Volcanic-Associated Massive Sulfide Deposits
Volcanic-associated massive sulfide deposits are, as a group, predominantly stratiform accumulations of sulfide minerals which formed on or near the sea floor by precipitation near the discharge site of hydrothermal fluids. The enclosing strata have prominent amounts of volcanic rocks, although in some cases the ores themselves may be enclosed in sedimentary rocks. The ores characteristically consist of more than 60 percent sulfide, most of which is pyrite and/or pyrrhotite plus variable amounts of sphalerite, chalcopyrite, or galena. The massive ore may be underlain by Cu-rich vein and disseminated sulfides (the stringer zone) and intensely altered rocks of the alteration pipe. The deposits are characterized by internal metal zoning, the most consistently described of which is a decrease upward and/or outward in Cu/(Cu+Zn+Pb) ratio. This zoning is particularly evident in those deposits which occur above their feeder pipes (e.g., Noranda deposits, Kuroko deposits). In other cases (e.g., Besshi, Kieslager, Bathurst, New Brunswick), the orebodies may be tabular with or without an underlying stringer zone. The massive ores may show evidence of reworking, and in some areas have been displaced, either partially or completely, from their discharge site.
On the basis of metal content, the deposits fall into two distinct groups, a Cu-Zn group and a Zn-Pb-Cu group. Many deposits in the Cu-Zn group, such as those in the Canadian Shield and, in part, those of the Scandinavian Caledonides, are contained in compositionally bimodal volcanic sequences; mafic volcanic rocks usually constitute at least 90 percent of the total volcanic complement, but felsic volcanic rocks are generally prominent very near the deposits. The ophiolite-associated Cu-Zn deposits, such as those in Cyprus, Turkey, Newfoundland, and Saudi Arabia, however, are usually at the contact between two pillowed mafic volcanic sequences. The Besshi, Kieslager, and some Fennoscandian Shield deposits are in areas composed of subequal amounts of mafic volcanic and clastic sedimentary rocks.
Deposits of the Zn-Pb-Cu group are in stratigraphic settings dominated either by felsic volcanic rocks, as in the Green Tuff belt of Japan and in Tasmania, or by subequal amounts of felsic volcanic and sedimentary strata, as at Bathurst, New Brunswick, and in the Iberian pyrite belt.
Oxide facies iron-formation, although not ubiquitous, is more commonly associated with the Zn-Pb-Cu deposits. Barite is abundant in some Zn-Pb-Cu deposits but is absent from a majority of the Cu-Zn deposits.
Although the two groups can be distinguished on the basis of their sulfur isotope compositions, this is not the case with lead isotopes. With one exception, the oxygen isotopic compositions of altered rocks that accompany volcanic-associated massive sulfide deposits are lighter than those of the unaltered equivalents. Strontium isotopic compositions of the altered rocks lie between those of primitive magmas and coeval seawater.
Alteration pipes under Zn-Pb-Cu deposits generally have a sericite + quartz core and a chloritic outer zone, whereas alteration pipes under Cu-Zn deposits typically have a chloritic core and sericitic outer zone. Large, semiconformable, regional scale alteration zones underlying many deposits in the Precambrian Shield of Canada and in ophiolite terranes may either enclose or lie below the alteration pipes occurring directly beneath individual deposits.
Although it is generally agreed that the volcanic-associated massive sulfide deposits were formed at or near the discharge sites of submarine hydrothermal systems, there are divergent views on other aspects of their genesis. The immediate source of the ore constituents may be either in the underlying rocks, contemporaneous magmas, or coeval seawater. The balance of the evidence and opinion seems to favor leaching of most of the metal and probably some of the sulfur, from the underlying rocks and derivation of the balance of the sulfur from coeval seawater.
The ore solutions were mobilized by either a convective hydrothermal cell or by a mechanism akin to seismic pumping. The former model postulates that the ore solution was mainly coeval seawater, whereas, in the latter model, the ore solution was mainly connate water which, in turn, originated mainly as trapped seawater. A minor contribution by magmatic or meteoric water is possible in both models.
The discharge sites of the ore fluids were focussed by faults or fractures, which are usually associated with local extensional tectonic activity. Precipitation of sulfides is due to cooling and oxidation brought about by mixing of the ore-forming solution with ambient seawater, or by boiling of the ore solution as it nears the sea floor.
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