Gold characterisation as a guide to bedrock sources for the Estero Hondo alluvial gold mine, western Ecuador

Abstract

Odin Mining International Limited was able to justify a major new diamond-drilling programme in the catchment area of their Estero Hondo alluvial gold mine in western Ecuador by using gold characterisation studies to support the opinion that the mine gold had been derived from two sources, not just one as originally thought. The gold characterisation studies involved reflected light microscopy, microprobe analysis and identification of mineral micro-inclusions on samples of gold grains panned from streams draining into the mine area and from productive gravel layers within the mine itself. Each sample was interpreted to contain a mixture of two principal gold types. VT-type gold was silver-rich (> 11% Ag), hosted a Pb-Ag-Te-S inclusion assemblage and appeared to be derived from known, skarn-hosted gold mineralisation in the Veletanga area. PB-type gold was silver-poor (< 11% Ag), hosted a Bi-Cu-Te-S-Pb-Sb inclusion assemblage and appeared to come from undiscovered gold mineralisation associated with copper bearing, hydrothermally altered, quartz-diorite intrusions in the Perros Bravos area. Combining the gold-type interpretation with grade and thickness estimates for the various gravel layers in the alluvial mine revealed that over half the gold was of PB-type. This result was interpreted as strong evidence for the possible presence of an undiscovered bedrock gold deposit at Perros Bravos and, therefore, provided adequate justification for a new diamond-drilling campaign. This programme eventually totalled 14 500 m in 61 holes; although it failed to find any bedrock gold deposit at Perros Bravos, it did result in the definition of an indicated, bedrock resource of 8 Mt at 0.45 g/t gold at Veletanga.