gems, jade, amphibole jade, nephrite, Canada, British Columbia, South Australia, jade geology

Two principal types of nephrite jade orebody are now recognized: (1) ortho-nephrite jade bodies associated with serpentinized ultrabasic igneous rocks and exemplified by those of the Canadian Cordillera, British Columbia; and (2) para-nephrite jade bodies associated with metasedimentary strata and exemplified by those of Eyre Peninsula, South Australia. Major-element geochemistry of nephrite jades from the two contrasting geological environments have much in common. However, trace-element geochemical signatures show marked dissimilarity with ortho-nephrites distinguished by high levels of the basic elements Cr, Co and Ni, and para-nephrites characterized by relatively subdued trace-element abundances. Whereas ortho-nephrite jade forms in folded orogenic belts and bears a mantle signature, para-nephrite jade may appear in a variety of tectonic settings but carries a crustal signature. The role of tectonic setting is important but subordinate to that of the composition of the source region in determining the typology of nephrite jades.