When the gold prospector found some signs or indications of gold on the surface, he must investigate the surface outcrop for signs of oxidations along the line of contact. The presence of iron oxide minerals is an indication of the limit. Also, it is possible to observe in the vertical joints leading down from the surface into the body of gangue minerals such as clays or iron stains. In these bedded deposits, prospecting on a large scale may be advantageously be done by drilling with diamond drills from the surface down through as many of the strata as are suspected of being ore bearing. The cores are analyzed and logged and it will possible to determine if a gold ore body was penetrated together with its approximate thickness at a certain point, and if this process is continued over a certain zone, the approximate areal limit of the ore body may be estimated.
It has been determined that this work may follow upon a close examination first of gold bearing minerals along the outcrop. It is sometimes done after an area has been exploited for some time with an idea of discovering new mineralized zones or continuations if the valuable ore. Whilst deep fault cracks may be filled by lava, those not descending to such great depths doubtless lay open until they were gradually filled by solutions carrying in earthy vein stone and metallic compounds. In other words, they were the channels of mineral or hot springs. It must not be assumed that these fault cracks were ever open chasms commensurate in width with the wide dykes and veins now found in them, but rather in some cases vary close fitting cracks mere lines of weakness, the walls appressed closely together by prodigious lateral pressure.
In the case of true fissure veins, the fissure or shattered zone was enlarged by the corroding, substituting power of acid mineral solutions until the final formation. In the shattered zone, this substituting process would go on easily and rapidly until nearly all the shattered fragments were replaced by gold bearing minerals, except a few pieces that if small, would cause what is called a brecciated vein. These fragments are not so much pieces that have fallen from into an open fissure gradually filling up with solutions of quartz and vein compounds in which they became entangled, but rather undigested and non-substituted fragments of the wall rock. The outline of fragments may be observed partially but not entirely replaced by quartz or veins compounds. Sometimes the breccias are surrounded by rings of quartz containing free gold particles.