Summary:
The Kemess Property is host to the Kemess South (KS), Kemess Underground (KUG) and Kemess East (KE) Deposits as well as the Kemess Offset Zone (KOZ) prospect.
The Kemess Underground property is predominantly underlain by a thick (>1,000 m) succession of andesitic flows (Takla Group). The Takla volcanic rocks host a significant portion of the copper-gold mineralization and display phyllic alteration at Kemess North. On the surface is mostly a distinctive feldspar porphyritic unit (bladed feldpsar porphyry – BFP) with some sections of a distinctive augite phyric unit (augite andesite porphyry – AAP). Mantling the northern and eastern limits of the Kemess Underground area is the Hazelton Group-Toodoggone formation. The structurally controlled phyllic sections of this polylithic fragmental dacite in the southeastern area of the deposit can carry anomalous gold concentrations. The evidence suggests that basement structures and conduits that allowed extrusion of the local Toodoggone volcanic assemblage underlie the Kemess Underground area.
The most prominent structure traversing the Kemess Underground area is the Kemess North (KN) Thrust Fault, an east-west trending south dipping reverse fault that truncates the Kemess North pluton and associated mineralization at depth. The KN Fault and the Kemess North pluton seem to be related with the KN Fault creating the pathway for the KUG pluton to intrude. Further movement along the KN Fault subsequently truncated the Kemess North Pluton. The KN Fault is complex in that there is a related north dipping secondary structure that demonstrates apparent normal offset with at least 1,000 m of southside up- apparent displacement. To the east of the Kemess Underground area the deposit is cut off by a northwest trending normal fault and north trending steeply dipping normal faults causing eastside down-apparent displacement. These north trending faults are interpreted as the youngest in the Kemess Underground area. To the south of the Kemess Underground area is another west trending fault forming the headwall in East Cirque, as drilling did not reaching adequate depths it is unknown the type of movement and displacement. Fault spacing ranges from 500 to 1,500 m.
Gold-copper mineralization forms an inclined tabular zone that is centred on the East Cirque (note: not the cirque in which the Kemess East deposit is located) porphyritic monzodiorite, which form structural contours, strikes east-west and dips 20° to the south. The quartz diorite/quartz monzonite intrusive exhibits an irregular upper contact with various peaks and troughs. The general east west strike and shallow south dip geometry is consistent for over 400 m (10,660E to 10,180E). Between 10,260E and 10,160E the tabular morphology disappears and the monzonite occurs as wide dykes (10–100 m) within the Takla volcanics. The change in geometry for the monzonite could be due to the effects of cross faulting which have down dropped the tabular upper contact present in the East Cirque, or the rheologic conditions during intrusion changed going towards the west whereby steep fracture infilling was preferred over stopping.
Alteration and mineralization is associated with and zoned both vertically and laterally from the quartz diorite/quartz monzonite intrusive and its associated dykes intersected at depth beneath the Central and East Cirques.
The sulphate leach zone, which consists mostly of iron oxides, sericite-chlorite-quartz-pyrite forms an extensive broken zone beneath bright orange-red outcrops at surface (hematite-limonite; McKinley 2006). Pyrite is common throughout (5–7%) as both disseminated and within vuggy quartz veining. This alteration zone is mostly barren of any significant copper and will often show a slight increase in gold with depth. The zone is a result of dissolution of the sulphate and carbonate minerals by highly acidic ground waters present currently and probably during the Jurassic.
Present over the entire area in all rock units except the late mafic dykes are barren pinkish zeolite-carbonate veins, which post-date and crosscut the above vein types and rock units. The zeolite-carbonate veinlets are a low temperature phenomenon.
Overall, sulphide mineralization throughout the deposit consists of 2–3% pyrite, with lesser amounts of chalcopyrite and traces of molybdenum. Pyrite occurs as disseminations, fracture fillings, and veins up to a few centimetres wide generally associated with quartz-gypsum-magnetite veins and zones of quartz- magnetite replacement. The mode of occurrence of chalcopyrite is similar except that veinlets are rare and significant disseminations occur in zones of stronger quartz-magnetite stockwork and quartz- magnetite replacements. Gold and copper grades variably diminish outward into the hangingwall and footwall. Total sulphide content in the core of the deposit averages 3–5%, rising to 10–12% in the phyllic halo.
Petrography shows a varying degree of accessory minerals throughout all rock types and alteration zones including: rutile, leucoxene, sphene, anhydrite, gypsum, epidote, zeolite, alunite, molybdenite, phlogopite, prehnite, and apatite.
Kemess North is a copper-gold porphyry deposit and is typical of calc-alkaline porphyry copper-gold deposits in the western cordillera. The deposit has a low-grade ore zone at a depth of 150 m below the surface on its western flank and a higher grade zone 300–550 m below surface on the eastern side, which forms the Kemess Underground project. Kemess Underground is hosted by potassic altered Takla Group volcanic rocks and Black Lake plutonic rocks. The deposit is centered on a mineralized porphyritic monzodiorite/diorite pluton and associated WSW trending dykes, which extend to the southwest. Higher- grade copper-gold mineralization is characterized by secondary biotite alteration in volcanic and the eastern plutonic host rocks.
Porphyry style copper-gold mineralization occurs within the Takla volcanic rocks and intermediate intrusive rocks associated with weak to pervasive propyllitic, phyllic, and potassic (biotitic) alteration assemblages. The latter is associated with higher copper and gold grades. Alteration of Toodoggone assemblages ranges from fresh to weak propyllitic and is generally barren of significant sulphides and ore grade mineralization.