Rare earth elements, protoliths, and alteration at the Hemlo gold deposit, Ontario, Canada, and comparison with argillic and sericitic alteration in the Highland Valley porphyry district, British Columbia, Canada

Abstract

Rare earth elements (REE), Sc, and Th are used to identify two volume-important protoliths for alteration and mineralization at the Hemlo gold deposit, Ontario: quartz-feldspar porphyry and greenstone (metasedimentary and metavolcanic rocks) of predominant intermediate-mafic composition. The REE geochemistry was essentially passive to the pervasive and characteristic microcline alteration at the deposit. However, using Al 2 O 3 as the immobile component, there is evidence for mobility of REE, particularly of light REE, TiO 2 , and Zr in other styles of alteration. Dilution of REE abundances by secondary minerals (pyrite, barite, quartz, and carbonates) averaged X 2.0, and loss of light REE to leaching averaged 56 percent in ores of the porphyry protolith, and X 1.6 and 68 percent, respectively, in ores of the greenstone protolith. Whereas most of the secondary minerals diluting REE abundances crystallized broadly contemporaneously with microcline, leaching of light REE most likely occurred later during the late calc-silicate alteration event. Depletion of light REE, as measured by a decrease in (La/Yb) N , increases with an increase in Au content for gold ores derived from both porphyry and greenstone protoliths. However, the REE geochemistry remains dominated by the protolith composition and, thus, is not a suitable primary tool for exploration for Hemlo-type gold deposits. Argillic alteration in the Highland Valley porphyry district, British Columbia, is uniformly enriched in REE (< or = X 2) compared with incipiently to intensely altered whole rocks and sericitic alteration. The argillic and sericitic alteration separates define a characteristic linear trend in the Al 2 O 3 -(CaO + Na 2 O)-K 2 O ternary diagram that is distinct from the composition field for aluminous rocks of the hanging wall of the Hemlo deposit. The geochemistry and regional occurrence of these latter rocks clearly indicate that they are metapelites.