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Type: Thesis
Title: An Investigation into Iron Isotope Systematics in Felsic Magmas and their Associated Magmatic-Hydrothermal Ore Deposits
Author: Wawryk, Christine Mary
Issue Date: 2017
School/Discipline: School of Physical Sciences : Earth Sciences
Abstract: The two possible valence states of the Fe ion have been shown to be a major control on the fractionation of Fe isotopes in both low and high temperature fluids. Magmatic-hydrothermal fluids can carry large amounts of Fe and economic metals leading to the formation of ore deposits, in which the metal association is determined by redox conditions of the magma chamber. This thesis examines the Fe isotope compositions of intrusive rocks and associated Fe-sulfides, oxides and carbonates, to assess whether a correlation exists between the oxidation state of magmas, and the isotope composition of hydrothermal minerals in ore deposits. Chapter 2 presents a case study of the Fe isotope values of granite and mineral separates from the Renison tin deposit in north western Tasmania. This tin deposit is associated with a reduced, S-type magma. Samples of granite and separates of pyrrhotite, pyrite, arsenopyrite, magnetite, chalcopyrite and siderite were measured. The δ⁵⁷Fe values of mineral separates are consistent with theoretical predictions of equilibrium fractionation based on spectroscopic and other parametric calculations. Mineral-mineral pairs yield temperatures of formation that are in agreement with prior detailed fluid inclusion studies, but are spatially inconsistent with declining fluid temperature with distance from the causative intrusion, limiting the use of Fe isotopes as a potential geothermometer. Mineral separates from the Renison deposit are isotopically heavier than those from the Xinqiao Cu-Fe skarn, contradicting the hypothesis that magmatic-hydrothermal minerals should be isotopically lighter than the source intrusion. Chapter 3 presents Fe isotope analyses from a differentiated series of magmatic intrusive rocks and coeval hypogene ore minerals from the Batu Hijau porphyry copper-gold deposit located on the Sunda Arc, Sumbawa, Indonesia. The variations of δ57Fe with major and trace element concentration is consistent with crystallisation of clinopyroxene, amphibole and magnetite being the primary control on isotopic evolution of the melt. These isotopic trends are supported by thermodynamic (rhyolite-MELTS) modelling of crystal fractionation using published mineral-melt fractionation factors, and demonstrate that the isotopic evolution of these hydrous melts was controlled by crystal fractionation. Magnetite from the Batu Hijau ore deposit is isotopically heavier than coexisting hypogene chalcopyrite and bornite, consistent with theoretical predictions. Models of Rayleigh type fractionation assuming an initial fluid composition that is isotopically light (between -0.5 and -0.1 ‰) produces mineral compositions that match our analytical data and are in agreement with experiments demonstrating that hypogene chalcopyrite has isotopic compositions reflecting that of the hydrothermal fluid. Chapter 4 presents results from the Río Blanco-Los Bronces deposit in Chile as a comparison to those of Batu Hijau. Despite the differences in tectonic setting (Andean margin and island arc, respectively), the results are strikingly similar, pointing to the local influence of fluid and mineral formation processes in determining the δ⁵⁷Fe values of minerals. The δ⁵⁷Fe values of chalcopyrite, when coupled with ³⁴S values result in grouping of minerals that highlight the potential of integrated studies to track metal formation processes. Chapter 5 presents a case study from the giant Olympic Dam and Hillside Iron Oxide Copper Gold (IOCG) deposits located on the Gawler Craton, South Australia. Deposit formation is spatially and temporally associated with the 1.59 Ga voluminous, bimodal Gawler Range Large Igneous Province. Iron isotope analyses of bulk whole rock A-type granites of Hiltaba Suite intrusions, and magnetite, siderite, pyrite, chalcopyrite, bornite and hematite were measured. Hiltaba Suite intrusions are relatively isotopically heavy consistent with published results from the literature for evolved high silica magmatic rocks. Mineral separates such as siderite, magnetite, hematite, chalcopyrite and pyrite have isotopic values that are consistent with theoretical mineral-mineral fractionation factors. Siderite from Olympic Dam and bornite from Hillside are the minerals with the lightest δ⁵⁷Fe values. Pyrite and magnetite returned ranges that are similar between the deposits, however the opposing signs of Δ⁵⁷Fepy-mgt are enigmatic and may be a result of preservation of isotopically light precursor minerals or quantitative dissolution of isotopically heavy precursor minerals. The δ⁵⁷Fe values of hematite and magnetite, sampled where clear overprinting relationships are evident at Olympic Dam, are isotopically indistinguishable within analytical error. This contrasts with previously published oxygen isotope results, demonstrating that Fe isotopes alone are unable to discriminate fluid mixing events at hydrothermal temperatures. The data from all four deposits demonstrate that that the range of δ⁵⁷Fe values between minerals, up to 2‰, is much greater than the range of δ57Fe values of suites of differentiated magmatic rocks (~0.3 to 0.7‰), suggesting that other mechanisms overprint and/or enhance fractionation controlled solely by temperature. Such mechanisms include Rayleigh-type precipitation, remobilisation of Fe²⁺ during coupled dissolution-reprecipitation reactions, and changing sulfur redox conditions in the fluid. Further experimental studies of precipitation mechanisms, coupled with in-situ Fe isotope analyses of zonation within minerals will help elucidate the complex overprinting of processes that render variability in natural samples difficult to interpret.
Advisor: Foden, John
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2017
Keywords: Iron isotopes
felsic magmas
ore deposits
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