Lexus Prospect, Ketchikan Gateway Borough, Alaska, USAi

Regional Level Types
Lexus Prospect	Prospect
Ketchikan Gateway Borough	Borough
Alaska	State
USA	Country

Location: The Lexus prospect was found in 2002. It is about 1.2 miles east of Mount Burnett and about 0.4 mile east of the southwest corner of section 21, T. 70 S., R. 87 E. The location of this prospect is known only from a small-scale map on a web page, but it is probably accurate to within 0.1 mile.
Geology: The Lexus prospect is in the Union Bay complex, the largest of numerous small, Cretaceous mafic-ultramafic intrusive bodies scattered in a belt along the length of southeastern Alaska (Ruckmick and Noble, 1959; Lanphere and Eberlein, 1966; Brew and Morell, 1983; Gehrels and Berg, 1992). Many of these plutons are concentrically zoned, an unusual characteristic that has led to their classification as 'Alaska-type,' or 'Alaskan,' complexes (Noble and Taylor, 1960; Taylor and Noble, 1960; Wyllie, 1967; Jackson and Thayer, 1972). As mapped by Ruckmick and Noble (1959) and reinterpreted by Himmelberg and Loney (1995), the Union Bay complex consists of an outer layer of gabbro that is succeeded inward by magnetite clinopyroxenite, wehrlite, and a core of dunite. The dunite forms a vertical pipe about a mile in diameter. It is bordered on the east by narrow, nearly-vertical shells of wehrlite and clinoproxenite, and on the west by a thick, layered sequence of wehrlite, clinopyroxenite, and gabbro that forms either a large recumbent fold or a lopolith. The complex intrudes probably Upper Jurassic and Lower Cretaceous argillite, tuff, and graywacke of the Gravina sequence (Gehrels and Berg, 1992). The bedded rocks are thermally metamorphosed to schist and gneiss for about 1,000 feet from the intrusive contact. Himmelberg and Loney (1995) suggest that the complex was emplaced during the last stages of Cretaceous regional folding, when the dunite underwent plastic deformation that resulted in a preferred orientation of the olivine. There has been considerable sporadic reconnaissance sampling of the Union Bay complex for magnetite and chromite since at least 1929 and several attempts to locate concentrations of platinum-group-elements (PGE) (See CR013 for details of exploration in the complex prior to 1995). In the early 1990's, Maas and others (1995) collected placer samples in several streams on the north side of the complex and reported anomalous platinum and palladium in the concentrates. They also identified ferroan platinum, native osmium, osmium-iridium, and hollingworthite (a rhenium-platinium-palladium arsenide) in the concentrates and suggested that the source was in clinopyroxenite and wehrlite on the north side of the complex. In 2000, Freegold Ventures Limited began exploring for PGE in the complex and they have located a number of prospects that they are currently working on in a joint venture with Lonmin PLC (www.freegoldventures.com/s/Home.asp; March 1, 2004). The Lexus prospect was found in 2001. While detailed geologic data or sample analyses are not available, the prospect is similar to the other PGE prospects that have been discovered since 2001 in the Union Bay mafic-ultramafic complex (see, for example, CR004-007). Van Treeck and Newberry (2003) studied these PGE deposits in detail and concluded that the PGE minerals are hydrothermal in origin and are associated with veins and lenses of magnetite that cut the mafic and ultramafic rocks of the complex. An early generation of hydrothermal magnetite associated with diopside dikes was formed from 575 to 700 degrees C; the PGE minerals are associated with an intermediate stage of hydrothermal activity marked by the deposition of magnetite and hornblende alteration that occurred between 475 to 575 degrees C; and a later generation of magnetite rimmed by interlayered chlorite and serpentine formed at less than 475 degrees C. They identified the following PGE minerals: ferroplatinum, erlichmanite, iridosmine, platiniridium, and several unnamed rhenium-arsenic-sulfur, rhenium-iron, platinum-antimony, and platinum-iridium-sulfur minerals. The source of the hydrothermal fluids is unknown but the absence of quartz suggests that the fluids are related to the mafic and ultramafic rocks.
Workings: Apparently only surface sampling.
Age: The PGE deposits are in a Cretaceous mafic-ultramafic complex and they are probably related to its emplacement.
Alteration: An early generation of hydrothermal magnetite associated with diopside dikes formed from 575 to 700 degrees C; the PGE minerals are associated with an intermediate stage of hydrothermal activity marked by the deposition of magnetite and secondary hornblende formed between 475 to 575 degrees C; and the last hydrothermal stage, marked by the deposition of of magnetite rimmed by interlayered chlorite and serpentine, formed at less than 475 degrees C.

Commodities (Major) - Ir, Os, Pd, Pt, Rh; (Minor) - As, Cr, Fe, Sb
Development Status: None
Deposit Model: PGE minerals associated with magnetite in pyroxenites of an Alaska-type mafic-u

Other Databases

Link to USGS - Alaska:	CR010

Other Regions, Features and Areas containing this locality

References