Heavy minerals weathering

ABSTRACT A geochemical analysis of major, trace and rare earth elements was carried out in beach sands collected from the Northeastern coast of Mexico in order to observe the spatial trends along three different beaches. Results show that major elements patterns along the beaches are controlled by heavy minerals and plutonic and sedimentary input towards the coast. In addition, trace elements tendencies indicate that the beach sands are influenced by the presence of magnetite. Finally, the differences in Eu anomalies indicate a mix of felsic to mafic and intermediate rocks and feldspar weathering. 

These deposits are formed in the natural process of weathering, transportation, and concentration at the site of heavy minerals originating from some primary source rock. Commercially, beach placers are the most important along with alluvial streams aeolian deposits are insignificant. 

Conservative elements (C,) include zirconium (Harden, 1987), titanium (Johnsson et al., 1993), rare earth elements, and niobium (Brimhall and Dietrich, 1987). Considerable disagreement occurs in the literature as to the relative mobility of these elements under differing weathering conditions (Hodson, 2002). Also, these elements are concentrated in the heavy mineral fractions and are often not suitable for describing weathering ratios in depositional environments where selective concentration and winnowing occurs. For such conditions, relatively inert minerals such as quartz can be considered (Sverdrup, 1990 White, 1995). 

The mineral zircon (ZrSi04) occurs in nature as component of acidic igneous rocks. Their weathering yields sediments containing small crystals (sand) of zircon and further heavy minerals. These deposits are resources for industrial production. 

Mineralogical investigations of soils may be undertaken with a number of different objectives in view. Analysis of primary minerals can show whether a soil is sedentary or transported, and if the latter, it can indicate the rock formations from which the parent material was derived. The nature and the extent of pedological weathering, and hence the age, of soils can be deduced from the secondary minerals produced as well as from the primary minerals that have survived. It is mainly in the study of the primary soil minerals that heavy mineral analysis is of importance. 

These may contain any of the heavy minerals, the species and proportions present depending on the nature of the rocks from which the sediments were derived, and on the ability of the minerals to withstand the processes of weathering, erosion, transport, deposition, burial, diagenesis, uplift, folding, and other tectonic effects and subsequent history up to the present time. These processes can have a considerable influence on the relative proportions of the heavy minerals, and the most resistant species such as zircon, tourmaline, and rutile may be the only survivors. 

Marshall [1941], suggested that the content of some of the most resistant heavy minerals would form more reliable indices for similar calculations in the study of soil profiles, if the analysis were carried out on the parent materials and on each of the soil horizons. He postulated that there should be a loss of material from the A horizon due to the removal of weather-able minerals or their alteration products, and a gain in the B horizon due to the accumulation of colloidal and precipitated clay and sesquioxides. Two years later, Marshall and Haseman [1943] reported on a study of a Grundy silt-loam profile, in which they used zircon as an index mineral and concluded that, despite a net loss in the A horizon, there had been a gain in weight for the whole profile due to accumulated organic matter, oxidation, and hydration. Later, the... 

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