Abstract

In the region of Peralonso, Ortega, Tolima, a detailed geochemical prospecting program was carried out, in which 518 fine fraction active sediment samples and 159 rock samples were taken in an area of 500 km2, to know the potential of a Cu, Pb, and Mo anomaly found during the Regional Geological and Geochemical Mapping program of the L-8 Quadrangle.

The predominant lithological unit in the area is part of the lbagué Batholith, which intrudes the metamorphic rocks of the Cajamarca Group. In addition, sedimentary rocks of the Luisa Formation and volcanic sedimentary rocks of the Saldaña Formation appear as small outcrops in the region. The Samaria-Colorada Fault contacts the Ibagué Batholith with the Honda Group and is the most important structural feature.

Fe, Mg, Ti, Cu, Mo, Pb, Ba, and Sr were chosen as indicator elements for the geochemical interpretation, both in the active sediments and in the rocks, and the normal and threshold values were calculated for each element by graphical method. From these graphs, a map with the geochemical distribution of the element was elaborated, to understand patterns, controls, and geochemical behaviors in the area.

In the Peralonso region, the elements subject to interpretation have a progressive increase in values in an east to west direction, recognized both in the active sediments and the rocks. Thus, high contents of Cu, Pb, and Mo occur in the Ibagué Batolith, west of the area, and low contents in the Cajamarca Group and in the area’s eastern edge.

By superimposing the geological map of the Peralonso region on the geochemical map of each of the elements studied, several areas were found with associations of major and minor elements in high contents, reflecting some lithological characteristics.

In the eastern fringe of the lbagué Batholith outcropping in the area, preferentially high Ba and Ti values were detected in the geochemistry of fine active sediments and rocks and relatively acid facies of the pluton (granodiorite, quartz monzonite, potassium feldspar, and quartz dykes) in the petrography.

In the metamorphic rocks of the Cajamarca Group, two main associations of elements (Mo and Pb, as constant elements; Ba and Mg, variables) were delimited in the fine fraction active sediments and an association composed by Cu and Pb in the rocks.

In the western sector of the area, several geochemical associations mainly composed of Fe, Mg, Cu, Pb, and Mo were distinguished. High contents of ferromagnesian minerals (biotite and hornblende) and disseminated magnetite were the main petrographic feature found for this sector of the batholith. This factor was taken as a basis to consider the copper and molybdenum anomaly of the Peralonso region as a lithological anomaly and the batholith as a non-mineralized intrusive.