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

Abstract

In the central York Mountains, carbonate rocks of Lower and Middle Ordovician age and aggregating at least 8,000 feet thick 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 certain 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. Dikes of granite, rhyolite porphyry, and lamprophyre were injected into some of these faults, the lamprophyres being younger. Trace elements in the lamprophyres prove they are mafic rocks probably derived from the lama, and that they cannot be related genetically to granite. Shortly after the intrusion of 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 mineralized for half its length for a distance of eight miles. The tin deposits contain cassiterite and stannite in topaz greisen with abundant sulfides 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, phenikite(?), todorokite and hematite. Beryl occurs sparingly 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 to banded skarns near granite, and which consist of magnetite and fluorite. Throughout the district, a strong zonation is displayed from tin deposits in greisen through transitional veins of sulfide 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 geochemical cycle of the trace elements Be, Sn, W, B, Li, Cu, Pb, Zn and Nb shows that these elements were enriched in the biotite granites and were strongly fractionated among the minerals of granites. Fran the granites, these rare elements moved outward into contact rocks and ore deposits. During the supergene cycle, clear geochemical anomalies were formed in stream sediments, soils and plants near ore deposits, and geochemical prospecting led to the discovery of the beryllium lodes. With the possible exception of zinc and niobium, the rare elements that are associated in the rocks and ores remain associated in the supergene processes. Fixation of zinc in clay soil and tundra plants may account for the relatively small amount of zinc in strew sediments. Datable Pleistocene events in the York Mountains begin with the Yarmouth Interglaciation when the York Terrace, a wide marine platform, was cut. In Illinoian time, the York Terrace was uplifted almost 400 feet, and during the Sangamon Interglaciation a second marine platform (Lost River Terrace) was cut and is not deformed. During Wisconsin time the widespread York Glaciation was followed by the more restricted Mint River Glaciation. Because uplift of the York Terrace extended into the Bering Strait, it is probable that prior to the uplift in Illinoian time the Bering Strait was a seaway and a barrier to land migration rather than a land bridge.