Heavy Mineral Sampling and Provenance Studies for Potentially Diamond-Bearing Source Rocks in the Jurassic Laberge Group, Atlin- Nakina Area (NTS 104N), Northwestern British Columbia

The min er al ogy of clastic sed i men tary rocks pro vides a re cord of the source re gions up lifted and eroded dur ing orog eny (Dickinson et al., 1983). The Ju ras sic Laberge Group is part of a fold and thrust belt ex posed in the north ern Cor dil lera (Fig. 1). The paleontological and sed i men tary re cord in the Laberge Group mark the up lift and ero sion of crust dur ing the early Jurrasic and the de po si tion and burial of de tri tus in a ma rine fore-arc ba sin (Eng lish et al., in press; Johannson et al., 1997). Gar net-rich ho ri zons of im ma ture wacke and con glom er ate oc cur ring in the Laberge Group south west of Sloko River were first rec og nized dur ing a re gional map ping and mag netic sur vey pro gram (Mihalynuk and Lowe, 2002). Sub se quent heavy min eral sam pling in the Atlin-Nakina area, aimed at trac ing the source of anom a lous di a monds in placer gold op er a tions of the north ern Cor dil lera (Casselman and Har ris, 2002), showed ev i dence for di a mond in di ca tor min er als, which were traced to a garnetiferous con glom er ate ho ri zon in the Ju ras sic Laberge Group ex posed near Sloko River (Canil et al., 2004). Fur ther de tailed study of heavy min er als from one com pos ite sam ple in the gar net-rich con glom er ate showed it to con tain clasts of eclogite, and gar nets and py rox enes from peridotite of man tle or i gin (Fig. 2, 3). Thermobarometric stud ies re vealed that the de tri tal gar nets and py rox enes in this sam ple equil i brated at man tle depths ap proach ing the di a mond sta bil ity field (MacKenzie et al., in press), on geothermal gradients expected in cratonic mantle lithosphere (Fig. 4). De tri tal man tle min er als are known in other clastic sed i ments (McCandless and Nash, 1996). The an gu lar na ture and lack of weath er ing of de tri tal grains in the Laberge Group re quire prox i mal sources and rapid de po si tion (McCandless, 1990). Po ten tial sources for the gar net and py rox enes in the Laberge Group could be the ero sion of peridotite and eclogite as xe no liths in an al ka line ig ne ous rock such as kimberlite or lamproite, or out crop-sized mas sifs ex posed by up lift and ex hu ma tion. Po ten tial source rocks of ei ther type that are the req ui site age (pre-Ju ras sic) are not known in the Atlin-Nakina area. The lack of picroil men ite in a heavy min eral sam ple of the Laberge con glom er ate ar gues against der i va tion from xe no liths in al ka line ig ne ous rocks (i.e., kimberlite; Mac Ken zie et al., in press). No eclogite or gar net peridotite, as ei ther xe no liths or mas sifs, has yet been rec og nized in out crop in the AtlinNakina area. Fur ther more, the source of peridotitic gar nets and py rox enes in the garnetiferous wacke unit of the Laberge Group is not likely to be from ophiolite and mélange that con sti tutes parts of the Cache Creek Terrane to the east. Man tle peridotite from ophiolite in the Cache Creek Terrane is of lower pres sure or i gin, in which spinel, not gar net, is sta ble. Fur ther more, the lat ter rocks have been in ves ti gated by field map ping, and al though blueschist as sem blages have been dis cov ered in the Cache Creek Terrane near Dease Lake (Fig. 1), no eclogite has been rec og nized (Fig. 1; Ghent et al., 1993); Mihalynuk et al., 2004).