Fingerprinting Source Fluids of Iron Oxide-Copper-Gold and Iron Oxide-Apatite Deposits Using Traditional and Non-Traditional Stable Isotope Geochemistry

Abstract

Iron oxide deposits have provided important amounts of metal to society since the dawn of the Iron Age. In 2017 alone, they supplied 2.4Gt of raw iron ore globally, in addition to other valuable elements (e.g. Au, Ag, Cu, and Co). In order to meet industry’s growing resource demand, it is imperative to test and refine current genetic models that explain the occurrence of these deposits. On the basis of their elemental contents, iron oxide deposits are divided into two distinct groups: the iron oxide–copper–gold (IOCG) and iron oxide–apatite (IOA) deposits. Nearly a century of geological research has produced several working models to explain how they formed, but agreement is lacking. Two predominant models invoke magnetite mineralization either directly from a magma or from hydrothermal fluids, where the occurrence of both types requires multiple fluid sources. Additionally, previous studies have hypothesized IOAs may form the deeper roots of some IOCG deposits, citing similarities among ore related minerals and the occurrence of both deposit types both spatially and temporally, and have proposed an IOA-IOCG continuum model. If we are to explore for more deposits of these types, it is imperative to test and refine these models, which is the objective of this thesis research. After a brief introductory chapter, Chapter II deals with models for the Proterozoic Pea Ridge and Pilot Knob IOA deposits of Missouri, USA. Stable Fe and O isotopes and trace elements in the ore forming magnetite from these deposits indicate a silicate magma source for the magnetite ore bodies and at least three generations of magnetite. Two generations grew from a hydrothermal fluid, while one high-Ti variety crystallized from a magma. These observations suggest a new genetic model that incorporates the occurrence of both hydrothermal and magmatic magnetite via magnetite microlite flotation where orthomagmatic magnetite may be enveloped by a buoyant Fe-rich fluid within a magma that further precipitates hydrothermal magnetite. This new model demonstrates that both magmatic and hydrothermal magnetite may originate from a single source. Chapter III focuses on the Jurassic Mantoverde IOCG deposit in Chile. It compares Fe and O isotopes and trace element contents of Mantoverde magnetite and hematite to the same minerals in the neighboring Los Colorados IOA deposits. The isotopes and trace elements indicate that both early magnetite and late hematite were sourced from a silicate magma. Similar isotopic signatures and cooling trends in trace element concentrations at Los Colorados support the previously proposed IOA-IOCG continuum hypothesis. In Chapter IV, triple O, H, and Fe isotopes were analyzed in the ore forming iron oxides of the enigmatic near-surface El Laco IOA deposits of Chile. This combination of isotopic measurements reveal that magnetite was sourced from a silicate magma and, when interpreted in combination with drill core data, indicate magnetite in these deposits formed from an evolving magmatic fluid that crystallized orthomagmatic and hydrothermal magnetite. Significantly, this supports the magnetite microlite flotation model that is compatible with all of these observations. This research effectively rules out other genetic models, and links IOAs and IOCGs in a continuum model. Future refinement of these models is key to better understanding of the petrologic and hydrothermal processes that form these important deposits, as well as to bolster exploration strategies for the IOAs and IOCGs.