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

Rare earth elements (REE), Sc, and Th are used to identify two volume- important protoliths for alteration and mineralization at the Hemlo, g old deposit, Ontario: quartz-feldspar porphyry and greenstone (metased imentary and metavolcanic rocks) of predominant intermediate-mafic com position. The REE geochemistry was essentially passive to the pervasiv e and characteristic microcline alteration at the deposit. However, us ing Al2O3 as the immobile component, there is evidence for mobility of REE, particularly of light REE, TiO2, and Zr in other styles of alter ation. Dilution of REE abundances by secondary minerals (pyrite, barit e, quartz, and carbonates) averaged x2.0, and loss of light REE to lea ching averaged 56 percent in ores of the porphyry protolith, and x1.6 and 68 percent, respectively, in ores of the greenstone protolith. Whe reas most of the secondary minerals diluting REE abundances crystalliz ed broadly contemporaneously with microcline, leaching of light REE mo st likely occurred later during the late calc-silicate alteration even t. Depletion of light REE, as measured by a decrease in (La/Yb)(N), in creases with an increase in Au content for gold ores derived from both porphyry and greenstone protoliths. However, the REE geochemistry rem ains dominated by the protolith composition and, thus, is not a suitab le primary tool for exploration for Hemlo-type gold deposits. Argillic alteration in the Highland Valley porphyry district, British Columbia , is uniformly enriched in REE (less than or equal to x2) compared wit h incipiently to intensely altered whole rocks and sericitic alteratio n. The argillic and sericitic alteration separates define a characteri stic linear trend in the Al2O3-(CaO + Na2O)-K2O ternary diagram that i s distinct from the composition field for aluminous rocks of the hangi ng wall of the Hemlo deposit. The geochemistry and regional occurrence of these latter rocks clearly indicate that they are metapelites.