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Fluid-rock interaction during progressive migration of carbonatitic fluids, derived from small-scale trace element and Sr, Pb isotope distribution in hydrothermal fluorite |
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| Authors: | B Bühn J Schneider AH Rankin |
| Institution: | 1 Instituto de Geociências, Universidade de Brasília, Brasília DF 70910-900, Brazil 2 Institut für Geowissenschaften und Lithosphärenforschung der Universität, Senckenbergstrasse 3, D-35390 Giessen, Germany 3 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany 4 School of Earth Sciences and Geography, Kingston University, Kingston Upon Thames KT1 2EE, UK |
| Abstract: | Associated with the Cretaceous Okorusu carbonatite complex (Namibia) is a hydrothermal fluorite mineralization hosted in Pan-African country rock marbles, which resulted from fluid-rock reaction between the marbles and orthomagmatic, carbonatitic fluids expelled from the carbonatite. Yellow fluorite I was deposited in veins up to 5 cm away from the wallrock contact, followed by purple and colorless fluorite II, smoky quartz and barite, a Mn-rich crust on early calcite, and pure calcite. This clear-cut sequence of mineral growth allows an investigation into fluid-rock interaction processes between the marble and the migrating carbonatitic fluid, and element fractionation patterns between the fluid and subsequent hydrothermal precipitates.Fluorite I shows a progressive change in color from dark yellow to colorless with purple laminations over time of deposition. Subsequent fluorite I precipitates show an increase in Ca, and a continuous decrease in F, Sr, REE, Y, Th, U and Pb contents. The ratios (Eu/Eu*)cn, Th/Pb and U/Pb increase whereas Y/Ho, Th/U and (La/Yb)cn decrease. The Sr-isotopic composition remains constant at 87Sr/86Sr = 0.70456-0.70459, but with varying, highly radiogenic Pb (206Pb/204Pb = 32-190, 238U/204Pb = 7-63). Fluorite II has 87Sr/86Sr = 0.70454-0.70459, 206Pb/204Pb = 18.349, and 207Pb/204Pb = 15.600, and a chemical composition similar to youngest fluorite I. The Mn-rich crust on early calcite accumulated REE, Ba, Pb, Zr, Cs, Th and U, developing into pure calcite with a prominent negative Ce anomaly and successively more radiogenic Sr. The calculated degrees of fluid-rock interaction, f = weight fraction of fluid/(fluid + marble), decrease from fluorite I and most fluorite II (f = 0.5) to calcite (f = 0.2-0.3) and hydrothermal quartz (f ? 0.1). A crush-leach experiment for fluid inclusions in the hydrothermal quartz yielded a Rb-Sr isochron age of 103 ± 12 Ma. Crush-leach analysis for the carbonatitic fluid trapped in the wallrock yielded a trend from the fluid leachate to the host quartz (206Pb/204Pb = 18.224 and 18.602, 207Pb/204Pb = 15.616 and 15.636, respectively) extending from carbonatite towards crustal rocks.Calculated trace element distribution coefficients fluorite/fluid are below unity throughout, and increase from La to Yb. Elements largely excluded from fluorite (Ba, Pb, LREE relative to HREE) were incorporated later into the Mn-rich crust on calcite. The trace element patterns of the hydrothermal minerals are related to changing aCO2 and aF− in the fluid during continued fluid-marble reaction. A predominance of carbonate over fluoride complexing in the fluid as reactions proceeded controlled the Y/Ho, Th/U and REE patterns in the fluid and the crystallizing phases. Deviations from these trends indicate discontinuous processes of fluid-rock reaction. |
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