Coal Metamorphism and Igneous Intrusives in Colorado

46 Coal Metamorphism and Igneous Intrusives in Colorado

Downloaded by GEORGETOWN UNIV on August 16, 2017 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch046

RUSSELL R. DUTCHER, DONNA L. CAMPBELL, and CHARLES P. THORNTON Department of Geology and Geophysics, University Park, Pa.

The Pennsylvania

State

University,

Two general areas in Colorado exhibit extensive alteration of coals by igneous intrusives. The first locality is 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.

^ u m e r o u s areas i n the U . S . contain examples of coal w h i c h has been altered by igneous bodies. These have been reported extensively i n the literature. In 1906 Fenneman a n d Gale reported on the effect of intrusions in the Yampa coal field of Colorado (4). M c F a r l a n e in 1929 reported on both coals of the Yampa field and others from the U . S. ( 1 2 ) . In 1939 Dapples worked extensively with altered coals from the Anthracite-Crested Butte area of Colorado (3). C l e g g (2) has studied coals i n southern Illinois which have been altered by intrusives. Johnson, working i n the Spanish Peaks region of Colorado has reported extensive areas of alteration by intrusives (9, J O ) . This paper reports the chemical and physical alteration of coals from two general areas i n Colorado. A n index map is included as Figure 1 to help i n 708 Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


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Coal Mefamorphism in Colorado

709

understanding the general areas from which samples were studied. The first area is i n the west central part of the state i n the vicinity of Somerset. T w o diamond drill core samples were available for study from this area. T h e coals in these two core samples have been altered by sills. Alteration i n the second area, the Spanish Peaks region, has been caused by sills on the south side of the peaks and by dikes on the north side. Material altered by both dikes and sills has been studied from the Spanish Peaks area. Samples i n this region were obtained from active mines on the north and from road-cut exposures and outcrops on the south. One exposure in the vicinity of M e d i n a Plaza, Colo., is of great interest i n that here a xenolith of natural coke occurs i n a sill of "basaltic" composition. T h e xenolith is approximately 16 inches thick and 9 % feet long. Experimental work is being conducted to determine the temperatures and pressures associated w i t h the intrusions and alterations of the samples described in this paper.

Figure 1. Index map. Insert in upper right corner shows location of two general regions in Colorado. Main map shows sample sites in the Spanish Peaks region. Modified from Johnson (9)

Given; Coal Science Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

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Somerset Area Samples Two diamond drill cores, designated D D H - A and D D H - B were obtained from central Colorado. D D H - B is from the vicinity of Redstone, Gunnison County, and core D D H - A is from the vicinity of Somerset in Delta County. These samples were particularly interesting since there is a transition from unaltered high volatile bituminous coal to natural coke at the igneous contacts. Both cores are of Cretaceous age coal. All core samples are from depths in excess of 1500 feet below the surface. These samples were studied by chemical and microscopic methods. Chemical data include moisture, ash, volatile matter, total carbon, and hydro gen. Microscopic data include reflectance measurements on samples from both cores and maceral analyses of core D D H - B . CORE - D D H - Β

CENTRAL C O L O R A D O

Volatile Matter 0

5

10

15

(daf) 20

25

30

1.2-

1.7·

2.2 3.0· w

φ

I

4.0-

I

5.0

Ζ

Ο CO

6.0-

7.0-

8.09.0

0

Figure 2.

1

2

3

4 5 6 7 8 9 10 11 12 13 14 Reflectance and Ash (%)

Volatile matter (daf), ash, and reflectance values in core DDH-B


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DUTCHER ET AL

Coal Metamorphism in Colorado


Table I.

