Hydrogen and oxygen isotope evidence for origin of MVT-forming brines, southern Appalachians

Abstract

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. δ 18O and δD 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 δ 18O = 2% and δD = −41% to δ 18O = 5% and δD = −87%0. Those in the Lower Ordovician paleoaquifer form a cluster at about δ 18O = 7% and δD = −35%. 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 paleoaquifer, which plot along the meteoric water line. Stable isotope compositions of fluids in barite form an array that extends toward very low SD 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 δD 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.