Geologic description
The 10-mile long, unnamed creek, referred to by Ucore Rare Metals, Incorporated as the Caribou Heights prospect, drains about one-third of the known extent of the Sithylemenkat pluton (Ucore, 2012, 2014). Geological investigations recognized regional tin potential as early as 1969 (Herreid, 1969; Patton and Miller, 1970, 1973; Barker, 1983; and in early 1980s tin was specifically recognized in the valley of this record (Barker and Foley, 1986; Barker, 2012). Additional regional geochemical sampling by the Alaska Division of Geological and Geophysical Survey was reported in Bachmann, and others (2013). The Sithylemenkat pluton is considered part of the broad northeast-trending peraluminous Ruby Batholith of central Alaska (Patton and Miller, 1970, 1973; Barker and Foley, 1986; Herreid, 1969). Generally, the granitic rocks are coarse-grained, equigranular to porphyritic, orthoclase-biotite-quartz monzonite varying to granite with subordinate phases of aplite, biotite aplite, tourmaline aplite, and fine-grained quartz monzonite. Tourmaline pegmatite phases have been recognized locally. The granitic rocks cut Paleozoic schist, phyllite, quartzite, and lesser greenstone and limestone. Mineralogical examination of heavy mineral concentrates from the bench channels and streambed samples confirmed abundant cassiterite and rare-earth minerals of monazite and xenotime (Barker and Foley, 1986). |
| Age of mineralization |
The prospect is Quaternary age. The Ruby batholith region is within the glacial ice-free Quaternary province of Beringia, defined as the non-glaciated intermontane region extending from eastern Siberia eastward across a then-dry Bering Sea floor, thence transecting interior Alaska, and extending into northwestern Canada. Across the Beringia region, the fluvial processes have generally continued since the Tertiary and the ongoing erosional and mineral concentration processes continue to concentrate valuable heavy minerals. A few small cirques are evident to the south in the higher elevations of the Ray Mountains, but evidence of glacial disruption of accumulating placer gravel deposits at this site was not seen. |
| Alteration of deposit |
The granitic pluton features alteration zones, including a large central area of argillic alteration enveloping chloritic and hematic greisen veins/bodies that are the apparent source of at least some of the detrital cassiterite, wolframite, and REE minerals. A three-mile long north-northeast trend of these intermittent and cross-cutting greisen veins/zones occurs along the ridges in sections 11, 14, 21, and 22, T. 14 N., R. 17 W., immediately south of this creek (see Figures 6 and 7 in Barker and Foley, 1986). Thermal alteration from the batholith is widespread and silicification extends well into the Paleozoic host rocks. Locally extensive, tourmalization, potassic, carbonate, and argillic styles of advanced alteration can be mapped and generally are associated with regional-scale faulting. Such zones of altered and weakened rock give rise to locally intense large-scale disintegration of the bedrock granite, which has released the interstitial or disseminated heavy minerals. The source of the placer tin and rare earth minerals appears to be widespread argillic alteration and greisen vein-like occurrences as described above, locally overprinted with chloritic and/or hematitic alteration (Herreid, 1969; Barker and Foley, 1986). Greisen samples will generally contain 100 ppm to as much as 2500 ppm tin and elevated rare earth elements.Cassiterite has also been found associated with quartz veins at a few locations in the batholith. |