711

Analyses of DDH-B

Sample

Moisture

Total Carbon (daf)

DDH-B-1.2 DDH-B-1.7 DDH-B-2.2 DDH-B-3.0 DDH-B-4.0 DDH-B-5.0 DDH-B-6.0 DDH-B-7.0 DDH-B-8.0 DDH-B-9.0

0.72 0.08 0.94 0.60 1.10 0.90 1.28 1.08 1.34 1.28

92.3 94.3 92.1 90.4 94.5 91.9 89.6 87.5 87.2 87.9

Hydrogen (daf) 0.80 0.94 0.78 1.07 1.76 3.16 3.60 5.18 4.96 5.30

Figure 2 shows the ash, volatile matter, and reflectance data of D D H - B plotted vs. the distance from the contact. There is a coke/sill contact at Sample 1.2 and between Samples 2.2 and 3.0 at position 2.6. The sill above Sample 1.2 is 15 inches thick, and that at position 2.6 is 3 inches in thickness. A l l these sample numbers are positions measured in feet along the core. As the distance from the sill contact increases, the volatile matter increases, and the ash decreases. Johnson, Gray, and Schapiro ( I I ) reported similar results for cores from the same general area. Moisture, carbon, and hydrogen analyses for D D H - B are listed in Table I. The most striking results are the hydrogen data. Hydrogen increases from 0.80% at the coke/sill contact to 5.30% at the 9-foot interval where essentially unaltered coal exists. Carbon data show relatively the same values throughout this range of samples. Hydrogen is possibly then an indicator of the degree of thermal alteration of a coal. However, reflectance values provide probably the most reliable indication of this alteration because they are less affected by the presence of mineral matter. This has also been suggested by Johnson, Gray, and Schapiro ( I I ) . Figure 2 shows the results of reflectance measurements made on polished pellets of the coal and coke samples using a L e i t z Ortholux microscope and Photovolt photometer. Reflectance increases as the distance from the coke sill contact decreases, with values from 1.23 to 13.2% . Maceral analyses—i.e., coal constituent analyses—were made on polished pellets of the coal samples from 9, 7, and 6 feet from the sill contact. A L e i t z Ortholux microscope at approximately 7 5 0 X magnification was used. A t a distance of less than 6 feet from the sill contact it was impossible to distinguish any specific macérais in the coal samples. A t 6 feet it was possible to distinguish the macérais, and a ratio of reactives to inerts of 2 0 : 8 0 % was found. At 9 feet there was an approximate ratio of 7 0 : 3 0 % of reactives to inerts. The bulk of the increase is caused by carbonization of other macérais. Figure 3 shows optically visible changes in the coal of D D H - B . Photograph a is of natural coke at a sill contact; vesicles are well developed. The coke appears completely opaque in transmitted light. Photograph b, a thin section of the coal 4 feet from the sill, appears slightly translucent red. F l o w structure is visible around the two large vesicles of the large grain in the center of the photograph. A t 6 feet from the sill the coal appears brown in transmitted light (photograph c ) . T h e essentially unaltered coal at 9 feet from the sill appears red in photograph d .



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Figure 3. Photographs of thin sections of altered coal from core DDH-B showing the gradual increase in translucency away from the contact with sill. X 46 (a) Opaque natural coke at the contact of coal seam and sill, (h) Highly altered coal at a distance of 4 feet from the sill; slightly deep red translucent; flow structure apparent around vesicles. (c) Altered coal at a distance of 6 feet from the sill; very few areas remain opaque; color is light brown. (d) Slightly altered coal at a distance of 9 feet from the sill; color is red and lighter than red areas of photograph h. D r i l l core D D H - B shows the effect of a sill on a coal bed over a distance of 9 feet. M c F a r l a n e (12) noted that, except where the intrusion represents a flow channel or conduit, the coal is carbonized to "anthracite" only for distances ranging up to one-third the thickness of a sill, both above and below it. T h e sill i n drill core D D H - B is just over 1 foot thick though the coal is coked for more than 4 feet from the contact, indicating that this sill must have been a flow channel or conduit. The greatest observable change i n hydrogen, volatile matter, and reflectance occurs in samples 5.0 and 6.0 which are 4 - 5 feet from the sill, respectively. A l l the carbonaceous material of core D D H - A is coked rather uniformly as can be seen by the reflectance measurements given i n Figure 4. This figure


46.

