Artur Deditius

IOCG deposits are an important source of Cu and other critical elements such Au, Ag, REE, U, P and Co. However, regardless of their economic significance, their origin remains controversial. In this study, we focus on the 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 collected from drill cores, and compare their chemistry with sulfides from iron oxide-apatite (IOA) deposits from the CIB in order to elucidate/evaluate the genetic links between these two styles of mineralization. EMPA and wavelength dispersive spectrometry (WDS) X-ray mapping performed on hypogene sulfides revealed very complex chemical zoning of Ni and Co in pyrite. The sectoral and oscillatory zoning, overprinted by brecciation and recrystallization of Co-rich pyrite confirms the presence of multiple pulses of cryptic mineralization. In general, pyrite has Co and Cu concentrations of up to 2.8 and 1.4 wt.%, respectively, whereas Ni can reach hundreds to thousands ppm. Three types of pyrite are recognized based on their Co/Ni ratios, i.e., Co/Ni<1, Co/Ni=1-20, and Co/Ni>90. Pyrite grains from the upper sections of the deposit have a Co/Ni<1, whereas pyrite grains with a Co/Ni>90 correspond to samples from the deeper sections of the deposit. The high Co/Ni ratio in some pyrites is consistent with a magmatic-hydrothermal origin associated with a mafic source. The concentration of other metals (e.g., Au, Ag, Pb, Cd, Zn, Bi) are usually low, with Ag reaching a maximum of 230 ppm. Concentration of metalloids (e.g., As, Sb) and Hg is generally below detection limit; however, some analyses indicate thousands of ppm As, Sb, Hg, suggesting the presence of micro to nano-sized inclusions. On the other hand, chalcopyrite has very low concentrations of the aforementioned trace elements; although As, Zn and Bi can locally reach up to 170 ppm. The EMPA-WDS results show that the concentrations of trace elements vary significantly in sulfides at Mantoverde (especially in pyrite), and their incorporation depends on the physicochemical conditions of the mineralizing fluid(s) of each stage. Furthermore, the geochemical characteristics of Mantoverde pyrite, including the high Co concentration, are remarkably similar to those reported for pyrite from Los Colorados and El Romeral IOA deposits in northern Chile, supporting a link between Andean IOA and IOCG deposits.

Rhenium-Os dating of molybdenite (MoS2), the most abundant molybdenum ore mineral, is commonly used to determine the age of mineralization by measurement of its 187Re and 187Os isotope concentrations. Unlike most sulfides found on Earth and in planetary materials, molybdenite is unique because it contains high Re (in the ppm range) and Os (at ppb levels), but almost no initial or common 187Os, hence all 187Os in molybdenite is of radiogenic origin; i.e., produced from the decay of 187Re. The potential of molybdenite as a single-mineral geochronometer was recognized years ago, although some researchers have suggested that 187Re and 187Os isotopes are not spatially linked at the micro-scale in molybdenite precluding the use of microbeam methods for Re-Os dating. It has been argued that this isotopic decoupling of Re and Os is caused by radiogenic 187Os diffusion which may accumulate in crystal deformation sites. Hence, to obtain accurate and reliable ages whole molybdenite crystals should be analyzed in order to overcome the inferred decoupling. In order to determine the crystallographic controls on the distribution of these elements within the molybdenite structure, elemental and isotope mapping of molybdenite grains using electron microprobe and nano-SIMS were performed. Our results show that Re is heterogeneously distributed in molybdenite and that 187Re and 187Os isotopes are not decoupled as previously thought. Both Re and Os are present in solid-solution and as nanoparticles in molybdenite, indicating a complex formation history that can compromise the accurate determination of mineralization ages.

