Alluvial clast with native gold from Montana, USA. (public display, Leadville Mining Museum, Leadville, Colorado, USA)

A mineral is a naturally-occurring, solid, inorganic, crystalline substrance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals.

To find a native element in nature, it must be relatively non-reactive and there must be some concentration process. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state as minerals.

Gold (Au) is the most prestigious metal known, but it's not the most valuable. Gold is the only metal that has a deep, rich, metallic yellow color. Almost all other metals are silvery-colored. Gold is very rare in crustal rocks - it averages about 5 ppb (parts per billion). Where gold has been concentrated, it occurs as wires, dendritic crystals, twisted sheets, octahedral crystals, and variably-shaped nuggets. It most commonly occurs in hydrothermal quartz veins, disseminated in some contact- & hydrothermal-metamorphic rocks, and in placer deposits. Placers are concentrations of heavy minerals in stream gravels or in cracks on bedrock-floored streams. Gold has a high specific gravity (about 19), so it easily accumulates in placer deposits. Its high density allows prospectors to readily collect placer gold by panning.

In addition to its high density, gold has a high melting point (over 1000º C). Gold is also relatively soft - about 2.5 to 3 on the Mohs Hardness Scale. The use of pure gold or high-purity gold in jewelry is not desirable as it easily gets scratched. The addition of other metals to gold to increase the hardness also alters the unique color of gold. Gold jewelry made & sold in America doesn’t have the gorgeous rich color of high-purity gold.

The Butte, Montana rock shown above is an alluvial clast (fluvial?) with patches of native gold. The black material is likely one or more manganese oxide minerals (pyrolusite?, etc.).

Butte has been described as the "Richest Hill on Earth" and was the world's top producer of copper for many decades. The Butte Mining District consists of numerous latest Cretaceous to earliest Tertiary hydrothermal veins (62 to 66 Ma) that cut through Cretaceous-aged Butte Quartz Monzonite (76 Ma). The veins contained a wide variety of metallic minerals - principally copper sulfides. The only currently active mine in Butte is the Continental Pit, which targets vein- & veinlet-filling molybdenite (MoS2 - molybdenum sulfide) and chalcopyrite (CuFeS2 - copper iron sulfide), plus disseminated molybdenite-chalcopyrite in the host quartz monzonite. Copper "mining" also occurs at the nearby Berkeley Pit, an inactive open-pit copper mine. The Berkeley Pit has been filling with iced tea-colored water for decades - the water is pumped out, copper is removed, and the water is returned to the pit.

The Butte District started in the 1800s when gold was found in stream gravels (placer deposits). The gold-bearing clast shown above is apparently derived from a Butte placer.