When magma intrudes to depths of less than a few kilometers it begins to interact with ground and meteoric water, heating and driving it back toward the surface. This water mixes with volatile phases (i.e. H2O, SO2, H2S, HCl, etc.) exsolving from the magma as it depressurizes and becomes hydrothermal fluid, which is able to efficiently dissolve and transport heavy metals as aqueous complexes. The fluid moves upward following fractures in the overlying rock body, while at the same time cooling and reacting with the exposed rock surfaces. Neutralization of acidic fluids, steam formation due to depressurization, and dilution of hydrothermal fluid by ground water are resulting in precipitation of heavy metals, the first to come out of solution is gold. Depending on the degree of the neutralization reactions, the fluid chemistry can range from strongly to moderately acidic. The figure above shows the pathways taken by this hydrothermal fluid and the pictures below, show some of the products of this complicated process.
The vent or crater of the volcano is where highly acidic steam and lava escapes. The crater-lake, located within the crater, is highly acidic due to interaction with gaseous magamatic emissions and evaporation induced concentration. There are occasionally concentrations of dissolved sulfur on the bottom of these crater lakes. As the hydrothermal fluid, containing magmatic emissions, migrates upward, it precipitates minerals and causes replacement within the country rock. This often creates a large area of hydrothermal alteration where the main rock type is porous silicastone. Close to the magma, at the center of the hydrothermal system copper and gold are precipitated.
Interaction between the fluid and the wall-rock causes neutralization and reduction within the hydrothermal fluid, limiting the extent of the alteration. Gold combines with sulfur within the hydrothermal fluid but remains dissolved as compounds such as, Au(HS)2. As the fluid cools or comes into contact with ground water, the hydrogen-sulfide escapes as steam and the fluid looses its ability to carry the gold. As the fluid cools, silica also becomes unstable and large amounts quartz is precipitated. This is why gold veins are predominantly quartz. Where the steam reaches the surface, neutral salt springs can occur and the exsolved hydrogen-sulfide and carbonate gases also fuel fumerolic activity.
Acidic Hydrothermal Systems
Gas Emission from Mount Augustine (1210m), Alaska, U.S.A.: The volcanic gas discharged from vents at the surface is over 600 degrees C and contains, along with water vapor, CO2, SO2, H2S, HCl, as well as the metal elements, zinc and copper. This gas is the result of exsolution of volitiles from the magma. The fact that there are heavy metals contained within the emissions, suggests that magmatic exsolution plays a role in the formation of ore deposits. Yet, at depth, most of the heavy metals are contained within the hydrothermal fluid, which is what is thought to be involved in the precipitation of these economic ore deposits, including epithermal gold. Mount Augustine is one of the most active volcanoes in Alaska. It has erupted six times in the last 200 years. After the last eruption, in 1986, a series of picture perfect gas emmisions occurred, expelling around 280 t/day of ash and gas at temperatures up to 870 C. These plumes contained, for every ton: 670 kg of water vapor, 47 kg of CO2, 160 kg of SO2, 15 kg of H2S, 100 kg of HCl, 4 g of iron, 0.2 g of copper, 1 g of zinc, 1 g of molybdenum, as well as minor concentrations of gold and other heavy metals. When gas composition was analyzed using a 1 m long silica glass tube, the result was the sublimation of various minerals as follows: from the high temperature end to the low temperature end, magnetite (Fe3O4), cristobalite (SiO2), molybdenite (MoS2), bornite (Cu5FeS4), halite (NaCl), pyrite (FeS2), and sphalerite (ZnS).
The Crater Lake of Mt. Maly Semiachik, Kamchatcka: When meteoric accumulates in craters of active volcanoes, it is common for the resulting lake to become extremely acidic. This is the result of interaction with volcanic gas, which bubbles up through the floor of the lake and contains sulfur-dioxide, hydrogen-sulfide, hydrochloric acid, and other acidic phases. These lakes are large reaction basins involving gas, water, steam, and rock, where unstable rock-forming minerals are quickly dissolved by the severely acidic environment. Within the lake water, sulfur is precipitated and settles to the bottom in a molten state. When gas bubbles up through this molten sulfur layer, hollow, spherulitic, sulfur bubbles are formed. Within the lower portion of these lakes, circulation of hot acidic water occurs. These and similar environments can be related to gold mineralization. Mt. Maly Semiachik (1560m asl.) is a basaltic stratovolcano and possesses a strongly acidic (pH<1) crater lake. The lake appears white due to entrained sulfuric silt and within every liter of acidic lake water there is 1.5 g of aluminum, 1 g of iron, 1.1 g of hydrochloric-acid, and 2 g of sulfate.
