Replacement Deposits

Replacement deposits are bodies of rock which have been replaced by ore. To make the situation clearerand more accurate, the fluids which form the ore have migrated out through the porous rock (in most cases it is the sedimentary rock that is used, but this is not always so) and have left behind sufficient mineralization in disseminations, fracture fillings, and true replacements to make the ore mineral. A large amount of gold deposits which are of a low grade in the southwestern United States, such as Carlin in Nevada, are an extraordinary example of the deposits mentioned in this section.

Replacement deposits are affected most of the time by supergene processes. The unoxidated primary ores very often have gold which is bound in pyrite, which prevents many metallurgical processes from having the gold extracted from them. The weathered ores, in which the pyrite has been devastated by oxidation, have free gold that can be extracted with much more ease.

Lode Deposits
Lode deposits are of a lot of importance as a source of precious metals, even though they can also contain base metals. They are the dominant gold deposits of the Precambrian Shields of Africa, South America, Australia and Canada – shear zones and veins, holding carbonate, quartz, pyrite and native gold minerals. They can also form dissemination areas or stockworks, or occupy “saddles” in the higes of folded strata.

Deposits of this sort are found in greenstone belts which are areas of sedimentary and metamorphosed volcanic rocks. Greenstone belts are the ones which are most numerous in Precambrian shields, but they occur throughout geological time. The deposits themselves can have practically all host rocks, but some of the most common rocks which can be found there are felsic intrusive rocks, mafic volcanic rocks and some sedimentary rocks. They occupy the contact between two different types of rock more often than not.

One characteristic which is shared by literally all the lode deposits is their occurence in zones that are tectonically deformed. The deposits cluster around large regoinal fault areas, and the mineralized bodies themselves are in areas of intense structural deformation, with shear zones, fault brecciation and fracturing.

Intense deformation makes for a wide variety of forms and shapes for these deposits. Gold deposits which are hosted by shears can be comparatively straight; shear zones have been very precisely described as zones of straightening in which all the planar features in an area – this includes beds, dykes, faults and fractures – are squeezed into near-parallelism. Veins can either be very sinuous or very straight, disseminated mineralization can have almost any shape, and saddles and stockwork most of the time form plunging, crudely pipe-like zones. In turn, whichever of these shapes can be folded into more complex shapes, or they can be cut off by a fault and appear again at some other place.

The wallrocks of the veins most of the time show very intense alteration. The mineralization in the veins themselves can be very irregular, making it absolutely necessary to drill and sample these deposits with keen interest before making any attempt to calculate their tonnage and grade.