Contents
- Introduction
- What a Volcanogenic Massive Sulfide Deposit Is
- Why Gold Is Usually a Byproduct in VMS Deposits
- How Seafloor Hydrothermal Fluids Move Metals
- Massive Sulfide Lenses, Stringer Zones, and Gold
- Gold, Silver, Copper, Zinc, and Lead Zoning
- Why VMS Gold Can Be Missed by Prospectors
- Examples of VMS Deposits With Gold Value
- What Prospectors Should Look For
- Conclusion
- Citations
1. Introduction
Volcanogenic massive sulfide deposits, usually shortened to VMS deposits, are ancient seafloor hydrothermal systems that formed when hot metal-bearing fluids vented into seawater or moved just below the seafloor and deposited large masses of sulfide minerals. They are best known as copper, zinc, lead, and silver deposits, but many also contain gold. In most VMS deposits, gold is not the main metal. It is usually a byproduct or co-product recovered while mining and processing the copper, zinc, lead, and silver ore. That distinction matters. A VMS deposit can contain valuable gold without looking like a normal gold vein, placer deposit, or quartz lode. The gold may be fine grained, locked in sulfide minerals, associated with pyrite, chalcopyrite, sphalerite, galena, tetrahedrite-tennantite, or enriched in certain parts of the massive sulfide body. For prospectors, VMS gold is important because it changes what a gold target can look like. Instead of chasing yellow metal in quartz, the target may be dark, heavy, rusty, sulfide-rich volcanic rock with copper, zinc, lead, silver, and gold together. [1][2][3]
2. What a Volcanogenic Massive Sulfide Deposit Is
A volcanogenic massive sulfide deposit is a sulfide ore body formed in close association with submarine volcanic or volcano-sedimentary rocks. The word “volcanogenic” means the system is related to volcanic activity, and “massive sulfide” means the ore contains a high proportion of sulfide minerals. These deposits commonly formed on or below the ancient seafloor where hydrothermal fluids discharged through vents similar in general concept to modern black smoker systems. Hot fluids leached metals from volcanic rocks, mixed with seawater, cooled rapidly, and precipitated sulfides. The main minerals are commonly pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena, with variable barite, silica, carbonate, magnetite, tetrahedrite-tennantite, and precious metals. VMS deposits can occur in Archean greenstone belts, Proterozoic volcanic belts, Paleozoic island arcs, back-arc basins, and younger volcanic terranes. The important point is that VMS deposits are not placer deposits and not ordinary gold-quartz veins. They are volcanic-hydrothermal sulfide systems where gold may be present as part of a larger base-metal ore package. [1][2][4]
3. Why Gold Is Usually a Byproduct in VMS Deposits
Gold is usually a byproduct in VMS deposits because the dominant ore minerals are base-metal sulfides, not native gold. A VMS mine is commonly developed for copper, zinc, lead, silver, or a combination of those metals. Gold may add important value, but it is often recovered during the same processing stream rather than mined as a separate gold ore body. This happens because VMS systems can concentrate several metals from the same hydrothermal fluid system. Copper may be strongest near hotter feeder zones, zinc and lead may be more abundant outward or upward, and gold and silver may concentrate in specific zones depending on temperature, sulfur activity, fluid mixing, oxidation state, and mineral assemblage. In some deposits, gold is enriched in the upper or cooler parts of the system. In others, it may occur with copper-rich stringer zones or precious-metal-rich lenses. Gold may be native, electrum, telluride-related, or finely included in sulfides. Because it is commonly fine and mineral-bound, gold in VMS systems may not be obvious in hand specimen. Its value is often revealed by assay rather than visible metal. [1][3][5]
4. How Seafloor Hydrothermal Fluids Move Metals
VMS systems begin when seawater circulates downward through fractured volcanic rock, is heated by magma or hot intrusive rocks, reacts with the volcanic pile, and rises back toward the seafloor carrying dissolved metals and sulfur. The heated fluid can leach copper, zinc, lead, iron, silver, gold, and other elements from the volcanic and sedimentary rocks it passes through. When that hot fluid reaches cooler seawater or mixes with pore waters beneath the seafloor, the chemistry changes abruptly. Sulfide minerals precipitate because metals combine with sulfur. This creates black smoker chimneys, sulfide mounds, replacement zones, stockworks, and eventually massive sulfide lenses preserved in ancient rocks. Gold behavior depends on the chemistry of the hydrothermal fluid and the precipitation environment. It may be transported with sulfur or chloride complexes and then deposited when the fluid cools, mixes, oxidizes, reduces, or forms sulfide minerals. The gold is therefore part of the same hydrothermal plumbing system that formed the base-metal ore. It is not added later by stream concentration unless the VMS body later erodes and contributes gold to placers. [1][2][4]
