Influence of Mineralogy and Geological Setting on Trace Metal Concentration within Subtropical Weathered Profiles, Bells Creek Catchment, Queensland, Australia

Abstract

The effects of mineralogy and geological setting on trace metal concentration and distribution within six weathered profiles developed sandstone and mudstone was assessed. Primary minerals occurring in the weathered profiles are quartz, plagioclase, and K-feldspar. Kaolinite is the most dominant secondary mineral followed by mixed layers of smectite-illite, illite, hematite, siderite, and occasional calcite. Metal concentrations within fresh and weathered samples were investigated by two methods of digestions: HF-based digestion and aqua regia. Results revealed that V and Cr are largely present in the primary aluminosilicate matrix and are not easily available to the environment; however, Cu, Zn, and Pb are present in extractable forms and readily leached. Iron occurs in both primary minerals and insoluble secondary minerals such as hematite. The mineralogical study also showed that drill hole material with more clay minerals tends to contain higher metal concentrations, demonstrating that mineral composition is the major control over trace metal content. Spearman's rank correlation matrix also confirmed the role of mineralogy on trace metal concentration (e.g., V and Cr correlated with kaolinite and Pb correlated well with mixed layers of illite-smectite). Effect of geological setting on trace metal concentration was assessed by examining the geomorphological location of drill holes with respect to paleochannels, surface topography, and water table position. Results revealed that depth of burial of the weathered profile does not have an important effect on weathering and trace metal composition of samples. However, samples located on flat terrain and with shallow water table are more prone to leach metals. Factors controlling degree of chemical weathering and subsequent trace metal distribution are summarized in order of importance: mineralogy > geological setting (topography and parent rock type) > water table depth > depth of profile burial.