A heavy mineral study of Pleistocene and Holocene sediments near Nome, Alaska

Abstract

A heavy mineral study was carried out for sand-size fractions of onshore Holocene sediments (modern beach and river sediments), nearshore Holocene and Pleistocene relict sediments, and Pleistocene and Pliocene sediments from several nearshore drill-holes. Heavy mineral assemblages of these sediments are dominated by garnet, chlorite, epidote, chloritoid, sphene, and staurolite. The nearshore region of Nome is mainly underlain by various types of Pleistocene sediments such as relict gravel that Mantles glacial drift, relict gravel over Nome River outwash fan, relict gravelly sand of submerged beaches, and relict gravel that mantles bedrock. In places these relict sediments are covered by Holocene sandy and muddy sediments. The high concentration of heavy minerals is expected for various relict sediments, because the latter were winnowed by several transgressions and regressions of the sea during Pleistocene time. Concentration of heavy minerals, however, is greater for Holocene sand than relict gravel, which mantles glacial drift and relict gravelly sand of submerged beaches. The high concentration of heavy minerals in Holocene sand suggests the winnowing of sand by strong bottom currents. The low concentration of heavy minerals in the relict gravel on glacial drift and relict gravelly sand of submerged beaches is probably due to the heterogenous nature of relict sediments. Sand fractions of the relict sediments probably have been introduced during the Holocene time. Also, contamination of samples of relict gravel from underlying glacial drift is suspected. A greater concentration of coarse gold particles (1 iiuii. or larger) is found in nearshore relict gravel that mantles glacial drift than in any other sediment type. Relict gravel on glacial drift, which carries high gold values, does not show a high concentration of heavy minerals or a high concentration of garnet. Two factors account for the lack of correlation between concentration of gold and the other heavy minerals: (1) contrast between hydraulic properties of the gold particles and other heavy minerals, and (2) the heterogeneous nature of relict sediments. Because of their extremely high specific gravity, coarse gold particles are not moved by longshore currents or bottom currents from relict gravel which mantles glacial drift, whereas the heavy minerals which are mostly medium to fine sand in size, are transported by longshore currents and strong bottom currents. The Holocene sand winnowed by strong bottom currents shows a high concentration of heavy minerals. Heavy mineral assemblages are more or less similar for the various sediments. Minor compositional variations mainly reflect the effect of sorting of mineral grains according to size and specific gravity. The frequencies of garnet and staurolite are slightly higher than average for modern beach and river sediments. In nearshore sediments, garnet is most abundant in Holocene sand winnowed by strong bottom currents. Holocene silty sediment which occurs in small patches is characterized by high concentration of micaceous minerals and low concentration of garnet, because the weak currents which deposit fine sediments usually carry light micaceous minerals in great abundance and minerals of high specific gravity like garnet in small amounts. Samples of Pleistocene glacial till and Pliocene marine silt from several nearshore drill-hole locations show high concentrations of micaceous minerals and low concentration of garnet. Holocene, Pleistocene, and Pliocene sediments of Nome are mostly derived from the same general metamorphic source rocks of the inland region. The majority of the minerals found in heavy mineral assemblages, such as garnet, chlorite, epidote, chloritoid, sphene, staurolite, hornblende, and tremolite-actinolite, are reported to occur in the metamorphic rocks of Nome and the adjacent region.