The structural features of rocks can be applied to study and explain the formation of some gold deposits, therefore, necessarily precedes any consideration of ore deposits or processes of ore deposition. For example, the arrangement of sediments in parallel and approximately horizontal layers is called stratification, and is an original property of sedimentary rocks. Successive strata of the same sedimentary bed commonly differ from each other in some minor characteristic, such as texture or color, which results in a bedded or stratified appearance.
The subjection of a rock to pressure develops within it parallel planes of weakness, called cleavage planes, along which the rock breaks into relatively regular slabs or blocks; the direction of these cleavage planes, which bear no relation to stratification, is determined by the direction of the pressure that produced them. Where there are more than one set of cleavage planes, one of them is likely to be more prominent than the others. In sedimentary rocks, the cleavage may coincide with the stratification, but more commonly cuts across it. Cleavage has been defined as the capacity present in some rocks to break in some directions more easily than in others. Cleavage does not imply the existence of subdivisions, but rather the tendency to subdivide along certain planes.
Fissility is a structure in rocks by virtue of which they are already separated into parallel laminas. Fissility may be regarded as a development of the property of cleavage, and is commonly expressed along closely set parallel planes. The long-continued stresses of regional metamorphism with accompanying recrystallization and rock flowage develop a banded structure in both sedimentary and igneous rocks; the various minerals contained by the rocks are arranged with their longer axes parallel and form planes of weakness to fracture that differ from cleavage planes and that are independent of any original stratification. A rock that has suffered this change is said to have a schistose structure.
Schists commonly exhibit cleavage that has no relation to their schistosity, and occasionally, also, traces of the original stratification are visible. A coarsely schistose structure is known as a gneissic structure. In gneisses the individual bands are more prominently developed than in schists, and the ease of fracture along these bands is relatively less.Planes of division through rocks that are the result of stresses insufficient to produce more than microscopic movement are called joints. Fragments of broken rock, both sedimentary and igneous, are commonly bounded in part by plane surfaces, which are joint planes. Joints are of several kinds, according to the nature of the stresses that produced them; they vary in expression from cleavage, or incipient jointing, to the well developed planes that bound the prismatic columns of certain basalts. Joint planes frequently preserve their general directions over long distances, and the angles between different joint systems are likely to be constant throughout large rock masses.
Where the joint planes are closely spaced and where the load of overlying rocks is light, they afford channels for the circulation of solutions and frequently become mineralized. During the contraction that results from cooling and solidification, igneous rocks separate into masses of roughly polygonal section bounded by tension or contraction joints. Such joints are best developed in certain flows, which upon solidification divide into regular prismatic columns. Sedimentary rocks upon drying not infrequently suffer contraction, which finds expression in similar, but usually less well defined planes of division. Fractures due to contraction are the result of internal strains in a rock mass, and do not pass outside of it into other rocks for any important distance. The outer members of folded strata undergo tensile stress during folding, which occasionally results in the formation of tension joints that follow a radial arrangement inward from the arc of the fold.