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.本研究提出的假说为：在塞罗马托索（Cerro Matoso）镍红土（Ni-laterite）矿床露天矿场采集的黏土与泥质物质，并非由橄榄岩风化形成的黏土，而是与热液系统相关的海洋沉积物。 数据显示，覆于橄榄岩之上的黏土岩-泥岩序列中的全岩沉积物及碳酸盐组分，其δ¹³C与δ¹⁸O值均呈亏损特征，表明该环境为受碱性pH影响且存在细菌活动的还原环境。 依据Zhang等（2001）提出的分馏因子公式，不同样品组的矿物沉淀温度均高于50℃，最高可达160℃，这与热液活动条件下的矿物沉淀特征相符，而非表生地下水作用的结果。 针对δ¹³C的多组模拟结果显示，以FeCO₃和CH₄分别作为产物1与产物2的反应组分，在H₂S储层中发生反应，这将进一步证实菱铁矿形成环境为富CH₄环境。 菱铁矿的同位素分析流程如下：称取10~48 mg干重的子样品（subsample），置于派热克斯（Pyrex）玻璃容器中与100%磷酸混合，于75℃下反应。反应持续2天后不再产生CO₂，即判定反应完全。收集逸出的CO₂，用于计算子样品中的菱铁矿含量，并开展氧、碳同位素组成分析。 同位素测试工作在西班牙格拉纳达扎伊丁实验站稳定同位素实验室（Estación Experimental del Zaidín, CSIC, 西班牙格拉纳达）完成，同位素比值通过Finnigan MAT 251型质谱仪测定。 为解读本数据集，首先对所有同位素结果进行计算，随后依据样品所属的岩相类型进行分组：丘状相或板状层相，其中板状层相对应于含金属沉积物。 数据集已预设计算公式，可通过调整动力学同位素分馏常数（KIE）、δ²或δ¹参数，观察图表的变化情况，且图表与数据已实现关联。 针对δ¹⁸O分馏因子，所有数据均已预设公式，可在第一张表中获取δ¹⁸O值与以开尔文为单位的温度T值；后续表格则依据样品归属类型（菱铁矿、黏土岩或含金属沉积物）进行分类整理。