How do Hydrothermal ore deposits form?

Question

How do Hydrothermal ore deposits form?

Answer 1

Magmatic and Magma-Hydrothermal Ore Deposits

The origins of platinum group element (PGE) enriched horizons in mafic layered intrusions are of great interest because such types of mineralization are the main resources of PGE. Bird et al. [1991] described a Au-Pd bearing horizon (Platinova reef) in the Middle zone of the Skaergaard intrusion of east Greenland. Based on textures, the Au appeared to have been trapped at a late magmatic stage as immiscible metal droplets within rims on cumulate silicates. They argued that three distinct fluids must have coexisted at the time of formation of the reef: silicate, sulfide and gold-rich. Boudreau and McCallum [1992] reviewed evidence for PGE enrichments in the reefs of the Stillwater layered mafic intrusion in Montana. They proposed a model in which a Cl-rich fluid phase exsolved from the intercumulus (interstitial to crystals) liquid and leached PGE from sulfide inclusions in cumulate phases, transporting both PGE and S upward and re-depositing these elements in mineralization fronts analogous to those in rollfront uranium deposits. These two studies are not contradictory; instead they show that multiple processes within crystallizing mafic magmas can influence the ultimate distribution of PGE observed within layered intrusions.

Base metal skarn (coarse calc-silicate) and porphyry deposits typically develop around crystallizing granitic plutons that have been emplaced at moderate to shallow crustal depths above subduction zones beneath their coeval volcanic arcs. A comprehensive review and bibliography on the geology of Au-bearing skarns was provided by Theodore et al. [1991]; they noted that most were calcic exoskarns (developed in wall-rock) associated with intense retrograde hydrosilicate alteration. Newberry et al. [1991] expanded and reinterpreted mineralogic, geochemical and isotopic data from the classic Darwin Pb-Zn-Ag skarn deposit in California, showing it to consist of several concentrically zoned sulfide skarn pipes whose ores precipitated in response to large shifts in fluid temperature, pH and oxidation state. Moreover, contrary to earlier studies, they showed that this deposit was genetically unrelated to the nearby, but older, Darwin pluton. Dilles and Einaudi [1992] described the geology and geochemistry of an exposed 5 km vertical section of hydrothermal alteration and mineralization associated with Ann-Mason porphyry copper deposit, one of three such deposits related to the Yerrington batholith in Nevada. From this unique section they were able to reconstruct the flow-paths and thermochemical evolution of hydrothermal fluids which formed the deposit. They identified a d**e swarm emanating from a deep granitic cupola as being responsible for the mineralization, and also identified argillic alteration in an adjacent mountain range as representing the paleosurface environment of the deep hydrothermal system.

Olympic Dam type deposits may represent the most significant new type of ore deposit whose geology became well-documented during the review period. As a follow-up to their 1990 paper on the Olympic Dam deposit in South Australia, Oreskes and Einaudi [1992] reported fluid inclusion and stable isotopic data from the unusual Fe-rich breccias and Cu-U-Au-Ag ores. They argued that primary magmatic fluids probably deposited early magnetite, but that the mineralized hematitic breccias were formed from the influx of cooler fluids having a more surficial origin. As noted above, Proterozoic Olympic Dam type deposits also occur in granite-rhyolite terranes of the U.S. midcontinent, as described by Nuelle et al. [1992] and Sidder et al. [1993]. These USGS authors concluded that saline magma-hydrothermal fluids derived from Fe-rich trachytes had initially emplaced Fe-silica ore and then subsequently boiled and explosively emplaced rare earth element

[4] (REE) bearing breccias into rhyolitic tuffs within a shallow eroded caldera complex.

Stable isotopes can be used to ascertain the degree to which magmatic volatiles contributed directly to volcanic-hosted ore deposits. Vennemann et al. [1993] used stable isotopic data to infer a direct role for condensed magmatic fluids in genesis of the Pueblo Viejo acid sulfate Au-Ag deposit (Dominican Republic), the world's largest bulk mineable deposit of this type. The late Cretaceous deposit was formed at shallow crustal depth within a maar-diatreme setting. Although metals and fluids were derived largely from magmatic vapors, it may be that shallow mixing with and cooling by convectively circulating meteoric or seawaters caused precipitation of the ores and acid alteration assemblages. The Pueblo Viejo hydrothermal system may have been similar to modern magma-hydrothermal systems such as White Island, New Zealand.

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U.S. National Report to IUGG, 1991-1994