Abstract

In the area of the municipalities of Salento, Quindío, and Cajamarca, Tolima, of interest for Au, Ag, Sb, and Hg, a regional geochemical prospecting program was carried out based on samples of active sediments complemented with samples of rocks and seams.

The region is dominated by metamorphic rocks of the Cajamarca Group, mainly intruded by small bodies of dacitic porphyry. This group was later affected by the Aranzazu and La Palestina faults. Finally, in recent volcanic events, lava flows, and pyroclastic materials were deposited.

The chemical composition of the different rock units is reflected by the variations of the different elements in the mean concentration graphs, correlation diagrams, and triangular diagrams. The active sediments, the main basis of the present study, are formed from materials contributed by the different rocks, according to their resistance to weathering and erosion. Unconsolidated materials such as tuffs contribute significantly to the formation of sediments, contributing, in this case the highest concentrations of Pb and Sr, which is why these two elements cannot be used individually as indicators of hydrothermal mineralization in the area.

Comparing the distribution of mineralizations and their associated anomalies with the location of igneous rocks and major faults, it is observed that there are possibly three different classes of hydrothermal mineral occurrences: a) Hg mineralizations related to the Aránzazu Fault, b) Sb occurrences associated with the Aránzazu and La Palestina faults, and c) Au and Ag seams genetically connected to dacitic porphyry intrusions. In the latter, two zones can be distinguished: 1) An internal high- temperature zone where dacitic porphyry apophyses frequently outcrop and where the AgX 100/Ag + Pb + Zn + Cu ratio is lower than the Cu/Zn ratio in seams, and 2) A marginal low- temperature zone where no igneous rocks outcrop and the AgX 100/Ag + Pb + Pb + Zn + Cu ratio is higher than the Cu/Zn ratio.

Relating the location of the mineralizations with the anomalies found, it was observed that the indicator elements of mineralizations in active sediments are a) Hg for cinnabar occurrences, b) Sb for stibnite seams, and c) superimposed anomalies of two or more elements such as Au, Sb, Ag, Pb, Sr, Zn, Ba, Cu and Mo for Au and Ag mineralizations.

The heavy fraction of the active sediments shows higher concentrations in most of the elements than the fine fraction, but comparing the anomalous drainages of these two fractions, it is noticed that they are very similar. Therefore, it is suggested to use only the fine fraction for future works of regional geochemical exploration in similar areas, due to the ease in the collection of the samples and the elaboration of the analyses.

The anomalous zones of interest are associated with both known mineralization and sites where mineral occurrences are unknown. The most important anomalous drainages are a) For Au and Ag occurrences, the Guayabal creek, the headwaters of the San Rafael and Campoalegre creeks, the headwaters of the Navarco and Tochecito rivers, and the upper part of the La Plancha creek; b) For Hg occurrences, the eastern tributary middle part of the Navarco river and the lower part of the Santa Lucía creek; and c) For Sb occurrences, the middle part, and eastern tributaries of the Aguas Claras creek.