Outcrop samples sections

Petrographic observations were made on polished thin sections, for 313 outcrop samples. Average sample spacing was one sample per 1 km within a radius of 15 km or less from the known porphyry Cu-Mo deposits, and one sample per 2 km outside this area. Five rock types are distinguished in the district andesites, diorites, granodiorites, dacites, rhyolites and sandstones. Rhyolites and dacites dominate in the zone of known porphyry Cu-Mo deposits. Sandstones are much less common and were not studied. 

Two general areao in Colorado exhibit extensive alteration of coalo by igneouo intrusives. The first locality it near Somerset in the west central part of the state, and the second area is the Spanish Peaks region near Trinidad and Walsenburg. Drill core samples, outcrop samples, materials from active mines, and thin sections of the intrusive rocks were studied. The results show that mean maximum reflectance of the altered coal or natural coke increases as the distance from an intrusive body decreases. Carbon and ash values increase as the distance from intrusive decreases whereas volatile matter values decrease. Sulfur data are variable. Hydrogen values increase as the distance from an intrusive increases. Hydrogen and reflectance are considered the most sensitive and reliable indicators of degree of alteration. 

Figure 5. Distribution of particle-size parameter (average diameter of cross-sectional area) for geothite (a-FeOOH) in samples of Middle Kittanning coal. Key A, high wall sample O, middle sample , outcrop sample and P, original pyrite sample. See Table / for definition of class limits.

Most of the analyses of Antarctic coal were based on blocks of transportable size taken from surface outcrops. In earlier work, sections of purer coal were cut from the blocks and submitted for analysis. In later work, selected blocks were crushed, and a float fraction of purer coal was submitted to provide analytic data from each deposit. These samples, while not useful for determining coal grade, may be adequate for indicating rank. Practically all of the Antarctic coal, with the exception of that at Amery (which may represent high volatile bituminous coal), corresponds to medium volatile bituminous or higher rank. The high apparent rank Antarctic coal may be classified according to conventional ASTM standards of rank based on proximate analysis, but it seems clear that these results serve only as a first approximation. 

Figure 4. Cross-section of Neptune s Grotto flowstone ( 73 cm thick), unconformably overlying Triassic marble. Black arrows indicate outcrop-scale disconformities (numbered I to IV). Numbered sections on the left correspond to text descriptions. The white arrow shows a broken stalactite resting on a disconformity surface. The open triangles to the right of the graph indicate stratigraphic locations of U-series samples.

On a regional basis the overall carbonate content of Lee and Breathitt sandstones is low, averaging approximately 3 vol% below the PMO and slightly less above (Table 1). Only around 10% of the 281 samples examined in the general petrographic survey (see Methods section above) contain carbonate in excess of 5 vol%. Carbonate enrichment is a highly localized phenomenon at the outcrop scale, and examples of carbonate-rich samples are found both above and below the PMO. 

Channel sampling is one of the best methods for sampling in-seam coal. When the coal sample is collected from an outcrop, the exposed area should be cleaned to avoid the weathered exposed coal surface. Normally, a small box cut is made at the coal outcrop exposing the entire thickness of the coal seam. For a relatively thin seam, only one coal section is recommended. However, if the seam is thick, two or more coal sections may be necessary to sample entire seam. 

Composite sample A recombined coalbed sample produced by averaging together thickness-weighted coal analyses from partial samples of the coalbed, such as from one or more bench samples, from one or more mine exposures or outcrops where the entire bed could not be accessed in one sample, or from multiple drill cores that were required to retrieve all local sections of a coal seam. 

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