Abstract
We constructed thermodynamic models of the formation of two types of gold-ore mineralization at the Kagan ultramafic massif in the Southern Urals (Russia). The first type of gold-mineralization is widely spread at the massif in the tectonic zones of schistose serpentinites containing typically ≤ 0.1 ppm Au. The second type of gold-ore mineralization is represented by veined massive, streaky and impregnated magnetite ores in contact with serpentinites. It contains to 5 vol.% sulfides and 0.2–1.2 ppm Au. Our thermodynamic calculations explain the formation of two types of gold-ore mineralization in the bedrocks of ultramafic massifs. Metamorphic water, which is the result of the dehydration of early serpentinites (middle Riphean) during high-temperature regional metamorphism (700 °C, 10 kbar) (late Precambrian), is considered as the source of ore-bearing fluid in the models. The metasomatic interaction of metamorphic fluid with serpentinites is responsible for the gold-poor mineralization of the 1st type at T = 450–250 °C and P = 2.5–0.5 kbar. The hydrothermal gold-rich mineralization of the 2nd type was formed during mixing of metamorphic and meteoric fluids at T = 500–400 °C and P = 2–3 kbar and discharge of mixed fluid in the open space of cracks in serpentinites. The model calculations showed that the dominant forms of gold transport in fluids with pH = 3–5 are AuCl2− complexes (≥450 °C) and, as the temperature decreases, AuHS0, or AuOH0. Mineral associations obtained in model calculations are in general similar to the observed natural types of gold mineralization.
Highlights
The Urals is one of the most important gold provinces in the Russia and in the world [1,2]
Scenario of metasomatic interaction of metamorphic fluid with serpentinites simulated the formation of the first type gold-ore mineralization in schistose serpentinites
At T below 550 ◦ C, olivine–enstatite rock transforms into serpentinites with a small amount of talc and magnetite, and below 400 ◦ C, carbonate appears in serpentinite and the content of talc increases (Figure 7A)
Summary
The Urals is one of the most important gold provinces in the Russia and in the world [1,2]. By the end of the 20th century, the Urals was one of the five largest regions in the world in gold mining. About 500 gold deposits of various scales were known [3]. The main proportion of gold is extracted as an accessory component from volcanogenic massive sulfide deposits localized in the Tagilo–Magnitogorskian zone (Figure 1A). These deposits are associated with the upper mantle. The largest gold-ore deposits (Kochkarskoe, Berezovskoe, Svetlinskoe, etc.)
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