Cool Metamorphism in Colorado

DUTCHER ET AL

713


also shows a correlation of ash a n d volatile matter data. These data suggest some effect caused b y mineralization, particularly b y carbonates. Unfortunately the presence of carbonate minerals was not known when the analyses were requisitioned, a n d therefore they are not corrected for the loss of CO2 i n the ash a n d volatile matter tests or for the enclosing of this CO2 i n the carbon contents. T h e peaks i n the volatile matter curve may be caused largely b y the lack of these corrections. However, the values i n Table II suggest that most of the analyses were not greatly affected b y the presence of carbonate. M o r e over, i t is difficult to see how there could be any marked trend i n carbon content

CORE - D D H - A

CENTRAL COLORADO

Volatile Matter ( d a f ) 5

555.5

10

16

20

556.5 558.5

559.5-1 560.5 561.5H 623.0 625.51 626.5 627.5 628.5

136.8

629.0 631.0 632.0 634.0 635.0· 637.5

54.2

638.5-1 640.5 641.9 671.7 676.0 677.0-1 678.oJ 2

4

'

8

1

1

1

1

1

1

1

ι

1

1

1

I

10 12 14 16 18 20 22 24 26 28 30 32

Reflectance and Ash (%) Figure

4.

Vofotile

matter (daf), ash, and reflectance core DDH-A

values in


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w i t h distance from the sill even if all values for percent carbon were fully corrected. This is i n contrast to the result w i t h the D D H - B core. A parallel contrast was found by G i v e n and Binder i n their study of the electron spin resonance properties of samples from the same two cores ( 5 ) .


Table II.

Analyses of DDH-A

Sample

Moisture

Total Carbon (daf)

DDH-A-555.5 DDH-A-559.5 DDH-A-623.0 DDH-A-628.5 DDH-A-632.0 DDH-A-634.0 DDH-A-637.5 DDH-A-640.5 DDH-A-641.9 DDH-A-676.0

0.56 0.54 0.64 0.98 0.50 1.14 1.68 3.60 2.78 0.54

90.2 90.5 84.2 91.1 84.4 93.5 84.8 89.8 70.9 93.6

Hydrogen (daf) 0.81 0.87 0.67 0.93 0.84 1.14 2.03 1.67 2.00 1.25

The hydrogen content is higher where the reflectance values noticeably decrease. This break i n the uniform reflectance curve occurs where there is a greater concentration of coked carbonaceous material, without a large admixture of igneous material. D r i l l core D D H - A offers an excellent opportunity for studying sill-coke relationships. Several of the contact relationships between coke and intrusive rock are shown i n Figure 5. The intrusive is intermittently mixed through the coal. The igneous material is a strongly carbonatized and hydra ted porphyritic mafic rock that rather intimately intrudes the coke. In the igneous rock, the approach of a contact is indicated by a noticeable darkening of the cryptocrystalline to microcrystalline, altered ground mass. Just at the contact, coke and igneous material often are separated by a thin zone of medium-grained, usually colorless carbonate mineral. Beyond the contact the coke is cut by numerous veinlets consisting mainly of colorless carbonate, but containing, i n addition, occasional crystals of quartz and a complexly twinned mineral, probably a zeolite; similar veinlets cut the igneous rock, but apparently less commonly than the coke. In addition, the coke is cut by rare apophyses (0.5-1 m m . thick) of the igneous rock itself. F i n a l l y , at least near the intrusion, the vesicles in the coke are filled by mineral matter apparently derived from the sill: by carbonate, in part iron-stained; by quartz; and by igneous rock. Spanish Peaks Region Black Beauty and M a i t l a n d Samples. Approximately three miles northwest of Walsenburg, Colo., coal beds of the Vermejo formation are cut by a northeast-trending, nearly vertical dike which is approximately 25 feet thick. Fortunately, at this locality two coal mines have operated—one on either side of the dike. Both mines have been developed by "drifting i n " from the outcrop of the seam. The Black Beauty mine on the south side of the dike has developed down d i p more or less parallel to the strike of the dike. The main slope of the Maitland N o . 2 mine on the north side has developed down d i p and



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DUTCHBR ET A i .

