W K FLETCHER / SCIENCE PHOTO LIBRARY W K FLETCHER / SCIENCE PHOTO LIBRARY
The confluence of a near neutral (pH 7.5) stream, on the left, with a strongly acidic stream (pH 3) that drains an oxidizing high sulfidation epithermal gold deposit. The brown colour of the acidic stream is caused by precipitates of iron (ferric) hydroxides. The ridge in the distance is the crest of the Andes and the boundary between Argentina and Chile.
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