In recent years, as a result of biological, environmental, and economic considerations, available copper in copper oxide ores that could not be recovered by pyrometallurgical methods was accumulated in so-called oxide dumps. Suitable material is treated with dilute sulfuric acid in a heap-leaching process, whereupon the copper content of the rock slowly dissolves in the acidic solution. The performed investigations show that one needs to consider the action of the acid on the copper oxide-containing rocks at the microscopic level. In this paper, we describe research carried out on oxide samples from the western dump of the Sarcheshmeh copper mine. Each sample was split into two parts and a portion of each was exposed to heap-leaching conditions in a column. Subsequently, polished sections, thin sections, and powdered samples were subjected to chemical analysis as well as petrographic and mineralogical considerations. Changes in the weight percentages of non-metal and metal minerals before and after acid treatment were measured. Microscopic studies have indicated that chemical analyses do not provide a complete picture of the effects of acid on the rock. Thus, microscopic studies on sections are shown to be a necessary requirement, neglection of which can have negative economic and environmental effects. (c) 2007 Elsevier B.V. All rights reserved.