The Cerro Quema project lies within an elongate belt of Upper Cretaceous to Paleocene island-arc volcanic and plutonic rocks characterized by andesitic and dacitic flow-domes, associated pyroclastic rocks, and diorite to granodiorite intrusions. Volcanic rocks lie atop Upper Cretaceous marine sedimentary rocks and submarine pillow basalts. Clusters of high-sulfidation epithermal prospects and deposits containing gold and copper mineralization are hosted by advanced argillic-altered volcanic rocks. District-scale alteration extends along a WNW trend for >10 kilometres within the limits of Orla’s 14,833 hectare concession. At the eastern part of the trend, the La Pava and Quemita gold deposits are at development stage for a proposed open-pit mine processed by heap leaching an oxidized mineral reserve of 0.5 million ounces gold. The remaining western part (>6 km) is at exploration stage for additional Au oxide and Cu-Au sulphide resources.
Zones of intense hydrothermal alteration that host the gold and copper mineralization consist of quartz-rich rock enveloped by quartz – dickite ± kaolinite ± alunite ± pyrophyllite ± diaspore (advanced argillic), illite – smectite ± pyrite (intermediate argillic), and a distal zone of chlorite – illite – smectite – calcite (propylitic). The quartz-rich rocks are predominately vuggy silica that formed by intense hypogene acidic leaching of the original volcanic rocks. Vuggy silica developed below the water table as rising magmatic vapors condensed and generated acidic fluids that leached feldspar and mafic minerals out of the rock leaving only a dense residual quartz matrix surrounding empty phenocryst sites. Other quartz-rich rocks include massive silica that developed where fluids cooled and mixed near the water table precipitating quartz (silicification) in breccia and vugs of the vuggy silica.
Oxidized and mineralized vuggy silica crops out on topographic highs at the La Pava and Quemita gold deposits. Oxide extends from surface to depths up to 150 metres containing gold with iron oxides and sulfates. The highest grades of gold mineralization occur within crackled and brecciated feeder structures containing abundant supergene jarosite-goethite-hematite after hypogene pyrite-enargite-chalcopyrite. Surface gold enrichment was noted in the feeder structures on the southern margin of the La Pava deposit.
Vuggy silica also crops out on topographic highs at the Idaida and Pelona prospects, 1.5 and 4.5 kilometres ESE of Quemita, respectively. Anomalous gold in vuggy silica with iron oxide is present at surface in both areas. Gold and copper mineralization in sulphide at depth below Idaida is associated with pyrite-covellite-enargite-chalcopyrite. Little is known about the sulphide mineralization at Pelona where limited drilling in oxide returned low gold grades.
Orla’s Caballito Cu-Au zone is a new mineral discovery in a topographic low situated 1.5 kilometres SE from the Quemita deposit adjacent Idaida and 4.8 kilometres ESE from the La Pava deposit. Caballito is a high-grade, copper-dominant, high-sulfidation style epithermal system hosted in Upper Cretaceous submarine dacitic volcanic rocks and may have been localized along permeable beds of dacitic hyaloclastites. Mineralization consists predominately of shallow Cu-Au sulphide from depths of ~40-200 meters below unaltered dacite and a thin veneer of lateritic soils. It is a blind discovery in steep jungle terrain guided by IP-EM geophysics and scout drilling (discovery holes include CQDH17-089, 104 and 116). Average copper grades range from 0.5 to >1.8% with gold from 0.2 to 0.4 g/t.
The Cu-Au zone developed as a stratiform hydrothermal breccia within a zone of quartz-rich rocks defined by a central core of vuggy silica that is progressively enveloped by quartz – dickite alunite to quartz – kaolinite ± dickite (advanced argillic), illite – smectite – pyrite (intermediate argillic), chlorite – illite – smectite – calcite (propylitic) to unaltered dacitic volcanic rocks. The breccia features sulfides with high-sulfidation states (pyrite-enargite-covellite) together with sulfides of intermediate sulfidation states (bornite-chalcocite) and contains fragments of vuggy silica with euhedral pyrite – enargite ± chalcopyrite (high temperature, low pH) cemented with quartz, colloform-textured granular and spheroidal pyrite (low temperature, increased pH), bornite and chalcocite. Preliminary ore microscopy at Caballito indicates that bornite replaced chalcopyrite, chalcocite replaced bornite, and covellite replaced rims of chalcocite. Adjacent the stratiform breccia, at the Idaida prospect, the hydrothermal breccia plunges sub-vertical and may represent a feeder zone for Caballito. Hypogene covellite replaced enargite in the Idaida breccia enhancing copper grades and reducing arsenic concentrations. Numerous dikes of quartz diorite, diorite and basaltic-andesite, variable altered to illite – smectite, intruded both the Idaida and Caballito breccia.
Orla is exploring a 6 kilometre WNW-ESE trend for high-grade Cu-Au sulphide from the Quemita Au-deposit through Caballito to Pelona, as well as, potential zones for Cu-Au sulfide in a quartz diorite intrusion to the south at La Prieta. No reserve or mineral resources have been calculated as the zone is currently in exploration mode.
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