The Bonaparte River magnesite prospect is located on the Mond Ranch, on the west slope of the Bonaparte River valley, nine kilometres northeast of Clinton between Clinton and Fiftyseven creeks.
Bedrock in the area is mainly basic volcanic flows, tuff, ribbon chert, limestone and argillite of the Permian to Triassic Cache Creek Group intruded by the Permian to Triassic Mika Ultramafic intrusion, composed of altered dunite and peridotite. The older rocks are unconformably overlain by nonmarine sediments (shale, sandstone, tuff, diatomite, conglomerate and breccias) of the Miocene Deadman River Formation, which is in turn overlain by plateau basaltic flows of the Miocene to Pleistocene Chilcotin Group.
Magnesite occurs as a surface alteration zone, 1 to 1.5 metres thick, associated with the Permian to Triassic Mika Ultramafic Intrusion. Dunites and peridotites form a sill-like intrusive, trending 340 degrees, within the Lower Permian metavolcanics and metasediments of the Cache Creek Group. The ultramafic rocks are generally altered to serpentine, and locally to talc. Magnesium carbonate is best developed in a broad, northwest trending zone about 600 metres wide by 2500 metres, along the northeast margin of the intrusive where it has a change of slope. Although the main showing is to the west of the river, ultramafics with magnesite have also been identified (along strike?) east of the Bonaparte River.
The ultramafics are zoned dunites and peridotites which are highly serpentinized and in places completely steatized. They are medium-grained, light to dark green on fresh surfaces and weather to a green or reddish green. Magnesium carbonate alteration occurs preferentially in the dunites but there is a gradation from serpent- inization and steatization, with remnant pyroxene and olivine, to a compact, cryptocrystalline, bone-white magnesite with no cleavage and conchoidal fracture. In the early alteration stages, brown ankeritic carbonates, possibly hydrous iron oxides, grains of magnetite, small veinlets of asbestos and black streaks of what may be manganese, are common. Further alteration results in the appearance of small grains and masses of magnesite within the serpentinized zones with associated ankeritic carbonates. In the most advanced alteration, usually near surface, magnesite replaces all serpentine and ankeritic carbonate and small, relatively pure veinlets of magnesite crosscut the alteration and parts of the less altered ultramafics.
Analysis of the Bonaparte River occurrence indicate 97.8 per cent MgCO3, 1.8 per cent CaCO3 and 0.4 per cent Fe2O3 with no alumina and only traces of insolubles. This sample was apparently collected from a surface exposure (Geological Survey of Canada Summary Report 1932).
In addition to magnesite, the same ultramafics are known to carry chromite as grains, small pods and veinlets within the dunitic units as well as locally within the magnesite. An area of about 150 by 450 metres immediately west of the magnesite showing, is host to variable amounts of chromite. Assessment Report 8111 states that the ultramafite contains seven known chromite occurences along three bands with an apparent total strike length of 500 metres. All chromite is within a dunitic section fo the ultramafite, with assayed grades of chromite ranging from 32% to 42.4% chrome oxide (Cr2O3).
In addition ot the magnesite and chromite, "harsh" asbestos veinlets, less than 1.6 millimetres wide are abundant throughout the serpentine (EMPR Open File 1995, page 25).
The earliest known work on the property was minor trenching and underground testing of chromite occurrences by Mr. W.N.D. McKay on the Winnifred claims in 1932 (Assessment Report 8111). In 1932, the property was staked and optioned to Western Asbestos but no work was completed. In 1957, new claims were staked and optioned to New Jersey Zinc Exploratin Company Limited, who completed a program of geological mapping, magnetometer surveying and bulldozer trenching to test the asbestos potential of the property in 1957 and 1958. Kaiser Resources is reputed to have drilled a hole to test for magnesite, probably in the early 1960's, however there is no public record of this work. The property was subsequently staked as the Jo claims, again as an asbestos prospect and prospected, geologically mapped and a dip-needle magnetic survey completed in 1968 (Assessment Report 1146) by Riviera Mines limited. The Mika claims were staked in 1979 and optioned to CCH Resources Limited (later Campbell Resources Limited) and prospected for chromite. They completed a program of geological mapping, geochemical surveying (194 soil samples, 18 rock chip samples and 12 "ore" samples - Assessment Report 8111) in 1979 and an additional 419 soil samples in 1980 (Assessment Report 8677).
