With the high market prices of gold and silver in recent months, a number of people are wondering about where these metals come from. Gold and silver metals are obtained from a variety of different types of rock ores. Most people think of gold nuggets and such as the source of gold, but the truth is that very little of the new gold produced comes from nuggets – nearly all newly mined gold comes from ores mined from the natural hard rocks that contain gold in tiny, even microscopic particles. This article is created to describe what these ores are like. Gold is found widely diffused in nature even though it is one of the scarcer metals in the earth’s crust. Very commonly, gold occurs as the native metal encased within a mineral known as quartz. Sometimes the gold is in a finely divided state, sometimes in particles of considerable size, as nuggets, grains, scales, plates, threads and wires in quartz rock. It occurs also in a finely divided state disseminated through schistose rocks, slates and some sedimentary rocks like limestone. In these cases the rock has been altered by the flow of heated and mineralized waters, often resulting in the impregnation of large amounts of rock with silica, iron and a certain amount of gold. Sometimes the silicified rock even replaces much if not all of the original country rock. While historically speaking vein deposits were the most productive, these disseminated deposits currently yield much of the worlds gold ore.
Within gold ores, the element itself occurs in nature chiefly in the form of native gold, which is by far the most common gold bearing mineral. In various gold ores, the native gold commonly occurs as tiny particles contained within sulfide minerals such as pyrite. Iron pyrite is an exceedingly common mineral associated with gold, but it also serves as a reducing agent. Therefore whenever gold is found encased in pyrite, it is always present as free gold and not as some type of gold sulphide. Gold is also found at times in chalcopyrite, galena and arsenopyrite and stibnite, but not as a rule in such large amounts as may be found in pyrite. Other minerals, like sphalerite, pyrrhotite, magnetite and hematite sometimes carry small amounts of gold as well. Gold also occurs as tellurides such as calaverite. Common gangue minerals in gold ores include quartz, fluorite, calcite and pyrite, but many others can be found in smaller amounts.
Gold Ore Minerals:
The most prominent is native gold – most of the native gold contains a small amount of silver, copper, platinum, etc. Telluride minerals are the most common minerals which contain significant gold in their make up. They include: Petzite (Ag,Au) 2,Te, with a gold content of about 25 per cent. Hessite (Ag2Te), with gold often present replacing a part of the silver. Sylvanite (Au,Ag)Te2 : typically about 25 per cent. gold. Calaverite (Au,Ag)Te2 : typically about 40 per cent, gold. Krennerite (Ag2Te,Au2Te3) : gold is about 35 percent. Nagyagite (Au2,Pbi4,Sb3,Te7,S7). Some samples of Nagyagite have given upon analysis 12.75 per cent gold. The gold containing sulphides, as well as the tellurides, are of primary formation, although auriferous chalcopyrite might also be formed by secondary enrichment processes. Native gold may occur in the primary, secondary enrichment, or oxidized zones. The tellurides, which are usually associated with pyrite, are widely distributed, though not so abundant, but not always recognized; indeed by some miners they are mistaken for sulphides.
General Types of Gold Ores:
Gold deposits are often classified according to their association. The first of these may be catalogued as quartzose. This implies that the gangue mineral is acid, that is, quartz, and that fluorite may abound, or even the other gangue minerals of the alkaline earth group. Not infrequently there appears within the quartz varying amounts of pyrite and even limited quantities of chalcopyrite and galena. These are free milling ores. By a free milling ore, it is meant one that the rock does not require roasting before the gold can be recovered from it. Dry ore is the term often used for this category. The second class of gold ores is auriferous copper ores. These are widely distributed throughout the United States and much of the chalcopyrite is gold bearing. These auriferous copper ores are especially abundant in Colorado, Utah, Montana and British Columbia. They are also present at Gold Hill, North Carolina and in Canada at Newfoundland. The third class of gold ore is auriferous lead ores. The percentage of lead in these rocks is large and the gold content is often small. They are refractory ores like the copper ores. By refractory ore is meant one that requires roasting before extraction processing. The heavy sulphides as copper, lead and antimony require this method of treatment, that is the condition of the gold in the mineral will not allow of its immediate capture with most recovery systems. The fourth class of gold ores comprises the gold-telluride group. The gold telluride ores occur with silver, or with silver, lead and antimony, or as native gold accompanied by other tellurides. These ores are often sent direct to the smelters for treatment. A fifth type is the disseminated ore type. They are often low in grade but large. They fill large fracture and fault zones or replace certain geologic horizons. They are the result of the circulation of large amounts of heated water deep underground.
Some folks are interested in learning to recognize gold ores and seeing if they might have accidentally found some. To see photos of various types of gold ores and learn more about how to prospect for them, and where they come from, take a look at this web page: Gold Ore Photos and Information
For more information about prospecting for gold, see my: Gold Prospecting Encyclopedia
Chris Ralph writes on small scale mining and prospecting for the ICMJ Mining Journal. He has a degree in Mining Engineering from the Mackay School of Mines in Reno, and has worked for precious metal mining companies conducting both surface and underground operations. After working in the mining industry, he has continued his interest in mining as an individual prospector.