La composición química y textura de la pirita pueden entregar información clave para dilucidar la evolución de los sistemas hidrotermales. Sin embargo, la relación entre las características composicionales y micro-texturales de la pirita con procesos fisicoquímicos específicos, como puede ser la ebullición, es un aspecto poco estudiado. En este estudio se examinan las características composicionales y micro-texturales de la pirita, y se relacionan con procesos formadores en el Sistema Geotermal Cerro Pabellón (SGCP), campo geotermal activo ubicado en el Altiplano del norte de Chile. Se integran datos de análisis composicionales obtenidos mediante EMPA y LA-ICP-MS con observaciones micro-texturales de pirita y minerales de ganga, en muestras obtenidas de un testigo de sondaje (~500 m) que atraviesa las zonas de alteración argílica, sub-propilítica y propilítica del SGCP. Además, se realizó un análisis de componentes principales para inspeccionar la estructura de la base de datos composicional de pirita. La pirita formada durante ebullición vigorosa se caracteriza por tener concentraciones mayores de As, Cu, Pb, Ag y Au, y menores de Co y Ni en comparación con pirita formada en condiciones diferentes. Estos granos de pirita son anhedrales a euhedrales, localmente con textura porosa con abundantes inclusiones minerales, sugiriendo una formación por cristalización rápida. Por otro lado, la pirita formada en condiciones de ebullición suave, o de no-ebullición, se caracteriza por tener concentraciones relativamente mayores de Co y Ni, y menores de As, Cu, Pb, Ag y Au. Texturalmente, estas piritas forman agregados de cristales euhedrales y prístinos, con escasos poros e inclusiones minerales, sugiriendo una formación bajo condiciones fisicoquímicas más estables. Nuestros resultados muestran que la pirita, además de poder registrar la evolución composicional de los fluidos hidrotermales, también puede proveer información crítica sobre procesos fisicoquímicos como ebullición y separación de fases. Dado que la ebullición de fluidos acuosos es un fenómeno común en sistemas geotermales activos y fósiles (depósitos epitermales de Au-Ag), estos resultados resaltan el potencial uso de la pirita como una herramienta complementaria para explorar la evolución geológica de sistemas geotermales activos, y también para vectorizar hacia mineralización de Au-Ag relacionada a ebullición en depósitos epitermales de baja a intermedia sulfuración.

Trace element concentrations in magnetite, hematite, pyrite and apatite, Fe and O stable isotope abundances of magnetite and hematite, and H isotopes of magnetite and actinolite from IOA and IOCG deposits in the Chilean Iron Belt are used to develop a new genetic model that explains IOA and IOCG deposits as a continuum produced by a combination of magmatic and magmatic-hydrothermal processes. Together, the data are consistent with a model where 1) magnetite and apatite crystallize as near-liquidus phases from silicate melt; 2) magnetite and apatite crystals serve as nucleation sites for gas bubbles and promote volatile saturation of the melt; 3) the volatile phase coalesces and encapsules magnetite and apatite, and scavenges metals, S and P from the melt; 4) the magnetite-apatite-fluid suspension ascends from the host magma during regional extension; 5) as the suspension ascends, magnetite and apatite grow from the fluid and record an evolving magmatic-hydrothermal composition; 6) during ascent, magnetite and apatite are deposited to form IOA deposits; 7) the further ascending fluid transports Fe, Cu, Au, S where hematite, magnetite and sulfides precipitate to form IOCG deposits. The model explains the globally observed temporal and spatial relationship between magmatism and IOA and IOCG deposits.

Arsenian pyrite is the main mineral host of gold in Earth's largest deposits, as well as a major phase contributing to arsenic in the environment. Here we report the discovery of a new form of arsenian pyrite in which As 3+ substitutes for Fe (Fe,As)S 2 (here called As 3+-pyrite), in contrast to the expected substitution of As 1-for S (here called As 1-pyrite). Unlike typical As 1-pyrite, As 3+-pyrite exhibits a linear anti-correlation between As and Fe...

Analyses by electron microprobe and secondary-ion mass spectrometry provide information on the relation of Se and Te to other trace elements in arsenian pyrite from Carlin-type and low-sulfidation type epithermal deposits. Correlations between abundances of Se and Te and the other trace elements probably reflect similar behavior in hydrothermal solution and possibly the presence of as yet unknown complexes. Strongest correlations are seen between Te and both Au and Ag in Carlintype deposits and between Te and Ag in epithermal-type deposits; correlations of Au and Ag with Se are uniformly poor. Although correlations with Te are strong, crossplots indicate low Te:Au and Te:Ag ratios of ~0.1 or less, suggesting that Te was not sufficiently abundant to complex either Au or Ag. These results suggest that Se complexes are not present and that Te complexes, though present, are of secondary importance as an agent of metal transport.