Igneous intrusions

In coal seams, igneous rock is commonly seen as a dike, which is a nearly vertical tabular mass cutting across the bedding of the sediments. If a zone of weakness exists between the bedding planes, the molten material may have been forced to spread as a broad sheet parallel to the bedding plane to form a sill. 

The size of such an igneous mass (and the temperature) may also play a prominent role in determining the rank of the coal. For example, coal adjacent to the intrusion may be either advanced in rank and has increased carbon content relative to the nonadjacent coal. The concept of higher-rank coal having 

FIGU RE 1.6 An igneous dike showing a thin zone adjacent to the igneous rock that has been thermally altered. 

The hardness of igneous rocks renders them a source of continued problems during the mining operations. 

---

V-Surficial, weakly coherent, alluvial deposits readily eroded by water. (Vj-<3 percent slope V2-<12 percent slope). I-Incompetent, or weakly coherent, bedrock such as shales and tuffs readily eroded by water and (or) prone to mass movement on steep slopes (li-<12 percent slope l2->12 percent slope). C-Competent, or strongly coherent, bedrock such as layered lava flow rocks and igneous intrusives not readily eroded by water, nor generally prone to mass movement except for rockslides and rockfalls from very steep slopes and cliffs (Ci-<12 percent slope). 

Pacific Cenozoic Wash. (Calif.) sbb to HVA low S, much influenced igneous intrusions at depth... 

Kerr R.G. and Tait S.R. (1986) Crystallization and compositional convection in a porous medium with application to layered igneous intrusions. /. Geophys. Res. 91, 3591-3608. 

Many authors have studied anthraxolite and related substances since Vanuxem s first account in 1842 (39). It has been reported as cavity fillings, veins, and dikes and as laminations in rocks of all geologic ages and types on all continents (e.g., I, 15, 20, 31, 32, 37, 38). Except for obvious cases of contact metamorphism near igneous intrusions, earlier workers have generally recognized a fluid origin and low temperature history for these substances. 

Exposures in the Black Beauty mine, near the surface in the main slope, show normal unaltered coal of a high volatile C rank. That this has most probably been affected by the intrusion of the dike is evidenced by high concentrations of spheroidal coal or coal apples. Johnson (JO) has reported on spheroidal coal and concluded that development of these structures is related in this area to the igneous intrusions. Further into this mine the entire seam becomes altered to natural coke in rooms headed toward the dike. In these areas mining had to be terminated for this reason. Large areas are exposed,... 

The coal bed from which this natural coke was formed has not been traced to an unaltered area. However, data reported previously indicated that the coal seams of this general area are in the high volatile range. This discrepancy between the reported volatility and the character of the coal particles in the coke may be a result of a relatively rapid increase in the stage of metamorphism immediately prior to coking of the coal by the heat from the igneous intrusion. 

A preliminary examination of coal materials associated with igneous intrusions from two general areas in Colorado has been completed. A study of the field relationships, petrographic examination, chemical analyses, and reflectance studies has been conducted. Results can be summarized as follows ... 

Reflectance is highest at contacts and decreases as distance increases from the igneous intrusion. 

Hydrogen increases as the distance from the igneous intrusion increases. 

The primary focus of research on secondary porosity formation has been on mechanisms for generating undersaturated formation waters. Because reactions that may result in undersaturation of waters with respect to carbonate minerals by consumption of calcium are unlikely to be quantitatively important, emphasis has been placed on reactions that may lower the carbonate ion concentration. Although not clearly documented in deep subsurface environments, mixing of waters of dissimilar composition can result in undersaturation with respect to calcite (see Chapter 7), and lead to secondary porosity formation. Acidic waters associated with igneous intrusions and thermal metamorphism can also cause carbonate dissolution that results in secondary porosity (e.g., deep Jurassic carbonates in Mississippi, U.S.A. Parker, 1974). 

McDougall, I. Potassium-argon ages from lavas of the Hawaiian islands. Geol. Soc. Amer. Bull. 75, 107 (1964) Determination of the age of basic igneous intrusion by the potassium-argon method. Nature [London] 190, 1184 (1961). 

Igneous intrusive bodies occasionally are observed to act as subterranean barriers or dams. Good examples are dikes (Fig. 3.11), sills (Fig. 3.12), and stocks. Fresh igneous rocks are nonpermeable, but with time they become fractured and may be somewhat conducting. Clay-rich weathering products may fill such fractures and improve their sealing properties. 

Gurba L. W. and Weber C. R. (2001) Effects of igneous intrusions on coalbed methane potential, Gunnedah Basin, Australia. Int. J. Coal Geol. 46, 113—131. 

Farrimond P., Bevan J. C., and Bishop A. N. (1999) Tricyclic terpane maturity parameters response to heating by an igneous intrusion. Org. Geochem. 30, 1011-1019. 

George S. C. (1992) Effect of igneous intrusion on the organic geochemistry of a siltstone and an oil shale horizon in the midland valley of Scotland. Org. Geochem. 18, 705-723. 

Contact metamorphism, or thermal metamorphism, occurs when heat from igneous intrusions, melted rocks that move upward, come in contact with cooler rocks above. The cooler rocks do not melt, but recrystaUize as a result of heating. The Palisades sill, an igneous intrusion, produced contact metamorphism in the rocks into which it intruded, and is well exposed beneath the George Washington Bridge near York City. 

Saxby, J.D., and Stephenson, L.C., In situ pyrolysis of oil shale at Rundle (Australia) by igneous intrusion, to be published in "Organic thermogeochemistry" R. Ikan and and Z. Aizenshtat (eds.,) Springer-Verlag... 

---