Two great ore bodies in the general vicinity of Franklin, N. J., are of unique interest, because they are mostly the red oxide of zinc called zincite. The ore deposits occur in white limestone along or close to its contact with metamorphosed (altered) strata and granite of early Paleozoic Age. It is not definitely known how the ore originated, but it was probably derived in solution from the hot granite and deposited in the limestone by replacement of the latter.
In Colorado the principal zinc mines are around Leadville, where lead ore is nearly always directly associated with the zinc ore. This district is above described in the discussion of lead.
Among many uses of zinc are for galvanizing; for making certain high-grade paints; brass and white metal; and for roofing and plumbing.
Gold. This precious metal has been used and highly prized by man for thousands of years. The discovery of gold in California in 1848 was one of the most important events in the history of the mining world. As early as 1852 that State reached its climax of production with an output of at least $81,000,000 worth of the metal. The Transvaal region of South Africa has for two decades been the world’s greatest gold producer. Though long known, the metal has there been worked only since 1886. In 1915 the peak of gold production in the world ($468,700,000) was reached and nearly maintained in 1916, but since that time there has been a great falling off. In 1916 South Africa produced gold to the value of about $200,000,000; the United States over $90,000,000; Australia over $40,000,000; Russia over $26,000,000; and Canada over $19,000,000.
In tiny amounts gold is really very widespread. It occurs in many stream gravels where so-called “color” may be obtained by washing gravel, and it is even dissolved in sea water. Gold-mining localities are also numerous in many parts of the world, but relatively few of them only have ever paid. The total amount of money spent in actual gold-mining operations; in hopeless but honest operations; and for stock in fake gold mines has no doubt exceeded the actual value of gold produced. In many a case acceptance of a report based upon a very brief examination of the ground by a competent geologist would have saved the cost of hopeless expenditure of money. Some one in nearly every community has a so-called “gold mine.”
Most of the commercially valuable gold occurs in nature as native gold, either mixed with gravel and sand (i.e., placer deposits) along existing or ancient stream beds, or in veins mechanically held in the mineral pyrite (described in the preceding chapter) in submicroscopic form, or visibly mixed with quartz in vein deposits. Another kind of ore which assumes considerable importance, as in parts of Colorado, is in the form of telluride of gold always found in veins. In deep vein deposits it is quite the rule to find free or native gold mechanically and visibly mixed with quartz in the upper levels, while deeper down the gold is mechanically, but invisibly, held in combination usually in pyrite, which latter is associated with quartz. This difference is due to the fact that the lower level ores are now just as they were formed, while in the upper levels the ores have been weathered, and the gold set free and often more or less further concentrated by solutions. Vein deposits, including also telluride ores, are found in many kinds of rocks—igneous, sedimentary, and metamorphic—of nearly all ages generally directly associated with igneous rocks. In nearly all cases the best evidence indicates that the vein fillings were formed by hot ore-bearing solutions from the igneous rock, which solutions deposited the ore plus quartz in fissures in either the igneous or adjacent rocks. Among the many localities where fissure veins of the kind just described are of great economic importance are the “Mother Lode” belt of the Sierra Nevada Mountains of California; Cripple Creek (telluride ore), Georgetown and the San Juan region of Colorado; Goldfield, Tonopah, and Comstock Lode of Nevada; and near Juneau, Alaska.
Placer deposits, that is, free gold mixed with gravel and sand, also yield much gold. They are most prominently developed in California and Alaska. These gold-bearing “gravels represent the more resistant products of weathering, such as quartz and native gold, which have been washed down from the hills on whose slopes the gold-bearing quartz veins outcrop, and were too heavy to be carried any distance, unless the grade was steep. They have consequently settled down in the stream channels, the gold, on account of its higher specific gravity, collecting usually in the lower part of the gravel (placer) deposit.” (Ries.) Such gold occurs as grains, flakes, or nuggets. When a chunk of gold-bearing vein quartz, with crevices filled by thin plates of the metal, is carried downstream pieces are gradually broken away, and the tough, very malleable gold bends or welds together into a single mass called a “nugget.” Nuggets varying in weight up to over 2,000 ounces have been found. Many placer deposits are along existing drainage channels, while others occur in abandoned and even buried former channels.
Most of the gold of South Africa comes from Witwatersrand district where the native metal occurs in a unique manner in beds or layers of conglomerate associated with other strata, all the rocks being considerably folded and somewhat faulted. Some of the mines are more than a mile deep (vertically), the deepest in the world. The gold either accumulated in placer form with gravel which later consolidated into conglomerate, or it was introduced into spaces between the pebbles subsequently by ore-bearing solutions.
Silver. For many years the United States and Mexico have been the world’s greatest silver producers, sometimes one and sometimes the other leading, with Canada third, and Australasia fourth. In 1918 the United States produced nearly 68,000,000 ounces of silver and Mexico over 62,000,000 ounces. In the United States in 1918 the four leading States were Montana, Utah, Idaho and Nevada with outputs ranging from over 10,000,000 to over 15,000,000 ounces each.