Using the Geochemistry of Magnetite and Apatite to Gain Insights Into the Genesis of Kiruna-type Ore Deposits and for Exploration in Densely Covered Terrains

Abstract

Minerals are a key constituent of ore deposits that are the source of metals and non-metals required for our global society to function. In this dissertation, I use field and laboratory methods to understand the processes that lead to the formation of iron oxide - apatite deposits — an important source of iron — and as a tool for discovering new ore deposits. The analytical methods used include backscattered electron (BSE) imaging, cathodoluminescence (CL) imaging, and energy dispersive X-ray spectroscopy (EDS) element mapping, as well as electron probe micro-analysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Chapter 2 describes an investigation of the chemistry of apatite from the Los Colorados iron - oxide apatite deposit, in Chile, that tested existing ore genesis hypotheses. My work indicates that apatite grains in the deposit contain distinct domains with chemistries that show statistically significant differences. The major, minor, and trace element chemistry of the apatite grains are consistent with growth from silicate melt and magmatic-hydrothermal fluid. The data are supportive of ore formation via magmatic/magmatic-hydrothermal fluids, according to a new model developed at the University of Michigan. In Chapter 3, I investigated the chemistry of magnetite and apatite from outcrop and drill core samples from five of the seven ore bodies at the El Laco iron - oxide apatite deposit, in Chile. Magnetite grains in the deepest samples have chemistries and textures consistent with growth of magnetite from a silicate melt, whereas the chemistries and textures of magnetite from the shallow samples and outcrops indicate growth of magnetite from magmatic-hydrothermal fluid. Apatite grains have major, minor, and trace element chemistry consistent with growth from a silicate melt or a magmatic-hydrothermal fluid. Magnetite and apatite grains contain mineral inclusions that preserve evidence for re-equilibration with hydrothermal fluids. Together, the data suggest that the ore bodies at El Laco formed via shallow emplacement and venting of magmatic-hydrothermal fluid suspensions that contained igneous magnetite microlites. In Chapter 4, I test the hypothesis that magnetite in stream sediment can be used as a tool for exploration in covered terrains, such as Guyana, located in the Guiana Shield. Similarities in the chemistries and textures of magnetite from outcrops and detrital grains in the same catchment indicate that the detrital grains can be used to gain insight about geological processes/sources in the catchments. However, the data also indicate changes in chemistry and loss of texture related to weathering and transport. Specifically, the concentrations of Mg, Ni, Cr, Ti, and Mn in magnetite are preserved in grains transported < 1.5 km in streams, whereas the concentrations of V and Al are preserved for transport distances up to 5 km. In order to test the hypothesis, I developed a new model that predicts the ore deposit sources of magnetite grains collected in streams in Guyana, using the compositions of magnetite from nickel-copper-platinum group element (Ni-Cu-PGE), orogenic gold, volcanogenic massive sulfide (VMS), iron oxide - copper - gold (IOCG), and porphyry copper deposits. The results indicate prospectivity for orogenic gold and Ni-Cu-PGE deposits in the sampled catchments. Additionally, sulfides present in detrital grains in the sampled catchments support the ore deposit source(s) inferred from magnetite geochemistry. This work demonstrates the utility of detrital magnetite geochemistry as a tool for exploration in covered terrains.