Primary and Secondary Vein Minerals - Secondary Enrichment
In many mineral veins, it is obvious that certain minerals belong to the original vein deposit while certain others have been formed subsequently. These two classes of minerals are known respectively as Primary and Secondary Minerals. The primary vein minerals are those which were originally deposited by the ascending waters in the vein fissure. The primary metallic vein minerals are comparatively few in number, the more important being pyrite, chalcopyrite, galena and sphalerite. The secondary vein minerals have been formed from the primary minerals by some subsequent chemical reaction. This change is ordinarily brought about through the influence of oxidizing waters which coming from the surface of the earth descend though the upper portions of the vein. Under these conditions, various new minerals are formed, many of them being oxidized compounds. As the descending waters lose their oxygen content within a comparatively short distance of the earth’s surface, the secondary minerals are only to be found in the upper part of a vein. Together with the formation of these secondary minerals, there is frequently a downward migration of the valuable metals in the vein. This is brought about by the solution of the minerals in the uppermost portion of the vein and a subsequent reprecitation a little farther down. As the surface of the earth is gradually lowered by erosion, the upper part of a vein is continually being worn away. But the metallic content of the uppermost part of the vein is always being carried downward by the descending oxidizing waters. In this way, the metallic content
of the upper of many veins has been notably enriched since there is concentrated in this short most of the original contents of hundreds, perhaps thousands, of fee of the vein which have been slowly worn away by the general erosion of the country. Consequently the zone of the secondary vein minerals is also frequently a zone secondary enrichment. This is an important fact to be borne in mind since, because of it, the upper two or three hundred feet of a vein are ordinarily the richest portion of a deposit. The ore below that depth gradually reverts on its original unaltered and unenrinched character and may frequently prove too low in value to warrant its being mined. The prevalent idea that the ore of a vein must increase in value with increasing depth is not true in the great majority of cases.
It will be of interest to consider the more important primary vein minerals and the secondary minerals that are commonly formed from them.
Iron Minerals. The common primary vein iron mineral is pyrite, FeS2. marcasite, FeS2, while not so common in occurrence is also a primary mineral. When oxidized, these minerals yield ordinarily the hydrated oxide limonite, Fe4O3(Oh)6. the upper portion of a vein that was originally rich in pyrite wil oftenshow a cellular and rusty mass of limonite. This limonite deposit near the surface is commonly termed gossan. They yellow rusty character of the outcrop of many veins enables one frequently to locate them and to trace them across the country.
Copper Minerals. The one common primary copper minerals is chalcopyrite, CuFeS2. At times, some of the other sulfides may be primary in their origin, but this is not generally the case. The secondary formation of bornite and chalcocite may be explained as follows. The copper sulfide existing in the original chalcopyrite is oxidized by the descending waters at the surface to copper sulfate which is then dissolved and carried farther down the vein. Here it comes in contact with unaltered chalcopyrite and a reaction takes place which enriches the sulphide, changing it to bornite, Cu5FeS4. Later, more copper sulfate in solution comes in contact with the bornite and a further enrichment takes place with the formation of chalcocite, Cu2S. In each case, there
Is an interchange of metals, the iron in the original sulphide going into solution as a sulfate thus taking the place of the copper which has been precipitated. If the copper deposit lies in limestone rocks, we commonly find the various carbonates and oxides of copper also formed in the upper parts of the deposit. The secondary copper minerals therefore include chalcocite, Cu2S, bornite, Cu5FeS4, native copper, Cu, cuprite, Cu2, malachite, (Cu. OH)2CO3, azurite, Cu(Cu.OH)2(CO3)2, chrysocolla, CuSiO3.2H2O, chalcanthite, CuSo4.5H2O:
Lead Minerals. The one primary lead minerals is galena, PbS. The secondary minerals of lead are all oxidized compounds and include the following; cerussite, PbCO3, anglesite, PbSO4, pyromorphite, Pb4(PbCl)(PO4)3, wulfenite, PbMoO4.
Zinc Minerals. Sphalerite, ZnS, is the only common primary zinc mineral. The chief secondary minerals are smithsonite, ZnCO3, and calamine, H2(Zn2O)SiO4.
Silver Minerals. Probably most of the sulfide minerals of sliver are primary in their origin. The following minerals are usually secondary, although native silver at times appears primary: native silver, Ag, cerargyrite, AgCl, embolite, Ag(Cl,Br), etc.
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