Abstract The Paleozoic strata of the U.S. mid-continent host several major Mississippi Valley-type (MVT) ore deposits and many trace occurrences of MVT mineralization. The present study sought to test the hypothesis that systematic differences in fluid composition led to the differences in scale of mineralization. To conduct this test, fluid inclusions in trace MVT occurrences were characterized and compared to previously characterized fluid inclusions in mid-continent MVT ore deposits. Primary sphalerite-hosted fluid inclusions in trace MVT occurrences from across the U.S. mid-continent were analyzed using microthermometry, Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry. Interquartile ranges (25th to 75th percentile) of trace MVT fluid inclusion properties were found to be as follows: homogenization temperature = 78 to 107 °C, salinity = 13.8 to 23.9 equivalent weight percent NaCl, atomic ratios of K/Na = 0.017 to 0.038, Ca/Na = 0.18 to 0.31, Mg/Na = 0.0081 to 0.013, Sr/Na = 0.0093 to 0.015, Ba/Na = 0.00013 to 0.00041. Compared to MVT ore deposits, fluid inclusions in trace MVT occurrences have lower homogenization temperatures and K/Na and Ba/Na ratios, similar salinities and Ca/Na and Sr/Na ratios, and slightly higher Mg/Na ratios (excluding Mg-rich fluid inclusions from the Upper Mississippi Valley district). In addition, fluid inclusions in trace MVT occurrences have low to undetectable methane contents whereas MVT ore deposits have high methane concentrations on the order of 100's to 1000's of ppm. Sphalerite matrix trace metal concentrations of trace MVT occurrences generally overlap those of MVT ore deposits. Differences in elemental concentrations and temperature between trace MVT and MVT ore-forming fluids are insufficient to account for the differences in deposit size. This suggests that differences in the local availability of sulfide, which could not be measured in this study but may correspond with the large observed differences in methane content, may have governed MVT deposit size.

中文翻译：

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美国大陆中部密西西比河谷型矿化痕迹的地球化学特征：对矿床增长的影响

摘要 美国大陆中部的古生界地层拥有几个主要的密西西比河谷型 (MVT) 矿床和许多 MVT 矿化痕迹。本研究试图检验流体成分的系统差异导致矿化规模差异的假设。为了进行这项测试，对微量 MVT 矿床中的流体包裹体进行了表征，并与之前在大陆中部 MVT 矿床中表征的流体包裹体进行了比较。使用显微温度测量法、拉曼光谱法和激光烧蚀电感耦合等离子体质谱法分析了来自美国中部大陆的微量 MVT 矿点中的原生闪锌矿承载流体包裹体。发现微量 MVT 流体包裹体特性的四分位距（25% 至 75%）如下：均质温度 = 78 至 107 °C，盐度 = 13.8 至 23.9 当量重量百分比 NaCl，K/Na 原子比 = 0.017 至 0.038，Ca/Na = 0.18 至 0.31，Mg/Na = 0.0081 至 S/Na = 0.013 0.0093 至 0.015，Ba/Na = 0.00013 至 0.00041。与 MVT 矿床相比，微量 MVT 矿床中的流体包裹体具有较低的均质温度和 K/Na 和 Ba/Na 比，类似的盐度和 Ca/Na 和 Sr/Na 比，以及略高的来自上密西西比河谷地区的流体包裹体）。此外，微量 MVT 矿床中的流体包裹体具有低到无法检测到的甲烷含量，而 MVT 矿床的甲烷浓度高，约为 100 到 1000 ppm。微量 MVT 矿床的闪锌矿基质微量金属浓度通常与 MVT 矿床的浓度重叠。微量 MVT 和 MVT 成矿流体之间元素浓度和温度的差异不足以解释矿床尺寸的差异。这表明本地硫化物可用性的差异，在本研究中无法测量，但可能与观察到的甲烷含量的巨大差异相对应，可能决定了 MVT 矿床的大小。