Deposit Type

Mouse Mountain, one of the two main targets in the western project area, is considered an alkalic porphyry copper-gold occurrence. Mouse Mountain is immediately next to, or above, small, high level, subvolcanic, magnetic, alkalic, quartz-poor, intrusive bodies that invade Nicola volcanic rocks. This setting closely resembles that of alkalic porphyry copper-gold-PGE deposits found in the Quesnel Trough in central BC. The following description of the deposit type is taken from Panteleyev (1995) and from his description of deposit type L03 in www.mapplace.ca.

The deposits consist of stockworks, veinlets and disseminations of pyrite, chalcopyrite, bornite and magnetite occur in large zones of economically bulkmineable mineralization in or adjoining porphyritic intrusions of diorite to syenite composition. The mineralization is spatially, temporally and genetically associated with hydrothermal alteration of the intrusive bodies and host rocks.

They occur in orogenic belts at convergent plate boundaries, commonly oceanic volcanic island arcs overlying oceanic crust. Chemically distinct magmatism with alkalic intrusions varying in composition from gabbro, diorite and monzonite to nepheline syenite intrusions and coeval shoshonitic volcanic rocks, takes place at certain times in segments of some island arcs. The magmas are introduced along the axis of the arc or in cross-arc structures that coincide with deep-seated faults. The alkalic magmas appear to form where there is slow subduction in steeply dipping tectonically thickened lithospheric slabs, possibly when polarity reversals (or 'flips')
take place in the subduction zones. In British Columbia all known deposits are found in Quesnellia and Stikinia terranes.

The environment of deposition is in high level (epizonal) stock emplacement levels in magmatic arcs, commonly oceanic volcanic island arcs with alkalic (shoshonitic) basic flows to intermediate and felsic pyroclastic rocks. Commonly the high-level stocks and related dikes intrude their coeval and cogenetic volcanic piles. Deposits in the Canadian Cordillera are restricted to the Late Triassic/Early Jurassic (215-180 Ma) with seemingly two clusters around 205-200 and ~ 185 Ma. In southwest Pacific island arcs, deposits are Tertiary to Quaternary in age.

Intrusions range from fine through coarse-grained, equigranular to coarsely porphyritic and, locally, pegmatitic high-level stocks and dike complexes. Commonly there is multiple emplacement of successive intrusive phases and a wide variety of breccias. Compositions range from (alkalic) gabbro to syenite. The syenitic rocks vary from silica- undersaturated to saturated compositions. The most undersaturated nepheline normative rocks contain modal nepheline and, more commonly, pseudoleucite. The silica-undersaturated suites are referred to as nepheline alkalic whereas rocks with silica near-saturation, or slight silica over saturation, are termed quartz alkalic (Lang et al., 1993). Coeval volcanic rocks are basic to intermediate alkalic varieties of the high-K basalt and shoshonite series and rarely phonolites. Deposit boundaries are generally determined by economic factors that outline ore zones within larger areas of low-grade, laterally zoned mineralization.

Deposit boundaries are generally determined by economic factors that outline ore zones within larger areas of low-grade, laterally zoned mineralization.

The principal ore minerals are chalcopyrite, pyrite and magnetite. Bornite, chalcocite and rarely galena, sphalerite, tellurides, tetrahderite, gold and silver are subordinate. Pyrite is less abundant than chalcopyrite in ore zones.

Alteration minerals include biotite, K-feldspar, sericite, anhydrite/gypsum, magnetite, hematite, actinolite, chlorite, epidote and carbonate. Some alkalic systems contain abundant garnet including the Ti-rich andradite variety - melanite, diopside, plagioclase, scapolite, prehnite, pseudoleucite and apatite; rare barite, fluorite, sodalite, rutile and late-stage quartz. Central and early formed potassic zones, with Kfeldspar and generally abundant secondary biotite and anhydrite, commonly coincide with ore. These rocks can contain zones with relatively high-temperature calcsilicate minerals diopside and garnet. Outward there can be flanking zones in basic volcanic rocks with abundant biotite that grades into extensive, marginal propylitic zones. The older alteration assemblages can be overprinted by phyllic sericite-pyrite and, less commonly, sericite-clay-carbonate-pyrite alteration. In some deposits, generally at depth in silica-saturated types, there can be either extensive or local central zones of sodic alteration containing characteristic albite with epidote, pyrite, diopside, actinolite and rarer scapolite and prehnite.

The main ore controls are igneous contacts between intrusive phases and with wallrocks, cupolas and the uppermost, bifurcating parts of stocks, dike swarms and volcanic vents. Breccias, mainly early formed intrusive and hydrothermal types are an important ore control. Zones of most intensely developed fracturing give rise to oregrade vein stockworks.

Porphyry deposits are subdivided on arbitrary economic criteria, mainly ratios between Cu, Au and Mo. Differences in composition between the host rock alkalic and calcalkalic intrusions and subtle, but significant, differences in alteration mineralogy and zoning patterns provide fundamental geologically based contrasts between deposit model types.