THE FE-P-REE DEPOSIT AT MINEVILLE, ESSEX CO, NY: MANIFESTATIONS OF PRECAMBRIAN AND MESOZOIC FLUID INFILTRATION EVENTS

The Kiruna-type Fe-REE deposit at Mineville, NY is hosted by Lyon Mountain gneiss and contains magnetite, hematite (martite), apatite, stillwellite-Ce, fluorian edenite, ferro-actinolite, titanite, zircon, allanite, in the main compact ore bodies and ilmenite (with hematite exsolutions), actinolite and titanite in the disseminated ore of the hanging and foot walls. The host rock shows Na and K-rich assemblages alternating with more basic, amphibole and biotite-rich assemblages; both lithologies are REE-enriched. Undeformed, REE-depleted pegmatite bodies with simple (quartz + plagioclase + magnetite) or complex (quartz + feldspar + scapolite + titanite + epidote + zircon + magnetite) mineralogies crosscut or rim the ore bodies and country rock. Both the host rock and ore commonly exhibit the same tectonic fabrics and layering, recording the general regional deformation of the wanning stages of the Ottawan orogeny.

REE ratios for primary apatite indicate that its source was not a highly fractionated fluid and it was strongly enriched in both LREE and HREE. Negative Sr and Eu anomalies characterize the apatite, as well. The comparable REE patterns for apatite and the host rock probably reflect a synchronous introduction of REE. Allanite, titanite and zircon are also REE-rich. Post-deformation ore-associated mineral assemblages include martite, and in places, ilmenite + magnetite + actinolite + titanite. The ores contain modally significant, mm-scale allanite. This allanite has been partially replaced by the assemblage thorite + monazite + kainosite + Y-dominant allanite. Twenty-two EMPA age determinations on secondary monazite yield age populations centering on 240 Ma (primary population) and 165 Ma (secondary population). This dates the allanite alteration as a Mesozoic event.

We interpret the origin and subsequent evolution of the ore body to include the following fluid-enhanced events: 1) Ore formation by segregation of immiscible Fe-P and silicate melts; 2) Partial melting, formation of REE-poor pegmatite, and mobilization of some magnetite that occurs as slightly discordant bodies; 3) Mesozoic hydrothermal alteration of primary allanite to monazite + thorite + kainosite. This late allanite alteration may be related to local emplacement of trachite and camptonite dikes.