Silver has been over $10 per ounce for most of a year, causing curiosity among some folks as to the where silver comes from. Silver is mined from a variety of rock like ores, commonly in combination with gold. In some ores the gold predominates, yet in others it’s the silver, while in still a third class these two precious metals may be mixed with various base metals, such as lead, copper, zinc, and iron. Few silver ores are absolutely free from gold, and vice versa, so that a separate consideration of the two is more or less a difficult task. For those ores where the most valuable element is silver, the silver is normally contained in minerals that are colored gray to black in appearance. These minerals range from those with a metallic sheen to others with an earthy soot-like appearance. Sooty black minerals are common in many very rich silver ore samples. Most of these sooty black deposits consist of acanthite or various complex silver bearing sulfides (a sulfide is a mineral containing sulfur in combination with one or more metals).
The most common valuable minerals which may constitute rich silver ores include native silver; acanthite (silver sulfide); pyargyrite (dark ruby silver or silver antimony sulfide); Proustite (light ruby silver or silver arsenic sulfide); Stephanite (brittle silver, also a silver antimony sulfide); Polybasite (also a silver antimony sulfide); Cerargyrite (Silver Chloride); Bromyrite (silver bromide) and Iodyrite (silver Iodide). Bonanza grade silver ore can also contain various silver telluride minerals including Calaverite, Sylvanite and Hessite. To a lesser extent, base metal sulfides including Galena, sphalerite, pyrite, chalcopyrite, and chalcocite may all be and frequently are silver bearing, but in most silver ore deposits, the silver is most concentrated in the minerals of the group first named above. Of the base metal ore minerals, the most common primary ones are argentiferous galena, sphalerite, and pyrite, while native silver and the sulphides and arsenides are less common.
Standard Types of Silver Ores:
There are two general classes of silver ore that have been mined: 1) lead-silver and 2) high-grade silver ores. Both have almost always varying amounts of gold. The lead -silver mines furnish also, as noted above, by far the greater portion of the lead produced in the United States. High grade silver ores normally have considerably less base metal content and often contain significant gold.
1. Dry or Siliceous Ores. These include: (a) The gold and silver ores proper, including bonanza epithermal silver ores; (b) fluxing ores carrying considerable quantities of iron and manganese oxides with small gold and silver contents; (c) precious-metal bearing ores with copper, lead and zinc in small amounts; and (d) disseminated low grade silver deposits. The states of Colorado, California, Nevada, South Dakota, and Alaska have been the largest producers of this category of silver ore The siliceous ores are in part free milling; in part simply concentrating, as parts of Colorado and Arizona; in increasing part all-sliming and cyaniding; and in part smelting. A great deal of the silver from the gold-silver siliceous ores is obtained with the gold by cyanidation, the silver being recovered by refining the mill bullion. The remainder is obtained by smelting rich ores and refining copper or lead bullion produced. The siliceous silver ores are of varying age, but most epithermal ores are young, typically post-Miocene age. Those found chiefly in Colorado, Nevada, and Montana, are associated with Tertiary lavas and characterized by Bonanza grade rich ores. Some of the most productive ones may carry fluorite and normally also tellurides. In some the value of the gold may predominate; in others, silver.
2. Copper ores, usually with over 1 per cent copper, but with less in the case of the western disseminated ores and those of Lake Superior. The largest gold producers are those of Utah, Arizona, Nevada, and Montana. The silver production comes from the electrolytic refining of blister copper produced by smelting. The great disseminated deposits of Utah, Arizona, Nevada, and New Mexico are yielding increasing quantities, while the vein deposits of places like Butte, Mont., have are also been important. The gold- and silver-bearing copper ores exhibit great differences in form and age; neither do all the occurrences yield much gold or silver, and, moreover, they are of more importance as gold producers, silver being less often associated with the copper.
3. Lead-bearing Silver ores: These are silver ores containing 4 percent or more of lead. The silver comes mainly from the lead-silver ores of Coeur d’Alene, Idaho, Utah (chiefly Park City and Tintic), Colorado (Leadville and Aspen). Most of the output is obtained by the de-silverization of lead bullion using molten zinc. The silver and lead ores form a large class, of rather wide distribution, and while the two metals characterizing the group are the most prominent, there may also be, and often is, present a variable quantity of other metals such as gold, zinc, and copper. The silver contents, though sometimes high, but are not necessarily visible, and may be contained within the galena as Ag2S partly replacing lead sulfide in the crystal. The ore bodies as a whole present a variety of forms, the ore having been deposited either by fissure vein cavity filling or replacement, or both. Most of the important occurrences seem to have been formed at intermediate depths. Oxidation zones frequently cap the ore bodies, and downward secondary enrichment has probably occurred in many cases. Silver-lead ores form a widely distributed class in the Cordilleran region of the United States and supply most of the lead mined in this country. Prominent deposits have been mined in Colorado, Idaho, and Utah, but are also known in New Mexico, Montana, Wyoming, Nevada, Arizona, California, and South Dakota. Canada supplies a small but steady production from British Columbia, while in other foreign countries districts worth noting for either commercial or historic importance are Broken Hill, New South Wales, Australia; Clausthal and Freiberg, Germany; Przibram, Bohemia; Sala, Sweden; Laurium, Greece; Mexico, etc.
4. Copper-lead or Copper-lead-zinc Ores. These are unimportant as compared with the others. The gold they supply is small, and the main silver production was from mines in Colorado and Nevada. Historically, the Eureka district in Nevada yielded significant amounts of copper-lead-zinc ore with large amounts of gold and silver. They are often designated as polymetallic ores in modern classification systems.
5. Zinc ores, containing at least 25 per cent zinc. These yield little gold, and the silver which is obtained mainly as a by-product from the smelting of zinc concentrates and in the US has been obtained chiefly from deposits in Nevada, Montana, and Arizona.