Coo/ Motamorphism in Colorado

715

Figure 5. Photographs of thin sections from core DDH-A showing some of the features along contact zone of the sill and natural coke. X 46 (a) Carbonate minerals filling large, irregular cavities in coke. Carbonate in cavity on the right is iron-stained; that in cavity on the left is clear. Altered igneous rock of the sill forms the lower third of the photograph. (b) Contact relations between altered igneous rock (mottled gray), coke (black), and carbonate minerals (white and uniform gray). (c) Vesicles in coke (black) filled with carbonate minerals (white) and quartz (gray). (d) Carbonate vein (left half) containing a single subhedral zeolite crystal (dark gray, top) cutting coke (right half). is oriented nearly at right angles to the strike of the dike. T h e general relationships of these mines to the dike and the samples areas are shown i n Figure 6. Exposures i n the Black Beauty mine, near the surface i n the main slope, show " n o r m a l " 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 a n d 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 i n rooms headed toward the dike. In these areas mining had to be terminated for this reason. Large areas are exposed,



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and the transition from coal to coke can be followed. The coke itself is a massive, dense, dark gray material. The original banding of the coal is readily visible; no evidence of flow or extensive vesicnlation was noted. There is a high mineral matter content. W i t h i n the Black Beauty mine the spheroidal coal has a reflectance value of 0 . 7 5 % . Reflectance of the coke described above is 2.86%. Representative samples taken from the face of the main slope (approximately 900 feet from the dike) of the M a i t l a n d N o . 2 mine (samples M - l and M - 3 ) and a third sample back approximately 100 feet ( M - 2 ) show reflectance values of 0 . 6 5 % , 0 . 6 1 % , and 0 . 6 5 % , respectively. T w o additional samples taken approximately 1600 feet from the dike ( M - 4 and M - 6 ) have reflectance values of 0.48% and 0 . 4 7 % , respectively. None of the coal material exposed at any place in the mine is visibly altered by the intrusion. However, the reflectance values are quite sensitive and have recorded some effect from the heat of the dike rock.

Coal Apples

Min* Entrance

Figure 6. Generalized schematic to show relationships of sample areas in the Black Beauty and Maitland No. 2 mines to the dike (hand with checks) Normally, alteration at this distance (approximately 900 feet) is not expected with a dike of this size. The miners " r u l e " is that one foot of coal w i l l be " c o k e d " on each side of a dike for each foot of thickness of the dike rock. The coal beyond these limits rarely appears to be altered at all. C l e g g (2) reports a very narrow zone of visible alteration in his work. Natural Coke at Sopris. At Sopris. Colo., approximately four and one-half miles west of T r i n i d a d , on Colorado State H i g h w a y 12, there are some striking exposures of a "basaltic" sill w h i c h has completely altered a coal seam and in some areas almost entirely replaced it. The natural coking process has been so extensive in this area that attempts were made years ago to actually mine "coke." Figure 7 shows an exposure at Sopris where the sill material has i n vaded the coal seam in several layers. The lighter, more massive beds are the



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Coal Motamorphism in Colorado

