile, or in any of the works cited in this review is there any mention of scheelite. In any event, the environment is really not correct for scheelite. Was someone confusing this property with another occurrence?
Giles Peatfield comments on the minerals reported:
The following comments, derived from several reports, give some details of the various minerals reported from the Gnat Pass porphyry copper deposit and immediately surrounding area.
Amphibole group: Several workers (e.g. Westervelt (1964), Jeffery (1967); Bowen (2013) and van Straaten et al. (2012) described hornblende, but gave no further specific data.
Ankerite?: Leitch (1989) reported ankerite (or ferroan dolomite) in polished thin section; Lang (2012) reported “Fe-carbonate” in polished thin section. I would regard this as tentative.
Apatite: Both Leitch (1989) and Lang (2012) identified apatite in polished thin sections.
Azurite: Westervelt (1964), describing copper mineralization at Gnat Lake, wrote that “Disseminated chalcopyrite mineralization is widespread and occurs, generally in trace amounts, in all the volcanic rocks. Only rarely are minute amounts of finely disseminated chalcopyrite observed within the intrusives. Where copper is present in any concentration, a thin surface skin of secondary malachite and azurite has developed.” Interestingly, subsequent writers have not mentioned this mineral.
Bornite: This was reported by several workers. Both Leitch (1989) and Lang (2012) identified bornite in polished thin sections.
Calcite: Both Leitch (1989) and Lang (2012) identified calcite in polished thin sections.
Chalcedony: Lang (2012) identified chalcedony in polished thin sections.
Chalcopyrite: This is the principal mineral of economic interest and was noted by all workers.
Chlorite group: Several workers noted “chlorite”, but none gave specific data.
Copper? Smith and Garagan (1990), describing the mineralization at Gnat Pass, noted that “Trace magnetite, bornite, cuprite, pyrrhotite and possibly native copper are also present.”
Cuprite: See note above for copper?.
Dolomite: Reported by Leitch (1989) in polished section. He distinguished between dolomite and ankerite in the various sections, based mostly on the relief.
Epidote: Westervelt (1964) reported epidote on joint planes in granodiorite. Bowen (2012) reported, on the basis of “limited petrographic studies”, that epidote was an important alteration mineral. Lang (2012) noted that “Propylitic alteration occurs on the northern margin of the deposit and comprises carbonate, sericite, chlorite, epidote, hematite and trace to minor pyrite.”
Feldspar group: Both plagioclase and potassium feldspar have been reported by various workers, but there do not seem to be any detailed petrographic data available. The K-feldspar is commonly noted as an alteration product, as evidenced by staining.
Hematite: Westervelt (1964) reported “heavy specular hematite” with magnetite-chalcopyrite mineralization in basic volcanic rocks. Leitch (1989) reported, in polished thin sections, hematite after magnetite. Other workers reported the mineral.
Limonite: Although this is probably common as a surface weathering product, only Jeffery (1967) reported iron-oxide staining with hematite.
Magnetite: This is common mineral here, mentioned by all workers. Westervelt (1964) described magnetite-chalcopyrite veins in volcanic rocks. Bowen (2012) reported that magnetite was common in altered Triassic Stuhini Group lavas, but much less so in intrusive rocks.
Malachite: See note above for azurite.
Mica group: Westervelt (1964) noted both hornblende and biotite in granodiorite. Jeffery (1967) found patchy sericite in altered rocks in the mineralized zones. Leitch (1989), in examining polished thin sections, reported both sericite and muscovite.
Molybdenite: This has been described by several workers as rare or sparse. Westervelt (1964) reported rare flecks of molybdenite in the magnetite-chalcopyrite mineralization in basic volcanic rocks. Leitch (1989) found a single grain of molybdenite in chalcopyrite from a tourmaline-chalcopyrite breccia. Lang (2012) found traces in polished thin sections.
Pyrite: This has been reported by most workers, as fine-grained and sparse. It is apparently not a major constituent of the mineralization.
Pyroxene group: van Straaten et al. (2012) described the Triassic Stuhini Group volcanic rocks as “augite-phyric”, but gave no specific compositional data.
Pyrrhotite: See note above for copper?.
Quartz: Although reported by several workers, it seems that quartz is not abundant at Gnat Pass. There are several reports of silicification, and a few of thin quartz veinlets, but the system seems to be relatively silica-poor. See note above for chalcedony.
Rutile: Leitch (1989) found trace amounts of very fine grained rutile in polished thin sections.
Tetrahedrite? Leitch (1989), describing a polished thin section of a highly altered porphyritic dacite, reported that “There is rare ?tennantite-tetrahedrite, as anhedral grains up to 0.1 mm across, with bornite.”
Titanite: Leitch (1989) noted sparse “sphene” in a polished thin section of highly altered porphyry, associated with minute grains of rutile.
Tourmaline: Nearly all workers have reported tourmaline, which appears to be a major constituent of a particular style of copper-bearing mineralization here. Leitch (1989), describing tourmaline in polished thin sections, called the mineral “schorl” (Fe-rich tourmaline) in most cases, but in one section, of altered dacite, reported that the pleochroism was in shades of green only, different from the schorl in other sections.
Zircon: Zircon grains from the Gnat Pass intrusive rocks, part of the Cake Hill Pluton, have yielded a crystallization age, by the U-Pb method, of ca. 217 Ma (van Straaten et al., 2012). See Anderson and Bevier (1992) for much more detail regarding zircons in the Triassic intrusive rocks.

Giles Peatfield comments on the rock types reported:
These rock names are derived from the several reports used in this review. Some may be alternate names for the same rock type, but I have chosen to list them all. Be aware that in most cases these are field names. The extended list is a reflection of the number of reporters and the protracted length of time of observation.

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

Other Databases

Link to British Columbia Minfile:	104I 001

Other Regions, Features and Areas containing this locality

References