The Bonanza mine, located just west of Granby Bay on Observatory Inlet, is a former copper, silver and gold producer.
The occurrence is located in a 14.4 by 9.6 kilometre roof pendant within the Eocene Coast Plutonic Complex. The roof pendant, consisting of volcanic and sedimentary rocks, has been correlated with the Middle-Upper Jurassic Hazelton Group and the Middle Jurassic Bowser Lake Group. The volcanics consist of variably chloritized pillow and massive basalt with minor mafic tuffs. The overlying sediments consist of argillite, siltstone and sandstone with minor chert and limestone. There are two observable phases of folding in the area, an initial north-northeast trending phase followed by a later east-northeast trending phase.
The deposit is situated on the western limb of a northeast trending, broad anticlinal fold (the Bonanza-Hidden Creek Anticline). The deposit forms a flattened cylindrical body 805 metres long, 61 metres wide and 9 to 12 metres thick trending 010 degrees. It lies near horizontal to the south, but gradually steepens northward toward a normal fault, dipping up to 30 degrees north. The fault strikes northwest, dips 50 degrees northeast and truncates the north end of the deposit.
The orebody consists of massive to disseminated layers and lenses of sulphides hosted in a zone of altered basaltic tuff and minor pelitic sediments. The hostrocks, up to 84 metres thick, occur within a sequence of tholeiitic pillow lavas. In the sulphide-rich strata, the hostrocks have been variably chloritized, sericitized and saussuritized. Due to the foliated nature of the hostrocks, the deposit was previously described as being hosted in a shear zone (Geological Survey of Canada Memoir 175).
Mineralization occurs as massive to disseminated crudely bedded layers of chalcopyrite, pyrite, sphalerite and quartz (up to a metre thick) and as disseminated chalcopyrite and pyrrhotite, minor pyrite and magnetite in schists. Gangue minerals consist of quartz, sericite, muscovite, actinolite, tremolite, hornblende and calcite.
Between 1928 and 1935, 656,974 tonnes of ore with an average grade of 0.13 gram per tonne gold, 13.31 grams per tonne silver and 2.17 per cent copper were mined.
Remaining reserve estimates vary from 226,800 tonnes grading 1.0 per cent copper (National Mineral Inventory card 103P5 CU5) to 65,116 tonnes (Property File - Sargent, 1942). The remaining reserves have more recently been classified as insignificant (Dr. W.J. Wolfe, Cominco Ltd., personal communication, 1989).
Reserve statistics compiled from original Granby and Cominco files by Taiga Consultants of Calgary are 10,620 tonnes grading 1.76 per cent copper, 0.16 gram per tonne gold and 13.71 grams per tonne silver (Report by Taiga Consultants Ltd., 1992). This report and Assessment Report 23582 has a good summary and history of exploration of the Anyox area.
