The degree of shattering, brecciation or straining of a gold deposit is next to the replaceability of the rock, the most important factor in determining the position of replacement deposits; the greater the area of the surfaces exposed in relation to the mass-of rock, the more rapidly and thoroughly does replacement take place, and the existence of these deposits may occasionally be predicted through tracing the shattered or strained zones. The passage of solutions through rocks is seen under the microscope to take place even where the existence of cracks or minute fissures is not discernable, and the invading mineral may form grains that are apparently completely surrounded by fresh, solid rock; a zone, therefore, within which the rock has been subjected to slight straining of the particles only, without rupture, may give access to the replacing solutions.
Basically, shattering is a less evident factor in the formation of replacement deposits in limestone than in other rocks, probably because of its ready solubility. Replacement deposits are rarely of great vertical extent, and very often the rock above them affords no indication of their existence. Outcrops of these deposits, therefore, are accidental, and relatively scarce, and the existence of one such body having been established, blind exploration for new gold deposits in the same horizon is more often justified than is the case with deposits of other types.
The boundaries of replacement of gold deposits are usually poorly defined, the ore merging gradually into the enclosing rock, where the boundary between ore and waste becomes a question of gold assay only. Occasionally, one mineral has penetrated the rock more easily than the others, and so preponderates in the outer parts of the deposits; in exploration, therefore, upon meeting such a lean, or perhaps barren, mineralization, it may indicate the existence of important gold grades beyond. Replacement gold ores frequently reproduce the structure of the replaced rock, certain bedding planes being followed in preference to others, and where this condition obtains the bedding planes are usually parallel to the greatest dimension of the ore-body.
Considering a special of gangue such as limestone, the principal alterations of limestone as an accompaniment of mineralization are silicification, marmorization, and to a less extent, the development of a disseminated pyrite mineralization, any of which may show the existence of ore-bodies. For example, in the Coeur D'Alenes, Idaho, large replacement deposits have formed along shattered zones in quartzite with variables concentrations of gold and silver. Other good example is the old Bunker Hill mine where the ore consists of galena and siderite, with small quantities of quartz, blende and pyrite, the gangue being the enclosing quartzite. It appears that siderite first replaced the quartzite, and that later argentiferous galena partially replaced the siderite, though the direct replacement of quartzite by galena is occasionally noted. The best ore consists of rather fine grained masses of galena with subordinate siderite, which grades into ore in which the siderite exceeds the galena and this into barren quartzite. The ore is principally a replacement of the t quartzite, but the replacement is closely connected with fissuring, and some of the galena was deposited in open spaces. In some of the important stopes, quartz and pyrite are usually most conspicuous in the transition zone from ore to country rock. The zone of fissured quartzite in which the ore-bodies occur has a maximum width of 100 m measured perpendicularly to the Bunker Hill fissure.