Summary:
The Morrison deposit is classified as a calc alkaline copper-gold porphyry with an alkalic trace element signature (Ogryzlo et al., 1995), which may reflect a mixed alkaline/calc alkaline parentage for the Babine Igneous Suite. The geologic settings of the host rock relationships, structural development, and the general style of the hydrothermal alteration and mineralization at Morrison are similar to other porphyry deposits in the northern Babine Lake area.
The Morrison deposit is situated on the northern edge of the Skeena Arch in a region underlain by volcanic, clastic, and epiclastic rocks ranging in age from the Lower Jurassic to Lower Cretaceous, including the Takla Group, Hazelton Group, Bowser Lake Group, Skeena Group, and Sustut Group (Carter, 1976). The rock units are disrupted by a series of dominantly north to northwesterly-trending faults into uplifted blocks, downfaulted grabens, and tilted fault blocks. This has resulted in older lithologic units being juxtaposed and locally truncated against younger rock units.
Intrusive rocks in the area include the Early Jurassic diorite and granodiorite Topley Intrusions, Eocene rhyolite and rhyodacite intrusions, and most importantly from an economic viewpoint, the Eocene Babine Igneous Suite which consists of quartz, hornblende, biotite, and plagioclase phyric intrusions (Carson and Jambor,1976).
The dominant geological feature on the Morrison Property is the Morrison Graben that transects the property in a north-northwesterly trend. The 1.5 to 2.0 km wide graben is spatially host to the siltstone, sandstone, and greywacke sedimentary sequence of the Upper Jurassic Ashman Formation on the northern half of the property, and younger sandstone, shale, and siltstone units of Lower Cretaceous Skeena Group to the south. Much of this southerly part of the graben is overlain by glacial overburden. These lithologic sequences have been down-faulted into the graben structure relative to the older volcanic and sedimentary rock units of Lower to Middle Jurassic Telkwa Formation, Saddle Hill Volcanics, and Smithers Formation that flank the Morrison Graben to the east and west. All of the above rock units are locally intruded by Eocene-age Babine Intrusions that occur as small stocks, plugs, and dyke-like bodies of biotite feldspar porphyry, quartz diorite, and granodiorite. More importantly on the Morrison Property, the copper-gold porphyry mineralization is developed in a BFP plug and related dyke-like bodies that intrude the siltstone/sandstone unit of the Ashman Formation.
Hydrothermal alteration at Morrison is similar to that at other Babine porphyry copper deposits (Carson and Jambor, 1974). Alteration is concentrically zoned with a central biotite (potassic) alteration core surrounded by a chlorite-carbonate zone. A third alteration facies, clay-carbonate alteration, is considered retrograde and associated with major faults and shears and subsidiary fracture zones. No well developed phyllic zone has been identified.
Sulphide mineralization at Morrison shows strong spatial relationships with the underlying intrusive (BFP) plug and associated alteration zones. The central copperrich core is hosted mainly within a potassically altered BFP plug with intercalations of older siltstone. This plug was initially intruded into the siltstone unit as a near-vertical subcircular intrusion approximately 700 m in diameter. It was subsequently disrupted by the East and West Faults and now forms an elongated body extending some 1,500 m in the northwest direction.
Chalcopyrite is the primary copper-bearing mineral and is distributed as fine grained disseminations in the BFP and siltstone, as fracture coatings or as stockworks of quartz veinlets in which the chalcopyrite occurs as coarse grains (1-3 mm) within veinlets that range from 1.0 mm to approximately 15 mm in width. Minor bornite occurs within the higher grade copper zones as disseminations and associated with the quartz-sulphide stockwork style of mineralization.
Polished-section studies have also shown that, in addition to chalcopyrite and pyrite, magnetite and minor bornite are present in the low-grade core of the deposit. Magnetite is a finely disseminated original constituent of the BFP and siltstones, and is most abundant in the western segment of the copper zone. Many magnetite grains are partly altered to hematite, which seems to be most abundant at the outer 0.2% Cu boundary. No iron oxides have been observed in the pyrite halo.
Diamond drilling, geological mapping, and detailed polished-sections studies performed by Caron and Jambor (1976) indicate that pyrite and chalcopyrite have a well-defined zonal relationship. Although pyrite predominates in the pyrite halo, the 0.2% copper isopleth precisely marks a change in pyrite-to-chalcopyrite ratios; chalcopyrite consistently exceeds pyrite in samples only from the inside of this boundary. Although the absolute abundance of pyrite decreases toward the centre of the Morrison deposit, disseminated grains of pyrite persist throughout the copper zone and in the low-grade core.
Molybdenum is present in smaller and somewhat spatially restricted amounts, particularly in the southeast portion of the deposit. Rare arsenopyrite and sphalerite have been noted locally in carbonate-cemented brecciated veins within and near the faults and in smaller parallel shears.
A pyrite halo is developed in the chlorite-carbonate altered wall rock surrounding the copper zone. The pyrite mineralization characteristically occurs as thin (0.1 to 5.0 cm) fracture-fillings and quartz-pyrite-minor chalcopyrite stringers in the form of stockwork within the halo. There is a crude zonation to the pyrite development with coarse (0.5 to 5.0 mm) disseminated crystals within the inner parts of the halo where pyrite content ranges from 5 to 15% by volume. Pyrite in the outer zone is predominantly developed as a stockwork and averages 1 to 2% by volume accompanied by weak copper mineralization (<0.1%). The pyrite halo is developed as a more extensive zone around the eastern and southeastern segment of the Morrison deposit. Drilling and geophysical surveys indicate that the halo at this position attains widths up to 500 m with up to 15% pyrite for the inner margin and decreasing abruptly to 1 to 2% in the outer two thirds of the halo. The pyrite halo is more restricted at the western and northwestern segments of the deposit where pyrite abundances decrease more gradually to the 3 to 5% range. The siltstone host rock at this location is intruded by large northerly-trending BFP and rhyodacite dykes.