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hydrothermal systems & ore deposits

Hydrothermal systems are responsible for a vast majority of economic metal deposits (e.g., skarn, porphyry, epithermal, mesothermal, VMS) and are also of great importance to the geochemical budgets of the oceans through time. Much work remains, however, in addressing outstanding questions such as: (i) what are the sources of precious (e.g., Ag, Au) and base (e.g., Cu, Zn) metals in these systems? (ii) what are the optimal processes and reaction rates/mechanisms for the formation of ore deposits? (iii) How do we exploit the most economically beneficial regions of these systems? And (iv) how have hydrothermal processes affected marine chemistry though geologic time?

To begin answering these questions, I constructed a geochemical model that shows that paleoseawater chemistry serves as a feedback on the composition of ocean-floor hydrothermal vent fluids, suggesting that less basaltic Ca and Sr are released into the oceans when seawater Mg and SO4 are low and that continental weathering fluxes (based on seawater 87Sr/86Sr) must be lower than previously assumed during the Cretaceous and Ordovician (Antonelli et al., 2017, PNAS). Stable isotopes are also critical for answering these questions, as reactive kinetic effects take place in both continental and ocean-floor hydrothermal systems and can be linked to reaction rates, mechanisms, and mineral supersaturation states. I am currently working on a project (in collaboration with C. Chelle-Michou, ETH-Zurich) investigating these effects during Pb-Zn skarn formation. We find significant variability in calc-silicate precipitation rates between skarn-forming fluid pulses that appear to correlate with crystal sizes (Antonelli et al., AGU Conf. 2021). I am also working on several projects (in collaboration with A. Giuliani and M. Schmidt) exploring stable and radiogenic Ca isotope fractionations in kimberlites from ab-initio (in collaboration with E. Schauble, UCLA), experimental (piston-cylinder centrifuge apparatus), and natural perspectives. These studies are essential for understanding the petrogenesis of these economically important mantle-derived melts.

A majority of economic deposits, however, result from rapid chemical reactions in hydrothermal systems. As discussed in the upcoming (invited) review by Watkins & Antonelli (2021, Elements), a great deal of information can be gained from the reactive kinetic isotope effects that accompany rapid unidirectional processes in these settings.

Dorian hydrothermal vent, East Pacific Rise (T = 344°C). Photo by Kang Ding, Nick Pester.

Grand Prismatic Spring in Yellowstone National Park. Photo Credit: E. Nordquist.

Relevant Publications:

Watkins, J.M. & Antonelli, M.A. “Beyond equilibrium: Kinetic isotope fractionation in high-T environments” (2021) Elements 17(6).

Antonelli, M.A., Hantsche, A.L., Schauble, E.A., Hoffman, J., Kouzmanov, K., Dini, A., Chelle-Michou, C. “Disequilibrium δ44Ca in calc-silicate minerals and kinetic isotope effects during skarn formation” AGU Annual Meeting, New Orleans, December 2021.

Antonelli, M.A., Pester, N.J., Brown, S.T., DePaolo, D.J. “Effect of Paleo-Seawater Composition on Hydrothermal Reactions in Mid-Ocean Ridges” (2017) Proceedings of the National Academy of Sciences, USA 114(47).

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