The Telkibánya precious metal mineralization was formed in the lowermost, oldest andesite and rhyolite mass cca. 12.5 M years before present. The fractures of approximately north-south striking, sometimes branching fault zones controlled the ascent of the water, which was heated with enhanced pressure in the vicinity of the melted rocks in the depth. This water became acidic by solving sulphur and other materials, also solving the precious and base metal content of the country rocks. During the ascent the temperature and the pressure decreased, so the solution became overfilled, and the solved materials were precipitated on the fracture walls - the fault zones were transformed into km-long veins. This overfill was reached at different heat-pressure conditions (that is, at different levels) for different metals, so various mineral parageneses were formed. Visible native gold enrichment can be expected in the upper (some 100 m deep from the paleosurface), epithermal (180-200 °C) zone, mainly in vuggy quartz vein rock.
The ascent, however, was not evenly distributed along the fault zones, but (as in recently active thermal springs, geysers and sulphur springs) chimneys and chimney groups were formed, which could be particularly rich. The outcrops of these chimneys (containing visible gold grains) were the ore bodies found by the ancient miners at first.
The vein quartzite is harder and more resistive against weathering and erosion than the volcanic country rocks. The veins therefore emerge as ridges; several north-south oriented chines (like Kánya Hill, Fehér Hill, Sinta Hill and, of course, Jó Hill) were formed along a resistive vein or set of veins. Nowadays, however, you can find series of pits instead of quartzite rock outcrops on these chines, witnessing the activity of the miners. The medieval miners opened pits on the top of the ore chimneys as deep as possible, called 'horpa' in their nomenclature. Subsurface mining started after exhausting these only. Recently you cannot find ore bearing outcrops anymore, but on the waste dumps of the pits here and there can occur some pieces of vein quartzite with ore minerals.
Source of info: infoboard on the site