Magmatic fluids, both vapour and hypersaline liquid, are a primary source of many components in hydrothermal ore deposits formed in volcanic arcs. These components, including metals and their ligands, become concentrated in magmas in various ways from various sources, including subducted oceanic crust. Leaching of rocks also contributes components to the hydrothermal fluid—a process enhanced where acid magmatic vapours are absorbed by deeply circulating meteoric waters. Advances in understanding the hydrothermal systems that formed these ore deposits have come from the study of their active equivalents, represented at the surface by hot springs and volcanic fumaroles.

The role of magmas in the formation of hydrothermal ore deposits

Fluid-phase relationships and thermodynamic reaction modelling based on published mineral solubility data are used to re-assess the Cu–Au-mineralising fluid processes related to calc-alkaline magmatism. Fluid inclusion microanalyses of porphyry ore samples have shown that vapour-like fluids of low to intermediate salinity and density (~2–10 wt% NaCl eq.; ~0.1–0.3 g cm−3) can carry percentage-level concentrations of copper and several ppm gold at high temperature and pressure. In epithermal deposits, aqueous fluids of similar low to intermediate salinity but liquid-like density are ubiquitous and commonly show a magmatic isotope signature. This paper explores the physical evolution of low-salinity to medium-salinity magmatic fluids of variable density, en route from their magmatic source through the porphyry regime to the near-surface epithermal environment, and investigates the chemical conditions required for effective transport of gold and other components from the magmatic to the epithermal domain. Multicomponent reaction modelling guided by observations of alteration zonation and vein overprinting relationships predicts that epithermal gold deposits are formed most efficiently by a specific succession of processes during the evolution of a gradually cooling magmatic–hydrothermal system. (1) The low-salinity to medium-salinity fluid, after separating from the magma and possibly condensing out some hypersaline liquid in the high-temperature porphyry environment, must physically separate from the denser and more viscous liquid, and then cool within the single-phase fluid stability field. By cooling under adequate confining pressure, such a vapour will evolve above the critical curve and contract, without any heterogeneous phase change, to an aqueous liquid of the same salinity. (2) High concentrations of gold, transported as stable Au bisulphide complexes supporting >1 ppm Au even at 200°C, can be maintained throughout cooling, provided that the fluid initially carries an excess of H2S over Cu+Fe on a molal scale. This condition is favoured by an initially high sulphide content in a particularly low-salinity magmatic fluid, or by preferential partitioning of sulphur into a low-salinity vapour and partial removal of Fe into a hypersaline liquid at high temperature. (3) Acid neutralisation further optimises gold transport by maximising the concentration of the HS− ligand. This may occur by feldspar destructive alteration along pyrite±chalcopyrite±sulphate veins, in the transition zone between the porphyry and epithermal environments. An alternative acid/base control is the dissolution of calcite in sediments, which may enable long-distance gold transport to Carlin-type deposits, because of the positive feedback between acid neutralisation and permeability generation. The three physical and chemical transport requirements for high-grade epithermal gold mineralisation are suggested to be the common link of epithermal gold deposits to underlying magmatic–hydrothermal systems, including porphyry-Cu–Au deposits. Both mineralisation types are the result of gradual retraction of isotherms around cooling hydrous plutons in similar tectonic and hydrologic environments. As magmatic fluid is generated at increasing depths below the surface the importance of vapour contraction increases, leading to the typical overprinting of potassic, phyllic and advanced argillic alteration and their related ore styles.

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The physical and chemical evolution of low-salinity magmatic fluids at the porphyry to epithermal transition: a thermodynamic study

The Rodalquilar caldera complex is located in the western part of the Cabo de Gata volcanic field in southeastern Spain and is the first documented example of epithermal gold-alunite mineralization within a caldera in Europe. The Rodalquilar caldera is an oval collapse structure having a maximum diameter of 8 km and formed at 11 Ma from eruption of the Cinto ash-flow tuff. The oval Lomilla caldera, with a diameter of 2 km, is nested within the central resurgent dome of the older Rodalquilar caldera. The Lomilla caldera resulted from the eruption of the Lazaras ash-flow tuff which was ponded within the moat of the Rodalquilar caldera. The last phase of volcanic activity in the caldera complex was the emplacement of hornblende andesite flows and intru- sions. This magmatic event resulted in structural doming of the caldera, opening of fractures and faults, and pro- vided the heat source for the large hydrothermal systems which deposited quartz-alunite type gold deposits and base metal vein systems. The gold-alunite deposits are enclosed in areas of intense acid sulfate alteration and localized in ring and radial faults and fractures present in the east wall of the Lomilla caldera. Like other acid-sul- fate type deposits, the Rodalquilar gold-alunite deposits are closely related in time and space to porphyritic, inter- mediate composition magma emplaced along caldera structures but unrelated to the caldera forming magmatic system.