717

Figure 7. Natural coke and "basaltic" sill at Sopris locality. Silver dollar, for scale, rests against band of well "fingered** natural coke. A band of the sill is lighter material beneath igneous rock. The darker, well "jointed" bands or layers are natural coke. The coke is well "fingered," and these fingers are oriented at right angles to the heating surface—the layers of the sill. This orientation of coke blocks is shown well toward the top of the photograph where a mass of the igneous rock roughly semicircular in outline is shown. The coke fingers along the upper surface of this conduit-like mass exhibit a clear radial pattern. In the photograph the very light circle is a silver dollar included for scale. The natural coke itself is not at all unlike commercial coke i n appearance although it obviously has a higher than desirable ash content. Associated with the igneous material are masses of carbonate—almost exclusively calcite. These occur as veinlets within the rock and as linings of cavities i n the sill material. Most of the calcite is massive, but some small crystals are present. W i t h i n one small cavity lined with carbonate, a small quartz crystal ("herkimer diamond") was encountered, which bridged from one side of the cavity to the other. Figure 8 is a photograph taken approximately 400 yards west along the road from that area shown in Figure 7. Here the sill is present as a series of conduits roughly circular i n outline and from 3*/2 to 4 feet in diameter. Some of these masses are bulged out at their base. These units, such as the one i n the center of Figure 8, are wholly within the upper and lower boundaries of



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the "coal seam" and are spaced from 15 to 30 feet apart along a 150-foot exposure. The seam at this locality was approximately 6 feet thick. The top and bottom of the seam are still readily located and these boundaries appear horizontal i n the photograph. The apparent uparching of material over the conduit is caused primarily by the coking action. The fingering here is not well developed, but that w h i c h exists radiates outward from the upper areas of the igneous mass. C o a l w h i c h has not reached the coke stage is present in the upper left hand portion of the photograph. C o a l w h i c h megascopically does not appear to be altered also occurs in some of the intervals between conduits. The direction of movement of the molten igneous rock was most likely toward the viewer, i n a southerly direction.

Figure 8. Coal seam highly altered hy "basaltic" sill at Sopris locality. Large conduit-like mass of sill rock in center with natural coke surrounding. Coke arches up and over the conduit-like mass. Seam is underlain by black shale and capped with a thin bfock shale and a massive light gray sandstone The presence of the mass of igneous rock in conduits, not as a sheet, coupled with the localized nature of much of the coking suggest that this area may be at the outer limits of the sill and the magma was moving as a cooler, more viscous mass with a higher concentration of solids than in other nearby areas. The sill is not present at the same horizon across the valley on the south side of the Purgatoire River. It is possible, however, that it may have "jumped section" and for that reason is not present to the south.


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Coal Motamorphism in Colorado

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Oriented vertically and passing through the center of the conduit is a dark zone shown i n Figure 8. This is a fissure that extends from the top of the coke over the igneous rock downward through the "basalt" and at least 12 inches into the underlying black shale. The fissure is 1-1 VA inches wide at the top and VS-VA of an inch wide at the base i n the shale. A l l of this fissure is filled w i t h a noncoked, nongraphitized coal material. The material must

Figure 9. Natural coke xenolith in sill, one-half mile east of Medina Plaza. Sill has replaced most of the coal seam in this area. The letters A and Β in the photograph indicate the longest dimension of the xenolith. It is approxi mately 9Vz feet between these stations necessarily have been forced into this fissure after cooling and solidification of the conduit. There are other similar occurrences—dikes of apparently un altered coal material, homogeneous and vitrain-like, cutting both the sill and underlying shale. One of these, at least 9 feet long, occurs at the xenolith locality east of M e d i n a Plaza. One feasible explanation is that these are the result of condensed volatiles from coal being pyrolized in adjacent areas. M e d i n a Plaza Xenolith. About one-half mile east of M e d i n a Plaza on Colorado H i g h w a y 12, a sill has replaced much of a coal seam in the Raton formation. This locality has been studied and illustrated by Johnson ( 9 ) . W i t h i n a part of this sill there is a large mass of natural coke completely sur rounded by igneous rock. This xenolith of natural coke is about 16 inches thick, 9% feet long and occurs in the base of the 8-foot thick sill. Figure 9 is a photograph of the exposure. Samples of the xenolith were taken in 2-inch increments from the top to the bottom. The coke had a definite columnar structure and was well "fingered." The fingers occurred in 1 V2-2-inch lengths.


