Artur Deditius

The ability for magmatic-hydrothermal fluids to scavenge and transport elements from silicate melt to form ore deposits is well accepted. Less well understood is the capacity for magmatic-hydrothermal fluids to sweep up and transport microlites that serve as nucleation sites for exsolving bubbles in silicate magma. Magnetite from the Los Colorados Kiruna-type iron oxide-apatite (IOA) deposit in the Chilean Iron Belt preserves evidence for the transport of igneous magnetite microlites by magmatic-hydrothermal fluid. IOA deposits are spatially and temporally associated with magmatic activity in arc environments. However, existing genetic models cannot successfully explain the geochemical signature of Kiruna-type IOA deposits, or their spatial association with magmatic activity. Here, we use trace element concentrations, and Fe, O and H stable isotope abundances in magnetite from Los Colorados to develop a new genetic model that explains IOA deposits as a combination of igneous and magmatic-hydrothermal processes. The novel genetic model invokes 1) near-liquidus crystallization of magnetite microlites from an intermediate silicate melt; 2) nucleation of gas bubbles on crystal faces of magnetite microlites; 3) coalescence of the volatile phase and encapsulation of magnetite microlites to form a magnetite-fluid suspension; 4) scavenging of Fe and other metals from the melt; 5) buoyant ascent of the suspension along structurally enhanced dilatant zones during regional extension; 6) growth of originally igneous magnetite microlites that source Fe from the decompressing magmatic-hydrothermal fluid; and 7) deposition of magnetite. The model explains the origin of Kiruna-type IOA deposits, and the globally observed temporal and spatial relationship between magmatism and IOA deposits, and provides a valuable conceptual framework to define exploration strategies.

The origin of magnetite-apatite " Kiruna-type " or iron oxide-apatite (IOA) deposits is still a matter of debate with models that range from a purely magmatic origin by liquid immiscibility to replacement of host rocks by hydrothermal fluids. More recently, a magnetite " flotation model " has been proposed to explain the genesis of Andean IOAs. In this model, fluids exsolved from the magma nucleate on the surface of primary magmatic magnetite. Due to its lower density this magnetite suspension migrates upward and accumulates forming the iron ore. Magnetite is a ubiquitous mineral phase in different geological environments and the specific forming conditions are reflected in its trace element geochemistry. Several trace elements can be incorporated within the magnetite structure. Hence, the presence and concentration of these elements can be used to construct discrimination diagrams and to explain the chemical variation of magnetite for any particular deposit. Here we discuss the trace element signatures of magnetite grains from three large IOAs in the Chilean Iron Belt: Los Colorados, El Romeral and Cerro Negro Norte. EPMA and LA-ICP-MS data coupled with micro-textural observations demonstrate that the magnetite ore formed by several stages, with an early magmatic event characterized by magnetite grains usually enriched in Ti, Mg, Al, Mn, V and Ga, followed by a high-temperature hydrothermal event with magnetite precipitated over the primary magnetite grains. In some cases, late hydrothermal magnetite veinlets form by dissolution-reprecipitation processes. The results present here show that the incorporation and concentration of trace elements in magnetite is a temperature-dependant process and that certain elements can be remobilized by superimposed hydrothermal events. Overall, the data obtained from these deposits are consistent with the " flotation model " proposed to explain the origin of Andean IOA deposits.

Chromite ores hosted in the upper mantle section of ophiolites contain (sub)-economic mineralizations of platinum-group elements (PGEs). These noble metals usually form platinum-group minerals (PGM) or are hosted within the structure of Ni-Fe-Cu sulphides, yet little is known about their potential occurrence as nanoparticulate phases. In this study, we used a combination of focused ion beam (FIB) micron sampling with transmission electron microscopy (TEM) to image the nanoscale occurrence of PGEs in chromite ores from Cuba and Mexico. High-resolution observation of Ru-rich pentlandite from unaltered Cuban chromite ores indicate the presence of the following nanoscale phases: (1) idiomorphic, needle-shape (acicular) Ir-Pt nanoparticles up to 500 nm occurring as oriented domains in Ru-rich pentlandite, and (2) nanospherical Ir-Pt inclusions (<250 nm) within pentlandite. These observations point to sub-solidus exsolution of the Ir-Pt alloy from Ru-rich pentlandite. However, the occurrence of nanospherical Ir-Pt inclusions is harder to interpret as the result of exsolution, suggesting the possibility that they formed within the silicate melt before sulfide liquid immiscibility. In contrast, metamorphosed chromitites from Mexico host laurite (RuS2) inclusions with abundant nanoparticles of Ru–Os–Ir alloys <50 nm in size. In this case, we argue that PGE nanoparticles can be exsolved and grown (or coarsen) from the sulphide matrix during prograde metamorphism, providing an alternative mechanism from the classical high-temperature (magmatic) formation. These two cases illustrate that natural PGE nanoparticles may form and be preserved under magmatic and metamorphic conditions.

In this study, we focus on the mineral chemistry of sulfides from the Mantoverde IOCG deposit, located in the northern section of the Chilean Iron Belt (CIB). Our purpose
is to characterize pyrite and chalcopyrite samples retrieved from drill cores, and compare their chemistry with sulfides from IOA deposits from the Chilean Iron Belt.

The formation of modern non-stoichiometric and highly disordered (proto)dolomite occurs mainly in evaporitic and marine-anoxic, organic-rich sediments dominated by bacterial sulfate reduction (BSR). The dissolution and subsequent recrystallization of the metastable (proto)dolomite into more stoichiometric and well-ordered dolomite during burial diagenesis results in the reset of their orginal (micro)textural and (isotope) geochemical signatures. Here, we investigated a succession of Upper Jurassic partly dolomitized limestone and dolostone deposited on a stable carbonate platform at Oker (Central Germany) in order to elucidate the role of trace elements (S, Sr, Fe, Mn, Na) and BSR during the formation, maturation and burial of dolomite in ancient marine settings. The δ 18 O, δ 13 C and δ 34 S CAS isotope data set (+1.4 to +2.9‰ for δ 18 O, V-PDB,-0.1 to +2.0‰ for δ 13 C, V-PDB and +17.9 to +19.7‰ for δ 34 S CAS , V-CDT) and the trace element distribution (300-400 ppm of Na, ~200 ppm of Sr and 100-900 ppm of Fe+Mn) of the Oker dolomite suggest that dolomitization progressed by the early diagenetic replacement of pre-existing magnesian calcite at moderate temperatures between 28°C and 39°C. The dolomitization fluid was a pristine-marine to slightly evaporitic and reducing seawater. The anti-correlation between decreasing carbonate-associated sulfate (CAS) contents in dolomite, from 1950 to 1050 ppm, and increasing ordering ratio, from 0.34 to 0.50, of the dolomite lattice structure indicates the important role of BSR during the formation and maturation of the dolomite. Thus, it is suggested that the CAS content of modern (proto)dolomites predefines its (meta)stability during burial diagenesis.