SPECULATIONS ON THE ORIGIN OF MASSIVE HEMOILMENITE DEPOSITS ASSOCIATED WITH ANDESINE ANORTHOSITE MASSIFS, GRENVILLE PROVINCE, QUEBEC

The Fe-Ti oxide phase hemoilmenite (an exsolution intergrowth of titanhematite in a ferrian ilmenite host) occurs only rarely in igneous rocks, yet it is the sole Fe-Ti oxide found in most andesine anorthosites. The predominance in certain massif anorthosites of this otherwise rare phase implies high fO2 conditions compared with those associated with the crystallization of typical mafic magmas. The hemoilmenite in andesine anorthosite also is characterized by a high Geikielite content (typically >10 mol% MgTiO3, but some reach values as high as 25 mol%), which likewise is not a feature of ilmenite found in common mafic igneous rocks. Locally, the hemoilmenite has been concentrated into massive ore deposits, such as found at Allard Lake, Labrieville, St. Urbain, and Morin. Typical accessory minerals include biotite, green spinel, plagioclase, and orthopyroxene. Small amounts of disseminated sulfides (Py ± Po ± Ccp) also occur. Such deposits typically are sharply bounded against their anorthositic host rocks but locally zones of mixed plagioclase-hemoilmenite rocks can be found. In some cases, dikes of hemoilmenite cut the anorthosite, and elsewhere inclusions occur in the ore that range from cm-sized plagioclase xenocrysts to m-sized anorthosite xenoliths. All of these features point towards the existence of a mobile ?ore magma? that was somehow intruded into the anorthosite. Previous attempt to explain these ore bodies as the products of crystal settling, segregation or filter-pressing of late-stage liquid, or an immiscible oxide-apatite melt, all meet with serious obstacles, and it seems appropriate to advocate a radical alternative. Specifically, the ubiquitous presence of biotite in these deposits (and local alteration in surrounding anorthosite) suggests a potential role for mineralizing fluids. The abundance of biotite ranges from a few to as much as 10% and, at St. Urbain, the contacts between certain ore deposits and surrounding anorthosites are decorated by a biotite selvage up to 10 cm wide in some cases! Experimental work by Gibson & Tuttle (1967, AmMin) on the system FeO-Fe2O3-SiO2-H2O indicated that water promoted the formation of an immiscible Fe-oxide melt. These experiments have not been pursued, but perhaps the development of an ilmenite-rich melt is mediated by the presence of an aqueous fluid.