Geology

Mouse Mountain geology is difficult; the rocks are fairly well exposed on the mountain, but they are fine grained and altered and hence ambiguous and difficult. The volcanic rocks are dominantly fragmental, but textures are obscure on fresh surfaces and saussuritization pervasive. Broadly Mouse Mountain is underlain by Late Triassic volcanic rocks with augite basalt at the base (?) and volcanic breccia above. Augite basalt is dark green, massive and fine grained but is distinguished by its stubby subhedral black augite phenocrysts to 5cm across. Volcanic breccia is massive dark green grey and purplish on fresh surfaces and immature. Angular fragments of a range of mafic to intermediate volcanic rocks and up to several cm across predominate. The matrix is of the same material but finer grained. Mostly the clasts are matrix supported.

The basalt -- breccia contact trends northwest, the general trend of layering in the region. Greywacke and slate are interlayered with the breccia locally as lenses of several metres. The greywacke is generally massive and very immature with angular grit sized volcanic debris in a dark volcanic matrix. Layering is seen rarely in the slate and greywacke; as these rocks occur only locally no general trend is seen. The thickness of the assemblage is unknown; it may be no more than two or three thousand metres.

Slate, like that intercalated with the volcanic rocks, occurs extensively east of Mouse Mountain. Its relationship to the volcanic-intrusive unit is not exposed. Most likely the slate and volcanic-intrusive assemblages are coeval and laterally equivalent; part of the eastern slate may predate the volcanic-intrusive rocks.

A plug of undersaturated very fine grained intrusive rock, under the high part of Mouse Mountain, intrudes the volcanic assemblate. It is thought to be Early Jurassic and broadly coeval with the Nicola Group.

Deformation is limited; the slate and greywacke are not folded where layering is observed. Observed faults are also minor and presumably of slight displacement. On the whole the rocks are competent and only fractured and jointed. Alteration is pervasive; volcanic and volcaniclastic rocks are strongly saussuritized. In many places, including near showings, rusty weathering iron carbonate alteration is seen as a late overprint of the rocks. The alteration is seen in the fragmental and intrusive rocks but not in the augite porphyry or greywacke.

Three generations of geological map are available for the property and all three are given here for comparison (Figure 11, 12, and 13). The first is from Sanguinetti (1989), the next from Donkersloot (1991, 92) and the most recent from this summer's work by Jonnes (2006). The three maps differ markedly illustrating the difficulty of mapping this area. The three generations of maps agree on the basalt-fragmental division and the location of the contact between these two groups, but they disagree markedly on the location and extent of the intrusive rocks and its phases. Also different are the interpretations of the fragmental rocks, their origin and relations.

Geology of Mouse Mountain according to Sanguinetti. This map portrays the geology as mapped by Placer Dome geologists and reported by Sanguinetti,(1989). The known showings are hosted in intrusive rocks according to this interpretation.

Geology of Mouse Mountain according to Donkersloot (1992). Known showings are restricted to the volcaniclastic rocks of the Nicola Group. Areas around the Valentine and Rainbow zones are altered over a considerable area as shown. The late volcanic or post-volcanic syenite to monzonite is exposed around the "high grade" showing.

Geological map of the same area as in the previous two figures showing the geology as mapped by Jonnes (2006). Note the agreement between the three authors on the augite basalt-fragmental volcanic contact and the general agreement on the intrusive rocks between Jonnes and Sanguinetti.