1991 Volume 41 Issue 226 Pages 77-93
El Roble Mine is located in Carmen de Atrato, Departamento del Choco, Colombia. The ore deposits are classed as submarine volcanogenic massive sulfide type.
The purpose of this paper is to describe geological features so far acquired during exploration. The features are summarized as follows:
(1) This area consists of basic volcanic rocks, sedimentary rocks of Cretaceous age and intrusive rocks of Miocene age. The basic volcanic and sedimentary rocks are distributed under the structural control of NW-SE and N-S fault systems. The ore deposits are located along the contacts between the basic volcanic rocks and sedimentary rocks.
(2) The basic volcanic rocks belong to tholeiite series, but their properties, especially the contents of trace elements and clinopyroxene compositions from the area around the deposits are different from the other areas. It seems that the basic volcanic rocks around the deposits are less differentiated than that of the other areas in magmatic differentiation. Supposing that is the case and ubiquitous, these analytical method can be applied as the useful clues to target the possible areas.
(3) Silicification and carbonitization are observed in the sedimentary rocks along the contacts between the basic volcanic and sedimentary rocks. Such alteration is notable particularly in black shale around the deposits. Also the forms of the ore bodies are concordant with the black shale. These facts suggest that the deposits have a genetical relation to the interaction between the presence of black shale or its tectonic environment and the tholeiitic igneous activity.
(4) The dykes are divided into two types: one is fresh andesite or lamprophyre, and the other is intensely altered andesite. But the K-Ar dating of these dykes results in the similar ages between 13 to 16 Ma, i.e. Miocene in age. The whole rock analysis indicates that these dykes have calc-alkalic or shoshonitic compositions.
(5) Main ore minerals are composed of pyrite and chalcopyrite, and classified into Type A and Type B according to the occurrence of pyrite. Type A ore is characterized by colloform or framboidal texture, and common occurence of electum, tetrahedite as gold and silver minerals, whereas Type B is composed of commom euhedral to subhedral pyrite crystals and chalcopyrite-pyrrhotite matrices.
(6) The ore deposits are massive in form and considered to have syngenetically formed in the rocks of Cretaceous age. But the high contents of Cu, Au and Hg in the dykes of Miocene age indicate the remobilization and addition of metal elements.
