Geochemical Analysis of Fluid Inclusions in Trace and Minor Mississippi Valley-type Zn-Pb Occurrences in the US Mid-continent: Implications for Crustal Metal Enrichment Processes

美国中部大陆微量和少量密西西比河谷型锌铅矿点中流体包裹体的地球化学分析：对地壳金属富集过程的影响

• 批准号: 1322018
• 负责人: Martin Appold
• 金额: $ 26.56万
• 依托单位: University of Missouri-Columbia
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-07-15 至 2017-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1322018&HistoricalAwards=false
• 关键词: Geochemical; Analysis; Fluid; Inclusions; Trace

The Mississippi River watershed of the central U.S. hosts numerous deposits of lead and zinc ranging from the world-class Southeast Missouri and Tri-State (Oklahoma-Missouri-Kansas) mining districts to widely dispersed non-commercial traces of mineralization. Collectively these deposits are referred to as 'Mississippi Valley-type' (MVT) and occur as sulfide minerals that were precipitated in limestone and dolomite host rocks by highly saline groundwater (about 10 times more saline than seawater) at temperatures between 80 and 150° C. The largest deposits are in the Southeast Missouri district, which currently account for about 70% of primary lead production and about 6.5% of primary zinc production in the U.S. The Tri-State district was mined commercially from 1848 to 1970 and during its peak in the early 20th century accounted for as much as about 60% of primary zinc and 11% of primary lead production in the U.S. Recent research carried out by the principal investigators and their collaborators has revealed the chemical composition of the groundwater that precipitated the MVT ores in the Southeast Missouri and Tri-State districts and other formerly commercial deposits in northern Arkansas and central Missouri. This research showed that large MVT ore deposits in the central U.S. formed from groundwater with high concentrations of lead, high Ca/Na and K/Na ratios, low Ca/Mg ratios, and that was chemically reducing. The purpose of the present research project is to determine whether groundwater that formed trace and minor MVT mineralization had the same composition as the groundwater that formed large ore bodies. This information would be useful in determining the commercial potential of new MVT deposit discoveries and in determining the geologic processes responsible for concentrating metals into large ore bodies. The project will be carried out by analyzing fluid inclusions, tiny ( 0.1 mm in diameter) samples of ambient groundwater trapped in the precipitated MVT deposit minerals. The locations of numerous trace occurrences of MVT mineralization have been compiled from the published literature and will be visited and sampled by the principal investigators. Slices of the rock samples will be cut and polished to 0.05 to 0.1 mm thicknesses to make the fluid inclusions visible by transmitted light microscopy. Fluid inclusions will first be studied using microthermometry to determine their bulk salinity from their melting point depression and their trapping temperature from the point of phase homogenization. The presence of methane, carbon dioxide, sulfate, and hydrogen sulfide will be sought in the fluid inclusions using Raman spectroscopy and will constrain the redox potential of the mineralizing groundwater and depth of mineral deposit formation. The concentrations of elements like Li, Na, Mg, K, Ca, Mn, Fe, Cu, Zn, Sr, Ba, and Pb will be determined from laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and will be used to see if groundwater that formed the trace MVT occurrences has the same chemical signature as groundwater that formed the large MVT ore deposits. More specifically, the data acquired from this research will allow two fundamental hypotheses for the origin of MVT deposits to be tested: (1) That anomalously metal-rich, high Ca/Na and K/Na, and low Ca/Mg fluids are needed to form large MVT ore deposits, (2) that redox conditions at the trace and minor occurrences of MVT mineralization were locally too oxidizing to allow much of the sulfur, whether resident or delivered by invading fluid, to exist as sulfide rather than sulfate, which would inhibit sulfide mineral precipitation and deposit growth.

美国中部的密西西比河流域拥有众多的铅和锌矿床，从世界级的东南部密苏里州和三州（俄克拉荷马-密苏里州-堪萨斯）矿区到广泛分布的非商业矿化痕迹。 这些矿床统称为“密西西比河谷型”（MVT），在80 - 150° C的温度下，高含盐地下水（约为海水的10倍）在石灰石和白云石寄主岩石中沉淀出硫化物矿物。 最大的矿藏在东南部的密苏里州地区，三州地区从1848年到1970年进行了商业开采，在世纪早期的高峰期，三州地区占美国原生铅产量的约70%和原生锌产量的约6.5%。主要研究人员及其合作者最近进行的研究揭示了沉淀密苏里州东南部和三州地区MVT矿石的地下水以及阿肯色州北方和密苏里州中部其他以前的商业矿床的化学成分。 这项研究表明，美国中部的大型MVT矿床是由铅浓度高、Ca/Na和K/Na比值高、Ca/Mg比值低且化学还原的地下水形成的。 本研究项目的目的是确定形成微量和微量MVT矿化的地下水是否与形成大型矿体的地下水具有相同的成分。 这些信息将有助于确定新发现的MVT存款的商业潜力，并确定将金属集中成大型矿体的地质过程。该项目将通过分析流体包裹体、沉淀MVT存款矿物中捕获的环境地下水的微小（直径0.1毫米）样品来进行。 MVT矿化的许多痕迹出现的位置已从已发表的文献中汇编，并将由主要调查人员进行访问和采样。 岩石样品切片将被切割和抛光至0.05至0.1毫米厚，以使流体包裹体通过透射光显微镜可见。 流体包裹体将首先使用显微测温法进行研究，以确定其整体盐度从他们的熔点下降和他们的捕获温度从相均匀化的点。 将使用拉曼光谱在流体包裹体中寻找甲烷、二氧化碳、硫酸盐和硫化氢的存在，并将限制矿化地下水的氧化还原电位和矿物存款形成的深度。 Li、Na、Mg、K、Ca、Mn、Fe、Cu、Zn、Sr、Ba和Pb等元素的浓度将通过激光烧蚀-电感耦合等离子体质谱法（LA-ICP-MS）测定，并将用于查看形成微量MVT发生的地下水是否与形成大型MVT矿床的地下水具有相同的化学特征。 更具体地说，从这项研究中获得的数据将允许对MVT矿床起源的两个基本假设进行测试：（1）形成大型MVT矿床需要富含重金属、高Ca/Na和K/Na以及低Ca/Mg的流体，（2）微量和次要MVT矿化处的氧化还原条件局部氧化性过强，不允许大量硫，无论是驻留还是由侵入流体输送，都以硫化物而不是硫酸盐的形式存在，这将抑制硫化物矿物沉淀和存款生长。