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

美国中部的密西西比河流域拥有大量的铅锌矿藏，从世界级的密苏里州东南部和三州(俄克拉荷马州-密苏里州-堪萨斯州)矿区到广泛分布的非商业性矿化痕迹。这些矿床统称为密西西比河谷型(MVT)，以硫化物矿物的形式赋存于石灰岩和白云岩中，在80至150摄氏度的温度下，由高盐地下水(大约是海水的10倍)沉淀在石灰岩和白云岩中。最大的矿床位于密苏里州东南部地区。目前约占美国原生铅产量的70%和原生锌产量的6.5%。三州地区于1848年至1970年进行商业开采，在20世纪初的高峰期，占美国原生锌产量的60%左右，占原生铅产量的11%。主要调查人员及其合作者最近进行的研究揭示了地下水的化学成分，这些地下水沉淀了密苏里州东南部和三州地区的MVT矿石以及阿肯色州北部和密苏里州中部的其他以前的商业矿床。这项研究表明，美国中部的大型MVT矿床是由地下水形成的，地下水中铅浓度高，Ca/Na和K/Na比值高，Ca/Mg比值低，而且正在化学减少。本研究项目的目的是确定形成微量和次要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矿床的商业潜力，并确定将金属浓缩到大型矿体中的地质过程。该项目将通过分析流体包裹体，即沉积的MVT矿床矿物中捕获的环境地下水的微小样本(直径0.1毫米)来实施。已根据出版的文献汇编了许多MVT矿化痕迹产状的位置，并将由主要调查人员走访和取样。岩石样品的切片将被切割和抛光到0.05到0.1毫米的厚度，以便在透射光学显微镜下看到流体包裹体。流体包裹体将首先用显微测温法从熔点下降确定其整体盐度，并从相均一点确定其捕获温度。利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，将限制成矿地下水的氧化还原潜力和矿床形成的深度。锂、钠、镁、钾、钙、锰、铁、铜、锌、锶、钡和铅等元素的浓度将通过激光烧蚀-电感耦合等离子体质谱(LA-ICP-MS)测定，并将用于观察形成微量MVT矿点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使对MVT矿床成因的两个基本假设得到检验：(1)形成大型MVT矿床需要异常丰富的金属，高Ca/Na和K/Na，以及低Ca/Mg流体；(2)MVT矿化微量和少量赋存的氧化还原条件局部过于氧化，使许多硫，无论是驻留在或由侵入流体输送的硫，以硫化物而不是硫酸盐的形式存在，这将抑制硫化物矿物的沉淀和矿床的生长。

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.