Geologic description
This occurrence consists of radioactive and rare-earth accessory minerals in the border phases of the Cretaceous, Zane Hills pluton. Miller and Ferrians (l968) note that the quartz monzonitic to monzonitic border phase of the generally granodioritic Zane Hills pluton is 5 to 10 times more radioactive than background. They also noted that large K-feldspar phenocrysts, with tourmaline-bearing aplite dikes, are common in the border phase. A sample of porphyritic quartz monzonite from the border phase contains 20 ppm uranium. Miller and Elliot (1977) subsequently described the border phase rocks as monzonite, syenite, quartz monzonite and hybrid diorite. They propose an intrusive contact between the border phase and the main Zane Hills granodiorite, and suggest that the monzonitic border phase is younger. Samples of border-phase rocks contain 11 to 99 ppm uranium, and 46 to 269 ppm thorium. The uranium and thorium are concentrated in grains of uranothorianite, thorite, betafite, allanite, zircon, and sphene, which occur as disseminated accessory minerals in the monzonite. They found no vein-type mineralization and concluded that the border phase intrusives are enriched in U and Th. Sphene from two samples gneissic monzonite (presumed to be border phase rocks ) from the Zane Hills pluton a few kilometers upstream of the Hogatza placer deposit (HU014) contains from 27,890 to 39,180 ppm REE, including 380 to 760 ppm thorium (Staatz and others, 1977). Later investigations, however, indicate that the border phase rocks contain no unusual concentrations of REE (Barker, 1991). |
| Alteration of deposit |
At the contact between border-phase, monzonite-quartz monzonite and the core granodiorite, the granodiorite is, at least locally, strongly altered to a bleached, fine-grained, sericitic rock. Although the alteration diminishes in intensity away from the contact, all of the original biotite is completely altered to sericite. The granodiorite exhibits cataclastic texture near the contact. |