COAL SCIENCE

720

These coke samples were studied by chemical, x-ray diffraction, and microscopic techniques. Chemical data include total carbon, volatile matter, ash, and sulfur determinations. Figure 10 shows the percent total carbon and percent volatile matter plotted vs. the distance across the xenolith. The total carbon ( d a f ) , ranging from 85 to 9 3 % , increases toward the center of the xenolith whereas the volatile matter, ranging from 6.5 to 1 6 % , decreases. This may be explained by the movement of volatiles outward to the top and bottom of the xenolith.


PERCENT VOLATILE MATTER

Figure 10. Carbon and matter distribution across of natural coke

(daf)

vofotile xenolith

Heating progressing inward may have coked the outer edges prior to the center and the volatiles from the central portion were condensed or trapped by the outer coke structure. T h i n sections show that translucent material is present in some vesicles and microfissures of the outer zones of coke. This also explains the increase in reflectance as being real in this case even though coupled with an increase in volatile matter. Figure 11 shows carbon and ash plotted vs. the distance across the xenolith. As expected, the ash decreases as the distance from the contacts increases. The ash content is extremely high, from 18.6 to 3 7 . 6 % . Sulfur data were variable, but the values were so small they were almost negligible. There was, however, a slight increase from the top to the bottom of the xenolith; the values ranged from 0.08 to 0.26%. The mineral matter i n the coke was extremely difficult to identify because it was finely disseminated throughout. X-ray diffraction techniques were used to gain some insight into the mineral content of the coke. In order to obtain x-ray diffraction patterns of the mineral matter, the interference of carbon had to be reduced to a m i n i m u m ; this was accomplished by grinding the coke to —200 mesh, sink-floating it in a 2.30-specific gravity liquid, and centrifuging.


46.

DUTCHiR ET AL

Coal Motamorphism

in

Colorado

721

This eliminated enough of the carbon to get readable patterns. T h e major minerals determined were quartz, calcite, kaolinite, and chlorite. The most obvious a n d abundant mineral, quartz, decreased i n relative amounts toward the center of the xenolith, and this trend was apparent i n all the other minerals. Diffraction patterns of the ash from the whole coke, i n general, showed the same mineral decrease trend except, of course, no calcite at all was detectable. Calcination or emission of carbon dioxide from calcite occurs at 8 9 8 ° C . ( 7 ) , significantly below the ashing temperature of 9 5 0 ° C .


PERCENT ASH

PERCENT TOTAL CARBON

(daf)

Figure II. Carbon and ash distribution across xenolith of natural coke In order to substantiate further the mineral matter content information received from the x-ray diffraction analyses, several coke pellets were scanned for sulfur, iron, calcium, silicon, aluminum and potassium in the electron microprobe. Electron probe results showed abundant silicon, suggesting quartz (S1O2), considerable aluminum, suggesting kaolinite [ A l j S i j O ^ O H ) - ! ] and, relatively speaking, little iron, sulfur, calcium, or potassium. Most of the mineral matter appeared to be of secondary origin. This was verified by the following. First, the mineral matter was more concentrated at the outer limits of the xenolith, increasing 2 0 % from the center of the xenolith to the contacts with the greatest increase at the top contact. Secondly, kaolinite which was found to be present, is always a mineral of secondary origin being derived by the alteration of aluminum silicates ( 8 ) . W i t h a few minor exceptions all igneous rock-forming minerals are silicates, and since all of the minerals present are silicates (except calcite), this tends to establish further the idea of secondary mineralization. Finally, M c F a r l a n e (12) suggests that coal near a basalt contact is heated to well over 1 2 0 0 ° C , and temperatures exceed 1 0 0 0 ° C . 1 foot from the sill. It would seem that most minerals present in the coal at the time of the intrusion would not be preserved in their original state.


