Gold-bearing veins in dykes are characterized for a decomposition or oxidation of the rock mass, which may present auriferous pyrite or not. Near the surface and usually considerable depth, the rock is decomposed and the gold bearing minerals are oxidized, liberating the gold, which forms wires, flakes or even small nuggets. As long as this decomposed or oxidation state continues, the ore is free milling, ut with depth the dyke is found in its primitive hardness.
In some gold deposits, there is a vertical deposit in joining or fracture plane in a dyke of quartz, rusty and much decomposed near the surface where it yielded free gold. The vein is from 0.20 m to 1.00 in width, stringers carrying ore extend into the rock mass, which is highly charge with gold bearing minerals that supplied the vein with mineral through the agency or surface waters. The dyke may be traversed by numerous small veins of white quartz which near the decomposed and rusty surface are rich in free gold. At slight depth the quartz veins become charged with non-oxidized material sufficiently rich in gold to merit concentration. The ore surface may be treated by a simple movement of material. The gold seems to be mostly confined to quartz veins.
There are some deposits more characterized by pyrite than others and these are generally gold producing districts. Some mines have strong pyritiferous veins in eruptive dykes. Also, it is possible to find large argentiferous veins with pyrite. It is important to mention that some gold deposits forming bedded deposits in limestone find the formation in solution through the vertical joints common to all water formed rocks, resulting from contraction in consolidating from a soft, muddy condition. Such veins and fissures are short and act like channels to a more important line of weakness occupied by the main body of the gold deposit, the dividing line between one stratum and another. Another line of weakness for the attack of mineral solutions is at the juncture of a dyke with some other rock. The interval between them is often occupied by a contact vein. When a fault descends to a very great depth, it may tap the molten rock reservoir supposed to lie beneath great mountain ranges and the molten lava rushes upward through the weak line of the fissure, fills it and after cooling becomes a dyke instead of a gold vein.