In the initial investigation of any gold deposit the first consideration is whether the mineralization is primary, secondary, or residual, as upon this rests all conclusions in regard to its persistency in depth. A primary mineralization is found in ore that has undergone no change since deposition. A secondary ore is an ore formed by secondary or surface, agencies. A residual mineralization is found in an ore that has remained after the solution and removal of associated minerals by secondary processes. In distinguishing between primary and secondary mineralization, the first criterion is the presence or absence of signs of oxidation; if an ore carries traces of oxidation, secondary action must be suspected, although it may be shown that it has had no effect in the distribution of gold.
It has been noted that a thin section under the microscope indicates the manner of intergrowth of primary minerals, and in this way primary and secondary ores may usually be distinguished from each other; evidence of structural intergrowth is rarely visible to the unaided eye. The presence of two generations of sulphides, the richer being in general the later, and coating the other as if precipitated upon it, is usually clear evidence of secondary enrichment. The presence of one sulphide, especially if the richer, through another sulphide, is likewise evidence of secondary enrichment. In deposits of massive pyrite that carry chalcopyrite and gold of undoubted primary origin, there is a tendency for the latter mineral to occur as veinlets through the pyrite. This is probably due to the greater solubility and the later crystallization of chalcopyrite over pyrite under conditions of regional metamorphism, to which most of the deposits of this kind have been subjected.
Also, the presence of fluid inclusions in an ore is evidence of primary origin unless secondary processes are seen to have been at work; a banded structure is another evidence of primary origin. While there are many doubtful cases where an gold deposit may not be assigned definitely to either primary or secondary processes, in a majority of ores the typical associations of primary minerals, their manner of intergrowth, the presence of fluid inclusions indicate a primary ore, while the absence of oxidation, of secondary minerals, or of secondary rearrangement of the primary minerals, indicate that surface agencies have played no part in the distribution of values; such an ore may be expected to continue in depth to the zone of primary impoverishment, which in most cases is deeper than the limits of profitable mining. No absolute rule may be formulated for field use, but an association of the following sulphides commonly indicates a primary origin for any ore that carries no trace of oxidation or typical secondary structure: galena, blende or chalcopyrite with pyrite, pyrrhotite or arsenopyrite. The most common primary occurrence of gold is either as native gold or as a telluride. Probably the most common primary occurrence of silver is as argentite. Auriferous or argentiferous tetrahedrite is a common primary mineral.