Campbell, Keith B., Christensen, Odin D., and Rasmussen, Hans J., Eaglecrest Explorations, Ltd., 1055 West Hastings Street, Suite 300, Vancouver, B.C., Canada V6E 2E9
Two of the newest gold discoveries in Bolivia are in Eaglecrest Explorations Ltd’s San Simon project, a 300 square kilometer concession group in eastern Bolivia. The concessions are centered on the San Simon plateau that rises approximately 300 meters above jungle of the western Amazon basin within 30 kilometers of the Brazilian border in the Department of Beni. Gold was first discovered in the area in the late 1600’s by Jesuit missionaries from Brazil and Paraguay. Small scale artisanal mining in the area has continued intermittently to the present. Modern exploration has located two gold deposits, the Doña Amelia zone and the Paititi zone. These gold deposits are hosted by structurally complex vein systems in Proterozoic meta-sedimentary rocks. Both deposits have been worked at the surface by local artisanal miners with the majority of each deposit still intact at depth. These deposits are geologically similar to deposits in western Brazil, Surinam and in Ghana that have been developed and are now being mined.
While access is made easy by plateaus rising above the jungle with grassy tops, exploration is challenging in the extreme tropical climate, with locally deep tropical weathering and laterite development. Steep topography along the edges of the plateau, while allowing access to a cross-section the stratigraphic section makes for difficult field conditions. Structural controls of the vein systems are complex and variable. Host rocks are quartz-rich siliciclastic sediments and these are cut by numerous generations of quartz vein systems, some related to gold mineralization. Hydrothermal alteration of the host rocks is sericite, with or without silicification.
Success at San Simon is being realized by an iterative program of sound geologic field observations and three-dimensional deposit modeling. Field mapping, critical core logging, and prospect-scale sub-laterite soil sampling have provided the framework upon which a three-dimensional deposit model has been constructed. The model has advanced understanding of the different structural domains within the fracture/vein systems. In turn, the three-dimensional model is proving to be a powerful guide for continuing exploration, for illustrating the behavior of structures within various lithologic units, and the overall distribution and orientation of gold bearing structures. As evidence, the three-dimensional model led to the most successful drilling program in the 14 year history of the project.