Evaporite and ores in high temperature systems
Metalliferous fluid indicators and ore deposits due to direct and indirect interactions between magma and evaporites at a regional scale are neither well documented, nor well understood. Mostly, this is because little or no actual salt remains once these high temperature interactions have run their course. On top of that, some hard-rock geologists with a career working in igneous and metamorphic terranes may not be well versed in sedimentary evaporites or their meta-evaporitic siblings. The term pyrometasomatic encompasses some, but not all, of the types of salt-magma interaction discussed (Warren, 2016; Chapter 16).
Styles of evaporites interacting with magma to form various economic metal accumulations are divided into two; 1) orthomagmatic (salt-assimilative and internal to the magma), and 2) paramagmatic (salt-interactive and external to the magma). Both outcomes, which include a variety of substantial ore deposits, are typically inferred not from the presence of evaporite salts, but from chemistry and mineralogy of the reaction products. Only where igneous sills and dykes have intruded salt masses, with contacts preserved, can direct effects of magma-salt interaction be documented.
Orthomagmatic and paramagmatic associations are distinct from occurrences of primary igneous/magmatic anhydrites, which precipitate from sulphate-saturated melts, as in the anhydrite crystals crystallised in trachyandesitic pumices erupted from El Chichón Volcano in 1982, dacitic pumices erupted from Mount Pinatubo in 1991 and acidic lavas in the Yanacocha district of northern Peru]. They are also distinct from fumarolic anhydrite, which precipitates where groundwaters and sulphur-bearing magmatic fluids interact, as on Usu Volcano, Hokkaido, many central American and Andean volcanoes, including El Laco . Likewise, they are distinct from the anhydrite precipitates (white smokers) in and below submarine vents across numerous mid-oceanic ridges.
Fumarolic and vent (white-smoker) anhydrites can form significant anhydrite masses that in the subsurface can be confused with metasedimentary anhydrite. Primary magmatic anhydrites are interesting in terms of defining particular magma chemistries, but do not form anhydrite in significant volumes of CaSO4. The maximum amount of sulphur (S) that can be dissolved in silicate melt is controlled by saturation of the melt with an S-bearing phase (A). At relatively low oxygen fugacities, this phase is either an immiscible iron-rich sulfide liquid (with as much as 10% oxygen), which occurs in high-temperature basaltic melts, or crystalline pyrrhotite (Fe1xS) in intermediate (andesitic) and silicic (rhyolitic) melts. A Cu-Fe sulfide mineral known as “intermediate solid solution,” may also crystallize from andesitic to rhyolitic melts. At higher oxygen fugacities, anhydrite crystallizes from a wide range of melt compositions. Silicate melts at intermediate oxygen fugacities can crystallize both pyrrhotite and anhydrite, an assemblage that occurs in the trachyandesite tephra from the 1982 eruption of El Chicho´n in Mexico.
Orthomagmatic ore deposits, tethered to the assimilation of evaporite masses into a magma, form prior to complete solidification of the igneous melt. That is, potential ore-forming liquid magmas assimilate salts and then fractionate/metasomatise in situ into ore segregations, before solidus. In contrast, paramagmatic-hydrothermal ore deposits form external to the melt. They are produced by country-rock/basinal brine interactions with magmatically-derived or thermally-driven super- or sub-critical fluid circulation under a general grouping sometimes called hydrothermal alteration.
Paramagmatic deposits are related to solutions and gases segregated and generated by magmas containing more dissolved volatiles (H2O, CO2, S, B, F, Li, Cl etc.) than the proportion that can be accommodated by silicates during their crystallisation. These volatile-rich fluids tend to convectively circulate in the adjacent country-rock strata and can drive alteration haloes, as well as produce further heat-generated volatiles and mineral phases via heating and interacting with labile phases in the country rock. Mineralogies in the country rock are indicative of the chemistry of the interacting lithologies and the parent (including juvenile) fluids. Compared to ores tied to orthomagmatic deposits, this style of deposit is more subject to controversy in terms of genesis and the relative contributions of magmatic versus country-rock volatiles and/or basinal brines.
Worldwide, the majority of paramagmatic and orthomagmatic ore deposits are likely not tied to evaporite interactions; but we will focus on a few of those that are (Table). The single most important point common to both high temperature and low temperature ore accumulations associated with evaporites is that the presence of evaporites enhances ore volumes, is does not create them. Evaporites or associated brine presence tend to increasing metal transport efficiency and/or precipitative focusing, rather than directly spawning a metalliferous ore deposit (warren 2016; Chapter 16). The role of evaporites in ore accumulations is to create and enhance the metal carrier system and locally contribute reductants that can fix various metal sulphides and oxides. For example, such sulphate evaporite enhancement explains the large ore volumes in the orthomagmatic supergiant Noril’sk Ni-Cu deposit in Siberia and the paramagmatic supergiant Muruntau gold deposit of Uzbekistan.
To illustrate the high temperature igneous evaporite association we will look at Noril'sk, the world's largest Phanerozoic Ni deposit.
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