Preliminary Map 56 portrays the geology of the Adams Plateau-Clearwater area. The map, which covers an area of approximately 3000 square kilometres, within the Shuswap Highland of south-central British Columbia. It is about 80 kilometres north-northeast of Kamloops and is bounded by latitudes 51o00' and 51o45' north, and longitudes
119o30' and 120o15' west. The topography is dominated by the North Thompson River valley in the north and west,
and the Adams Lake valley in the east. The map is accompanied by six cross-sections. It is the result of geological mapping by Ministry staff during the 1978 to 1981 field seasons. The map-area is largely underlain by Paleozoic metavolcanic and metasedimentary rocks of the Eagle Bay and Fennel Formations which host numerous mineral deposits. Note, for detailed descriptions, see Paper 1987-2.
The map area covers a belt of structurally complex low-grade metamorphic rocks that lies along the western margin of the Omineca Belt. It is flanked by high-grade metamorphic rocks of the Shuswap Complex to the east and by rocks of the Intermontane Belt to the west. The area is underlain mainly by Paleozoic metasedimentary and metavolcanic rocks of the Eagle Bay Assemblage and the Fennel Formation. Late Devonian granitic orthogneiss locally intrudes Eagle Bay rocks. The Paleozoic rocks are cut by mid-Cretaceous granodiorite and quartz monzonite of the Raft and Baldy batholiths, and by Early Tertiary quartz feldspar porphyry, basalt and lamprophyre dykes. They are locally overlain by Eocene sedimentary and volcanic rocks of the Kamloops Group and by Miocene plateau lavas.
The Paleozoic rocks occur in four structural slices separated by southwesterly-directed thrust faults. The upper three fault slices contain only Eagle Bay rocks, while the lowest slice comprises Eagle Bay strata structurally overlain by the Fennell Formation.
Rocks assigned to the Eagle Bay Assemblage range in age from Early Cambrian to Late Mississippian. They are in part correlative with Paleozoic successions in the Kootenay Arc and in the Barkerville-Cariboo River area. The oldest Eagle Bay rocks comprise quartzites and quartzose schists overlain by a unit of predominantly mafic metavolcanic rocks and limestone which, at one locality, contains Lower Cambrian archaeocyathids. An undated package of grit, phyllite, carbonate and metavolcanic rocks overlies the Early Cambrian succession. It is locally overlain by calcareous phyllite and associated calc-silicate schist and skarn or by mafic metavolcanic rocks. The upper part of the Eagle Bay Assemblage comprises a Devono-Mississippian succession consisting of felsic metavolcanic rocks overlain by intermediate, locally alkalic, metavolcanics and fine to coarse-grained clastic metasediments. These Devono-Mississippian rocks may be separated from older portions of the Eagle Bay Assemblage by a significant unconformity. Late Devonian orthogneiss that intrudes Eagle Bay rocks is probably related to the felsic metavolcanics.
The Fennell Formation comprises imbricated oceanic rocks of Slide Mountain terrane that were tectonically emplaced onto Mississippian clastic rocks of the Eagle Bay Assemblage prior to synmetamorphic southwesterly directed folding and thrusting. The formation comprises two major divisions. The lower structural division is a heterogeneous assemblage of bedded chert, gabbro, diabase, pillowed basalt, sandstone, quartz-feldspar-porphyry rhyolite and intraformational conglomerate. Conodonts extracted from bedded chert range in age from Early Mississippian to Middle Permian, while zircons extracted from quartz feldspar porphyry yield a Devonian uranium-lead age. The distribution of dated units indicates that the lower division is segmented into at least three and locally four imbricate thrust slices. The upper division consists almost entirely of pillowed and massive basalt, together with minor amounts of bedded chert and gabbro. Conodonts from two separate chert lenses within the division are respectively Early (?) Pennsylvanian and Middle Permian in age. The two divisions are therefore the same age, at least in part, and are inferred to be separated by a thrust fault.
Rocks of the Fennel Formation accumulated in a deep oceanic basin an unknown distance west of partially coeval rocks of the Eagle Bay Assemblage. Sandstone within the Fennell is very similar to Mississippian sandstone of the Early Bay Assemblage and may have been derived from it. Similarly, Devonian quartz-feldspar-porphyry rhyolite within the Fennell is like Devonian felsic volcanic rocks of the Eagle Bay Assemblage and may be an expression of the same igneous activity. The Fennell succession is inferred to comprise an imbricated marginal basin suite that was originally not far removed from the Eagle Bay terrane.
Deformation is predominantly Jura-Cretaceous and early Tertiary in age. The earliest macroscopic structures are the thrust faults that imbricate the Fennell Formation and separate it from Mississippian clastic rocks of the Eagle Bay Assemblage. East-verging, pre-metamorphic mesoscopic folds within the Fennell Formation probably also formed at this time. Tectonic emplacement of the Fennell Formation was followed by synmetamorphic southwesterly directed folding and associated thrust faulting, which gave rise to several large overturned folds, and also the thrust faults which define the structural/stratigraphic panels that dominate the map pattern. The associated synmetamorphic schistosity is the dominant mesoscopic fabric within the area. These early structures are cut by post-metamorphic northwest-trending mesoscopic folds with associated steeply dipping crenulation cleavage and axial crenulation lineation, and by later west trending macroscopic and mesoscopic folds which are synchronous with intrusion of the mid-Cretaceous Raft and Baldy batholiths. The youngest structures recognized are northeast-trending strike-slip faults and later northerly trending faults and associated folds that are Eocene in age.
Stratabound massive to semimassive sulphides containing mainly silver, lead and zinc occur on Adams Plateau where they are hosted by metasedimentary rocks within a succession of dominantly mafic metavolcanic Eagle Bay rocks of probable Early Cambrian age. Similar deposits occur within a correlative (?) interval near Mount McClennan.
Polymetallic precious and base metal massive sulphide occurrences are hosted by Devono-Mississippian felsic to intermediate metavolcanic rocks of the Eagle Bay Assemblage. They include the Homestake prospect and a number of other occurrences in the Sinmax Creek - Adams Lake area, as well as several showings along Birk Creek. Overlying alkalic rocks host similar mineralization at the recently discovered Rea deposit southwest of Johnson Lake.
Large-tonnage disseminated pyrite-pyrrhotite-chalcopyrite mineralization, such as that at the Harper Creek deposit, occurs within Eagle Bay rocks where they are intruded by Devonian orthogneiss. This spatial relationship suggests that the mineralization is Devonian in age and related to the orthogneiss, although it has been remobilized during later deformation and metamorphism.
Uranium-thorium-fluorite mineralization is hosted by a trachytic intrusive-extrusive complex within Devono-Mississippian Eagle Bay rocks along Foghorn and Lute creeks (Rexspar and Bullion showings).
The CC deposit is a cupriferous pyrite volcanogenic massive sulphide deposit hosted by oceanic basalts of the Fennel Formation east of Chu Chua Mountain.
The area also contains numerous vein deposits, as well as small pods of skarn mineralization near Cretaceous and Devonian granitic intrusions. The veins contain mainly silver-lead-zinc mineralization although gold was produced from the Sweet Home and Gold Hill veins within the Fennell Formation east of Dunn Lake.