Gold-ore Rocks from Acidic Hydrothermal Systems: Gold deposits, which form within acidic hydrothermal systems, have certain identifying characteristics. First, they are located in areas where most of the elements which compose major rock forming minerals (Al, Ca, Mg, Na, K, etc.) have gone into solution; except for SiO2, which is concentrated into porous silicastone. Second, gold and copper sulfides are found precipitated within the pores of the silicastone. Third, the precipitation of the ore minerals occurs after that of the silicastone, when the pH of the hot acidic water has risen and acidity level has dropped. The rock in the photograph is a disseminated gold-ore rock, which was collected from Kasuga Mine in Kagoshima Prefecture. The irregular shaped pores in the rock are due to dissolution of pumice fragments, which existed within the protolith, an andesitic welded tuff breccia. Later the pores were lined with, chalcedony, black, columnar enargite, gold colored pyrite, blue covellite, along with sulfur. The gold is associated with enargite but due to the small size of the crystals they are difficult to distinguish with the naked eye. Within every ton of ore-rock of this type, there is between 2 and 10 grams of gold. The gold is not very concentrated but evenly dispersed throughout the massive silicified zone. For this reason, open-pit mining techniques are being applied.
Neutral Hydrothermal Systems
Neutral hydrothermal systems have highly variable surface expressions. Neutral pH sodium chloride springs discharge steam and gas, which results in sublimation of sulfur and occasional formation of geysers. This is also affected by the introduction of meteoric water into the hydrothermal system, which along with boiling, causes fluctuations in the salinity of the fluid. Mixing of sulfuric acid, created by oxidation of steam, into the fluid can cause the formation of a strongly acidic, sulfur spring.
When a spring erupts boiling water on to the surface at relatively regular intervals, it is referred to as a geyser (below). High temperature water from deep within the system expelled onto the surface, brings with it large quantities of dissolved SiO2. As the water and steam come into contact with the atmosphere they are instantly cooled, inducing precipitation of amorphous silica sinter. The conduits or fractures within the country rock also create a temperature gradient, where the walls of the conduit are cooler than the water traveling through it. This induces precipitation of silica as well; sometimes causing blockages resulting in high pressure steam and hot water explosions. The resulting crater then resumes spring activity, the explosion having cleared the water pathway (second from top). In some cases, devolitilization is not completed within the conduit and underground hydrothermal system, which enables minerals containing heavy metals to be precipitated on the surface (second from bottom).
Old faithful, in Yellowstone National Park in the United States was given its name because of the regularity of its impressive eruptions. For at least the last hundred years the geyser has erupted every 40-80 minutes, shooting a water column between 35 and 50 m into the air. The area surrounding the geyser is a cone of silica sinter, precipitated from the discharge.
A hydrothermal explosion crater, which has resumed spring activity. The crater, which is also located in Yellowstone National Park, U.S.A., is surrounded by white, silica sinter.
The hydrothermal precipitates from a spring within the caldera of Mt. Osore-zan, Aomori Prefecture. Neutral, chloride-rich high temperature waters reach the surface and mix with acidic surface water, precipitating a vivid yellow substance. The reaction between arsenic and hydrogen-sulfide accelerates as the fluid pH drops, producing arsenic-sulfides (i.e. As2S3). The precipitation of large quantities of arsenic-sulfides indicates that hydrogen-sulfide-rich hydrothermal fluid reached the surface and that gold deposits are most likely located somewhere within the system because gold is effectively dissolved within sulfide complexes in such water.
The brilliant white stripe within the dark colored rock is a gold bearing quartz vein, which represents a filled hydrothermal water pathway. The inverted cone shape of the vein indicates that it began to travel horizontally as it went up toward the top of the picture. The area where this is occurring marks the boundary (a) between the underlying, Mesozoic sedimentary country rocks and the overlying Quaternary volcaniclastic country rocks. Within the veins are white bands composed of quartz and adularia inter-layered with darker bands containing gold and sulfide minerals (b). The layers represent minerals, which were precipitating at the same time. Pure white, quartz veins (c), which cross-cut the gold veins containing black layers, suggest that there was continued hydrothermal activity along the newly formed fractures (d) after the precipitation of the ore. The Hishikari gold deposit was formed between 600 Ka and 1.25 Ma and stands out as the highest yield deposit, with respect to gold, in Japan containing over 250t of high grade (10-1000g/ton) ore.