Sainsbury reported in his dissertation to Stanford University in 1965 and published by the U.S. Geological Survey as Bulletin 1287, that in the central York Mountains in western Seward Peninsula, Alaska, 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 consumption 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 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 50 percent CaF2 they constitute a large reserve of fluorite, which, together with fluorite ores associated with the tin deposits, totals some 1 million short tons of fluorite. In addition, the known tin deposits contain approximately 62,000 short ton units of WO3, and important but as yet unassessed amounts of silver, lead, and zinc.
The geochemical cycle of the trace elements beryllium, tin, tungsten, boron, lithium, copper, lead, zinc, and niobium shows that these elements were enriched in the biotite granites and were strongly fractionated among the minerals of the granites. From 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 of these anomalies 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 stream sediments.
Datable Pleistocene events in the York Mountains begin with the Yarmouth Interglaciation when the York Terrace, a wide marine platform, was cut. In
Illinoisan time, the York Terrace was uplifted almost 400 feet, and during the Sangamon Interglaciation a second marine platform (Lost River Terrace),
which, if not deformed, was cut. During Wisconsin time the widespread York Glaciation was followed by the more restricted Mint Biver Glaciation. A pre Wisconsin glaciation of much greater extent than the York Glaciation has also been recognized. Because uplift of the York terrace extended into the
Bering Strait, it is probable that prior to the uplift in Illinoisan time the Bering Strait was a seaway and a barrier to land migration rather than a land bridge.
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