The Allendale Lake occurrence is located 1.5 kilometres west of Allendale Lake, 18 kilometres east-northeast of Okanagan Falls.
The occurrence is underlain by a small oval-shaped stock of the Eocene Coryell intrusions and is informally referred to as the Allendale Lake stock. This stock is roughly 2.5 kilometres diameter (8 square kilometres) and occurs at the intersection of the Eocene hornblende granodiorite to the west, the Okanagan Gneiss to the southwest and northwest, and granite of the Cretaceous Okanagan batholith.
The Allendale Lake stock consists of three phases. The main phase is biotite pyroxene monzonite. The rock is typically porphyritic with a spongy framework of smoky grey, perthitic textured high temperature orthoclase and orthoclase-anorthoclase phenocrysts, 1 to 2 centimetres diameter with interstitial diopsidic augite and biotite. These mafic minerals occur either as individual grains or as clusters with apatite, magnetite and sphene.
The syenite phase is hosted in small pockets in the monzonite phase. Rhomb-shaped anorthoclase phenocrysts are distinctive. Apatite and magnetite are also locally abundant. The syenite is weakly propylitic altered in isolated fracture zones. Epidote and calcite veins comprise alteration minerals. Local zones of strong secondary biotite replacement occur adjacent to pegmatite dikes. Argillic alteration of feldspars is very weak. Partially assimilated aplite xenoliths are common within the syenite. They range from less than 1.5 to 6 metres length. However, angular fragments of gneiss are also present.
A shonkinitic border phase is exposed along the west and southwest margins of the stock where it forms a continuous zone ranging from 50 to 300 metres wide. The phase is relatively mafic-rich and probably is a basic differentiate of the monzonite. The fine to medium-grained rock is composed of intermixed anorthoclase and orthoclase perthite (80 per cent) and pyroxene (15 per cent). The pyroxene contains accessory biotite and hornblende in clots with apatite and magnetite or as poikilitic inclusions in large augite grains. Small, partly altered nepheline grains, one-half to one millimetre diameter, are sparingly disseminated throughout the rock.
The main fractures within this Coryell stock have a mean strike of 035 degrees and dip 80 degrees southeast. Strong subsidiary fractures strike 245 degrees dipping 80 degrees northwest. Two weaker sets strike 190 degrees dipping 55 degrees northwest and 135 degrees dipping vertical.
Pegmatite dikes crosscut the syenite and monzonite phases in the north, east central and south parts of the stock. The pegmatites are quartz-rich and feldspars consist of very coarse albite. Biotite and actinolite comprise mafic minerals. Sphene, allanite and magnetite comprise accessory minerals. Mineralization at the Allendale Lake occurrence consists of a pegmatite dike within syenite. The dike contains betafite, cyrtolite, and perhaps brannerite and euxenite in association with magnetite and hematite. Somewhat higher than normal radioactivity occurs in this Coryell stock.