Metallic Minerals and Archaeological Geology

Economic geology had its inception in the ancient utilization of rocks and minerals. The first economic materials were nonmetallic and include flint, quartz, diabase, rhyolite, obsidian, jade, and other stones, which were sought for weapons, implements, adornment, and even art. Beginning with the Upper Paleolithic Aurignacian period, clay began to be widely used for simple figurines, then brick and finally pottery. S. H. Ball identifies 13 varieties of minerals—chalcedony, quartz, rock crystal, serpentine, obsidian, pyrite, jasper, steatite, amber, jadite, calcite, amethyst, and fluorspar—as economic within the Paleolithic. Add to this list the use of ochres and mineral paints together with nephrite, sillimanite, and turquoise. In the standard reference on the nonmetallic deposits, "Industrial Minerals and Rocks", 6th edition published in 1994, deposits are classified by use and the minerals and rocks described as commodities. The fourteen use groups include such items as abrasives, constructions materials, and gem materials; the 48 commodities include clay, diamonds, feldspar, etc. Metalliferous minerals as ore deposits are unevenly distributed throughout the world. The formation of a mineral deposit is an episode or series of episodes in the geological history of a region and reflects three broad categories: (1) igneous activity, (2) sedimentary processes, and (3) metamorphism. Table 12.1 summarizes general features of the three categories of mineral deposits. Admixtures of metals are by far the most common form of mineral deposits. Gold, silver, and copper occur either as native metals or admixed with other metals and compounds. Most ore deposits are actually mixtures of metals: silver commonly with lead, zinc with cadmium, iron with copper. Many metallic ore deposits are products of igneous activity. Conditions change in the magma chamber as the principal rock-forming minerals crystallize, temperature falls as the magma cools, pressure is lowered as the magma rises in the crust, and volatiles increase in the magma chamber.