Unraveling Shear-Related Mineralization Processes and Tectonic Overprint from Investigations in the Dipilto Block of NW Nicaragua

Ben M., Colorado School of Mines
Bradley Squires, Colorado School of Mines

Abstract

Orogenic gold deposits represent structurally controlled systems that form due to contractional orogenic events and represent one of the primary global sources of gold and tellurium. Along the cordilleran margin of North America, Phanerozoic deposit systems are variably preserved. These relatively young (<150 Ma) systems provide an opportunity to investigate the genesis and exhumation of orogenic gold deposits within a more clearly defined tectonic framework, when compared to ancient Archean systems. While such Phanerozoic deposits are well-defined in Mexico and California, correlative systems in Central America are poorly delineated and the timing of mineralization relative to tectonism is unclear. This study is focused the Dipilto block of Nicaragua and Honduras where orogenic gold deposits are expressed as shear vein systems within carbonaceous metasedimentary host rocks. Micro- to macro-scale structural investigations constrain top to the SE shear along moderately NW-dipping domains that parallel regional foliation and are coincident with the limbs of isoclinal folds in the host schist. Mineralization is expressed as 1-2 m wide domains of banded quartz-carbonate vein sets that contain high-grade gold (>100 g/t) and host massive sulfides (primarily pyrite, arsenopyrite, sphalerite, and galena). Pit-scale mapping indicates that the shear vein sets are locally affected by late thrust stacking and local-scale folding. Furthermore, the vein sets are affected by a complex history of post-emplacement faulting. Statistical analysis of the vein and fault orientations along with petrographic analysis indicates progressive shear vein emplacement occurred at greenschist facies conditions, with native gold largely being emplaced after initial vein development. Cathodoluminescence and fluid inclusion data indicate that earlier quartz and sulfides were emplaced at lithostatic conditions, while late-stage quartz and gold was emplaced at hydrostatic conditions. LA-ICP-MS U-Pb zircon dating of the dike sets that cross-cut mineralization indicate emplacement at ~96 Ma. This suggests that mineralization occurred before the end of the Cretaceous and is likely to have occurred after the emplacement of the regionally extensive ~120-115 Ma Dipilto Batholith. The dike sets are affected by normal faults associated with a complex zone of faulting as part of a NNE-trending fault zone, the regionally extensive Guayape fault zone. These new data define a protracted history of tectonism that spans from the Cretaceous to active systems due to the development of the Caribbean Plate, with mineralization related to the earlier collisional events. This study shows how fundamental structural studies provide critical context for understanding the distribution and definition of critical Earth materials.

Session

Convergent and transpressional orogens