Berg Cu-Mo deposit, Ootsa Lake, Omineca Mining Division, British Columbia, Canadai

Regional Level Types
Berg Cu-Mo deposit	Deposit
Ootsa Lake	Lake
Omineca Mining Division	Mining Division
British Columbia	Province
Canada	Country

The Berg porphyry copper-molybdenum deposit is located in the Tahtsa Mountain Range, about 48 kilometres west of the west end of Ootsa Lake, and 108 kilometres south of Smithers, B.C., in the Omineca Mining Division.

There is an extensive description of the property on the British Columbia Minfile site, current to 2022. This includes a summary of the exploration history of the property as well as a comprehensive description of the geology, selected parts of which are quoted below. There is much more information available on the Minfile site, to which interested readers are referred. Many of these details are treated in the ‘Minerals reported’ and ‘Rock types reported’ sections below. The deposit has been studied extensively – refer to the various papers and reports listed in the reference section. In abbreviated summary, focusing on the mineralization:

“Copper and molybdenum mineralization occur primarily in potassically altered rocks. The intrusion of the quartz diorite likely acted as a ground preparation event, hornfelsing and rendering andesitic rocks brittle. The subsequent intrusion of the Berg stock and the resultant hydrothermal system fractured the enclosing andesite and quartz diorite adjacent to the contacts of the stock. The strongest fracturing and best mineralization in the system developed where the hydrothermal system interacted with the previously hornfelsed volcanic rocks and quartz diorite in the North Shell.

Most hypogene mineralization occurs in several generations of quartz-sulphide veining. The earliest veins contain much of the copper and molybdenum mineralization. Associated alteration envelopes are either potassic or non-existent. Later veins are typically poor in copper plus/minus molybdenum sulphides and are associated with phyllic and propylitic alteration assemblages. Calcite plus/minus gypsum plus/minus quartz-sphalerite-pyrite plus/minus galena veins are a common late vein type and contain up to 1020 grams per tonne silver [Harris (2012)]. This argentiferous mineralization is particularly prevalent within the coarse-grained plagioclase-biotite-quartz porphyry unit in the West Shell and to a lesser extent with coarse grained plagioclase-biotite-quartz porphyry unit in the North Shell. Disseminated mineralization is only important in the central part of the stock and in the adjacent quartz diorite where fracture intensities are low.

A well-developed supergene enrichment blanket is superimposed on hypogene mineralization. Three mineralogically distinct supergene zones have been recognized: supergene sulphide (covellite, chalcocite and digenite), supergene oxide (malachite/azurite, cuprite, tenorite and native copper) and leached capping.”

Giles Peatfield comments:

The Berg deposit is a large porphyry copper-molybdenum deposit, of very low grade, that is still in the exploration stage. Norton et al. (2021) presented a “Mineral Resource Estimate” for the Berg deposit, effective as of March 9, 2021. The estimate adhered to guidelines set forth by National Instrument 43-101 and the CIM Best Practices and Definition Standards. The combined measured and indicated resource was given, as 609,986,000 tonnes grading 0.34% Cu, 0.03% Mo and 3.02 grams per tonne Ag. The report, to which interested readers are referred, contains much more detail.

Regarding the age of the deposit, Carter (1981) presented K-Ar ages for biotite from five samples of porphyries, and one whole rock sample from hornfels, from the Berg property. These ranged from 52.0±3 to 46.8±1.5 Ma (million years before present). The oldest date, 52.0±3 Ma, was from the hornfels, suggesting that the alteration processes in the intrusive rocks continued for some time after the initial intrusion.

Berg is an interesting deposit from a British Columbia perspective, in that it is one of the few deposits in the province with very well-developed supergene and oxide zones, extending to considerable depth. The consensus seems to be that the weathering and oxidation has happened after the glaciation some 10,000 years ago.

Comments on the minerals reported:

The identities of the minerals given here are from various sources, as detailed below. A few of these comments are, where deemed necessary, somewhat extensive. Wherever possible, the comments refer to instances where there is some analytical work to confirm the identifications. For details of the rock types mentioned in this section, refer to notes below in the ‘Rock types reported’ section.

