M. L. Rivas-Sanchez

Berthierine and chamosite hydrothermal : genetic guides in the Peña Colorada magnetite-bearing ore deposit, Mexico

We report the first finding of berthierine and chamosite in Mexico. They occur in the iron-ore deposit of Peña Colorada, Colima. Their genetic characteristics show two different mineralization events associated mainly to the magnetite ore. Berthierine is an Fe-rich and Mg-low 1:1 layer phyllosilicate of hydrothermal sedimentary origin. Its structure is 7 Å, dhkl [10 0] basal spacing and low degree structural ordering. The phyllosilicate has been identified by a lack of 14 Å basal reflection on X-ray diffraction (XRD) patterns. These data were supported by High Resolution Transmision Electron Microscopy (HRTEM) images that show thick packets of berthierine in well defined parallel plates. From the analysis of Fast Fourier Transform (FFT), we found around [1 0 0] reflections of berhierine 7.12 Å and corresponding angles of hexagonal crystalline structure. Berthierine has a microcrystalline structure, dark green color, and high refraction index (1.64 to 1.65). Birefringence is low, near 0.007 to null and it is associated to nanoparticles (<15 nm) and microparticles of magnetite (<25 μm), fine grain siderite, and organic matter. Its texture is intergranular-interstratified with colloform banding. The chamosite Mg-rich is of hydrothermal epigenetic origin affected by low-degree metamorphism. It is an Fe-rich 2:1 layer silicate, with basal space of 14 Å, dhkl [0 0 1]. The chamosite occurs as lamellar in sizes ranging from 50 to 150 μm. It has intense green color and refraction index from 1.64 to 1.65. The birefringence is near 0.008, with biaxial (-) orientation and a 2V small. It is associated mainly to sericite, epidote, clay, feldspar, and magnetite. Chamosite is emplaced in open spaces filling and linings. Mössbauer spectra of berthierine and chamosite are similar. They show the typical spectra of paramagnetic substances, with two well defined unfoldings corresponding to the oxidation state of Fe+2 and Fe+3. Chemical composition of both minerals was obtained by an electron probe X-ray micro-analyzer (EPMA). The radio Fe+Mg+Mn vs Si and Al show similar chemical compositions and different XRD patterns in the crystalline structure provoked by the environmental conditions of emplacement. A hydrothermal environment was predominant, occurring before, during, and after the magnetite mineralization. The identification of magnetite nanoparticles supports the hypothesis of a marine environment, specifically exhalative sedimentary (SEDEX) for the berthierine.

Integrated magnetic studies of the El Romeral iron-ore deposit, Chile: implications for ore genesis and modeling of magnetic anomalies

Abstract Mineralogical and rock-magnetic studies of iron ores and host rocks in El Romeral Mine are carried out to characterize the magnetic mineralogy and the processes that affect the natural remanent magnetization during emplacement and evolution of the iron-ore deposit. Extremely important is the identification of magnetic mineralogical composition (magnetite and/or titanomagnetite, hematite and/or titanohematite, and titanomaghemite) and grain size. These data permit investigation of magnetic domain state and magnetization acquisition processes and to assess their significance as a source of magnetic anomalies. Chemical remanent magnetization (CRM) seems to be present in most of investigated ore and wall-rock samples, substituting completely or partially the original thermoremanent magnetization (TRM). Magnetite (or Ti-poor titanomagnetite) and titanohematite are commonly found in the ores. Although hematite may carry a stable CRM, no secondary components are detected above 580 °C, which probably attests that oxidation occurred soon after the extrusion and cooling of the ore-bearing magma. The microscopy study under reflected light shows that magnetic carriers are mainly titanomagnetite with significant amounts of ilmenite–hematite minerals. Magmatic titanomagnetite, found in igneous rocks, shows trellis texture, which is compatible with high temperature (deuteric) oxy-exsolution processes. Hydrothermal alteration in ore deposits is indicated by goethite and hematite oxide minerals. Grain sizes range from a few microns to >100 μm, and possible magnetic states from single to multidomain, in agreement with hysteresis measurements. Thermal spectra, continuous susceptibility measurements, and isothermal remanent magnetization acquisition suggest a predominance of spinels as magnetic carriers, most probably titanomagnetites with low-Ti content. For quantitative modeling of the magnetic anomaly, we used data on bulk susceptibility and natural remanent intensity for quantifying the relative contributions of induced and remanent magnetization components, and this allows greater control of the geometry of source bodies. The position and geometry of these magnetic sources are shown as ENE-striking tabular bodies, one steeply inclined (75°) to the south and another lying horizontal.

Rock magnetism and microscopy of the Jacupiranga alkaline-carbonatitic complex, southern Brazil

This study of the Cajati deposit provides evidence that the ore was neither purely hydrothermal, nor volcanic in origin, as previous workers have proposed. The ores were formed from magnetite-rich magmas, hydrothermally altered and intruded at an indicated crustal depth in excess of 500 m. The mineralogical and textural association between magnetite and magnesioferrite in the carbonatite, and between the titanomagnetite and magnesioferrite-Ti mineralization in the pyroxenite of hedenbergite, seems to be analog mineralizations strongly related to the ionic substitution of Fe2+ by Mg. Relatively high Q ratios (≥5) for Jacupirangite-pyroxenite may indicate a thermo remanent magnetization (TRM) by the ore during post-metamorphic cooling, however it can also be developed from chemical remanent magnetization (CRM). Vector plots for the pyroxenite samples show reasonably linear and stable magnetic components. The intensity decay curves show that only two components of magnetizations are likely present. Continuous susceptibility measurements with increasing temperature show that the main magnetic phase seems to be magnetite. Maghemite is probably produced during the cooling process. Susceptibility recorded from low temperature (liquid nitrogen (-196°C)) to room temperature produces typical curves, indicating Verwey transition of magnetite. Hysteresis parameters point out that nearly all values fall in a novel region of the Day plot, parallel to but below magnetite SD + MD mixing curves.

Microscopy and rock magnetism of fine grain-size titanomagnetite from the Jacupiranga Alkaline Complex, Brazil: unearthing Ti-magnesioferrite nanoparticles

Resumen Se seleccionaron muestras muy finas de zonas mineralizadas del complejo Jacupiranga de la mina Cajati para efectuar la identificacion cristalografica de nanoestructuras de titano-magnesioferrita (TMf) embebidas en titanomagnetita (TM) usando microscopia de transmision de alta resolucion (TEM). Se redujo un concentrado magnetico a partir de muestras de piroxenita (sitios 4 a 7), despues se dividio en fracciones de rangos de tamano distintos: 26±2 μm, 19±1 μm, 13±1 μm, 9±1 μm, 6±1 μm and 6-0.1 μm. Las muestras mineralizadas de piroxenita y carbonatita se caracterizaron por: difraccion de rayos-X, microscopia de luz transmitida y reflejada, y microscopia electronica de barrido con analisis multielemental. La muestra de concentrado mas fino (MC6) se analizo por microscopia TEM y campo anular obscuro de angulo alto y espectroscopia Raman. Se midieron las propiedades magneticas de las distintas fracciones granulometricas, mostrando cambios drasticos cuando los tamanos de grano pasan de tamanos micro a nanometricos. El porcentaje de susceptibilidad magnetica dependiente de la frecuencia (χfd%) arrojo valores altos (10.2%) para las fracciones mas finas (6±1 μm y 6-0.1 μm), lo que se atribuyo a las fracciones dominantes de particulas superparamagneticas. Los tamanos de grano nanometrico y