Summary:
Deposit Geology
The Cerro Blanco deposit is a classic hot springs-related, low-sulphidation quartz-adularia-calcite vein system. It is localised along a complex fault intersection created during late Miocene-Pliocene tectonic extension within the active Central American volcanic arc. Local igneous activities that drove the Cerro Blanco hydrothermal system include a vesicular andesite dike swarm and mineralization stage rhyolite / dacite flow dome eruption and cryptodome intrusion.
The Cerro Blanco vein systems are best developed (widest and most continuous) within the 300 to 500 m elevation ranges. Principal host rocks include a lithic tuff–calcareous shallow marine- volcaniclastic sequence and, to a lesser extent, the overlying volcaniclastic-hydrothermal breccia sequence of probable Pliocene age. Vein zones often appear to transition to barren calcite beneath the ±300 m elevation in the northern half of the deposit. To the south, high-grade quartz-adularia-calcite vein zones continue at least another 100 m down to 200 m elevation. Some veins remain open at depth.
Mineralization
The Cerro Blanco gold deposit occurs within a large hydrothermal alteration zone covering an area about 5 km long and 1 km wide. This zone exhibits the effects of strong, pervasive hot spring type hydrothermal alteration.
Gold mineralization is hosted within a broadly north-south striking sequence of westerly-dipping siltstones, sandstones, and limestones (Mita Group) that are capped by silicified conglomerates and sediments with contemporaneous dacite/rhyolite flow domes or cryptodomes (Salinas Unit). The Salinas rocks are synmineral and believed to have accumulated progressively in a low-relief graben characterised by a shallow groundwater table. The Salinas conglomerate was presumably derived by erosion of the flanking horst blocks as relief was created during active faulting. The topographic inversion required to explain the current prominent position of the graben fill is ascribed to the silicic character of the Salinas unit and its consequent resistance to erosion.
The west and east sides of the Cerro Blanco ridge consist of flat agricultural plains characterised by Quaternary basalts, interbedded with boulder beds and sands. These rocks also appear down-faulted to lower elevations, implying major post-mineral extensional movements on such faults; and they may be neotectonic (active).
The current gold resource occurs under a small hill and is confined within an area about 400 m x 800 m. The gold deposit is characterised by both high-angle and low-angle banded chalcedony veins, locally with calcite replacement textures. High-angle mineralised faults and discontinuous stockwork zones host some of the highest gold grades. Gold-bearing structures in the Cerro Blanco project area extend 2 km to the northwest of the gold deposit and occur largely confined within the hydrothermal alteration zone. Exposures are poor and locally covered by alluvium and post-mineral rocks. Gold-bearing structures extend at least 1 km south and southwest of the deposit under valley fill and post-mineral rocks. Geothermal well MG7, located about 0.5 km east of the deposit, encountered a 27 m zone averaging 6.3 g Au/t and 22 g Ag/t at a depth of 634 m. The upper 6 m of this zone averages 23.9 g Au/t and 79 g Ag/t
Gold and silver occur almost exclusively in quartz-dominated veins of low-sulphidation epithermal origin and in low-grade disseminated mineralization within the Salinas conglomerates and rhyolites.
Deposit types
The low sulphide content and near absence of base metals in the Cerro Blanco veins confirm it as a classic hot springs-related low-sulphidation epithermal deposit. In common with most low-sulphidation deposits, it appears to be linked to compositionally bimodal, basalt-rhyolite volcanism, the hallmark of intra- and backarc rift settings worldwide. The hydrothermal system seems likely to have been initiated during rhyolite dyke and cryptodome emplacement, at the base of the Salinas unit, with the rhyolitic magma and magmatic input to the mineralising fluid both being derived from the same deep parental magma chamber.
Adularia-sericite epithermal gold-silver deposits characteristically occur as banded fissure veins and local vein / breccias which comprise predominantly colloform banded quartz, adularia, quartz pseudomorphing carbonate, and dark sulphidic material termed ginguro bands. Examples of adularia-sericite epithermal gold-silver deposits include Waihi and Golden Cross, Pajingo, Vera Nancy, Cracow, Hishikari, Sado, Konamai, Tolukuma, Toka Tindung, Lampung, Chatree, Cerro Vanguardia, Esquel, El Peñon.
At near surficial levels, many are capped by eruption breccias and sinter deposits. Eruption (phreatic) breccias, which form by the rapid expansion of depressurized geothermal fluids, are characterised by intensely silicified matrix and generally angular fragments including sinter, host rock and local surficial plant material. Although sinter deposits formed distal to fluid upflows commonly associated with eruption breccias, sinters tend to be barren with respect to gold but may be anomalous in other elements such as boron, arsenic and antimony.
Cerro Blanco shows all the characteristics of a completely preserved, non-eroded epithermal deposit. The occurrence of hot springs (sinters, silicified reeds, pisoliths) directly above the presumed feeder veins at Cerro Blanco implies a high water table and swampy conditions (cf. McLaughlin, California). In areas of high topographic relief, outflow springs (sinter) are usually found several kilometres from the upflow zones. The widespread occurrence of lacustrine and fluvial clastic sediments in the Salinas Group and accretionary lapilli, typical of water-rich pyroclastic surges, supports this interpretation. Sedimentation probably kept up with subsidence. Mudstone dykes and geopetal structures—open fractures filled by horizontally bedded chalcedonic and sulphide-rich sediment—reinforce the interpretation. The hydrothermal breccia in the south part of the South Ramp may be a diatreme.