Akaganeite?: Panteleyev (1981) wrote that “Other minerals observed in small amounts in the zone of oxidation are cuprite, tenorite, native copper, malachite, azurite, brochantite, chalcanthite, ferrimolybdite, possibly akaganeite (molybdenum-bearing beta limonite), and black amorphous iron-manganese oxides formed from lead-zinc-bearing carbonate veins.” It is interesting that Panteleyev described that mineral as “molybdenum-bearing”, which is not how it is described in Mindat or in the Handbook of Mineralogy. It must be regarded as tentative here.

Amphibole Group: Reported as a rock-forming or gangue mineral by Sutherland Brown (1967) and by Owens (1968), who wrote that “The gangue minerals [in the ore samples] consist chiefly of quartz and dolomite, with small amounts of garnet, mica, amphibole, epidote and chlorite.”

var. Hornblende: Panteleyev et al. (1976) identified hornblende in thin sections of porphyritic intrusive rocks.

Anhydrite: Panteleyev (1981) wrote that “Orthoclase, quartz. sericite, and biotite are main alteration minerals [in the potassic alteration zone] together with some anhydrite, sulphide minerals, chlorite, magnetite, kaolinite, and montmorillonite.”

Antlerite: Heberlein (1995) reported antlerite in the supergene oxide zone – this is the only reference to the mineral in the various papers reviewed.

Apatite: Panteleyev, describing the intrusive breccia, wrote that “Sulphide and accessory minerals including apatite, zircon, topaz, rutile, and possibly sphene constitute 4 to 6 per cent of the rock, mainly in the matrix.”

Arsenopyrite: Sutherland Brown (1967), describing the primary mineralization at Berg, wrote that “Primary mineralizing sulphide minerals include chalcopyrite, molybdenite, and pyrite with minor sphalerite, galena, and arsenopyrite.” Owens (1968) identified arsenopyrite in a polished section prepared from the 65 to 250 mesh material screened from the crushed “head sample” of the ore. It is important to note that Owens (1968) had a relatively limited number of small samples to work with.

Azurite: See note above for akageneite. Owens (1968) identified azurite in the “head sample” material.

Biotite: See note above for anhydrite.

Bornite: Owens (1968) wrote that “The only bornite . . . found during the examination of the ore consisted of a few inclusions in pyrite. These inclusions are very small and range from 2 to 10 microns in size.”

Brochantite: See note above for akageneite. Note that Heberlein (1995) mis-spelled the mineral “brochanthite”.

Calcite: This was noted by several workers. Heberlein (1995) reported that it occurs in type 3a and 3b veins.

Chalcocite: Owens (1968) wrote that “Only a very few grains of chalcocite . . . and delafossite
. . . were found in the head sample. Their presence is consistent with the remainder of the ore assemblage.”

Chalcanthite: See note above for akageneite.

Chalcopyrite: Primary (hypogene) ore minerals are mainly pyrite, chalcopyrite, and molybdenite. In addition to the three main ore minerals, magnetite, sphalerite, tennantite, and galena are present as well as trace amounts of arsenopyrite, pyrrhotite, scheelite, ilmenite, hematite, and rutile.

Chlorite group: See note above for anhydrite. Panteleyev (1981) expanded somewhat, writing that “Supergene alteration caused by weathering, oxidation, hydration, hydrolysis, recrystallization, and leaching by acidic solution is extensive and has resulted in a thick leached capping containing mainly quartz, limonite, sericite, chlorite, and clay minerals. Thin section and X-ray diffraction analyses of clay-sized minerals in the capping reveal marked increases in amounts of quartz, kaolinite, sericite (illite), chlorite, and amorphous hydrates compared to rocks subjected only to hypogene alteration.”

Covellite: Owens (1968) wrote that “The chalcopyrite [in polished sections] is frequently rimmed by digenite and covellite.”

Cuprite: See note above for akageneite.

Delafossite: See note above for chalcocite.

Digenite: See note above for covellite.

Dolomite: Owens (1968) wrote that “The gangue minerals [in the ore samples] consist chiefly of quartz and dolomite, with small amounts of garnet, mica, amphibole, epidote and chlorite.”

Epidote: This is common, reported by numerous workers.

Ferrimolybdite: See note above for akageneite.

Fluorite: Heberlein and Godwin (1984) wrote that “A unique feature of this subzone [their biotite-anhydrite subzone] is the presence in vein envelopes of trace amounts of fluorite and topaz . . . .” This comment was based on an original observation by Stewart (1967).

