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Reports: DNI256766-DNI2: Fluid+Thermal History Reconstruction of Basins and Fault Zones, from Combined Fluid-Inclusion and (U-Th)/He Analyses Applied to Hematite

Matthew Steele-MacInnis, University of Alberta

The record of paleofluid flow through rocks and fractures is commonly enigmatic. Fluid inclusions trapped in minerals can provide information on the properties (temperatures, compositions) of paleofluids, but information is commonly lacking to be able to link these fluid inclusion properties with specific episodes of fluid flux, and particularly the timing of these episodes. Similarly, information on timing of fluid pulses can be obtained from radiometric dating techniques applied to hydrothermal minerals, but information on the fluids that formed these minerals is commonly lacking.

In this study, we are focusing on unraveling the history of fluid fluxes and the properties of the fluids, using combined fluid-inclusion analysis and (U-Th)/He dating of a single mineral: hematite. Hematite is uniquely suited as a record of past fluid fluxes, because it is nearly ubiquitous in rocks and fractures, it can be dated using the (U-Th)/He approach, and it commonly hosts fluid inclusions. However, regarding the latter point, fluid inclusions in hematite have only rarely been studied in previous work because hematite is opaque to visible light. Here, we use infrared light to observe and analyze fluid inclusions in hematite.

Our ongoing work has so far targeted two specific settings: Hematite mineralization along a detachment fault in western Arizona (the Buckskin-Rawhide detachment fault), and hematite veins hosted within enigmatic clastic dykes from Colorado. In both cases, our results indicate that hematite was deposited from hot, highly saline groundwater that fluxed through these rocks. Moreover, our (U-Th)/He dating on hematite from the clastic dykes places new constraints on the timing of hematite crystallization, and by extension, on clastic dyke injection into Proterozoic basement rocks. This work represents the first application of combined fluid-inclusion and (U-Th)/He analyses of hematite from faults and veins, and shows that this approach is highly promising for obtaining new insights into paleo-fluid-flow systems.