ON THE DISTRIBUTION AND DISPERSION OE ' PLATINUM IN THE SOILS OF THE TULAMEEN ULTRA . MAFIC COMPLEX , SOUTHERN BRITISH COLUMBIA
INTRODUCTION Exploration for platinum in British Columbia is hampered by a lack of data on its distribution and behavior in the surficial environment. To obtain some of the information required and suggest practical guidelines for exploration, this study, involving systematic sampling and analysis of soils and other media in the vicinity of a platinum deposit in the Tulameen district of southwestern British Columbia, was initiated in 1988 (Fletcher. 19x9). Preliminary results of work in progress are described. Tulameen River, approximately 25 kilometres west of Prince-The field area, on Grasshopper Mountain north of the ton, is divided into the main study area, on the southern slope of the mountain, and a second, smaller area near the summit (Figure 6.2-I). The lower part is accessible by pack trail from the Tulameen River road. Several kilometres of recently constructed drill road on the north and west sides of g e o l o g y , Tulameen ultramafic c o m p l e x. B. C. Figure 6-2-1, Location of the study areas and generalized (1) Nicola Group (2) syenogabhrn. (3) dunite. (4) olivine clinopyroxenite,(5) homblendr~linopqmxenite. (6)rnyloni-tic rocks. (7) Eagle granodiorile (modified after Nixon and Rublee. 1988). Grasshopper Mountain connect with the Lawless 'Creek forestry road and provide access from both the village of Tulameen and the Coquihalla Highway. BEDROCK GEOLOGY dissected dunite core of the Tulameen ultramafic complex, a Grasshopper Mountain is the northern segment o' the Late Triassic zoned Alaskan-type ultramafic-gabbroic intrusion within metasedimentary and metavolcanic rocks c,f the Upper Triassic Nicola Group. Geology of the complex, described by Findlay (1969) and Nixon and Rublea (1988). comprises a dunite core surrounded by crudely concentric shells of olivine clinopyroxenite, hornblende c h o p r o x-enite and gabbroic rocks ((Figure 6-2-1). The focus of the current study is within the dunite :ore. The dunite is typically Fine grained, extensively sepen-tinized, and weathers buff-brown on exposed surfaces "lati-num mineralization is restricted to chromite-rich dunite (Nixon e t a /. is best exposed in the summit region and in the sub\ertical randomly distributed massive to discontinuous pods, segre-gations. schlieren and disseminated grains. Schlierel are randomly distributed thrcmughout the dunite and are thought to represent foliation-concordant remnants of former cumu-late layers (Nixon and Rublee, 1988:1. Several cons of central region of the cor'? (Bohme, 19x7). but till-covered mineralization have beer1 defined within the well-exposed areas are largely untested. platinum-iron alloys and platinum antimonides …
ALASKAN-TYPE ULTRAMAFIC ROCKS IN BRITISH COLUMBIA : NEW CONCEPTS OF THE STRUCTURE OF THE TULAMEEN COMPLEXY
Alaskan-type ultramafic rocks in British Columbia are potential hosts forcommerciallyexploitabledeposits ofplatinummetals(Rublee, 1986;Evenchicketa/., 1986)aswellas other commodities (for example, chrome, nickel, cobalt, asbestos and jade). In 1987 the British Columbia Geological Survey initiated a program designed to investigate the mineral potential of these intrusions through detailed geologic mapping and lithogeochemical sampling specifically for platinum group elements. In the first year of this project, a concerted effort is being made to more thoroughly understand one of these intrusions, the Tulameen ultramafic complex in southern British Columbia. The Tulameen complex warrants careful attention as it has been the largest producer of platinum in the province and bas been taken as a typical example of a zoned Alaskan-type ultramafic intrusion (Findlay, 1963, 1969; Evenchick et a / . , 1986). The general structure of Alaskan-type complexes is characterized by a crudely concentric outward zonation of rock types ranging from olivine-bearing to hornblende-rich or magnetite-rich clinopyroxenites about a steeply dipping dunite core (Taylor, 1967). Lesser proportions of hornblendite and mafic pegmatite usually occur as isolated masses toward the periphery of the intrusion. Typical cumulate minerals include forsteritic olivine, diopsidic augite, chromite and magnetite; orthopyroxene is characteristically absent, indicating an alkalic affinity. Gabbroic rocks associated with the intrusion are commonly tholeiitic, and in this respect the syenogabbros and syenodiorites of the Tulameen complex are unusual (Findlay, 1969).
ORIGIN OF THE ULTRAMAFIC COMPLEX AT UNION BAY, SOUTHEASTERN ALASKA
An ultramafic complex at Union Bay, in southeastern Alaska, is an exceptionally well-exposed example of a series of similar complexes forming a belt parallel to the Coast batholith. 
 
A body of gabbro, approximately circular in plan and about 6 miles in diameter, intrudes folded sedimentary rocks of probably Triassic and Cretaceous age. A moderately low grade of regional metamorphism in the sedimentary rocks is increased to almandite-zone grade adjacent to the gabbroic contact. Intrusive into the gabbro is a remarkable ultramafic complex which comprises a vertical pipe approximately 1 mile in diameter, to which is attached a lopolithic offshoot approximately 5 miles long and 3 miles wide. The ultramafic units range through hornblende pyroxenite, pyroxenite, olivine pyroxenite, peridotite, and dunite, and both the pipe and the lopolith show a well-developed concentric zoning with dunite in the center and pyroxenite or hornblende pyroxenite on the periphery. Magnetite is a primary constituent of the pyroxenite unit. 
 
The most plausible explanation of the structure and petrology of the ultramafic complex is a hypothesis of successive injections of an ultramafic magma changing in composition with time and intruded in reverse of the order of crystallization of the component minerals. It has not been possible to explain the gabbro (which is olivine-free) by the same mechanism, yet the space and time relationships here, as elsewhere in the world, are probably not accidental. It is proposed, therefore, that a body of gabbro was emplaced, and into this was injected a plug and associated lopolith first of diopside-magnetite magma with minor olivine, then diopside-olivine magma, and finally olivine magma, each new magma intruding the central, incompletely solidified portion of the earlier magma. Minor mixing and melting explain the composition and distribution of the intermediate units of the complex. Lack of contact metamorphism of grade that might be expected to accompany intrusions of such high temperature is explained first by the fact that the ultramafic magmas were almost everywhere in contact with gabbro, not sedimentary rocks, and second by the fact that, being anhydrous, the ultramafic magmas absorbed, rather than expelled, volatiles. 
 
The only likely source for magmas of these compositions would be melting portions of the mantle, but no proposed mechanism for supplying the energy for this process is suggested herein. With an available source of energy, however, it can be postulated that magmas would be generated in the succession proposed. The gabbroic magma probably would originate as a result of melting at a shallower level.