Sodic-Calcic Alteration at the White Rock Pluton: Constraining Fluid Sources with Oxygen Isotopes
Sophia Hopp ’20, Chicago, Illinois
Major: Environmental Geology
Rocks on the eastern margin of Whiterock Pluton (29 Ma) in the southwestern Elk Mountains, have been locally affected by sodic-calcic metasomatism due to the flow of moderate to high salinity fluid along joints and fractures. In this process, sodium and calcium were added to the rock at the expense of other elements. This form of alteration is widespread in contact aureoles and is commonly associated with iron oxide copper gold deposits (IOGs) and iron oxide apatite deposits (IOAs). Mineral samples taken from the Whiterock Pluton, and surrounding country rock, were analyzed for oxygen isotopes to determine the source of altering fluid. The alteration cuts across grandorite and Maroon Formation metasedimentary rocks, and occurs across several zones of varying intensity of alteration. Some igneous plagioclase persists through the alteration zones, but much was replaced by metasomatic Na feldspar. Alteration products include titanite and albitized feldspar, which are present in the most altered zones. The oxygen isotopic compositions of several minerals in these zones have been analyzed to constrain the origin of the alteration fluids. The δ18O of a mineral is a function of the mineral’s fractionation factors, the δ18O of the altering fluid, and temperature.
Based on these new mineral data, δ18O values of water in equilibrium with these samples fall within the range for normal igneous-derived fluids (+6 ‰ to +10 ‰) for a range of alteration temperatures. Previous studies of the Whiterock Pluton have shown evidence for meteoric water sourced alteration. This particular alteration zone seems to be dominated by igneous fluid, and suggests inhomogeneous fluid sources at the scale of the contact aureole.