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Mineralized and unmineralized calderas in Spain; Part II, evolution of the Rodalquilar caldera complex and associated gold-alunite deposits

A geochemical study of stream sediments near Rodalquilar (South-east Spain) was undertaken to investigate the impact of processing activities associated with nearby gold mining. Despite the semi-arid nature of the area, visual inspection of tailings indicates that considerable fluvial erosion occurs during rainfall events. Geochemical analysis demonstrates that Mo, Sb, Te, Bi and As are all found at elevated levels in stream sediments downstream of the tailings site. Elevated levels of Pb, Cu and Zn were also noted in stream sediments. Statistical analysis demonstrates that four factors control trace-element distribution. Factor 1 displays a strong association of Mo, Sb, Te and Bi and As and is attributed to the mine tailings. Factor 2 displays an association of Pb and Cu, and is most easily attributed to anthropogenic pollution. Factor 3 distinguishes Au and reflects the selective nature of ore-processing methods employed at the site. Factor 4 only highlights Zn, reflecting that it is derived from several, probably unrelated, sources.

The impact of unconfined mine tailings and anthropogenic pollution on a semi-arid environment – an initial study of the Rodalquilar mining district, south east Spain

The Neogene volcanic region of Cabo de Gata, Almeria, SE Spain, is dotted with many outcrops of bentonite, some of them of significant economic interest. The bentonites have their origin in the hydrothermal alteration of pyroclastic rocks (15-7 Ma). The deposits are usually associated with fractures. The major mineral is a dioctahedral Fe- and Mg-smectite (89-75%) and this is accompanied by minor amounts of feldspars, quartz, amphiboles, pyroxenes, biotite, zeolites, disordered tridymite, calcite, etc. This paper describes the geological background, the general characteristics of the bentonites and major aspects of their formation, e.g. type of low-temperature hydrothermal solutions, mass balance, chemical evolutions of the smectites and geochemistry of trace elements. Finally, the characteristics of three of the most important deposits are described.

Bentonites from Cabo de Gata, Almería, Spain : a mineralogical and geochemical overview

At Rodalquilar gold mineralization is found in Late Tertiary volcanic rocks of the Sierra del Cabo de Gata and is related to a caldera collapse. Radial and concentric faults were preferred sites for gold deposition. Hydrothermal activity produced a specific alteration zoning around gold-bearing vein structures, grading from an innermost advanced argillic via an argillic into a more regionally developed propylitic zone. Advanced argillic alteration with silica, pyrophyllite, alunite, and kaolinite extends down to several hundred m indicating a hypogene origin. High-grade gold mineralization in vein structures is confined to the near-surface part of the advanced argillic alteration. Fine-grained gold is associated with hematite, jarosite, limonite, or silica. At a depth of about 120 m, the oxidic ore assemblage grades into sulfide mineralization with pyrite and minor chalcopyrite, covellite, bornite, enargite, and tennantite. Two types of fluids from different sources were involved in the hydrothermal system. Overpressured and hypersaline fluids of presumably magmatic origin initiated the hydrothermal system. Subsequent hydrothermal processes were characterized by the influx of low-salinity solutions of probable marine origin and by interactions between both fluids. Deep-reaching, advanced argillic alteration formed from high-salinity fluids with 20–30 equiv. wt% NaCl at about 225°C. Near-surface gold precipitation and silification are related to fluids with temperatures of about 175°C and 3–4 equiv. wt% NaCl. Gold was transported as Au(HS)
 2
 −
 , and precipitation resulted from boiling with a concomitant decrease in temperature, pressure, and pH and an increase in fO2. All features of the Rodalquilar gold deposit reveal a close relationship to acid-sulfate-type epithermal gold mineralization.

Fluid evolution, wallrock alteration, and ore mineralization associated with the Rodalquilar epithermal gold-deposit in southeast Spain

The operating Rodalquilar gold deposit and the abandoned Triunfo and Maria Josefa gold mines are located within the Sierra del Cabo de Gata volcanic field some 40 km east of Almeria in SE Spain. While the gold mineralization at Rodalquilar is mainly controlled by caldera-tectonics, vein structures at Triunfo and Maria Josefa are not. Wall-rock alteration at Triunfo and Maria Josefa is characterized by argillic alteration (illite/sericite, kaolinite). The alteration zonation around the gold-mineralized vein structures at Rodalquilar ranges from advanced argillic alteration (porous quartz, alunite, pyrophyllite, dickite) over argillic alteration into a regionally developed propylitization. Fluid inclusion studies from all three mines indicate that gold was deposited from low-salinity fluids (2–5 wt.% NaCl equivalent) between 170° and 250 °C. However, the hydrothermal system at Rodalquilar was fed by a second fluid source. High-salinity, halite and/or sylvite-bearing, liquid-rich, and vapour-dominated, CO2-bearing fluid inclusions are assumed to be of magmatic origin. High sulfidation ore mineral assemblages at depth (covellite, enargite, tennantite) and part of the advanced argillic alteration can be related to these fluids. Thus, part of those features which attribute the Rodalquilar gold deposit to the acid-sulfate or high sulfidation type of epithermal gold deposits, stem from magmatically derived fluids which are typical for a porphyry environment, whereas gold mineralization at all three localities is associated with low-salinity fluids, probably of marine origin.

P. Sänger-von Oepen

Papers

Fluid evolution, wallrock alteration, and ore mineralization associated with the Rodalquilar epithermal gold-deposit in southeast Spain

Comparison between the fluid characteristics of the Rodalquilar and two neighbouring epithermal gold deposits in Spain