USGS Science⌕ Search

Geology topics

H. L. James

Publications and source records attributed to H. L. James.

At least 19 recordsLinked to original sources
USGS1984

Banded iron-formations of late Proterozoic age in the central eastern desert, Egypt: Geology and tectonic setting

In the central Eastern Desert of Egypt, deposits of iron-formation of the Algoma type occur as sharply defined stratigraphic units within layered volcanogenic rocks of late Proterozoic age. The volcanic sequence is characterized by interfingering and repetition of rocks of dominantly andesite-basalt composition and by tectonically juxtaposed ophiolitelike assemblages; it is intruded by voluminous syntectonic granodiorite-tonalite and post-tectonic potassic granite. The iron deposits were formed as chemical precipitates during lulls in dominantly subaqueous, calc-alkaline volcanism, apparently in an intraoceanic island-arc environment. During subsequent complex telescoping of the sequence, the rocks were transported along thrust faults and are now allochthonous.

Economic Geology↗
USGS1981

Bedded Precambrian iron deposits of the Tobacco Root Mountains, southwestern Montana

Bedded deposits of iron-formation are minor components of the thoroughly metamorphosed and deformed Precambrian rocks that make up the core of the Tobacco Root Mountains. The rocks are Archean in age; they predate a major Precambrian orogeny that affected all of southwestern Montana about 2,750 m.y. ago. The principal bed of iron-formation occurs within a metasedimentary sequence that has dolomite marble at the base and rests on quartzofeldspathic gneiss of uncertain origin. The stratigraphic thickness of the preserved part of the metasedimentary group cannot readily be established because of structural complexities, including both thickening and attenuation, but it probably does not exceed 300 m. The true (original) thickness of the iron-formation is even more difficult to determine because of the structural incompetence of the rock, but it ranges from 15 to 30 m. All the rocks, with the exception of a few younger Precambrian (Proterozoic Y) diabase dikes, are metamorphosed to amphibolite or hornblende granulite facies. The iron-formation typically consists of quartz and magnetite, with subordinate amounts of iron silicates, mainly hypersthene, garnet, clinopyroxene, and grunerite. The principal deposits of iron-formation are in the Copper Mountain area, an area of about 13 km 2 in the west-central part of the Tobacco Root range that has been mapped in some detail. The structure consists of an early set of tight isoclinal folds, trending north-south and overturned to the east, that are deformed by later crossfolds that trend and plunge northwest. The most prominent belt of iron-formation is on a tight anticlinal buckle within the northsouth-trending Ramshorn syncline, a major structure of the first fold set. This belt of iron-formation is estimated to contain about 63 million t of potential low-grade ore (taconite) to a depth of 100 m. The rock contains about 35 weight percent Fe, mostly in the form of magnetite. Iron-formation occurs in many other localities in the region as distinctive but thin and discontinuous units. Some deposits probably are stratigraphically equivalent to the iron-formation of the Copper Mountain area, but others have quite different lithologic associations and probably different ages.

Professional Paper↗
USGS1980

Age of the basement rocks of southwest Montana

Rb-Sr analyses of a suite of quartzo-feldspathic gneisses that are interlayered with beds of marble, quartzite, and amphibolite in the Ruby and Tobacco Root Ranges and the Gallatin River canyon of southwest Montana show that the age of metamorphism of these strata occurred about 2,750 m.y. ago. The 13 samples analyzed are from rock units that have in the past been assigned stratigraphically to the Pony Group, Cherry Creek Group, and Dillon Granite Gneiss. Except for two samples of anomalous composition, the data define a linear array on an isochron diagram that has a best-fit value of 2,762 ± 113 m.y. Inclusion of other published data for the Tobacco Root Range yields a best-fit value of 2,730 ± 85 m.y. This age corresponds closely to that of the principal metamorphic-plutonic epoch of the Bear-tooth Mountains, to which the term “Bear-tooth orogeny” has been applied. It also demonstrates that the major Precambrian metasedimentary sequences of the region are of Archean age.

