Open systems fractionation factor Cerro Matoso
The research hypothesis is that clayish and muddish material collected at open pit mine in Cerro Matoso Ni-laterite deposit, correspond to marine sediments associated at hydrothermal systems instead of clays formed by the peridotite weathering. The data shows depleted δ13C and δ18O values in bulk sediments and carbonates that composed a succession of claystone and mudstone overlaying peridotites, sign a reducing environment with the influence of alkaline pH, and bacterial activity. The fractionation factor following the Zhang et al. (2001) equation, indicate in different set of samples precipitation temperatures over 50ºC and reaching 160ºC which fit with a hydrothermal activity instead of supergene groundwaters during the mineral precipitation. Different modeling of δ13C show the reacted fraction assuming FeCO3 and CH4 as products 1 and 2 respectively, in a H2S pool, and will confirm an environment rich in CH4 for the siderite formation.Isotopic analysis of siderite was accomplished by reacting a subsample (10–48 mg dry weight) with 100% phosphoric acid in a Pyrex vessel at 75°C. Reaction was terminated after 2 days, at which time CO2 was no longer produced, and the reaction was considered completed. The evolved CO2 was used for calculation of siderite abundance in the subsample and for analysis of oxygen and carbon isotope compositions. Isotope measurements were carried out at the Stable Isotope Laboratory of the Estación Experimental del Zaidín (CSIC, Granada, Spain). Isotopic ratios were measured by a Finnigan MAT 251 mass spectrometer. To interpret dataset the calculation was done first for all isotope results, and then grouping the data δ13C or δ18O, according to the lithofacies to which they correspond: mound facies or tabular bed facies, the later correspond to metalliferous sediments. Data are formulated, so it is possible to change the constant KIE, delta 2 or delta 1 to see how is the graphs changes. Graphs also are linked with data. For δ18O fractionation factor all data are formulated to obtain δ18O and T in Kelvin degrees in the first table and the according with their affinity (siderite, clasystone or metalliferous) in tables below.