Precious Ore Minerals (Gold, Silver, and Copper) and Their Characteristics and Genetics

Ore is a metalliferous minerals, more or less mixed with gangue, which form the standpoint of the miner can be won a profit, or from the standpoint of the metallurgist can be treated at a profit.

There are several terms that commonly used to ore:

  • Syngenetic – denotes ores and minerals that formed at the same time as their host rocks (most often applied to sedimentary rocks and ore)
  • Epigenetic – ores were emplaced into pre-existing rocks of any origin (e.g. veins, metasomatic ore)
  • Hypogene – ores that were formed by ascending solutions (e.g. Mississippi Valley type lead-zinc)
  • Supergene – ore formation by descending solutions (meteoric water interacting with rocks during surficial weathering processes)
  • Lateral secretion – concentration of metals by abstraction from surrounding rock
  • Endogenetic – concentration caused by processes in the Earth’s interior (magmatism or metamorphism)
  • Exogenetic – concentration caused by processes at the Earth’s surface (sedimentation, weathering).

Ore are usually regarded as being composed of two categories of material: ore minerals and gangue minerals. Stanton (1972) divided ore minerals into 3 principal classes:

  • Native metals and semimetals (gold group, platinum group, and arsenic group).
  • Sulphide and sulfosalts (argentite group, chalcocite group, galena group, sphalerite group, wurzite group, niccolite group, pyrite group, marcasite group, krennerite group, and skutterudite group).
  • Oxides (zincite group, spinel group, hematite group, rutile group, and uraninite group).

Geochemistry of Gold and Silver

The geochemical character of gold is strongly siderophile, in contrast to silver, which is chalcophile. Gold contents in magmatic rocks correlate positively with Fe and S, yet fractionation of felsic melt may lead to economic gold concentrations. Dissolution behaviour of these minerals controls liberation of gold from source rocks; in passing fluids, sulphides probably dissolve first. Some sediments concentrate gold. Sandstones and conglomerates average 0.03 ppm, coal ashes reach 0.1 and black shale 2 ppm Au. Arsenian iron sulphides of mid-oceanic ridges concentrate gold and silver (28–140 ppm Au, 800–2400 ppm Ag: Halbach et al. 2003).

If we consider the origins of epithermal deposits we can distinguish three classes based on the fluids that formed the deposits: two formed dominantly from end-member fluids, and one from a combination:

  • Magmatic
  • Magmatic-Meteoric
  • Meteoric 

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There are 2 forms of deposits :

Low-Sulfidation

High-Sulfidation

Open-space veins dominants

Veins subordinate, locally dominant

Stockwork ore common

Stockwork ore minor

Disseminated ore mostly minor

Disseminated ore dominant

Replacement ore minor

Replacement ore common

 

Geochemistry of Copper

The geochemical properties of copper are dominated by its great affinity to sulphur, which characterizes the “chalcophile or thiophilic elements” Cu, Zn, Ag, Cd, In, Hg, Tl, Pb, Bi, As, S, Se, Sb and Te. Although Cu is redox-sensitive, both Cu. and Cu2. are mobile cations under oxidizing conditions. Copper forms stable complexes with organic substances. Copper is adsorbed by clay and Mn-Fe oxyhydroxides (e.g. the manganese nodules of the deep sea). Its average abundance in the crust is 68 (14–100) ppm, 100 in mafic magmatic rocks and 10ppm in felsic rocks. Acidic and sulphur-poor ore-forming hydrothermal solutions transport copper mainly in the form of chloride complexes, such as (CuCl)o >250o C and CuCl32+ or CuCl2- at lower temperatures.