Abstract

The mineralogical and chemical composition of the <180 μm fraction of 90 stream sediments collected along the Rapel, Cachapoal and Tinguiririca rivers were studied in order to assess the influence of natural and anthro- pogenic factors. In order to distinguish major and secondary chemical patterns the data was analyzed using an unsupervised artificial neural network technique. The studied rivers present different chemical signatures, which are associated with geological formations and anthropogenic factors such as mining, agro-industry and urban- ization. The upper-middle reaches of the Cachapoal River are characterized by elevated Cu–Mo–As–Sb con- centrations that reflect input of mining-derived material. This signature, which is further amplified in the middle reaches of this river due to the influence of the city of Rancagua, masks other signatures such as organic matter- related high B concentrations. The sediments of the Tinguiririca River are strongly depleted in heavy metals, but present high P2O5 concentrations, which reflects the extensive use of phosphates in its floodplain. At the upper reaches of the Rapel River the transition from granitic to sedimentary-dominated bedrock coincides with a marked decrease in P2O5–La–Ce–Y abundances. Similarly, the lower reaches of this river coincide with a marked CaO–MgO increase, which reflects an increase in the availability of CaO-rich detritus associated to a lithological transition. Enrichment of strongly correlated Fe2O3–TiO2–MnO–P2O5–Pb–V–Ni–Cr–Y–Ce–La in the middle-lower reaches of this river is interpreted as reflecting heavy mineral accumulation in point-bar deposits. This is further supported by the mineralogy of the associated samples. Most of the suspended load of the Cachapoal and Tin- guiririca rivers settles in the Rapel Lake. Thus the quantity of metals and P2O5 contained in the sediments of this lake is considerable and, together with the very high Cu–Mo–As–Sb concentration of the sediments of the Cachapoal River constitute a potential hazard to the catchment environment.