OKANAGAN VALLEY REGION 1 Regional Geology

The main physiographic components of the area are the Shuswap-Okanagan Highlands, representing the area east of the dry belt boundary shown on Figure 2 and the Okanagan Valley region, which occupies most of the area to the west of this boundary.

The Shuswap metamorphic terrane contains the oldest rocks in the area. Rock types include gneisses, quartz! t es, schists and limestones within the Monashee and Mount Ida Groups. Metavolcanic-metasedimentary sequences of Palaeozoic-Mesozoic age, consisting of greenstone-quartzite-argillite-limestone assemblages, occur as isolated roof pendants on the Shuswap metamorphic and Mesozoic intrusive complexes.

Major intrusive activity probably began in Early to Middle Jurassic times with the emplacement of the West Okanagan intrusions consisting of the Okanagan, Oliver, Shorts Creek and Vernon bodies in the Okanagan Valley region. The activity continued into Late Cretaceous times with the emplacement of the Okanagan Highlands intrusive complex, apophyses of which also occur in the Okanagan Valley region. These intrusives consist mainly of porphyritic granite and quartz monzonite, with lesser amounts of granodionte.

Volcanic-sedimentary rocks of Eocene age formed in isolated basins in the southwestern half of the region. These rocks consist mainly of basal sediments and intermediate (alkalic) and acid volcanics. The Coryell alkaline intrusions are coeval with the Tertiary volcanics.

The structural pattern observed in Figure 2 is largely the result of Tertiary tectonics. Subsequent to emplacement of the main intrusive phases, the period between Late Cretaceous and Early Eocene appears to mark a transition between waning compressional tectonics on a NE-SW axis (Laramide Orogeny) and the development of

Figure 1

Location of south central dry belt region {solid line), British Columbia. Numbers refer to major urban centres mentioned in text. 1. Kamloop, 2. Kelowna, 3. Summerland, 4. Penticton. Dashed line represents Okanagan

Valley and Highlands Region outlined in Figures 2, 3, and 4.

extensional tectonism on a probable E-W axis. The peak intensity in extensional tectonism is marked by the extrusion of the Eocene volcanics and emplacement of coeval Coryell intrusives. The tectonic processes that accompanied and followed the Eocene volcanism (late Pliocene uplift) were probably responsible for extensive block faulting and the development of major N-S fault lineaments such as the Okanagan Valley Lineament.

2.2 Glaciation and Geomorphology

At least two glacial advances have been recorded for the Okanagan region [1 ], both apparently covering the entire region with continental-type ice sheets. Stratified drift and varved glaciolacustrine deposits with thicknesses of up to 1 200 m occur in lowland areas such as the Okanagan valley.

The dry belt region in Figure 2 consists of a valley floor occupied largely by Okanagan Lake and to the east, a

"midlands-type" of topography, characterized mainly by benches along the major valley, rolling hills and by small isolated basins perched above the valley.

2.3 Climate and Vegetation

The weathering history of this region, as it pertains to the formation of surf icial uranium deposits, is related largely to postglacial climatic conditions. Glacial activity ceased about 10000 years ago [1]. Significantly different climatic conditions exist for many areas of the Okanagan. Annual precipitation in the uplands region is generally in the order of 400 to 700 mm, most falling in the winter months, leading to considerable spring runoff. The dry belt region is characterized by mild winters and hot summers, giving rise to a sparsely vegetated semi-arid landscape. Annual precipitation trends over a 30 year period for the dry belt region are relatively uniform and range from about 250 mm(Kamloops) to 380 mm (Princeton). In the Oliver area, the annual precipitation is about 300 mm. Unlike most semi-arid regions, winter precipitation falls mainly as snow and there is, therefore, an important meltwater period during the spring. Much of this meltwater enters a groundwater regime severely depressed by the semi-arid conditions of the previous summer. The spring period thus marks an important influx of labile uranium into the depositional sites containing surficial uranium deposits.

Temperatures in the dry belt may vary from a low of-20 °C in the winter months to a high of 40 °C in the summer period (June to September). During the summer, the rate of evaporation easily exceeds the rate of precipitation.

PLATEAU BASALT FORMATION 181CORYELL INTRUSIONS

g EARLY TERTIARY VOLCANIC id SEDIMENTARY SEQUENCE

CRETACEOUS GRANITOIDS JURASSIC-EARLY CRETACEOUS GRANITOIDS METAVOLCANIC-META-SEDIMENTARY SEQUENCE

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Figure 2

Generalized geology of the Okanagan Valley and Highlands Region. Heavy dashed line represents eastern boundary of the south-central dry belt 1. Oliver area; 2, Summerland area; Regions 3 (Lassie Lake area) and 4 (Hydraulic Lake area) outline areas containing basal type uranium deposits (B-Blizzard, T-Tyee deposits; see

Boyle (8)). Geology after Little (10, 11) and Jones (12).

2.4 Regional Geochemistry

The entire Okanagan region has been covered by a hydrogeochemical survey with a sampling density of one sample per 12 square kilometres [2]. An interpretation of the uranium data from this survey may be found in Boyle and Ballantyne [3]. In addition, detailed studies in various parts of the region have helped in explaining the various environmental factors affecting dispersion and concentration of uranium in the surficiàl environment [3, 4, 5].

A contour plot of pH (Figure 3) outlines those areas in the Okanagan region with highly alkaline stream waters.

The dry belt region is characterized by stream waters having a pH generally in excess of 7.5 and as high as 9.0.

Bicarbonate contents of these waters are generally of the order of 50 to 600 ppm. These conditions have been shown to severely affect the ability of the sediments along the valleys to concentrate uranium [3,4], thus allowing greater amounts of the element to reach the closed basins and other types of traps in which surficial uranium deposits will form.

The regional distribution of uranium in stream waters is shown in Figure 4. The dry belt region is characterized by highly anomalous concentrations of the element in stream waters; in particular, the region south of Summerland

120°

Figure 3

Aplicon contour plot of the pH of stream waters in the Okanagan Region, Data from Geological Survey Canada, 1977.

is typified by surface waters generally containing uranium in excess of 5 ppb. Values as high as 300 ppbin stream waters and 2 000 ppb in alkaline lake waters have been reported [4].