Elucidating Igneous and Ore-Forming Processes with Iron Isotopes by using Experimental and Field-Based Methods.

Iron (Fe) is a vital resource and the fourth most common element in the Earth’s crust, but variations in the Fe isotope composition of igneous rocks were only recently identified. This dissertation uses experimental and field-based methods to demonstrate the utility of Fe isotopes in tracking igneous and ore-forming processes. Chapter II presents the first experimental data that measure directly Fe isotope fractionation among phases in a fluid-bearing magmatic assemblage. The results, some of which contradict theoretical predictions, indicate that Fe isotopes fractionate during crystallization of magnetite from a melt and that Fe isotope fractionation between melt—fluid is influenced by the Cl content of the fluid. This is important considering the frequent extrapolation of data obtained from Fe-Cl complexes that are unrealistic for magmatic systems. Chapter III applies Fe isotopes to natural ore samples since Fe is globally mined from the rocks of iron oxide—apatite (IOA) deposits, which are a globally important source of Fe and other elements such as the rare earths but lack a genetic model. I focus on the world-class Los Colorados IOA, Chile as a case study and combine the Fe and O isotope composition of magnetite to investigate their formation. The data are consistent with a high-temperature (i.e., magmatic/magmatic-hydrothermal) origin for IOA deposits, and contributed to the development of a published novel IOA model. Iron is also abundant in layered mafic intrusions, and Chapter IV focuses on the uppermost portion of the world’s largest exposed mafic magma chamber, the Bushveld Complex, South Africa. These Fe isotope data demonstrate that fractional crystallization is reflected in the Fe isotope signature of the uppermost Bushveld. Stratigraphically, over the top ~2.5 km of this 9 km-thick intrusion, there is little variation in both whole rock and magnetite Fe isotope compositions, revealing that, despite theoretical predictions for the crystallization of magnetite to shift the isotopic composition of the whole rock, the presence of other Fe-bearing phases can buffer that effect. By incorporating published fractionation factors to model the measured data, this study provides the first benchmark for Fe isotope evolution during the crystallization of a large magma chamber.PhDEarth and Environmental SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113582/1/bilenker_1.pd

Abiotic Oxidation Rate of Chalcopyrite: Implications for Seafloor Mining

In situ mining of seafloor massive sulfide (SMS) deposits will have consequences thus far not quantified. On land, interaction of mined sulfide minerals with surface and groundwaters yields acid mine drainage. Pulverization of SMS on the ocean floors will produce highly reactive sulfide mineral surface areas, leading to the localized potential for seafloor acid generation. Chalcopyrite (CuFeS2) is one of several ore minerals found in SMS deposits whose oxidation kinetics need to be quantified to estimate the significance of acid production. To constrain the oxidation rate of chalcopyrite in seawater, the initial rate experimental method was employed and combined with the isolation method to derive a rate law. Data collected from batch reactor experiments without abundant precipitates (pH <4.5), between 7°C and 25°C, and PO2 from 0.10 to 0.995 atm were incorporated into the rate law. The molal specific rate law is:Rsp = - 10-9.38(PO2)1.22(H+)0.36Chalcopyrite oxidizes slowly in seawater relative to other sulfide minerals like pyrrhotite (Fe1-xS), so data from this study establishes a minimum rate of abiotic SMS weathering by oxidation. The slow rate of oxidation of chalcopyrite observed here has positive implications for seafloor mining. Not only will this sulfide not be the main culprit for acid production, but the copper ore will arrive at the surface with minimal dissolution and loss of metal value. Constraining the oxidation rates of individual sulfide mineral species will be useful in modeling SMS mining repercussions, as well as quantifying rates of natural chemical weathering in the oceans over geologic time. This information will be applicable to interpreting the Cu/Fe ratios of VMS deposits.The potential for local acid generation can be viewed as a microcosm of the global problem of ocean acidification caused by dissolution of anthropogenic atmospheric CO2. Data show sulfide mineral oxidation rates increase with lower pH, implying that a worldwide drop in ocean pH may amplify the dissolution of SMS deposits, changing the marine ecosystem