Geochemical and micro-textural fingerprints of boiling in pyrite

© 2018 Elsevier Ltd The chemical composition, textures and mineral associations of pyrite provide key information that help elucidate the evolution of hydrothermal systems. However, linking the compositional and micro-textural features of pyrite with a specific physico-chemical process, e.g., boiling versus non-boiling, remains elusive and challenging. In this study we examine pyrite geochemical and micro-textural features and relate these results to pyrite-forming processes at the active Cerro Pabellón Geothermal System (CPGS) in the Altiplano of the northern Chile. We integrate electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data with micro-textural observations of pyrite and associated gangue minerals recovered from a ∼500 m long drill core that crosscuts the argillic, sub-propylitic and propylitic alteration zones of the CPGS. Additionally, we carried out a Principal Component Analysis (PCA) in order to inspect and un

New contributions to the understanding of Kiruna-type iron oxide-apatite deposits revealed by magnetite ore and gangue mineral geochemistry at the El Romeral deposit, Chile

Iron oxide-apatite (IOA) or Kiruna-type deposits are an important source of iron and other elements including REE, U, Ag, and Co. The genesis of these deposits remains controversial, with models that range from a purely magmatic origin to others that involve variable degrees of hydrothermal fluid involvement. To elucidate the formation processes of this deposit type, we focused on the Chilean Iron Belt of Cretaceous age and performed geochemical analyses on samples from El Romeral, one of the largest IOA deposits in northern Chile. We present a comprehensive field emission electron microprobe analysis (FE-EMPA) dataset of magnetite, apatite, actinolite, pyroxene, biotite, pyrite, and chalcopyrite, obtained from representative drill core samples. Two different types of magnetite grains constitute the massive magnetite bodies: an early inclusion-rich magnetite (Type I); and a pristine, inclusion-poor magnetite (Type II) that usually appears as an overgrowth around Type I magnetite. High V (similar to 2500-2800 ppm) and Ti concentrations (similar to 80-3000 ppm), and the presence of high-temperature silicate mineral inclusions (e.g., pargasite, similar to 800-1020 degrees C) determined by micro-Raman analysis indicate a magmatic origin for Type I magnetite. On the other hand, high V (2300-2700 ppm) and lower Ti (50-400 ppm) concentrations of pristine, inclusion-poor Type II magnetite indicate a shift from magmatic to hydrothermal conditions for this mineralization event. Furthermore, the composition of primary actinolite (Ca- and Mg-rich cores) within Type II magnetite and the presence at depth of fluorapatite and high Co:Ni ratios (> 1-10) of pyrite mineralization are consistent with a high temperature (up to 840 degrees C) genesis for the deposit. At shallow depths of the deposit, the presence of pyrite with low Co:Ni ratios (< 0.5) and hydroxyapatite which contains higher Cl concentrations relative to F record a dominance of lower temperature hydrothermal conditions (< 600 degrees C) and a lesser magmatic contribution. This vertical zonation, which correlates with the sub-vertical shape of the massive iron ore bodies, is concordant with a transition from magmatic to hydrothermal domains described in several IOA deposits along the Chilean Iron Belt, and supports a magmatic-hydrothermal model for the formation of the El Romeral. The dose spatial and temporal association of the deposit with the Romeral Fault System suggests that a pressure drop related to changes in the tectonic stress had a significant impact on Fe solubility, triggering ore precipitation.FONDECYT 1140780 Millennium Science Initiative (MSI) NC130065 University, State and Commonwealth Government

Nanoscale partitioning of Ru, Ir, and Pt in base-metal sulfides from the Caridad chromite deposit, Cuba

We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum-group elements (PGE)-rich base-metal sulfides. The Ni-Fe-Cu base-metal sulfides (BMS) studied are millerite (NiS), pentlandite [(Ni,Fe)9S8], pyrite (FeS2), and chalcopyrite (CuFeS2). These BMS were found forming composite inclusions (<60 mm across) within larger unaltered chromite from the Caridad chromite deposit, which is hosted in the mantle section of the Mayarí-Baracoa Ophiolite in eastern Cuba. Electron probe microanalysis of BMS revealed PGE values of up to 1.3 wt%, except for pentlandite grains where PGE concentrations can reach up to 12.8 wt%. Based on the amount of Ru, two types of pentlandite are defined: (1) Ru-rich pentlandite with up to 8.7 wt% of Ru and <3.5 wt% of Os, and (2) Ru-poor pentlandite with Ru <0.4 wt% and Os <0.2 wt%. Ru-rich pentlandite contains Ir-Pt nanoparticles, whereas the other sulfides do not host nanometer-sized platinum-group minerals (PGM). The Ir-Pt inclusions are found as: (1) idiomorphic, needle-shape (acicular) nanoparticles up to 500 nm occurring along the grain boundaries between Ru-rich pentlandite and millerite, and (2) nanospherical inclusions (<250 nm) dispersed through the matrix of Ru-rich pentlandite. HRTEM observations and analysis of the selected-area electron diffraction patterns revealed that nanoparticles of Ir-Pt form domains within Ru-rich pentlandite. Fast Fourier transform analyses of the HRTEM images showed epitaxy between Ir-Pt domain and PGE-poor millerite, which argues for oriented growth of the latter phase. These observations point to sub-solidus exsolution of the Ir-Pt alloy, although the presence of nanospherical Ir-Pt inclusions in some other grains suggest the possibility that Ir-Pt nanoparticles formed in the silicate melt before sulfide liquid immiscibility. These Ir-Pt nanocrystals were later collected by the sulfide melt, preceding the formation of Ru-rich pentlandite. Early crystallization of the Ru-rich pentlandite and Ir-Pt nanoparticles led to the efficient scavenging of PGE from the melt, leaving a PGE-poor sulfide residue composed of millerite, pyrite, chalcopyrite, and a second generation of PGE-poor pentlandite

