Gold deposits present gold bearing minerals and gangue, which contain sulphide and non-sulphide minerals. The latter is characterized by the presence of rocks made up of certain minerals such as quartz, feldspar and other minerals. Without any doubt, quartz scarcely needs description being well known. Basically, quartz is too hard to be scratched with a knife and will scratch glass. This aspect is a difference respect to calcite and barite, for which it might be mistaken in the field or during core logging.
The feldspars are nearly as hard as quartz and their colors are white, grayish and flesh color. Feldspars are rarely transparent, being usually opaque and their form of crystallization is different from quartz. In a vein they shoe one smooth face of crystals, whilst quartz is more like crushed sugar. In porphyry deposits the feldspar crystals are very distinct and show a characteristic spotted appearance to the rock. Two varieties of feldspar are characteristics of crystalline rocks, one known as orthoclase and other oligoclase. The former is very characteristic of granitic rocks as well as of igneous rocks, the latter is rather more characteristic of igneous rocks such as diorite and basalt. Orthoclase is usually found as large crystals, oligoclase is small. When the crystals are small, it makes take a microscopic examination to determine the variety of feldspar.
Other specimen is hornblende, which differs from mica in being of a duller luster and a different form of crystallization. The color is a greenish black. The greenish tint is distinct when the crystal is struck by a hammer. Pyroxene is scarcely distinguishable from hornblende. In Colorado, the rock is confined to two kinds of rock, basalt and andesite, both of comparatively volcanic origin. Talc among miners means almost soft, sticky, slippery, decomposed rock, but strictly talc is silicate of magnesia. Chlorite is another magnesia mineral of a green and soft character. Chlorite is again a name given to almost any greenish rock of a schistose and soft decomposed character. Dolomite or carbonate of lime and magnesia is very like calcite and is the element of dolomitic or magnesian limestone. Dolomite effervesces with much greater difficulty than true limestone.
Granite is massive, shapeless or amorphous and shows no bedding planes or other signs of former stratification. The crystalline amorphous condition is result of extreme metamorphism of originally bedded rocks such as gneiss or schist, which being traced down through a gradual change in granite. The crystalline structure differs both in character and appearance from porphyries and other igneous rocks in the fact that its crystals are all jumbled up and crushed together like sugar and none of the crystals are set like plums in a pudding, distinctly in a backing or paste of very small crystals of amorphous or glassy material. Probably, granite is the oldest and deepest rock known. It is often traversed by sparse veins, both great and small, which consists of quartz or feldspar or both, in a more sparse condition than when diffused through the parent rock.
Quartzite was originally sandstone composed of quartz grains, which by heat were partially fused together at the edges. Quartzite differs from quartz in being a rock made out of pieces of quartz and not the original mineral itself. Quartzite may be white, grey, brown or rusty, and shows a stratified structure.