Paleo-fluid composition determined from individual fluid inclusions by Raman and LIBS: Application to mid-proterozoic evaporitic Na-Ca brines (Alligator Rivers Uranium Field, northern territories Australia)

摘要

Detailed determination of the composition of individual fluid inclusions in the H2O-NaCl-Ca(Mg)-Cl-2 system from microthermometry data may be difficult because (i) the melting of salt hydrates is often unclear, (ii) salts or salt hydrates are often metastable, and iii) eutectic temperatures are often difficult to relate precisely to a unique salt-water system. These problems can be circumvented by using a Raman microprobe to identify the salt hydrates and to measure the chlorinity, by using Laser Induced Breakdown Spectroscopy (LIBS) to determine cation ratios, and by cross-checking the data from these independent techniques at each step in the determination of the fluid inclusion composition. Data from individual inclusion analysis are then completed by crush-leach technique data on bulk samples. This methodology was applied to a case study, the mineralized faults and associated breccia located in the Alligator Rivers Uranium Field (middle Proterozoic Kombolgie Subgroup, Northern Territory, Australia). Faults are cemented by euhedral quartz containing a variety of fluid inclusions with a wide range of ice-melting temperatures, from near 0 degrees C down to about -50 degrees C. The approach has permitted the identification of three types of fluids: an Na-fich brine with variable Mg concentrations, a Ca-rich brine with low Mg concentrations, and a lower salinity fluid. The Ca and Na-brines mixed with each other and a less saline end-member. On the basis of Cl/Br and cation ratios, the Na-brine is interpreted as a primary brine, resulting from the evaporation of seawater. It is inferred that the Na-brine evolved chemically during its interaction with the Ca-rich lithologies from Proterozoic basement, forming subsequently a Ca-brine through Na-Ca exchange reactions. Thus, the fault system, which affects both the base of the Kombolgie sandstone and its basement, represents active drainage zones where different fluid reservoirs were connected, and thus a place where fluid mixing was highly favored. (c) 2006 Elsevier B.V. All rights reserved.