Hydrogen and oxygen isotope compositions of fluid inclusion hosted
brines from Mississippi Valley type (MVT) deposits in the southern
Appalachians were analyzed to test whether the dominant water in the
brines was seawater as suggested by previously reported Na-Cl-Br
relations. Measurements were made for deposits in the two main
paleoaquifers in the area, the Lower Cambrian Shady and Lower Ordovician
Knox, and included the large East Tennessee ore field and the
Austinville district. delta(18)O and SD values for sphalerite-hosted
brines in the two main paleoaquifers cluster in separate fields. Those
in the Lower Cambrian paleoaquifer form an elongate array that extends
from delta(18)O = 2 parts per thousand and delta D = -41 parts per
thousand to delta(18)O = 5 parts per thousand and delta D = -87 parts
per thousand. Those in the Lower Ordovician paleoaquifer form a cluster
at about delta(18)O = 7 parts per thousand and delta D = -35 parts per
thousand. Stable isotope compositions of fluid inclusion-hosted brines
in fluorite from the Lower Ordovician paleoaquifer form an array that
extends from the Lower Ordovician hosted sphalerite cluster toward the
composition of modern formation waters in the Lower Ordovician
paleoquifer, which plot along the meteoric water line. Stable isotope
compositions of fluids in barite form an array that extends toward very
low delta D values typical of organic matter or gas. These data permit,
but do not require, the interpretation that the mineralizing brines
contain seawater with hydrogen and oxygen isotope compositions that were
modified by reaction with country rocks, particularly with organic
matter and/or hydrocarbon gas. When the Appalachian data are plotted
together with those from other MVT districts in the mid-continent, the
combined data form a broad array that extends downward from hydrogen and
oxygen isotope compositions typical of seawater, toward low delta D
values such as would form by fluid interaction with organic matter or
gas. Consideration of published isotopic compositions of brines from
other MVT deposits suggests that interaction with organic matter and/or
gas may be an important process in the evolution of ore-forming brines.
Copyright (C) 1997 Elsevier Science Ltd.