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

University Of Missouri-Columbia, Columbia MO

Investigators

Abstract

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.