Formation of volcanogenic massive sulfide deposits: The Kuroko perspective

The main objective of this paper is to identify the geochemical, hydrological, igneous and tectonic processes that led to the variations in the physical (size, geometry) and chemical (mineralogy, metal ratios and zoning) characteristics of volcanogenic massive sulfide deposits with respect to space (from a scale of mining district size area to a global scale) and time (from a < 10 000 year time scale to a geologic time scale).All volcanogenic massive sulfide deposits (VMSDs) appear to have formed in extensional tectonic settings, such as at mid ocean spreading centers, backarc spreading centers, and intracontinental rifts (and failed rifts). All VMSDs appear to have formed in submarine depressions by seawater that became ore-forming fluids through interactions with the heated upper crustal rocks. Submarine depressions, especially those created by submarine caldera formation and/or by large-scale tectonic activities (e.g., rifting), become most favorable sites for the formation of large VMSDs because of hydrological, physical and chemical reasons.The fundamental processes leading to the formation of VMSDs include the following six processes:

1. (1) Intrusion of a heat source (typically a 10³ km size pluton) into an oceanic crust or a submarine continental crust causes deep convective circulation of seawater around the pluton. The radius of a circulation cell is typically 5 km. The temperature of fluids that discharge on the seafloor increases with time from the ambient temperature to a typical maximum of 350°C, and then decreases gradually to the ambient temperatures in a time scale of 100 to 10 000 years. The majority of sulfide and sulfate mineralization occurs during the waxing stage of hydrothermal activity.

2. (2) Reactions between low temperature (T < 150°C) country rocks with downward percolating seawater cause to precipitate seawater SO²⁻₄ as disseminated gypsum and anhydrite in the country rocks.

3. (3) Reactions of the “modified” seawater with higher-temperature rocks at depths during the waxing stage cause the transformation of the “seawater” to metal- and H₂S-rich ore-forming fluids. The metals and sulfide sulfur are leached from the county rocks; the previously formed gypsum and anhydrite are reduced by Fe²⁺-bearing minerals and organic matter, providing additional H₂S. The mass of high temperature rocks that provide the metals and reduced sulfur is typically 10¹¹ tons ( 40 km³ in volume). The roles of magmatic fluids or gases are minor in most massive sulfide systems, except for SO₂ to produce acid-type alteration in some systems.

4. (4) Reactions between the ore-forming fluids and cooler rocks in the discharge zone cause alteration of rocks and precipitation of some ore minerals in the stockwork ores.

5. (5) Mixing of the ore-forming fluids with local seawater within unconsolidated sediments and/or on the seafloor causes precipitation of “primitive ores” with the black ore mineralogy (sphalerite + galena + pyrite + barite + anhydrite).

6. (6) Reactions between the “primitive ores” with later and hotter hydrothermal fluids cause transformation of “primitive ores” to “matured ores” that are enriched in chalcopyrite and pyrite.

Variations in the mineralogical and elemental characteristics, the geometry, and the size of submarine hydrothermal deposits are controlled by the following four parameters:

1. (A) The chemical and physical characteristics of seawater (composition, temperature, density), which depend largely on the geographical settings (e.g., equatorial evaporating basins),

2. (B) The chemical and physical characteristics of the plumbing system (lithology, fractures),

3. (C) The thermal structure of the plumbing system, which is determined largely by the ambient geothermal gradient, and the size and temperature of the intrusive, and

4. (D) The physical characteristics of the seafloor (depth, basin topography).

For example, the submarine hydrothermal deposits developed in basaltic plumbing systems are generally poor in Pb and Ba compared to those developed in felsic plumbing systems. The lower temperature systems are generally poorer in sulfides, but richer in iron oxides and sulfates. The higher temperature and larger hydrothermal systems tend to produce chalcopyrite and pyrite rich ores. Contrasts in the metal ratios between the Noranda-type Archean VMSDs and the younger VMSDs reflect the differences in the geothermal gradient of the plumbing systems. The submarine hydrothermal deposits developed in the near equatorial regions tend to form large continuous bedded type ores because of the likeliness of creating large stratified basins.The basic processes of submarine hydrothermal mineralization have remained essentially the same throughout the geologic history, from at least 3.5 billion year ago to the present.