Economic Minerals Associated with Black Smokers

James Whittington

ES 767 Global Tectonics, Emporia State University, Spring 2006

Black Smoker

Venting


image acquired from:
http://www.gannon.edu/resource/dept/enviro/EnvGeo/projects2003/smokers/Black_Smokers.html.

Abstract

The discovery in the mid 1970s of seafloor hydrothermal vents associated with undersea tectonic boundaries led to a revolution in the interpretation of many metallogenic provinces. Many massive sulphide deposits were reinterpreted as fossilized hydrothermal vent systems. Research into the newly discovered vents, or ‘black smokers’ as they have come to be known, has expanded the relationship between tectonics and hydrothermal mineral deposits.

One of the more interesting features of black smokers is specific mineral facies they exhibit. The sequence of zoning and mineral assemblage found in undersea hydrothermal vents (a form of volcanogenic massive sulphide or VMS deposit, Guilbert and Park, 1986) is unique.

How do Black Smokers become Enriched in Valuable Minerals?

Black smokers are not so much water or mineral vents as heat vents. Heat generated from below by magma formed at plate boundaries is released to the sea through black smokers. Seawater, being cool, sinks through the substrate along the flanks of the vent system. As it sinks, it is heated and caused to circulate (via convection) towards the vent system. As the now heated water nears the magma chamber in begins to dissolve material from its surroundings. One of the common elements in this environment is sulfur. As sulfur is dissolved the pH of the water decreases allowing it to dissolve metal ions.

The metals most commonly dissolved are iron, copper, zinc, lead, and barium. In addition silver, gold, cobalt, nickel, arsenic, and a host of trace metals are also swept up in the hot metallic soup. The hot water, now pregnant with metal ions, begins to move upward towards the vent. Initially it travels through pore-space in the substrate, but as it gets closer to the vent it begins to flow in voids and fissures towards the primary vent pipe.

As the water rises, it cools. As it cools some of the material begins to precipitate within the substructure of the black smoker. The rate at which material precipitates is based on solubility and temperature. Copper and gold frequently fall out in the plumbing or stockwork beneath the chimney. As the fluid rises, much of the iron is precipitated at the base of the pipe as pyrite. The chimney itself is made of zinc and lead sulphides, as well as barite and anhydrite (sulphates). The ‘smoke’ from a black smoker is composed of tiny particles of sulphide minerals the flash precipitate when the scalding hot hydrothermal fluids hit the near freezing seawater. These particles cascade to the seafloor near the vent forming a layer of what is called exhalite. All of the metals discussed so far can appear in the exhalite.

A generalized depiction of the process by which a black smoker concentrates sulphide minerals. Hydrothermal Enrichment

Image by James Whittington

The mineral deposits left behind for geologist to examine often bear little resemblance to the vent area that remains after hydrothermal activity has ceased. Sediments cover and bury the deposit. The activity associated with plate movement can warp and twist the structure. Often the vent complex will undergo metamorphism during the collision and rending of the plates.

The deposit described below is a generalized snapshot of what a hydrothermal VMS would look like in pristine conditions. Even after the often torturous geologic treatment the complex may receive, the basic mineral assemblage helps geologist determine the black smoker VMS for what it is.

The deepest portion of the vent system is the stockwork. This appears as veining and brecciation with associated minerals such as chalcopyrite, quartz, chlorite, sericite (both chlorine minerals) and potentially gold. Moving upward is a zone of massive pyrite and chalcopyrite; this is the primary copper containing ore in deposits worked for copper. The upper portion of the ore body, sometimes called the black ore, is primarily composed of sphalerite, galena, and pyrite. (An active black smoker chimney dredged up by Australian researchers was composed almost entirely of sphalerite, or zinc sulphide). Most of the silver will be found here associated with galena.

Along the outer flanks of the deposit will be a zone containing many of the sulfate and carbonate minerals such as anhydrite and barite. This zone is dominated mostly by pyrite and cherts. The exhalite will be interbedded with the chert zone, and depending on currents may enrich an area considerably larger then the main vent structure.

These are only some of the more common minerals. Many other sulphides such as marcosite, arsenopyrite, and pyrrhotite are also found. Most of the copper minerals can occur either as primary or alteration products. Chlorine is commonly exhaled along with the sulfur and chlorite and sericite are commonly noted within the stockwork. When observed in the field oxides and hydrates of all the above minerals are also common (Guilbert 1986, Evans 1995, Paleozoic-Volcanic Hosted Massive Sulphide Deposits).

Schematic of a generalized Volcanogenic Massive Sulphide Deposit. VMS Deposit

Image by James Whittington

The tectonic setting that hosts these deposits often alters them significantly. Faulting and folding can rearrange the order and structure of the complex and thermal and physical metamorphism can change the mineralogical composition(Guilbert, 1986).

Boulder from a hydrothermal VMS deposit in Canada. Note the banding and quartz derived from low grade metamorphism. The sulphides are still quite obvious. Banded with sulphides


image acquired from:
http://www.barkerminerals.com/images/ace_boulders_3_big.jpg.

In order to determine the origin and mineralogy of hydrothermal sulphide deposits a detailed examination of the rock is made. Polished sections are studied microscopically and chemically for clues about the depositional setting. This information is used not only to determine the viability of a deposit, but also to assist in future exploration (SME, Volcanogenic massive sulphide deposits).

A polished slab of a hydrothermal VMS ore. Note the volume of metallic minerals.

VMS Ore


image acquired from: http://www.geo.ucalgary.ca/~tmenard/ores/snowlakeSeg.html.

Summary

There are thousands of known hydrothermal VMS deposits throughout the world (Evans 1995). Most of them are typically small ranging from a few thousand to a few million tons of ore. Because of the nature of the ore (sulphide) and the fact that most small deposits can only be worked economically from the surface, VMS mining often causes environmental degradation. Scientist are now trying to determine if active or recently dormant vent pipes can be harvested and ‘mined from the sea. This would pose minimal environmental risk and could be considered a renewable resource (some pipes grow as much as 3 feet per day). Currently, due the depth that black smokers appear in the sea this is not feasible, but time and escalating metal prices may open the door to a new way of looking at deep sea hydrothermal vents(Evans 1995, Hartmen 1992).


References

Palaeozoic Volcanic-hosted Massive Sulphide Deposits J. Bruce Gemmell, Ross R. Large & Khin Zaw, Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001, Australia. URL:
http://www.ga.gov.au/image_cache/GA5406.pdf.

How to Build a Black Smoker Chimney URL:
http://www.whoi.edu/oceanus/viewArticle.do?id=2400&archives=true.

SME, Volcanogenic massive sulphide deposits URL:
http://www.smenet.org/opaque-ore/ix_t_4.htm.

Evans, Anthony M., 1995; Introduction To mineral Exploration, Blackwell Science

Guilbert, John M. and Park, Charles F., 1986; The Geology of Ore Deposits, W H Freeman Company

Hartman, H.L. (editor), 1992; Mine Engineering Handbook, SME press


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