The Nipple Mountain Opal showing is located on Nipple Mountain, 35 kilometres southeast of Kelowna.
The showing occurs in the historic Beaverdell mining camp. The Beaverdell mining camp is centered on a nunmber of silver, gold and copper prospects discovered in 1897 and a few rich veins of silver-lead ore mined from 1913 to 1991. In 1995, Don Sandberg of Kelowna discovered opal on the upper slopes of Nipple Mountain in the Eocene volcanic rocks underlying the Queen and Glory claims. The showing was visited by B.N. Church of the BC Geological Survey Branch in September, 1996.
Interest in opal occurrences in British Columbia has increased significantly since 1993 when the Klinker deposit was discovered near McGregor Creek northwest of Vernon. Similar smaller occurrences are known in the Kamloops, Salmon Arm, Spences Bridge, Keremeos and Kelowna areas (Read, 1995 and Church, 1996).
The volcanic rocks consist of dacite, andesite, trachyte and basalt lavas and breccias of the Eocene Penticton Group, probably equivalent to the Kettle River Formation. This is Reinecke's Nipple Mountain Series.
Don Sandberg initially discovered opal in a logging road cut in the area now covered by the Queen claims on the ridge extending north from Nipple Mountain, 1500 metres east of the Dale Creek road. Subsequently two other localities were found; one on a rock bluff 150 metres to the northwest of the road on the west slope of the ridge, and the other 400 metres to the east on the east side of ridge.
At the three localities opal occurs in flow banded dacite filling cavities in the bands, in brecciated structures, and on cross joints. The opal associated with banding is commonly 1 to 3 centimetres in diameter, almond shaped and roughly elongated in the direction of flow. The opal on cross-fractures includes translucent coatings a few millimetres thick, covering areas up to 0.5 square metres on the walls of the fissures. The largest opals occur on the east side of the ridge. Blocks of opal with rock inclusions weigh as much as 23 kilograms and opal fills breccia cavities several centimetres thick.
The opal is commonly waxy amber coloured but ranges to flesh, peach, honey-hues and less commonly grey and rarely green. Some of the watery fissure-lining opal displays a weak play of colours.
In some instances white plume opal is associated with quartz and chalcedony that forms variegated horizontal or concentric bands on cavity floors or walls. The chalcedony is believed to form within gas cavities of volcanic host rocks when microcrystalline chalcedony fibres nucleate on vug walls and grow inward (O'Donoghue, 1983). Oscillatory zoning and iris banding, as seen in thin section, is the result of variations in silica concentrations in solutions at the tips of the growing chalcedonic fibers forming smooth and regular or botryoidal surfaces parallel to the banding (Heaney and Davis, 1995; Church, 1996).
The most probable source of the silica-rich solutions is the host Nipple Mountain dacite. Analyses of the rhyodacite shows marked excess silica based on norm calculations. For example, a fresh dacite sample from the Glory claims contains 74.06% SiO2, 0.24% TiO2, 14.13% Al2O3, 2.00% Fe2O3, 0.02% MnO, 0.49% MgO, 1.76% CaO, 3.65% Na2O and 3.65% K2O (major oxides recast to 100) that yields 34.3% free silica/quartz (CIPW norm). Thin sections reveal an estimated 7 per cent plagioclase phenocrysts, 1 per cent amphibole microlites and 1 per cent opaque minerals in a glassy and devtrified fine grained matrix, leaving a large amount of unaccounted (excess) silica. It is concluded that part of the excess silica, accompanied by fluids and gases, moved from the dacite lava to gas cavities and fracture openings, during the original magma cooling process, to form the opal, quartz and chalcedonic fillings.
