Las mineralizaciones de uranio en las rocas volcánicas de Macusani. Puno (Perú)

Abstract

The Macusani U deposits, located 150 km. NNW of the Titicaca lake, in Perú, are characterized by certain petrological, mineralogical and tectonic features which make them a unique type of U mineralization among those normally associated with pyroclastic rocks. These features can be outlined as follows:

1. The U minerals occur almost exclusively in the upper levels of the volcanic pile.

2. The host rocks, Plio-Quatemaryi,gnimbrites of rhyolitic to rhyodacitic composition, consist of quartz, sanidine, oligoclase and biotite, and occasionally muscovite and andalusite. These minerals occur within a partially devitrified g1assy matrix that contains numerous xenoliths of pelitic material.

3. Biotite, smoky quartz and andalusite are specially abundant in the U-bearing tuffaceous layers.

4. The ore minerals consist of massive pitchblende and very scarce Fe sulphides, Due to the intense weathering, gummites and secondary U minerals, mainly phosphates and silicates, predominate in the outcrops and mining works.

5. The pitchblende fills fractures which range from a few centimeters to several meters in length, and from less than 1 mm. to 10 cm. in width. Some of these fractures are subvertical and due to contraction, and coincide with the columnar jointing of the ignimbrites. Others are subhorizontal and parallel to a conjugated, ductile shear joint system which was developed by the compaction and adjustment of the competent layers which host the U mineralizations.

According to these features, a preliminary model to explain the origin of this particular type of U deposits is proposed in this paper.