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

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. Chemical weathering of rocks is one of the major processes that modify the earth’s surface and is one of the key processes contributing to the geochemical cycling of elements (Berner and Berner 1996). The effect of geological setting (e.g., parent rock type and topography), climate, and position of water table on chemical weathering of rocks is well documented (e.g., Jenny 1941, Loughnan 1969, Summerfield 1991, Macias and Chesworth 1992, Thomas 1994, Hill et al. 2000, Taylor and Eggleton 2001). For example, the subtropical climate of southeastern Queensland is a paramount factor in chemical weathering throughout the study area, as it readily produced highly weathered profiles such as those beneath the Tertiary land surfaces, which have undergone deeper weathering and induration than the interfluves (Grimes et al. 1986). In addition, by controlling weathering Pacific Science (2005), vol. 59, no. 3:421–438 : 2005 by University of Hawai‘i Press All rights reserved 1 Manuscript accepted 8 August 2004. 2 School of Earth and Geographical Sciences, The University of Western Australia, Crawley, Western Australia, Australia. 3 Corresponding author (phone: þ61-7-3864 4185; fax: þ61-7-3864 1535; e-mail: [email protected] .au). 4 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia. processes, topography can also influence the trace metal composition of both weathered bedrock and the underlying layers (Boggs 1995). The mobilization and redistribution of trace metals during weathering are substantially affected by various processes such as dissolution of primary minerals and formation of secondary phases. Other factors that are important are pH and Eh conditions (e.g., Harris and Adams 1966, Chesworth et al. 1981, Cramer and Nesbitt 1983), soil texture (Richards et al. 2000), biological activities (Welch and McPhail 2003), and amount of organic matter (Kaschl et al. 2002, Udom et al. 2004). During weathering of crystalline rocks, primary mineral phases are partly dissolved, and hydrolysis and hydration take place. Secondary minerals such as illite and smectite are commonly the earliest to be formed, followed by halloysite and kaolinite. In the final stage, as leaching intensifies, partial desilicification occurs and kaolinite is converted to gibbsite. Feldspars, for example, often weather directly to an amorphous phase (Fields and Swindale 1954), then to kaolinite (McCaleb 1959, Exley 1976), and ultimately to gibbsite or boehmite (Helgeson et al. 1969, Parham 1969, Lodding 1972). Reported studies investigating the roles of mineralogy, topography, and parent material on the trace metal concentration in sedimentary weathered profiles are limited due to either the lesser abundance of these rocks (only 5% of earth crust [Carroll 1970]) or their limited economic value. The aim of this study was therefore to investigate the extent to which mineral composition, topography, and parent material control trace metal distribution within sedimentary weathered profiles occurring in a subtropical setting. This investigation also increased our understanding of weathering mechanisms and trace metal distribution/mobility. In addition, although some of the profiles are present near the land surface, others are buried under thick layers of young unconsolidated sediments; the effect of this aspect of the setting on weathering and trace metal contents of the samples was therefore investigated. Stratigraphic and hydrogeological drilling in Bells Creek catchment, southeastern Queensland (Figure 1) provided the opportunity to investigate several types of buried and exposed weathered profiles. Digestion methods such as HF-based total digestion and aqua regia (partial digestion) are used to determine the amount of heavy metals potentially available in total rock in comparison with the same metals present in extractable forms. Such comparisons can be of importance to the environmental assessment of areas under development pressure, because studies of trace metals in soil typically represent bulk concentrations rather than comparisons between total and extractable metal concentrations.