Gold bearing minerals and free gold may be left after the decomposition of the pyrite, and the removal in solution of soluble substances. Below this, at the water level of the lode, the brown iron ore is entirely lost in the soluble iron salts and decomposing pyrite. And immediately beneath, again, the solid and unaltered pyrite is found; in highly mineralized lodes this zone of decomposition is very clearly marked by a line of black friable decomposing sulphides with iron in the form of soluble sulphates immediately overlying the still unaltered mineral matter.
The gold in this mineral matter in the decomposing zone and in the gossan is found in irregular and, more or less, crystalline particles; the sizes of the particles found under similar circumstances vary greatly in different lodes. Sometimes the gossan is very rich in gold, sometimes it appears as if some of the gold had been dissolved out, sometimes it is blank, which naturally leads to the supposition that whatever gold it contained had, at some previous time, been carried off in solution by water; nearer the surface there is strongly confirmative evidence for this supposition. As the lode is traced upwards, the gossan is found if gold-bearing to bear gold and be of the nature described, until the cap is met. The more or less hard compact iron oxide forming the cap of the lode is found to be impregnated with gold, in entirely different forms to that below.
The joints of the stone are frequently found covered with a gold film, so that any one not versed in the subject might often mistake it for a purple iridescence left by an iron-impregnated water; again, the shape and size of the deposited films are such as to preclude all possibility of their having found their way, through the natural filter-bed of the lode, from the gold-bearing pyrites below, unless carried in solution by water and there precipitated. The impregnation of the hard compact masses of the cap by microscopically fine gold is accounted for by the water carrying a solution of both iron and gold, and on evaporation under atmospheric action, the gold-impregnated solidified iron oxide is found.
A peculiarity of such formation is that the caps of these lodes are much wider, more confused and indefinite than the lodes beneath; this is accounted for by the corrosive action of the mineral waters destroying the original confining walls of the lodes and eating back into the country rock. In the distribution of gold through this cap there is another peculiarity observable. Although the lode underneath may be 60 or 90 cm wide, the cap may be 6 m, 12 m, 30 m wide or more; and yet not, proportionately dilute in gold, as compared with the narrow body of lode from which it derived its origin. It is generally as rich as the lode beneath, in many cases far richer in patches, yet there are in these great mass many blank patches also. The formation of gold-bearing patches of greater or less richness is accounted for by the belief that from great lengths of the lode, the drainage waters, bearing gold in solution, find their way to the surface in comparatively constricted outlets and precipitate the metal from its solutions, throughout the masses of rock in these outlets which had already been corroded and altered by solvent agencies. Although in nature the solvent solutions for gold may be indefinitely dilute, the geological ages are indefinitely long, during which the result of accumulating action brings about tangible results, and though chlorine is of this class, the most widely-spread element, bromine, and iodine, in lesser quantities are continually acting in a similar manner.