Marlowe\u27s religious attitude

Magnetite from hydrothermal ore deposits can contain up to tens of thousands of parts per million (ppm) of elements such as Ti, Si, V, Al, Ca, Mg, Na, which tend to either structurally incorporate into growth and sector zones or form mineral micro- to nano-sized particles. Here, we report micro- to nano-structural and chemical data of hydrothermal magnetite from the Los Colorados iron oxide-apatite deposit in Chile, where magnetite displays both types of trace element incorporation. Three generations of magnetites (X-Z) were identified with concentrations of minor and trace elements that vary significantly: SiO2, from below detection limit (bdl) to 3.1 wt%; Al2O3, 0.3-2.3 wt%; CaO, bdl-0.9 wt%; MgO, 0.02-2.5 wt%; TiO2, 0.1-0.4 wt%; MnO, 0.04-0.2 wt%; Na2O, bdl-0.4 wt%; and K2O, bdl-0.4 wt%. An exception is V2O3, which is remarkably constant, ranging from 0.3 to 0.4 wt%. Six types of crystalline nanoparticles (NPs) were identified by means of transmission electron microscopy in the trace element-rich zones, which are each a few micrometres wide: (1) diopside, (2) clinoenstatite; (3) amphibole, (4) mica, (5) ulvospinel, and (6) Ti-rich magnetite. In addition, Al-rich nanodomains, which contain 2-3 wt% of Al, occur within a single crystal of magnetite. The accumulation of NPs in the trace element-rich zones suggest that they form owing to supersaturation from a hydrothermal fluid, followed by entrapment during continuous growth of the magnetite surface. It is also concluded that mineral NPs promote exsolution of new phases from the mineral host, otherwise preserved as structurally bound trace elements. The presence of abundant mineral NPs in magnetite points to a complex incorporation of trace elements during growth, and provides a cautionary note on the interpretation of micron-scale chemical data of magnetite.MSI Millennium Nucleus for Metal Tracing Along Subduction NC13006

Formation of giant iron oxide-copper-gold deposits by superimposed episodic hydrothermal pulses

Abstract Iron oxide-copper-gold (IOCG) deposits are a globally important source of copper, gold and critical commodities. Despite their relevance, IOCG deposits remain an ill-defined clan, with a range of characteristics that has complicated development of the general genetic model. Here we focus on the Candelaria IOCG deposit in Chile and reveal that by using micro-textural and compositional variations in actinolite, a common alteration mineral found in many IOCG deposits, we can constrain the evolution of these systems. We demonstrate that Candelaria formed by the superposition of at least two pulses of mineralization with a late Cu-rich event overprinting and superimposed over an early, and probably higher temperature, iron oxide-apatite (IOA) mineralization event. These distinct pulses were likely caused by episodic injections of magmatic-hydrothermal fluids from crystallizing magmas at depth. Our data provide empirical evidence of grain-to-deposit scale compositional and potentially temperature changes in an IOCG system. The results support the use of actinolite chemistry as a novel approach to understand the formation of IOCG deposits and a potential tool for vectoring in exploration

Formation of massive iron deposits linked to explosive volcanic eruptions

Abstract The genetic link between magmas and ore deposit formation is well documented by studies of fossil hydrothermal systems associated with magmatic intrusions at depth. However, the role of explosive volcanic processes as active agents of mineralization remains unexplored owing to the fact that metals and volatiles are released into the atmosphere during the eruption of arc volcanoes. Here, we draw on observations of the uniquely preserved El Laco iron deposit in the Central Andes to shed new light on the metallogenic role of explosive volcanism that operates on a global scale. The massive magnetite (Fe3O4) ore bodies at El Laco have surface structures remarkably similar to basaltic lava flows, stimulating controversy about their origin. A long-standing debate has endured because all proposed models were constructed based exclusively on samples collected from surface outcrops representing the uppermost and most altered portion of the deposit. We overcome this sampling bias by studying samples retrieved from several drill cores and surface outcrops. Our results reveal complex lithological, textural and geochemical variations characterized by magmatic-like features and, most notably, a systematic increase in titanium concentration of magnetite with depth that account for an evolving system transitioning from purely magmatic to magmatic-hydrothermal conditions. We conclude that El Laco, and similar deposits worldwide, formed by a synergistic combination of common magmatic processes enhanced during the evolution of caldera-related explosive volcanic systems