Development of Proxies for Vent Fluid Trace Metal Concentrations and pH through Study of Sulfide Chimney Linings

Analyses of trace elements (Ag, As, Co, Mn, and Zn) hosted in the chalcopyrite linings of `black smoker' chimneys using secondary ion mass spectrometry (SIMS) have been combined with data for trace metal concentrations in corresponding vent fluids to investigate fluid-mineral partitioning of trace elements. Goals of this research include development of proxies for fluid chemistry based on mineral trace element content. The use of SIMS allows for the measurement of trace elements below the detection limits of electron microprobe and at the necessary spatial resolution (20 microns) to examine fine-grained and mixed-mineral samples. Results indicate that the chalcopyrite linings of many `black smoker' chimneys are homogeneous with respect to Ag, Mn, Co, and Zn. Minerals picked from samples exhibiting homogeneity with respect to specific elements were dissolved and analyzed by solution inductively coupled plasma mass spectrometry (ICP-MS) for use as working standards. Results also document a strong correlation between the Ag content of chalcopyrite and the Ag:Cu ratio of the corresponding hydrothermal fluid. This supports systematic partitioning of Ag into chalcopyrite as a substitute for Cu, providing a proxy for fluid Ag concentration. Additionally, the Ag content of chalcopyrite correlates with fluid pH, particularly at pH>3, and thus represents an effective proxy for fluid pH. Application of these proxies to chimney samples provides an opportunity to better identify hydrothermal conditions even when fluids have not been sampled, or not fully analyzed.