The impact of sulfide mineral weathering on the isotopic signatures of dissolved iron and copper in metal-rich streams
The oxidative weathering of sulfide minerals, like pyrite, at the Earth's surface controls the formation of acid mine drainage sites as well as the Fe isotope composition of these areas and the main processes taken place. In this study, water and rock samples were collected from acidic drainages (most from abandoned mining sites) from the St. Juan Mountains in Colorado. The water samples were measured in-situ for dissolved oxygen, dissolved Fe(II), pH, and conductivity. Filtered acidified, unfiltered acidified, and Fe-oxide samples were analyzed for their metal concentrations. Ion-exchange chromatography was applied to both water and digested oxide fractions in order to separate Cu and Fe for isotopic analysis. An Nu Instruments Multicollector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) was used for the measurement of the isotopic signatures of Fe and Cu. Regarding the Fe isotope analysis, the water samples were characterized by <4 and>4 pH values, including δ56Fe values from −0.38‰ to +0.17‰ as well as −1.38‰ to −0.52‰ resulting from weathering dominated and precipitation dominated fractionation respectively. The weathering dominated group includes two water samples of −1.39‰ and −1.14‰ δ56Fe which result due to either unique precipitation or biologic process taken place. The isotopic signature of the Fe(III)-oxides vary from +0.05‰ to +0.62‰ confirming the precipitation dominated fractionation process. The δ56Fe average value of the initial rock as well as continental crust is ∼0.00‰. The comparison of the initial rock value with the δ56Fe values from the weathering dominated samples helps explain that the specific process causes limited fractionation to the metal. The precipitation dominated fractionation is the dominant process in the samples studied. Limited Cu concentrations in the samples resulted in a lack of isotope data. Further investigation could include enhanced Cu isotope analysis as well as calculation of weathering rate of the sulfide minerals.
Gutierrez Najera, Ana Lourdes, "The impact of sulfide mineral weathering on the isotopic signatures of dissolved iron and copper in metal-rich streams" (2011). ETD Collection for University of Texas, El Paso. AAI1498293.