1993 Volume 43 Issue 238 Pages 93-106
The Noya area is located in a geothermal field where early Pleistocene volcanic rocks are distributed. As a result of a drilling programme, promising quartz veins of 30-100 cm width were discovered at a vertical depth of 200 m, grading 28.6-63.9 g/t Au and 12.9-16.7g/t Ag. The vein directions were inferred from supersonic log data and observed fractures to strike E-W and WNW-ESE and dip 75°N. The vein gangue minerals consist of quartz, adularia and calcite with minor chlorite and sericite. The fluid inclusion temperature of the calcite is 177°C and the NaCl equivalence percentage 0.2 wt%. The ore minerals consist of electrum (Au 55-59 atm%, Ag 41-45 atm%), pyrite, marcasite and trace sphalerite and chalcopyrite.
The alteration in the mineralized zone consists of a surface and near surface cristobalite-smectite-halloysite zone (Zone 1), a quartz-smectite zone (Zone II) below it, and around the veins, an interstratified clay zone (Zone III) and a chlorite-sericite zone (Zone IV) are present. With regard to the geochemical anomalies, the Hg soil and Hg gas anoma-lies correspond exceptionally well with the surface clay alteration zones, with the gas anomaly closely resembling the direction of the blind veins.
The resistivity distribution shows that the veins are located just below the low resistivity zone (<5 Ω) of the up-per horizon in the low to medium resistivity zone of the lower horizon. According to some geothermal drilling, it is suggested that these veins were located in a very high temperature hydrothermal discharge zone, where the thermal gradient suddenly changed at a depth of about 200 m.
From these observations, it is inferred that at around 200m depth, the surficial groundwater and hydrothermal fluids met, and with the resultant decrease in temperature and oxidation of the hydrothermal fluid, Au was deposited in the veins.
