Geology Reference
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In a study of four widely separated coal basins, Hower and Gayer (2002) determined that coal metamorphism is
generally controlled by increased temperature related to depth of burial. However, there are sufficient exceptions to
this to attribute some increases in coal rank to other causes, such as igneous and tectonic activity, and to the
movement of hydrothermal fluids.
Igneous Activity and Tectonism In the Illinois basin, Damberger et al. (1999) correlated the rank of most of the
coal seams with maximum depth of burial. A rank increase in the coal seams of SE Illinois that exceeds the
expected increase was attributed to a heating event related to a paleo-geothermal anomaly.
In Indonesia, Paleogene coals are generally bituminous in rank, while Neogene coals are subbituminous (Daulay
and Cook, 2000). However, in some areas Neogene coals in geologically young basins are bituminous. In this area,
increased rank is attributed to uplift and igneous intrusions.
Tectonic displacement of coal seams in China has resulted in communition of coal in the footwall (Cao et al.,
2000). Only slight differences in reflectance and other chemical properties were observed in bituminous and
anthracite samples collected from undisturbed and from deformed layers. However, lower molecular weight
hydrocarbon fragments were concentrated in the deformed samples, indicating that there was some modification
of chemical structure due to exposure to tectonic pressure.
Isotopic ratios of authigenic clay minerals indicated that two episodic, short-lived thermal events were responsible
for increases in the rank of coals in the Bowen Basin of Australia (Uysal et al., 2001). Rather than gradual
temperature increase due to progressive burial, the increased maturity of the coals is related to igneous activity
associated with the breakup of Gondwana.
Hydrothermal Fluids Anomalous variations in rank unrelated to depth of burial or igneous heat flow have been
attributed to the transient geothermal gradients due to the migration of hydrothermal fluids (Hower and Gayer,
2002). This model suggests that coal maturation is due to long term (
2my)
regional high-temperature fluid flow. However, a simple causal relationship between coal metamorphism and fluid
flow has not yet been demonstrated; rather a variety of parameters have been cited to support the increase in coal
rank by hydrothermal fluids.
>
10my) burial and to short lived (1
-
Numeric heat flow models of a transition zone between the Alps and the Pannonian Basin were used to evaluate
heat flow in Paleogene and Neogene sediments (Sachsenhofer et al., 2001). Oligocene vulcanism was the main heat
source for the Paleogene sediments, and magmatic activity was partially responsible for Miocene heat flow. But
igneous rocks were absent in at least one area of very high heat flow, and local increases in the rank of coals may be
due to migrating fluids expelled from sediments beneath the Alpine front (Sachsenhofer and Rantitsch, 1999).
Pyrite from 14 samples of lower Pennsylvanian coals of northwestern Alabama was examined by ion microprobe/
SEM (Kolker et al., 1999). Epigenetic pyrite was found to be enriched with arsenic. Arsenic-rich coals are
prevalent in fault zones, implying that hydrothermal fluids were limited to fault zones and that the hydrothermal
activity was post coalification.
The vertical distribution of coal rank in the South Wales coalfield was found to deviate from the relationship of
Hilt
s Law (Fowler and Gayer, 1999). Variations in vitrinite reflectance were correlated with the intensity of
tectonic deformation. The complexity of a detailed model of faults necessary to explain the vertical rank profile by
post coalification faulting renders this model improbable. Shear stress, frictional heating, and localized fluid flow
are considered more probable mechanisms for this vertical rank profile.
'
Daniels et al. (1990) collected 15 coal samples from the anthracite fields of Pennsylvania and analyzed minerals
from three distinct locations with the coal: in the coal matrix, in the systematic cleat, and in a poorly mineralized
nonsystematic joint set. Mineral assemblages in the three were significantly different; the differences are attributed
to differences in the composition of fluids during late stage diagenesis. Enrichment in Mg and Na suggests that the
minerals were derived from migrating hydrothermal fluids in the higher permeability systematic joint sets of the
coal seams which acted as regional aquifers. The authors suggest that the hydrothermal fluids at temperatures
between 250 and 300°C caused the increase in coal rank. Depth of burial is discounted as the primary mechanism
due to the necessity of assuming that the coal was covered by 6 km of overburden, an unusually large value. Since
igneous activity is absent in this region, the heat flow necessary to produce anthracite would have to have come
from another source.
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