Abstract

The California region of Santander, Colombia, has been a center of gold and silver mining since colonial times. These are hypogene deposits in which mineralizing solutions have filled fractures and metasomatically altered the adjacent rock.

In the area of the deposits referred to in this study, the rock is a porphyry consisting of quartz and feldspars and outcrops over a relatively large area. Therefore, the mineralization is assumed to be related to this hypabyssal l rock, at least in the restricted sector covered by the present study.

Depending on mineralization and other characteristics, the California deposits were formed under moderate to low temperature and pressure conditions, ranging from mesothermal to epithermal. Mineralization, considering the region, is characterized by the common sulfides s iron, zinc, and lead and compounds of the sulfosalts group, mainly argentiferous. The economic metals that have been exploited have been only gold and silver, but locally, and especially in the sector of La Baja, there are zinc-lead and copper that deserve interest as by-products, apart from uranium.

The present study refers exclusively to the deposits or mineralized zones that, according to the radiometric surveys carried out in 1953 by the National Geological Institute in combination with the Atomic Energy Commission of the U.S.A., revealed to contain radioactive mineralization worthy of investigation. Such are: the San Celestino lode, the San Antonio area and the San Cristobal and Las Animas lodes, all of them in the mining sector of La Baja. In addition, the results of a preliminary radiometric reconnaissance in the old mining sector of La Francia to the S. of La Baja are given.

The San Celestino vein is made up of a fracture filling that gives rise to a mineralized breccia and, in addition, by the adjacent rock affected by hydrothermal solutions. This vein was formerly mined for precious metals with unknown intensity. The mined zone is only accessible 15 m along strike and 20 m down dip. By outcrop, the lode is known for about 70 m.

The mineralization consists mainly of quartz, pyrite, sphalerite, tetrahedrite, and galena. The quartz contains almost all the uranium, of which two compounds were identified: uraninite and cofinite. The gold is associated with the sulfides, and the silver is in the tetrahedrite, at least for the most part. The average ore tenor in the accessible zone is as follows: Uranium (at U308), 0.22%; gold, 30.84 g/t; silver, 730.32 g/t; zinc, 4.42%; lead, 0.91%; copper) 0.10%. Computing gold and silver at their current price in the country and converting to Colombian pesos the value of uranium at the U.S. price, the metric ton of this mineral is worth in situ, for these three metals, $ 294.74. Based on the U.S. market prices, the value of the same metric ton is US$ 92.73.

Under current conditions, the tonnage of ore "in sight" is not economically significant. However, a general assessment of expectations is that, as this is a phyllo-type deposit in a mining area characterized more by an abundance of mineralized structures with high-grade ore shoots than by the size of the deposits, it should be considered a medium to small prospect, but with flattering prospects in terms of grade.

In the San Antonio area, near the San Celestino vein, there are several manifestations of radioactivity, some of which are related to small structures through which hydrothermal solutions ascended, and others are due to the redeposition of secondary compounds precipitated from meteoric waters. Except for one or two that are worth exploring, the others are of no interest in themselves but as evidence that there has been uraniferous hydrothermal mineralization in this area, the focus of which is supposed to be the Pié de Gallo vein that crosses this area, and which was formerly exploited for precious metals. This vein cannot be known today in the part that was exploited. Nevertheless, in places that are supposed to correspond to the extension of this structure, slight radioactivity can be appreciated. The expectations of this area are therefore related to this vein whose exploration by drilling is justified. In addition, there are other radioactive series related to other independent structures in this area. Therefore, some preliminary explorations using cuts and exploratory pits are also justified.

The San Cristobal and Las Animas veins have also been worked for precious metals and today can be seen in a reduced sector of the exploited area. Their uranium and precious metal contents are much lower than those of San Celestino. The uranium content in both seams varies between 0.04 and 0.10 % U308 according to the samples taken. It is necessary to explore these seams, especially the one of Las Animas, advancing the present fronts, to be able to estimate its possibilities based on a wider knowledge of its characteristics. This exploration is justified by the possibility that these seams can be economically exploited by the joint exploitation of uranium, copper, and precious metals.

There are so far no prospective deposits in the La Francia region, but as there are signs of uraniferous mineralization and geological conditions like those of La Baja, a systematic radiometric survey for uraniferous filonian deposits is warranted.

Finally, the California region is the only one in which promising mineralization as a source of industrial uranium has been found so far and is characterized by the multiplicity of mineralized structures. Therefore, it is of interest not only to explore of known deposits but also to continue the search for new ones. In addition, it is also important to study the problems of precious metal mining in this region, not least because the economic exploitation of uranium may depend to a large extent on the simultaneous exploitation of these metals.