HALOGEN.BEARING ALLANITE FROM THE WH|TE RIVER GOLD OCCURRENCE, HEMLO AREA. ONTARIO

Halogen-bearing allanite (containing up to 0.95 ft.qo Cl and 0.76 wt.%o F) occurs as a minor constituent in the zone of anomalous gold values at the White River gold occur-rence of LAC Minerals Ltd., Hemlo area, Ontario. It is present as fine-grained ageregates associated with monazite, apatite, rutile, zircon, clinozoisite, and epidote and exhibits a well-developed chemical zonation with an Fe- and R.EE-poor core and an Fe- and R.EE-rich margin. REE-bearing and R.EE-free clinozoisite also are locally present as nuclei to the R.EE-poor core. Halogen-bearing allanite has minor Mg and Mn and an extremely low content of ferric iron. Clinozoisite and halogen-bearing allanite exhibit a complete solid-solution through the coupled substitution: Ca42 + All,rn = REEAI + Fe2+ pn. Textures and chemical com-position indicate that the halogen-bearing (F, Cl) allanite from the study area crystallized from a H2O-rich fluid during the formation of the calc-silicate (skarn) assernblage. Hydrothermal fluid, probably directly or indirectly related to plutonic activity, introduced F and Cl and remobilized and locally concentrated REE. The restricted occurrence of halogen-bearing allanite and its close correlation with high gold concentrations in the Anomalous Zone support a genetic link between calc-silicate alteration and gold mineralization in this Archean terrain, analogous to that in Phanerozoic auriferous skarn deposits