Secondary alkali feldspars in porphyry systems

CIM Bulletin, Vol. 74, No. 831, 1981

CRAIG H. B. LEITCH, Research Geologist, Western Division, Newmont Exploration of Canada Limited, Vancouver, B.C.

In describing the alteration of porphyry systems, the term "secondary K-feldspar" appears to be an oversimplification. A better term would be "alkali feldspar" alteration, because in actual fact the replacement feldspars show gradational changes from an albite, which is important in some systems, to an alkali feldspar containing minor orthoclase molecule, to true orthoclase.There are several textural varieties of secondary alkali feldspar present in the porphyry systems studied from British Columbia, Australia and New Zealand. These include, in apparent increasing order of alteration intensity: clear albite (An10); patchwork albite (An5-10); chessboard albite (An0-10); "irregular albite" (An0Ab100 to Ab90Or10); untwinned "anoralbite" (Ab90Or10 - Ab70Or30); and orthoclase (Ab40Or60 - Or100).The exploration significance of these varieties of alkali feldspar is that their gradation in composition, from sodic to potassic, may be linked to a corresponding increase in intensity of alteration. Thus, there is a tendency for more sodic alteration feldspars to characterize weak, sub-economic deposits, or the fringe zones of stronger deposits; the more potassic alteration feldspars may provide an exploration vector toward more highly mineralized zones.
Keywords: Mineralogy, Feldspars, Alkali feldspars, Porphyry systems, Alteration, Mineralization, Exploration, Albite, Orthoclase.