Tectonic evolution and origin of the Golfo Dulce gold placers in southern Costa Rica

Abstract

A new model of the geologic evolution of the Golfo Dulce gold placers is presented. It is shown that the placers were formed on or about an Early Miocene to Late Pleistocene island, prior to its tectonic break-up between Late Pleistonce and Recent times. The island had its origin in a post-Cretaceous swelling or anticlinal doming of the deep oceanic crust, from the crest of which the older pelagic sedimentary cover (Golfito Formation) was removed by submarine erosion, and was preserved only on its flanks. Younger pelagic sediments (Salsipuedes-Pavones Formation) were depostied directly over the older crust in the central part of the structure; and over the Golfito Formation on the flanks. Simultaneously, gold-bearing basaltic intrusive and extrusive activity took place in the tensionally weakened central part of the structure, giving rise to a younger Basement Complex or crust. By Early Miocene, this basement rose above sea level, and formed the island. Later it began subsiding, allowing the Charco Azul Formation to gradually encroach upon it -if not actually cover it-by Late Pliocene time. The first concentrations of gold were formed in the basal strata of this formation. Thereafter, a period of stanstill or possible slight uplift followed, allowing the erosion of the Charco Azul strata in certain parts of the island, and the depositionof the Armuelles Formation in a brackish/near-shore to neritic enviroment. Second cycle gold concentrations were formed in its basal strata. Post-Armuelles tectonic movements caused third-cyclegold concentrations. The structurally high younger basement was thrust over the strata of its own detritic sedimentary cover by the Osa Overthrust; and the older basement was thrust over the same sediments in the area of Golfo Dulce; and over younger basement rocks in the area of the isthmus. This later faulting was brought about by the Golfo Dulce Oriental Fault and associated structures to the northwest. Therefore, no pull-apart Pliocene basins exist in the Golfo Dulce are, nor do certain right-lateral-strike-slip faults related to he Panama Fracture Zone.