GEOCHRONOLOGY OF IOA-TYPE DEPOSITS: WHAT ARE WE DATING? (Invited Presentation)

Fe-oxide – apatite (IOA) deposits are increasingly recognized as important and economically viable sources for critical rare earth elements (REE). The origin of such deposits, which are called Fe-oxide – copper – gold (IOCG) or Kiruna-type deposits, , are widely debated and enigmatic. Two end member models are magmatic or metasomatic origin. Therefore, geochronology is required to determine the origin model. The eastern Adirondack Mountains (that be rocks exposed in GAJ) are host to numerous IOA-type depositsthat are associated with the emplacement of the post-kinematic, leucogranitoid of the ca. 1.05 Ga Lyon Mountain granite gneiss. The ore bodies contain widespread Na-fluid alteration in surrounding host rocks indicative of Kiruna-type deposits. We present in-situ major and trace element data, and geochronology from monazite, apatite, and zircon, which demonstrates that fluid-mediated recrystallization of ore phases cause several generations of geochronometers that span as much as 60’smy?). One example from the Cheever Mine, NY, consists of symplectite monazite intergrown with allanite, apatite, and quartz. Textural evidence suggests that monazite and apatite formed during multiple stages to form several generations of compositionally distinct monazite, apatite, and allanite, with a likely starting composition of britholite-Ce. Geochronology of monazite symplectite suggests formation at ca. 0.98 Ga, ca. 60 my after ore formation. However, no zircon has been found in-situ within the ore, and thus its petrogenetic significance remains unknown. Therefore, in order to utilize geochronometers within such deposits, detailed textural and in-situ analyses are critical to establish the processes pertaining to their formation. Analysis of one type of mineral may yield age data indicative of secondary processes after formation of the ore. These data suggest that both magmatic and metasomatically enriched ore of Kiruna-type deposits in the Adirondack Grenvillian rocks have a component of both origin models.