Geological Society of America Bulletin↗
USGS1965

Ages of minerals from metamorphic and igneous rocks near Iron Mountain, Michigan

More than 100 independent isotopic ages have been determined for minerals from an area in northern Michigan about 35 miles square. Granites, pegmatites, and metamorphosed sedimentary and volcanic rocks have yielded Rb-Sr ages for feldspar, muscovite, and biotite, K-Ar ages for hornblende, muscovite, biotite, and feldspar, and U-Pb and Th-Pb ages for zircon. It was anticipated that we would learn from the measurements both the intrusive and metamorphic history of the area and would be able to place limits on the age of the Precambrian sediments in this area. The conclusions may be summarized as follows: (1) Granites and pegmatites with approximate ages of 2,700 and 1,900 m.y. have been found. (2) The major mineral -forming metamorphic event occurred between 1,800 and 2,000 m.y. ago. (3) The Precambrian sedimentary rocks called Animikie are older than 1, 900 m.y. and younger than 2,700 m.y. (4) Biotite Rb-Sr and K-Ar ages and muscovite K-Ar ages were strongly modified by the equivalent of a rise in temperature approximately 1,350 m.y. ago, although no mineral -forming event in this interval has been observed. (5) The K-Ar system was further affected by a thermal rise at about 1,100 m.y. This later event is probably recorded geologically by a few diabase dikes.The resistance to post-formation thermal events shown by the various minerals and decay systems tested may be classified as follows (lowest to highest): feldspar K-Ar, biotite K-Ar, muscovite K-Ar, biotite Rb-Sr, zircon U 238 -Pb 206 , zircon Th-Pb 208 , zircon U 235 -Pb 207 , muscovite Rb-Sr, feldspar Rb-Sr. The few hornblende K-Ar measurements indicate that the resistance of this system is about comparable to that of muscovite and feldspar Rb-Sr. The Pb 207 -Pb 206 age derived from the two U-Pb ages is somewhat more resistant to change than the feldspar Rb-Sr ages .

Michigan↗
USGS1954

Sedimentary facies of iron-formation

The sedimentary iron -formations of Precambrian age in the Lake Superior region can be divided on the basis of the dominant original iron mineral into four principal facies : sulfide, carbonate, oxide, and silicate. As chemical sediments, these rocks reflect certain aspects of the chemistry of the depositional environments. The major control, at least for the sulfide, carbonate, and oxide types, probably was the oxidation potential. The evidence indicates that deposition took place in restricted basins, which were separated from the open sea by thresholds that inhibited free circulation and permitted development of abnormalities in oxidation potential and water composition. The sporadic distribution of metamorphism and of later oxidation permits description of the primary facies on the basis of unoxidized, essentially unmetamorphosed material. The sulfide facies is represented by black slates in which pyrite may make up as much as 40 percent of the rock. The free-carbon content of these rocks typically ranges from 5 to 15 percent, indicating that ultra-stagnant conditions prevailed during deposition. Locally, the pyritic rocks contain layers of iron -rich carbonate. The carbonate facies consists, in its purer form, of interbedded iron -rich carbonate and chert. It is a product of an environment in which oxygen concentration was sufficiently high to destroy most of the organic material but not high enough to permit formation of ferric compounds. The oxide facies is found as two principal types, one characterized by magnetite and the other by hematite. Both minerals appear to be of primary origin. The magnetite-banded rock is one of the dominant lithologies in the region; it consists typically of magnetite interlayered with chert, carbonate, or iron silicate, or combinations of the three. Its mineralogy and association suggest origin under weakly oxidizing to moderately reducing conditions, but the mode of precipitation of magnetite is not clearly understood. The hematite-banded rocks consist of finely crystalline hematite interlayered with chert or jasper. Oolitic structure is common. This facies doubtless accumulated in a strongly oxidizing, probably near-shore, environment similar to that in which younger hema-titic ironstones such as the Clinton oolite were deposited. The silicate facies contains one or more of the hydrous ferrous silicates (greenalite, minnesotaite, stilpnomelane, chlorite) as a major constituent. Granule structure, similar to that of glauconite, is typical of some varieties; others are nongranular and finely laminated. The most common association of the silicate rocks is with either carbonate- or magnetite-bearing rocks, which suggests that the optimum conditions for deposition ranged from slightly oxidizing to slightly reducing. The relationship between the iron -rich rocks and volcanism, stressed by many authors, is considered by the writer to be structural, not chemical: in the Lake Superior region both iron -deposition and volcanism are believed to be related to geosynclinal development during Huronian time. In Michigan, the lower Huronian rocks are iron -poor quartzite and dolomite-typical "stable-shelf" deposits; much of the upper Huronian consists of iron -poor graywacke and slate with associated volcanic rocks -a typical "geosynclinal" assemblage. Thus the iron -rich beds of the middle Huronian and lower part of the upper Huronian were deposited during a transitional stage in structural history. The major environmental requirement for deposition of iron - formation is the closed or restricted basin; this requirement coincides in time with what would be a normal stage in evolution of the geosyncline: namely, structural development of offshore buckles or swells that subsequently develop into island arcs characterized by volcanism.

Lake Superior↗