Experimental determination of the partitioning of Zn, Pb, and Cu between brine and dolomite at temperatures and pressures of sediment-hosted base metal ore deposit formation

世界上许多最重要的金属矿床，如铅（Pb）、锌（Zn）和铜（Cu），包括美国的矿床，都是从多孔沉积岩中的热地下水（即热液流体）中沉积下来的。从以前的研究中，这些流体的一些典型特征现在已经众所周知，例如它们的温度和最丰富的溶解元素的浓度。然而，在大多数矿床中，流体中许多不太丰富的元素，如Pb、Zn和Cu的浓度并不为人所知。了解这些金属浓度是很重要的，因为它有助于回答有关矿床起源的基本问题，例如，是否需要异常富金属的流体来形成矿床，形成矿床需要多长时间，以及什么化学反应导致金属溶解到流体中并最终从流体中沉淀出来。此外，对矿床如何形成的充分了解对于成功的矿产勘探和评估美国国内资源基础至关重要。因为大多数Pb、Zn和Cu矿床形成于遥远的地质过去，所以现在出现在矿床中的流体不再是形成矿床的相同流体。然而，这些古代流体沉淀的一些矿物质提供了古代流体的一些特性的记录，包括它们的金属浓度。例如，矿物白云岩是许多Pb、Zn和Cu矿床的共同组成部分，它倾向于将微量的这些金属结合到其晶体结构中。一般来说，流体中这些金属的浓度越高，白云石中这些金属的浓度就越高。如果这种比例因子，称为分配系数，可以量化每种金属，那么古代流体中每种金属的浓度就可以根据其在白云岩中测量的当前浓度来计算。该研究的目的是确定在典型成矿温度范围内水和白云岩中Pb、Zn和Cu的分配系数。拟开展的研究将分三年进行，第一年重点研究Zn，第二年重点研究Pb，第三年重点研究Cu。每种金属的实验将在125°，150°和200°C下进行，持续时间为10,20和40天，金属浓度为50,100和1000 ppm，压力为10 MPa， pH为5.5。实验流体将具有典型的成矿沉积热液的主要元素组成。x射线衍射（XRD）证实了实验中白云石的析出。用激光烧蚀-电感耦合等离子体质谱（LA-ICP-MS）测定白云石晶体的组成。在每次实验结束时，将使用电感耦合等离子体原子发射光谱（ICP-AES）测量实验流体的元素组成。该研究的预期结果是一套与温度相关的分配系数，可用于根据流体沉淀的白云岩中这些金属的浓度来确定热液流体中的Zn， Pb和Cu浓度。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Many of the world’s most important ore deposits of the metals, lead (Pb), zinc (Zn), and copper (Cu), including those in the U.S., were deposited from hot groundwater (i.e. hydrothermal fluids) in porous sedimentary rocks. Some typical characteristics of these fluids are now well known from previous research, such as their temperature and the concentrations of the most abundant dissolved elements. However, the concentrations of many less abundant elements in the fluids like Pb, Zn, and Cu are not well known for most deposits. Knowledge of these metal concentrations is important because it helps to answer fundamental questions about the origin of the deposits, such as whether anomalously metal-rich fluids were needed to form the deposits, how much time was needed to form the deposits, and what chemical reactions caused the metals to dissolve into and eventually precipitate from the fluids. Further, sound knowledge of how ore deposits form is essential for successful mineral exploration and assessing the U.S. domestic resource base. Because most Pb, Zn, and Cu deposits formed in the distant geologic past, the fluids that are present at the ore deposits now are no longer the same fluids that formed the deposits. However, some of the minerals that these ancient fluids precipitated provide a record of some of the properties of the ancient fluids, including their metal concentrations. For example, the mineral, dolomite, which is a common constituent of many Pb, Zn, and Cu deposits, tends to incorporate trace amounts of these metals into its crystal structure. In general, the higher the concentrations of these metals in the fluid, the higher the concentrations of these metals in the dolomite. If this proportionality factor, called a partition coefficient, can be quantified for each metal, then the concentration of each metal in the ancient fluid can be calculated based on its measured present concentration in the dolomite. The purpose of the proposed research is to determine the partition coefficients for Pb, Zn, and Cu for water and dolomite over a range of typical ore-forming temperatures. The proposed research would be carried out over three years, with the first year focused on Zn, the second year focused on Pb, and the third year focused on Cu. Experiments for each metal would be conducted at 125°, 150°, and 200° C for durations of 10, 20, and 40 days at metal concentrations of 50, 100, and 1000 ppm, a pressure of 10 MPa, and a pH of 5.5. The experimental fluid would have a major element composition typical of ore-forming sedimentary hydrothermal fluids. The precipitation of dolomite in the experiments would be confirmed by X-ray diffraction (XRD). The composition of the dolomite crystals would be determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The elemental composition of the experimental fluid at the end of each experiment would be measured using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The expected results of the study are a suite of temperature-dependent partition coefficients that could be used to determine the Zn, Pb, and Cu concentrations of hydrothermal fluids based on the concentrations of these metals in dolomite that the fluid precipitated.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.