The Allendale dimension stone prospect is located 1 kilometre north 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. 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.
This Coryell stock consists of four 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.
This unusual type of stone prospect forms a round hill with a scattered boulder field along its edges. When cut and polished, this stone has a dark blue colour with occasional light iridescence in some feldspar grains. The rock is a very coarse grained, dark grey syenite. The colour and texture of the stone varies slightly in individual boulders and rock outcrops. The presence of many small boulders indicates a high fracture density. Therefore, in spite of its very attractive appearance in finished slabs, potential development of this site will probably be limited to monument work and interior projects only.
The Allendale stone is a distinctive, dark grey to black, rhomb-shaped anorthoclase syenite. The rock is very coarse with large (1-2 centimetres) phenocrysts of grey anorthoclase and black augite. It has a poorly developed linear fabric defined by the augite crystals. The rock is partially altered with pseudomorphs of chlorite after augite and some chloritization of biotite. Quartz is significant in its absence and nepheline may be present as a minor constituent of the fine matrix. Minor constituents are apatite, magnetite and pyrite.
The rock takes a good polish (7-8/10) with some pitting on chlorite or biotite grains. There are tight intergranular cracks throughout the rock and individual grains show some cracking as well. Grains are well interlocked and there is no iron staining from either the pyrite or magnetite (2-3 per cent) (Fieldwork 1994, pages 367-368).