Ore Deposits

ORE DEPOSITS. The word "ore" is defined in the Oxford Dictionary as "a native mineral containing a precious or useful metal in such quantities and in such chemical combination as to make its extraction profitable," and this is the sense in which it is used in commerce and by miners, although the word is also ap plied to metalliferous minerals without the above-mentioned limitations. (See also PHYSICAL RESOURCES.) Ore deposits are accumulations in Nature which have arisen by some form of concentration, either chemical or mechanical, or both. Prof. Kemp has made a useful comparison between the average percentages of the metals in the earth's crust and those necessary to constitute an ore suitable for profitable working. The i.wo tabulations, slightly modified in the case of manganese and copper to conform with later improved practice, are here put side by side :— Average percentage Percentage necessary Metals in Earth's crust for profitable working Aluminium 8.13 30 Iron 4.71 35-65 Manganese . . . 0.07 25-5o Nickel o.oi 2-5 Tin o.000X- -0000X 1.5-3 Copper o.0000X Lead o.0000X 2-2.5 Zinc o.0000X 5-25 Silver o-000000X 0.03-0.16 Gold o.0000000X o•003-0 00016 Platinum . . . . o.00000000X 0.00005 The letter X indicates some undetermined digit.

A

comparison of the two tabulations gives an idea of the degree of concentration that has been instrumental in the formation of ore deposits from the materials of the earth's crust. Research has shown that it is to the igneous magmas that we must look for the original source of these concentrations of the metals. The agents by which the metallic components of the magmas are segregated, or extracted from the rocks to which the magmas consolidate, and finally concentrated, are numerous. In the case of the molten magmas it is by the process known as differentiation, or by the escape of gases and vapours carrying volatile metallic compounds to the adjacent rocks—a process which is known as pneu matolysis. From the consolidated igneous rocks extraction is effected by hydrothermal solutions, whether of deep-seated or of meteoric origin. Further concentration takes place (r) by oxida tion, hydration and solution in the zone of weathering and re-pre cipitation in the zone of secondary enrichment ; (2) by the me chanical agents of erosion and sedimentation, and (3) by chemical and bacterial agents leading to precipitation in seas, lakes and swamps.

It will be seen from these considerations that ore deposits can be divided into two great classes according as they are of primary or of secondary origin.

The Primary Deposits

may be subdivided into : (a) Magmatic differentiations within the boundaries of the igneous mass; (b) Deep-seated injections of materials extracted from the mag ma by differentiation; (c) Infillings and replacements at moderate depths by magmatic waters or by meteoric waters heated by descent; (d) Infillings and replacements near the surface by waters of meteoric origin. The Secondary Deposits are : (a) Oxidation and reduction ores in the zones of weathering and secondary enrichment; (b) Residual ores ; (c) Detrital ores; (d) Precipitated ores.

The form assumed by ore deposits will be referred to later. As regards their mineral composition, it is necessary in the first place to distinguish between the ore and the gangue, the latter being the name given to the minerals that accompany the ore.

In the deep-lying parts of the deposit, or what is of ten called the primary sulphide zone, the ore-forming metals occur in corn bination with sulphur, tellurium, arsenic and antimony (as sul phides, tellurides, arsenides and antimonides of one or other of the metals—gold, silver, mercury, lead, vanadium, zinc, nickel, cobalt, etc.), or as silicates (of iron, manganese), or oxides (of iron, tin, chromium). Occurrence in the native state is in this zone confined to a few of the metals (gold, platinum, iridium, palladium, etc.). In the upper portion of the lodes, in the zone of oxidation, the metals occur as carbonates, sulphates, phosphates, arseniates, anti moniates, chromates, silicates, chlorides, oxy-chlorides and drates. Where special conditions involving strong reduction prevail certain metals appear in the native state (copper, silver, mercury).