Feldspar Group: Heberlein (1995), discussing supergene alteration, wrote that “Wall rock reactivity is low because the dominant gangue minerals are quartz and feldspar.”

var. Albite: Panteleyev (1981), discussing the plagioclase-biotite porphyry, wrote that “Plagioclase phenocrysts and matrix grains are sericitized but twinned crystals, mainly albite and combined Carlsbad-albite types are common and only moderately affected by alteration.”

var. Andesine: Panteleyev (1981) reported andesine in thin sections. Note that other plagioclase species may well be present, but it seems pointless to list these in detail.

var. Orthoclase: See note above for anhydrite.

Galena: See note above for chalcopyrite.

Garnet: See note above for dolomite. Owens (1968) provided no specific information regarding the garnet.

Goethite: See note below for limonite.

Gypsum: Panteleyev (1981) wrote that “This study is concerned primarily with defining hypogene alteration types and zoning patterns. At Berg deposit the distinction between rocks with solely hypogene or combined hypogene-supergene alteration is indicated clearly in drill core by fractures filled with gypsum that mark the limit to which present day groundwaters have circulated. Supergene alteration has taken place only near surface where gypsum is leached. In the zone with gypsum-healed fractures, groundwater flow following hydrothermal activity has been minimized and all alteration can be assumed to be hydrothermal and hypogene.”

Hematite: See note above for chalcopyrite.

var. Specularite: Harris (2012), describing the West Zone supergene zone, wrote that “Also of particular note is the abundance of significant Ag mineralization in this zone related to quartz-calcite-sphalerite-pyrite-specularite±galena veining.”

Ilmenite: See note above for chalcopyrite.

Jarosite: See note below for limonite.

Kaolinite: See notes above for anhydrite and chlorite.

Limonite: See note above for chlorite. Panteleyev (1981) wrote about the “limonite” at Berg in some detail; in summary, his conclusion was that “Breakdown of sulphide minerals near surface has resulted in a leached capping consisting of mainly quartz, sericite, clay minerals, limonite, and opaline silica. Limonite is composed primarily of amorphous ferric hydroxide [hydrogoethite] . . . goethite . . . and minor hematite . . . with locally developed jarosite . . . .” Further, Panteleyev (1981) wrote that “The most abundant accumulation of limonite is ferricrete, a relatively homogeneous deposit of ferric hydroxide. Ferricrete is transported amorphous hydrogoethite and goethite precipitated on surface or in overburden as soft, porous, friable limonite. Ferricrete deposits commonly contain plant and rock fragments and other detritus. Some ferricrete forms a matrix in soil and talus deposits resulting in cemented soil and breccialike deposits. At Berg, ferricrete is being deposited actively in creek gullies and along the base of slopes where groundwater discharges. Such deposits mantle much of the lower slopes along the north fork of Bergeland Creek over hundreds of square metres and locally are up to 2 metres in thickness.”

Magnetite: See notes above for anhydrite and chalcopyrite.

Malachite: See note above for akageneite.

Molybdenite: See note above for chalcopyrite. Owens (1968) concluded that “. . . the molybdenite found in the ore is of a very fine grain size, and difficulty may be encountered in achieving liberation of many of the grains.”

Montmorillonite: See note above for anhydrite.

Muscovite: Sutherland Brown (1967) and Panteleyev (1981) both reported muscovite. Heberlein and Godwin (1984) suggested that the muscovite might in fact be fluoromuscovite.

var. Illite: See note above for chlorite.

var. Sericite: See note above for anhydrite.

Native Copper: See note above for akageneite.

Pyrite: See note above for chalcopyrite.

Pyrrhotite: See note above for chalcopyrite.

Quartz: This is ubiquitous, as a vein constituent, as a rock-forming mineral and as an alteration product.

Rutile: See note above for chalcopyrite.

Scheelite: See note above for chalcopyrite.

Sphalerite: See note above for chalcopyrite.

Tenorite: See note above for akageneite.

