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To better constrain the origin of the chromitites associated with the Esker Intrusive Complex (EIC) of the Ring of Fire Intrusive Suite (RoFIS), a total of 50 chromite-bearing samples from the Black Thor, Big Daddy, Blackbird, and Black Label chromite deposits have been analysed for major and trace elements. The samples represent three textural groups, as defined by the relative abundance of cumulate silicate phases and chromite. To provide deposit-specific partition coefficients for modeling, we also report on the results of laboratory experiments to measure olivine- and chromite-melt partitioning of V and Ga, which are two elements readily detectable in the chromites analysed. Comparison of the Cr/Cr+Al and Fe/Fe+Mg of the EIC chromites and compositions from previous experimental studies indicates overlap in Cr/Cr+Al between the natural samples and experiments done at >1400oC, but significant offset of the natural samples to higher Fe/Fe+Mg. This is interpreted to be the result of subsolidus Fe-Mg exchange between chromite and the silicate matrix. However, little change in Cr/Cr+Al from magmatic values, owing to the lack of an exchangeable reservoir for these elements. A comparison of the composition of the EIC chromites and a subset of samples from other tectonic settings reveals a strong similarity to chromites from the similarly-aged Munro Township komatiites. Partition coefficients for V and Ga are consistent with past results in that both elements are compatible in chromite (DV = 2-4; DGa ~ 3), and incompatible in olivine (DV = 0.01-0.14; DGa ~ 0.02), with values for V increasing with decreasing fO2. Simple fractional crystallization models that use these partition coefficients are developed that monitor the change in element behaviour based on the relative proportions of olivine to chromite in the crystallizing assemblage; from 'normal' cotectic proportions involving predominantly olivine, to chromite-only crystallization. Comparison of models to the natural chromite V-Ga array suggests that the overall positive correlation between these two elements is consistent with chromite formed from a Munro Township-like komatiitic magma crystallizing olivine and chromite in 'normal' cotectic proportions, with no evidence of the strong depletion in these elements expected for chromite-only crystallization. The V-Ga array can be explained if the initial magma responsible for chromite formation is slightly reduced with respect to the FMQ oxygen buffer (~FMQ- 0.5), and has assimilated up to ~20% of wall-rock banded iron formation or granodiorite. Despite the evidence for contamination, results indicate that the EIC chromitites crystallized from 'normal' cotectic proportions of olivine to chromite, and therefore no specific causative link is made between contamination and chromitite formation. Instead, the development of near- monomineralic chromite layers likely involves the preferential removal of olivine relative to chromite by physical segregation during magma flow. As suggested for some other chromitite-forming systems, the specific fluid dynamic regime during magma emplacement may therefore be responsible for crystal sorting and chromite accumulation.

This paper reviews briefly the important techniques used for site selection and for explorations including the use of pedological, geological, and topographic maps—the role of aerial photographic interpretation techniques—and the importance of seismic, electrical resistivity, and ordinary drilling procedures in the collection of subsurface data. The paper contains a selected group of references for the guidance of those not entirely familiar with the details of these involved procedures including some limitations and ramifications of each. The outstanding contributions of ASTM through Committee D-18 on Soils for Engineering Purposes as well as the role of the Highway Research Board's Department of Soils, and some of the Committees of the American Association of State Highway Officials are indicated. In the production of plans for a new structure—an airport, a bridge, a building, a highway, a dam, a sanitary disposal plant—there should be no priority higher than that of explorations to develop information for selection of the best site available and to obtain data on the properties of the foundation materials for the proposed structure so that an adequate and efficient design can be made.

This paper summarizes some of the information available covering the distribution of mineral aggregates. Because of the scope of the paper, it was considered necessary to compile a selected bibliography, which is included. The origin of aggregates is discussed on the basis of the method of occurrence, that is, glacial or water-deposited granular materials such as naturally occurring sands and gravels, and igneous, metamorphic, or sedimentary rock as regard solid-rock materials. Gravels and sands occur in glaciated regions in the form of kames, eskers, terraces, outwash plains, beaches, and moraines. The distribution of these materials, in turn, is confined largely to the northern tier of states—all of the North east, much of the Middle West, and the northern portions of the Great Plains as well as portions of the northern states of the Far West. In contrast, other forms of water-deposited materials are found extensively in the Atlantic and Gulf Coastal Plain, the Great Plains, the filled valleys of the West, and to a limited extent in the mountainous regions of both the eastern and western sections of the country. Bedrock materials, used for the production of mineral aggregates, occur in the West in the Columbia Plateau, the Colorado Plateau, and the mountainous regions; while in the East, outcrops of rock are widespread between the Coastal Plain in the east and south and the Great Plains to the west. Even in many sections of the glacial-covered region to the north, bedrock materials are quarried extensively, particularly where the glacial cover is thin. Igneous rocks, used for the production of mineral aggregates, are to be found in widely scattered sections of the country. Traprock, as well as other igneous material, is quarried extensively in the Connecticut Valley, in several sections of the Piedmont in the East, in the Columbia Plateau, and in certain sections of the Rocky Mountains in the West. Certain metamorphic rocks, such as quartzites, are found in South Dakota and Minnesota, and other desirable metamorphic materials are found in the Piedmont sections in the East and in many of the mountainous regions of the country. Limestone is the most important sedimentary rock used for aggregate production and occurs extensively throughout the “Ridge and Valley” region of the East, throughout the Middle West, Kentucky, Tennessee, the Ozark Plateau, and widely scattered areas in the Great Plains and some small areas of the Far West. Since blast-furnace slag is a by-product of the steel industry, its occurrence and distribution is mentioned only briefly. Likewise, lightweight aggregates— such as pumice, volcanic cinders, and vermiculite—are not discussed.