Saturday, November 28, 2015

Six Fluids in Ore Forming Magmatic Hydrothermal Systems

Magmatic hydrothermal systems are the Earths most vigorous environments of endogenic heat and mass transfer, and arguably the most efficient settings for hydrothermal ore formation. The generation of giant anomalies with extreme trace element enrichment is due to chemical interaction and physical separation of up to six mobile fluid phases with vastly different transport properties: (1) slab fluid, (2) silicate melt, (3) sulfide melt, (4) dense liquid brine, (5) low density gas like vapor and (6) aqueous solution.

Ore forming magmatic hydrothermal system
Magmatic hydrothermal system illustration (img source : geology.gsapubs dotcom).

The aim of this post is to discuss some key questions regarding the consecutive steps in the generation, the physical separation, and the chemical evolution/interaction of these fluids with each other and with minerals. Focus is on calcalkaline magmatic systems associated with convergent plate margins.

Generation of Calcalkaline Melts
The generation of significant quantities of calcalkaline melt requires addition of H2O from subducted lithosphere to hot asthenospheric mantle in the overlying wedge. The main unknown is the physical state and chemical composition of the fluid ascending from the slab. Contributions of Cl and S to the melting wedge may determine subsequent steps towards hydrothermal ore formation. Partial melting in absence of a sulfide melt is probably critical for effective extraction of chalcophile and precious metals into silicate melt. Specific geodynamic requirements (such as upwelling of hot asthenosphere, e.g., due to slab break off) may be instrumental to the generation of sulfide undersaturated hydrous magmas.

Ascent of Magmas
Ascent of magmas through continental crust is associated with variable degrees of assimilation, as evident from isotopic data. Sulfur isotope variations along the American Cordillera suggest crustal assimilation of sulfate into porphyry related magmas. A tentative association of Au-rich porphyry deposits with calcalkaline magmas intruding continental basement may indicate an essential crustal contribution of gold by assimilation.

Magma Chamber Dynamics
Large (100-1000 km 3) upper crustal magma chambers are essential to source adequate quantities of metal and magmatic volatile phases for porphyry style ore deposits. Ongoing case studies of Sn-W and Cu-Au mineralized systems using microanalysis of fluid and melt inclusions demonstrate that the metal budget of these deposits is primarily controlled by the magmatic fluid source. In Sn-W systems, different Cl/F/H2O ratios in final melts control stepwise enrichment of Sn or W and the composition of the exolved metal rich brine. In contrast, Cu and Au are progressively depleted in ore forming magmatic systems through partitioning into a sulfide melt. Whether the generation of a metal rich volatile phase requires sulfide undersaturation in part of the magma chamber or destabilization of existing sulfides is unclear. Basic questions concern the process by which magmatic hydrothermal fluids extract metals from a large magma volume.

Separation of Magmatic Volatiles
Volatiles exsolving from a magma may be single phase or coexisting brine + vapor. Comparison of barren and Cu-mineralized porphyries in the southwestern USA suggests that coexistence of melt with brine + vapor is critical for ore formation, but data from other Cu-Au deposits indicate that salinity (and Cl/H2O ratio in the melt) may be more important than phase state itself.

Ore Mineral Deposition
While the bulk metal budget of deposits seems to be dominated by the magmatic fluid source, their distribution depends on ore mineral types. Cassiterite precipitation in Australian and Bolivian deposits has been clearly linked to mixing between magmatic brine and cooler meteoric water. Co-precipitation of Cu and Au at Alumbrera can be attributed to cooling of magmatic brine below a sulfide saturation temperature near 400 C. Indications for a deposit-scale zonation of Cu/Au at Bingham suggest a more complex process. In all deposits studied by LA-ICP-MS of fluid inclusions, boiling occurred before, during or after ore deposition but was not a dominant cause for mineral precipitation.

Magmatic Meteoric Interaction
Transfer of magmatic volatiles and metals to the near-surface is evident in volcanic eruptions and fumaroles, but processes explaining the common association of porphyry style and epithermal ore deposits are debated. Micro analysis shows that the characteristic element suite of high sulfidation epithermal ores is selectively enriched in the vapor phase of boiling fluids in porphyry type deposits. Condensation of such a vapor phase to an epithermal aqueous liquid is possible by near isobaric cooling. Thermodynamic modeling is limited by experimental data for low density fluids but indicates that vapor condensate with a molal excess of S2- over Fe2+ + Cu+ could transport very high concentrations of AuHS to epithermal temperatures, where gold saturation may occur by low pressure boiling or mixing. (Reference: C. A. Heinrich, in Eleventh Annual V. M. Goldschmidt Conference).

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