NEWLY DISCOVERED INTRUSIVES AT THE LEMHI PASS THORIUM – REE IRON OXIDE DISTRICT, IDAHO: CAMBRIAN SYENITE AND MYSTERY ULTRAMAFICS – SIGNATURES OF A BURIED ALKALINE COMPLEX OR TWO SYSTEMS?

Thorium and rare earth element (REE) mineralization in the Lemhi Pass District, Idaho and Montana, is characterized by quartz-thorite-hematite veins and monazite-thorite-apatite-bearing shears and replacements with specularite, biotite and alkali feldspar. REE are unusually enriched in neodymium. Mineralization is hosted in Proterozoic metasediments of the Gunsight/Apple Creek formations (1420 Ma detrital zircons) but has not previously been linked to specific intrusives in the district. New Idaho Geological Survey mapping discovered a small syenite outcrop, locally cut by nonradioactive, specular hematite veins. The syenite (80% feldspar, 63% SiO2) has thin albite rims on orthoclase; mafics are locally converted to specular hematite. SHRIMP geochronology gives a U-Pb crystallization age on zircon of 529.1 +/- 4.5 Ma, or lower Cambrian. Paleozoic ages are also reported in the literature for granitic plutons further south in the Beaverhead Range, and thorium enrichment has been noted at the Leadore pluton.

A suite of mafic dikes, previously interpreted as Tertiary, include altered pyroxene porphyry lamprophyres. A newly discovered, ultramafic sill (41-45% SiO2 and 10% MgO with 3% Na2O + K2O) has possible carbonatitic affiliation (abundant dolomite “phenocrysts” or replacements of olivine phenocrysts; 3% total carbon). The sill contains Ba and Sr minerals found in the thorite veins, suggesting it predates or is contemporaneous with them. At least one lamprophyre dike is cut by base metal mineralization; many dikes show propylitic alteration. Ages of the mafic rocks are unknown.

Previously reported electron microprobe ages on monazites suggest Th-REE mineralization had a Proterozoic (800-1100 Ma) and a mid-Paleozoic component at the Lucky Horseshoe prospect nearest the sill. A single Re-Os age (1050 Ma) on molybdenite at a copper mine supports Proterozoic mineralization. Non-plateau 40Ar/39Ar ages on hydrothermal biotite from the Lucky Horseshoe Th-REE prospect are interpreted to indicate an older (> 200 Ma) component of biotite growth followed by the regional Cretaceous resetting (~140 Ma saddle age). Field relations are consistent with ancient and overprinted mineralization. It could be associated with a buried Precambrian or Cambrian alkaline intrusive complex – or both.