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

Abstract

Abstract 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, d hkl [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, d hkl [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.