Tetrahedrite Subgroup: See note above for chalcopyrite. Owens (1968) wrote that “. . . although a number of tennantite . . . grains are present in the head sample, none were found in the polished sections of the rock fragments, and therefore their grain size and textural relationships to the other minerals could not be fully assessed. However, several grains of tennantite in the head sample were found to be veined by covellite. Because of the number of grains in the head sample, and the general mineralogical assemblage of the ore, the tennantite is believed to be a valid constituent in the ore, rather than having been introduced by contamination.”

Titanite: Sutherland Brown (1967), describing the quartz bearing monzonite porphyry, wrote that “Sphene is a common accessory phenocryst.”

Topaz: See note above for fluorite.

Zircon: See note above for apatite.

Comments on selected rock types reported:

Breccia: There are numerous examples of breccias of various types reported for the property, of which the most noteworthy is located a few hundred metres south of the deposit. It was described by Panteleyev (1981) as follows: “The centre of the intrusive breccia pipe is about 750 metres to the southeast of Berg camp. The breccia mass is elliptical in plan, approximately 580 metres along its longer east-southeast axis and about 175 metres wide. It intrudes quartz diorite and pyritic volcanic rocks at the outer edge of the zone of mineralization. Outcrops of breccia are
deeply weathered, pale cream to yellow, and contrast sharply with surrounding darker brown outcrops and debris.”; he went on to write that “The breccia pipe is thought to be explosive in origin and formed by venting of volatiles related to magma intrusion or by phreatic explosion of groundwater. Abundance of porphyry fragments and presence of a few myrmekitic quartz grains indicate that the breccia is related genetically to the mineralized stock. C. S. Ney (1969, personal
communication) and Sutherland Brown (1967) have suggested that the breccia pipe is a late structure related to emplacement of young intrusive phase(s) possibly 'quartz latite porphyry' (now called hornblende quartz feldspar porphyry). However, the breccia may be older. Brecciation postdates at least one period of mineralization as quartz veinlets with molybdenite were seen in one milled breccia fragment.”

Dacite: The only reference to dacite seen in the papers reviewed was by Heberlein (1995), who reported dacite flows in the Middle to Upper Cretaceous Kasalka Group volcanic flow rocks on Mount Ney, about a kilometre north of the main Berg deposit.

Felsite: Panteleyev et al. (1976), describing the breccia unit, wrote that “In one drill hole, a creamy buff-coloured felsite that has a chilled contact against the breccia appears to have intruded the pipe. Its significance is not fully understood.”

Monzonite: The only reference to monzonite seen in the papers reviewed was by Heberlein (1995), who wrote that “A post mineralization monzonite dike cuts the Berg stock along a northeast axis. Large cream-coloured phenocrysts of plagioclase and black biotite books make up 35% of the rock. These become sparse close to the margins of the dike in a distinct chilled zone. Quartz phenocrysts are typically rounded and show evidence of resorption. The groundmass is deficient in quartz compared to the other monzonite phases.”

Skarn: Sutherland Brown (1967), describing the contact metamorphism of the andesitic tuffs, wrote that “Such rocks in the hornfelsic aureole may be converted into rusty-weathering purply-brown biotitic hornfels, or into metasomatized rocks in which the origina1 texture is scarcely visible, if at all. The most common skarn is composed of a mosaic of fine new quartz, biotite, and potassium feldspar with palimpsest remnants of original grains shown by varying proportions of these minerals and earlier chlorite, plagioclase, and kaolinite.” Panteleyev (1981) expanded on this subject, writing that “At Berg deposit, Sutherland Brown (1967) distinguished between biotitic hornfels in a thermal aureole and hydrothermally metasomatized rocks he termed skarn, containing recrystallized quartz, biotite, and K-feldspar. He further subdivided hydrothermally altered rocks into a third group of mottled 'greisen-like' (quartz-sericite) rocks. However, origin of biotite cannot always be ascribed on the basis of appearance to either purely contact metamorphism or metasomatism and the term 'hornfels' will be retained in this report for
purely descriptive purposes. Thus, biotite hornfels refers to all the massive, dark, fine-grained
metamorphosed rocks surrounding Berg intrusions. While some epidote and rare garnet occur in altered calcareous beds along the quartz diorite contact or in pendants in quartz diorite, no skarn assemblages are present within Berg deposit [my emphasis].”

Research by Giles Peatfield, Courtenay, British Columbia.
Edited by Doug Scott, Ottawa
Posting prepared 18 December, 2025.

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