KIRUNA-TYPE IRON DEPOSITS IN THE ADIRONDACK MOUNTAINS: MINEVILLE, ESSEX CO., NY

The Kiruna-type Fe-REE deposit at Mineville, NY, belongs to a large metallogenetic event that characterizes the Grenville crust in New York and New Jersey. It is hosted by Lyon Mountain gneiss and has the following mineral composition: magnetite, hematite (martite), apatite, fluorian edenite, titanite, zircon, allanite, monazite (in places) 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 compositions alternating with more basic, amphibole and biotite-rich compositions, both with high REE amounts. REE depleted pegmatite bodies with simple (quartz + plagioclase + magnetite) or complex (quartz + feldspar + scapolite + titanite + epidote + zircon + magnetite) compositions cross cut 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 waning stages of the Ottawan orogeny. Mineral reactions such as the formation of martite, probably by metamorphic fluids, or ilmenite – magnetite + actinolite + titanite are common and created the post deformation mineral associations.

(Ce/Yb)N, (Nd/La)N and (La/Yb)N ratios for apatite indicate that its source was not a highly fractionated fluid and it was strongly enriched in both LREE and HREE. Low Sr and Eu anomaly characterize the apatite. The comparable REE patterns for apatite and the host rock probably reflect a synchronous introduction of REE. Allanite, titanite and zircon also have high REE compositions.

The general features of the deposit are: (a) occurrence as lenses and layers in acid (albite + quartz + K-feldspar) and disseminations in basic (amphibole + feldspar + biotite) lithologies; (b) the ore shares the same metamorphic fabrics with its host; (c) association with undeformed pegmatite bodies; (d) high enrichment in REE of the ore and host rock; (e) the host rock is strongly enriched in Na, K, P, F, etc.

The mineralogical and geochemical data suggest a hydrothermal origin for this deposit, but the source of the generating fluid cannot be recognized.