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Polished pellets and thin sections were made of each incremental fraction across the xenolith for reflected and transmitted light microscopic examination. T h i n sections revealed little because of the opacity of the coke. However, the pellets, which were made b y mounting the coke i n plastic, were invaluable to the study. Reflectance readings of the polished pellets were made on a l l fractions across the xenolith, utilizing a L e i t z Ortholux microscope a n d a Photovolt photometer. Figure 12 shows the mean reflectance values of each fraction plotted vs. the distance across the xenolith. These values generally decrease toward the center of the xenolith as the distance from the sill increases. T h e mean reflectance values range from 5.7 to 8 . 1 % .

Û

PERCENT M E A N REFLECTANCE

Figure 12. Variation in mean maximum reflectance across xenolith of natural coke In addition to reflectance measurements, the pellets were studied microscopically employing a gypsum plate. This is one of the most effective ways of observing coke differences without relying on mechanical measurements. T h e appearance of the various constituents of the coke, whether flat, mottled or striated and the degree of anisotropy readily indicated the type of coal from which the coke is produced ( I ) . U t i l i z i n g the gypsum plate, microscopically this natural coke appeared striated a n d anisotropic. Production coke from a low volatile coal has this same appearance. The coal b e d 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 i n the high volatile range. This discrepancy between the reported volatility a n d the character of the " c o a l " particles i n the coke may be a result of a relatively rapid increase i n the stage of metamorphism immediately prior to coking of the coal b y the heat from the igneous intrusion.


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Coal Metamorphism in Colorado

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Conclusions


A preliminary examination of coal materials associated with igneous intrusions from two general areas i n Colorado has been completed. A study of the field relationships, pétrographie examination, chemical analyses, and reflectance studies has been conducted. Results can be summarized as follows: (1) A s h values are greatest at contacts a n d decrease with increasing distance from the contacts. (2) Reflectance is highest at contacts and decreases as distance increases from the igneous intrusion. (3) Hydrogen increases as the distance from the igneous intrusion increases. (4) Reflectance and hydrogen content are the best parameters for correlating distance from an igneous contact within a coal bed. (5) Volatile matter is a very poor parameter for correlations because of the possible interference of inorganic materials, especially carbonates, and because volatiles from adjacent areas can be trapped i n coke structures. (6) Use of reflectance most probably can detect alteration at greater distances than previous techniques allowed. (7) Fissures or dikes of homogeneous vitrain-like coal cutting sills a n d underlying rock are probably caused by condensed volatiles. (8) T h e conduits encountered at Sopris probably represent the outer limits of this particular sill where the magma was cooler and more viscous. Acknowledgments The authors wish to acknowledge the support of the National Science Foundation (Grant N o . N S F G P - 1 3 7 ) . Ross Johnson of the U . S . Geological Survey has been of great assistance i n all phases of the work. H i s help is gratefully acknowledged. W e also wish to thank the owners of the R e d A s h C o a l C o . who helped i n collecting samples from the Maitland N o . 2 mine. Literature Cited (1) Benedict, L. G., personal communication. (2) Clegg, Κ. E., Illinois StateGeol.Surv., Rept. Invest. 178 (1955). (3) Dapples, E. C., Econ. Geol. 34, 369 (1939). (4) Fenneman, N. M., Gale, H. S., U.S. Geol. Surv., Bull. 297 (1906). (5) Given, Peter H., Binder, C. R., Proc. Conf. Organic Geochem., Paris, 1964 (in press). (6) Hurlbut, C. S., Jr., "Dana's Manual of Mineralogy," 15th ed., John Wiley & Sons, New York, 1941. (7) Ibid., p. 382. (8) Ibid., p. 303. (9) Johnson, Ross B., U.S. Geol. Surv., Bull. 1112-E, 129 (1961). (10) Johnson, Ross B., U.S. Geol. Surv., Profess. Papers 424-C, C20 (1961). (11) Johnson, Vard, Gray, R . J., Schapiro, N., "Abstracts of Papers," 145th Meeting, ACS, September 1963, p. 3K. (12) McFarlane, George C., Econ. Geol. 24, 1 (1929). RECEIVED March 16, 1966.


Coal Metamorphism and Igneous Intrusives in Colorado

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