XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 S1_026

Chemical and Mineralogical characterization of the River (Norte Chico, ).

Valdés, A.1,Polvé, M.1, Oliva, P.1, Morata, D.2

(1) Laboratoire des Mécanismes et Transferts en Géologie - UMR 5563 UR 154 CNRS Université Paul-Sabatier IRD Observatoire Midi-Pyrénées - 14, avenue Edouard Belin - 31400 Toulouse. (2) Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla, Santiago. [email protected]

Geographical Context

The area of study is located south of the third region of Atacama between the 28 and 29th southern latitude, corresponding to the Valley. The origin of this river is located at the confluence of the rivers “El Tránsito” and “El Carmen”, which flows into the northern part of the city with the same name. The river Huasco extends for 88 kilometres and presents a basin of 9850 km2, with a general orientation east west.

Geographically the total surface of the basin is equivalent to a 13% of the regional surface area (III region of Atacama). Through the Huasco River valley different urban centers can be found, for example, Puerto Huasco, , y . According to the 2002 census, there exist 15 sites, from which two are cities and the rest rural areas. The cities found on the basin are Vallenar with 48040 inhabitants and Huasco with 7945.

Objectives

Considering geologic, geographic and meteorological characteristics of the study area, the goal of the present study is to characterize chemically and mineralogically the river Huasco through the analysis of water and sediment samples, collected between the cities of Alto del Carmen and Huasco. This allow us to know the origin of the elements analysed, the determination of the transport mode of a metal by a surface drainage, and to know some of the factors that control the dynamics of the river Huasco.

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XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Methodology

Two field work sessions were organised within the framework of this project. The first field work was done on April 2007 and the second was performed on January 2008, each corresponding to 10 days of work.

The data obtained in the first field work period allows us to have a general knowledge of the study area. From these results the general distribution of the elements allowed a more detailed sampling exercise during the second field work campaign.

To accomplish the detailed sampling, two sections of the Huasco valley were defined. The first between Alto del Carmen and Vallenar, and the second between Vallenar and Huasco. A total of 11 locations were sampled, with one river water sample and one sediment sample taken at each location). Both sample groups were analysed by ICP-MS (Inductively Coupled Plasma – Mass Spectrometry) at the LMTG, Toulouse, France.

Results

For each station, water and sediment are compared, and sediment mineralogy taken into account. Evolution of water and sediment chemical compositions from upstream to downstream traces the local influence of each formation, as the two sections sampled cross cut all the N-S geological formations. Water analysis from the two field sessions show inter seasonal variations as they have been sampled during two different flow rates.

As a general trend, major element concentrations in waters are roughly constant from Alto del Carmen (east) to Vallenar then increase from Vallenar to the ocean (fig. 1 and 2). When normalised to SO4 and plotted versus either Si/SO4, Ca/SO4, Cu/SO4, trace elements concentrations show clear patterns which allow identifying sources for the elements: calcareous formations for Ca, different hydrothermal formations for trace metals.

River sediments are compared with the mean continental crust and show their strong lithogenic origin. But some elements (As, Ag, Sb, Pb, U, Cd and Bi) display important enrichment factors.

These two observations, both in waters and sediments, can be related to human activities, both in the well spread mines and in relation with towns and industry.

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XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Acknowledgements

This work was possible by the support of the ALBAN EU grant to A.V. and the IRD (Chile and France). The first field work session was realized with the logistic support of professors Rodrigo Riquelme and Arturo Jensen of the Universidad Católica del Norte, Chile. In addition, the authors thank Maria Eliana Lorca from the Universidad de Chile for her assistance during field work.

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XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009

The diagrams 1, and 2 show the variations of the major elements. In this case we can appreciate a uniform distribution in the east region with values around 50 ppm for sodium, 100 ppm for calcium, 20 ppm for Cl and 300 ppm sulphate. In these graphs, in the proximity of Vallenar, an increase is observed in the concentration of most elements. These plots confirm a difference in concentration for the years of 2007 and 2008. A higher concentration can be therefore affirmed for 2008.

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