Although micron-size particles of metallic gold are observed at the Zarshuran gold deposit, northwest Iran, the quantities do not account for the gold concentrations determined by chemical analyses. The presence of invisible gold has been established by means of trace element electron microprobe analyses of pyrite, arsenian pyrite, orpiment, realgar, stibnite, getchellite, sphalerite, and lead sulfosalts. Quantitative point analyses indicate that invisible gold is present in anhedral pyrite, arsenian pyrite overgrowth rims on gold- and arsenic-free euhedral pyrite, in massive, network, and colloform arsenian pyrite, and in massive and colloform sphalerite intimately intergrown with colloform arsenian pyrite. Gold in these forms adequately explains the measured gold concentrations at Zarshuran. The invisible gold owes its origin to solid-solution deposition and/or encapsulation of submicron-size particles of metallic gold.

Steel is an alloy of iron and carbon that is vital to the global economy. Its unique combination of strength, formability, versatility, recyclability, and low cost, make it an ideal material for the construction industry, shipbuilding, motor vehicle manufacture, railway construction, bridge building, heavy industry, machinery manufacture, and engineering applications. Based on the World Steel Association statistics shown in Figure 1.1, the world's crude steel production has almost doubled since 2000.

Iron oxide copper (-gold) deposits consist of dominant magnetite or haematite, with one or more copper sulphides and pyrite, with associated K-feldspar or sericite or albite or biotite and chlorite predominant in the ore host rocks. The deposits display a unique association with host successions characterised by an absence of, or by very minor occurrence of, elemental carbon or reduced-carbon compounds and reduced-sulphur minerals. The relatively oxidised nature of the ore host succession is reflected in the "magnetically active" signature that usually defines iron oxide copper (-gold) mineralised districts. This signature shows that magnetite is ubiquitous and variably abundant within ore host successions. Host successions with discrete domains respectively characterised by (a) by rocks with an absence or rarity of carbon or reduced carbon minerals, and (b) by a predominance of rocks containing carbon or reduced carbon minerals, however, contain iron sulphide-copper (-gold) deposits on or near the boundaries of the domains. Examples of the iron sulphide-copper (-gold) deposits are the Mt Isa and Gunpowder deposits, many small occurrences in the Eastern Fold Belt (Mt Isa Inlier), the El Soldado deposit, and others.