I have data on coal here in Alberta and SE British Columbia that might
bear on occurences in Montana.
There are distinct regional trends. Shallow buried coals out on the plains
containing 0.5 % S or less , have delta 34S values of +10 per mil or even
higher. As one goes westward and southward into the mountains, burial is
deeper, S-contents can excede 1 % and the delta 34S values are near zero.
These ranges of delta values refer to samples which are dominately
organic-S. In some occurences, there may be associated pyrite in varying
amounts and ranging widely in isotopic composition and also trace sulfate
which in most cases appears to be the product of pyrite oxidation in situ
or perhaps even during sample storage.
Increase in S-content and decrease in delta value reasonably correlate to
ash content (based on residue of combustion) One explanation which we
offer is that the distribution of volcanic emissions and/or weathering of
volcanic rocks may have contributed to these regional trends.
There is also a trend in C-isotopes with the more deeply buried coal to the
south and west having delta 13C values about 2 to 3 permil higher than
those of coals on the plains. This possibly relates to losses of 13C
depleted organics with deeper hotter burial but there may have been some
differences in the isotopic composition of the parent material.
I believe that the answer to your question upon which you can confidently
base decisions requires measurements on the materials of interest ( which
are not so difficult these days in contrast to former times).