Geology and associated mineral deposits of some ultrabasic rock bodies in southeastern Alaska

Abstract

In connection with a Geological Survey project to investigate the occurrence of nickel, chromium, and refractory-grade olivine in southeastern Alaska, the authors examined several ultrabasic rock bodies during the summer of 1943. Among these were the bodies in the Lituya Bay-Mount Crillon area, central Baranof Island, Red Bluff Bay, the Blashke Islands, Kane Peak, and Mount Burnett In the Lituya Bay-Mount Crillon area the oldest group of rocks is a thick sequence of highly metamorphosed sedimentary and volcanic rocks which are separated by a major fault from a younger group of bedded slate and greenstone. Overlying the slate and greenstone is a thick sequence of Miocene rocks consisting of unsorted conglomerate, sandstone, shale, a few thin seams of coal, and some tuffaceous beds. A large layered basic igneous body with the bulk composition of a hypersthene gabbro or norite intrudes the metamorphosed sedimentary and volcanic rocks. This body is composed for the most part of alternating layers of norite, olivine norite, dunite, troctolite, anorthosite, bronzitite, and ilmenite. Beach sands in the vicinity of Lituya Bay have yielded a limited amount of gold and some platinum; the platinum may have been derived from the basic intrusive. The lowermost outcrops of the basic body contain abundant sulfide minerals, principally chalcopyrite and pyrrhotite, and the ilmenitic sheets in the body are rich in sulfide minerals. No chromite was observed in place, although chromite float has been reported on the glaciers. In the central part of Baranof Island, in the area between Red Bluff Bay and Silver Bay, numerous discontinuous sills of highly serpentinized dunite crop out. Chromite is abundant in the sills, occurring as scattered grains, as crystal aggregates as much as 3 inches long, and as streaks and seams as much as 3 feet long and a few inches wide. No concentration of chromite was found of sufficient size and grade to constitute an ore body. At Red Bluff Bay, Baranof Island a body of chromite crops out in the northeastern part of the ultrabasic mass. This chromite contains sufficient iron to be appreciably magnetic. A series of magnetometer observations was made in a small area immediately south of the northern segment of the deposit to detect, if possible, a southward extension of the body beneath a fault. A distinct high was noted in the magnetometer readings, which tends to indicate subsurface southward extension of the northern segment of the ore body. A small chromite deposit, hitherto unreported; crops out in a gulch a few hundred feet west of the deposit explored by the magnetometer. This deposit contains about 80 tons of chromite with an average grade of 43.20 percent of Crs03. 65 66 MINERAL RESOURCES OF ALASKA, 1943 AND 1944 The Blashke Islands are a group of 16 small islands made up of deformed rocks, which are intruded by an ultrabasic body of roughly circular outcrop. The bedded rocks are largely graywacke and pyroclastic rocks of Silurian age, which generally strike northwestward, but locally they roughly conform in attitude to the margin of the ultrabasic intrusive body. The core of the ultrabasic intrusive is an oval dunite mass about 6,000 by 8,000 feet in outcrop dimensions. Encircling the dunite core is a ring of pyroxenite and wehrlite 500 to 2,000 feet wide, which in turn is encircled by a ring of gabbro with local variants of hornblendite, diorite, augitite, and anorthosite. The gabbro phase is gradational into the surrounding rocks. Contacts between the various units of the ultrabasic body are approximately vertical. A small percentage of sulfide minerals, principally pyrrhotite and chalcopyrite, is locally present in the pyroxenite and the gabbro. Three analyses of samples of the sulfide-bearing material indicate that it contains 0.04 to 0.1 ounce of platinum-group metals per ton. In the Kane Peak area, Kupreanof Island, an ultrabasic intrusive rock body is surrounded by grayw.acke of Cretaceous age and by biotite-quartz gneiss and monzodiorite. The ultrabasic rocks range in composition from gabbro to dunite. Wehrlite, pyroxenite, hornblendite, and mica-rich variants of these rocks are locally abundant. The distribution and rock types in the Kane Peak area are roughly similar to those at the Blashke Islands: In the Mount Burnett area an ultrabasic rock mass about 7 miles long and 1 to 2 miles wide intrudes phyllite and schist of the Wrangell-Revillagigedo belt of metamorphic rocks. Some Tertiary conglomerate overlies the ultrabasic rock body. The ultrabasic rocks form a large composite stock made up of diorite, gabbro, hornblendite, pyroxenite, wehrlite, and dunite. A crude banding was noted within the ultrabasic body. In some places the dunite is highly serpentinized; elsewhere in the body it is very fresh, with no alteration. The olivine approximates forsterite in composition. A few tons of chromite of inferior grade are present. Ultrabasic rocks in southeastern Alaska are known to contain nickel, copper, platinum, and chromite. In most of the ultrabasic rocks in this region nickel and copper are present in minor quantities and are believed to be of no economic importance. A further investigation of the platinum content of some of the ultrabasic bodies may reveal minable quantities of platinum-bearing rock. A few small bodies of chromite are known, but these are of relatively little importance. Large quantities of serpentine-free dunite are known, and these may be of value as a refractory. Almost unlimited quantities of dunite are available, and will be of value if efforts to perfect a process for obtaining metallic magnesium from olivine are successful.