Geology and ore deposits of the Central York Mountains, western Seward Peninsula, Alaska

Abstract

In the central York Mountains, carbonate rocks of Early, Middle, and Late Ordovician age aggregating at least 8,000 feet in thickness are thrust northward over slate and argillaceous limestone of pre-Ordovician age which were intruded by gabbro in pre-Ordovician time. Normal faults of four distinct systems cut .the thrust plates, and, in Late Cretaceous time, stocks of biotite granite, abnormally rich in beryllium, tin, boron and other trace elements pierced the thrust plates. In part following the intrusion of the granites, a strong set of normal faults developed striking N. 60°-85° E. through the central York Mountains, and locally these faults cut the granites. Still later, dikes of granite, rhyolite porphyry, and finally, lamprophyre were injected into some of these faults. Trace elements in the lamprophyres prove that they are maflc rocks, probably derived from the sima, and that they cannot be related genetically to granite. Shortly after intrusion of the lamprophyre dikes, ore deposits of tin, beryllium, and fluorite were formed from solutions probably derived from deeply buried hot granite where the granite was ruptured by normal faults. Ore shoots were localized beneath thrust faults where the faults are intruded by dikes, and a major ore-bearing structure, the Rapid River fault, is 50 percent mineralised for half its length, or a distance of 8 miles. The tin deposits contain cassiterite and stannite in topaz greisen with abundant sulfldes of copper, lead, zinc, and iron, as well as wolframite. The beryllium deposits contain fluorite, chrysoberyl, diaspore, muscovite, and tourmaline, with trace to small amounts of euclase, bertrandite, helvite, phenakite(?), todorokite, and hematite. Beryl occurs sparsely in late veins of quartz and fluorite. Chrysoberyl is the earliest and commonest beryllium mineral, followed by euclase and bertrandite, and then phenakite(?) and beryl. Helvite is restricted near granite to banded skarns which consist of magnetite and fluorite. Throughout the district, a strong zonation is displayed from tin deposits in greisen through transitional veins of sulflde minerals with fluorite and chrysoberyl to fluorite-beryllium deposits and thence to barren veins of silica and fluorite with trace amounts of beryllium. This zonal arrangement of deposits probably will be found elsewhere in the world where greisen tin deposits occur in carbonate rocks. The ore deposits of tin and beryllium are of considerable economic and strategic value. Although the known tin deposits contain only about 23,000 short tons of tin metal less than one-half year's supply at the 1965 consumtion rate they represent by far the largest lode reserves of tin in the United States. The known beryllium deposits, though not in production, contain about 1 2 GEOLOGY, ORE DEPOSITS, YORK MOUNTAINS, ALASKA 4,500 tons of beryllium metal, more than 10 times the beryllium contained in all known domestic pegmatite deposits of substantially lower grade. As the beryllium ores contain more than percent CaF2 they constitute a large re...

Supplemental Material: Detrital zircon ages from upper Paleozoic–Triassic clastic strata on St. Lawrence Island, Alaska: An enigmatic component of the Arctic Alaska–Chukotka microplate

“…970 Ma (Amato et al, 2014) and metasedimentary rocks that generally have MDAs of late Neoproterozoic/Cambrian to Devonian (one sample had an MDA of Pennsylvanian; Amato et al, 2009;Till et al, 2014). Strata on the northern part of Seward Peninsula (York Mountains and Teller regions) preserve sedimentary structure but are generally older than Mississippian (Sainsbury, 1969;Dumoulin et al, 2014a). Thus, no clear stratigraphic correlations can be made between Seward Peninsula and St. Lawrence Island, despite their current proximity, other than the presence of latest Neoproterozoic DZ ages in samples from both Seward Peninsula and St. Lawrence Island that can be matched with ages of igneous rocks on Seward Peninsula and across the Bering Strait in Chukotka (Amato et al, 2009;Gottlieb et al, 2018).…”