The occurrence of gold long mineralized structures is influenced by several factors, which must be analyzed by the gold explorer. The segregation of gold bearing minerals in any mineralized structure must be referred to some condition, or combination of conditions, local to the mineralized structure compared with the remainder of the deposit of low-grade or barren minerals. Among the localizing factors that have been recognized are, in filled fissures, the amount and distribution of open spaces available for ore deposition, and in replacement deposits, the degree of brecciation giving access to the replacing solutions; the intersection of veins with other veins, dikes, sheeted zones, or porous strata; the impounding of solutions by impervious strata; and the differing precipitation influences of enclosing rocks. A large proportion of important and well defined ore-shoots, especially in deposits of deep-seated origin, are not assignable to any of these factors.
While knowledge of the character of mineralized structure in general is important, a knowledge of the behavior of the known structures in the property or in the district under investigation is even more significant. Accurate and complete gold assay and geological maps of the areas already mined are essential to an intelligent investigation of mineralized structures, and it is strange that such maps are so rarely to be found at even important gold mines. The distribution of gold in structures is the greatest factor in the risk of mining, and the factor of most vital importance to the mine owner and mining engineer. Not infrequently, a practical man from long experience in a particular deposit can predict its behavior in advance of exploration with a valuable percentage of accuracy; this reliability of prediction would in many cases be greatly increased by a technical study of complete assay and geological maps, upon which relations become definite.
It is important to mention that mineralized structures vary with commercial as well as with natural conditions, and material left in place as waste may in later years constitute a valuable gold ore. It is natural to extract gold ore and to be satisfied as long as the ore lasts, but all mineralized structures come to an end, and then, when a search is commenced for further reserves, the data of most value in conducting that search will have been lost unless carefully recorded on a map, and the exploration must be conducted by a process of elimination which may or may not yield results. In the study of any gold deposit the first question to decide is whether the mineralized structures are due to primary segregation of values or to secondary enrichment by surface agencies. It is much easier to recognize a secondary gold ore with certainty than to determine a gold ore to be certainly primary, for the reason that the criterion of a primary ore is the known lack of secondary additions. With a majority of ores microscopic study in thin section will reveal primary character, but there are many instances where the assumed primary nature of an ore is open to question, especially with ores of gold or silver.
Even below the zone of recognized secondary enrichment, deep exploration usually shows a falling off in value in the primary gold ore in depth, and the actual influence of surface agencies may extend to greater depths than is generally supposed. It often happens that commercially valuable gold ore ceases at the depth reached by secondary agencies, and the data is likely to be scanty in regard to the primary distribution of values were not obscured by surface agencies.