The substances most frequently mistaken for gold are iron pyrites, copper pyrites and mica. The precious metal is easily distinguished from these by its malleability (flattening under the hammer) and its great weight, sinking rapidly in water.
Searching for Silver.—This metal is usually found with lead ore and native copper. Slates and sandstones intersected by igneous rocks as trap and porphyry, are good localities. Pure silver is often found in or near iron ores and the dark brown zinc blende. The Colorado silver lodes are porous at the surface and colored more or less red or green. Any rock suspected of containing silver should be powdered and dissolved in nitric acid. Pour off the liquid and add to it a solution of salt. If a white powder falls to the bottom which upon exposure turns black, there is silver in it. Silver mines increase in value as in depth, whereas gold diminishes as we descend.
Searching for Copper.—The copper ores, after exposure, or after being dipped in vinegar, are almost invariably green on the surface. They are most abundant near trap dykes. The pyrites is generally found in lead mines, and in granite and clay-slate. Copper very rarely occurs in the new formations, as along the Atlantic and Gulf borders, and in the Mississippi Valley south of Cairo.
Searching for Lead.—Lead is seldom discovered in the surface soil. It is also in vain to look for it in the coal region and along the coast. It must be sought in steep hills, in limestone and slate rocks. A surface cut by frequent ravines or covered by vegetation in lines, indicates mineral crevices. The galena from the slate is said to contain more silver than that from the limestone. The purest specimens of galena are poorest in silver; the small veins are richest in the more precious metal. A lead vein is thickest in limestone, thinner in sandstone and thinnest in slate.
Searching for Iron.—Any heavy mineral of a black, brown, red or yellow color may be suspected to be iron. To prove it, dissolve some in oil of vitriol and pour in an infusion of nut-gall or oak-bark; if it turns black, iron is present. If a ton of rich magnetic ore costs more than $4 at the furnace, good hematite more than $3, and poor ores more than $1.50 or $2, they are too expensive to pay, unless iron is unusually high. Deep mining for iron is not profitable. Generally speaking, a bed of good iron ore, a foot thick, will repay the cost of stripping it of soil, etc., twelve feet thick. Red and yellow earths, called ochres, contain iron. Magnetic ore is easily found by a compass.
CHAPTER V.
ASSAY OF ORES.
WHEN AN ORE WILL PAY—WASHING FOR GOLD AND PLATINUM—HOW TO ASSAY GOLD IN THE SIMPLEST WAY—TO TEST ANY ROCK FOR GOLD AND SILVER—TO FIND THE PURITY OF GOLD—TO DETECT AND ASSAY SILVER ORES—ASSAY OF COPPER, IRON, ZINC, TIN AND LEAD ORES—READY METHOD OF TESTING GRAPHITE.
ONE of the first questions asked after the discovery of a metallic ore, is—“will it pay?” We propose to state in plain words a method of determining the character and value of the principal ores, so that any intelligent man, however unscientific, may answer his own question. The chemical analysis or exact assaying of ores is too complicated, and must be left to professional assayers.
“Will it pay?” is an important query; for many ores of even precious metals, are not “paying.” Whether an ore is profitable depends not so much upon the relative value of the metal as upon the ease of separating it from the rock or “gangue” as it is called. Thus the minimum percentage of metal, below which the working of the ore ceases to be profitable is—
| Of | Iron, | 25 | per cent. |
| Zinc, | 20 | “ | |
| Lead, | 20 | “ | |
| Antimony, | 20 | “ | |
| Copper, | 02 | “ | |
| Tin, | 01½ | “ | |
| Quicksilver, | 01 | “ | |
| Silver, | 1/2000 | “ | |
| Platinum, | 1/10000 | “ | |
| Gold, | 1/100000 | “ |