5. Massive Sulfide Lenses, Stringer Zones, and Gold
A typical VMS deposit may include a massive sulfide lens above a feeder or stringer zone. The massive sulfide lens is the main stratiform or mound-like sulfide body, commonly rich in pyrite, sphalerite, chalcopyrite, galena, or barite depending on the deposit type. The stringer zone lies beneath or beside the massive lens and consists of veins and veinlets formed as hydrothermal fluids moved upward through fractured footwall rock. This stringer zone can be copper-rich because it is closer to the hotter feeder system. Gold can occur in either part, but its distribution is not always simple. Some VMS deposits show precious-metal enrichment in upper, cooler, or more distal portions, while others have gold linked to copper-rich zones, barite-rich zones, or late overprinting fluids. This is why one sample from a VMS body may not represent the whole deposit. A rusty gossan cap may show gold and silver enrichment after sulfides weathered away, while deeper primary ore may be dominated by base metals. Understanding the vertical and lateral zoning is essential before judging gold potential. [1][2][5]
6. Gold, Silver, Copper, Zinc, and Lead Zoning
Metal zoning is one of the most important features of many VMS deposits. The hottest feeder areas commonly favor copper-bearing minerals such as chalcopyrite, while cooler or more distal zones may contain more sphalerite, galena, barite, silver, and sometimes gold. This pattern is not perfect, but it is useful. VMS systems are chemical gradients frozen into rock. Temperature, acidity, sulfur, oxygen, salinity, and mixing conditions changed from the feeder zone to the seafloor mound and outward into sediments or volcanic layers. Those changes controlled which minerals formed. Gold and silver can be enriched in certain zones because precious metals respond strongly to boiling, cooling, mixing, sulfide precipitation, and late fluid overprints. In some deposits, gold may be associated with pyrite-rich ore. In others, it may occur with copper-rich chalcopyrite, zinc-lead-silver zones, tetrahedrite-tennantite, barite, or tellurium-bearing phases. This is why VMS gold should not be evaluated only by looking for visible yellow metal. The better approach is to understand the whole metal suite. Gold in VMS deposits often travels with a base-metal and silver story. [1][3][5]
7. Why VMS Gold Can Be Missed by Prospectors
VMS gold can be missed because the rock may not look like a familiar gold target. Many prospectors are trained by experience to look for quartz veins, free gold, placer flakes, iron-stained vein quartz, or bedrock cracks in streams. A VMS target may instead appear as dark sulfide-rich rock, rusty gossan, altered volcanic rock, chert, barite, pyritic shale, greenstone, or heavy black to brassy material. The gold may be microscopic or locked in sulfides, so panning crushed material may not always show obvious free gold. Weathering can make the situation even more confusing. Near the surface, pyrite, chalcopyrite, sphalerite, and galena may oxidize, leaving iron oxides, boxwork textures, silica, clay, copper stains, or a gossan cap. Some gossans can be enriched in gold and silver after base metals are leached, but many rusty gossans are barren or weak. The only reliable way to know is to combine geology with assays. A VMS prospect should be judged by sulfide textures, volcanic setting, alteration, copper-zinc-lead-silver geochemistry, barite or chert association, and gold assays, not by visible gold alone. [1][2][6]
8. Examples of VMS Deposits With Gold Value
Several well-known VMS districts show that gold can be economically meaningful even when the deposit type is not primarily a classic gold vein. The Greens Creek deposit in southeast Alaska is a major polymetallic massive sulfide deposit with silver, zinc, lead, and gold value. The Bathurst Mining Camp in New Brunswick contains VMS deposits, and the Murray Brook deposit was historically mined in its gossan zone for gold and silver before later attention turned to base-metal potential. The Flin Flon and Noranda districts in Canada, the Iberian Pyrite Belt in Spain and Portugal, and the Kuroko deposits of Japan also show the global importance of VMS systems as base-metal and precious-metal sources. These examples should be used carefully. They do not mean every pyrite-rich volcanic rock is gold-bearing. They show that gold can be part of the economic package in VMS systems and may be recovered as a byproduct from ores mined mainly for other metals. For exploration, that means gold assays should not be ignored in copper-zinc-lead sulfide systems, especially where silver, barite, alteration, and gossan development are also present. [1][4][6]
9. What Prospectors Should Look For
A prospector looking for VMS-related gold should first look for the right geological setting. Favorable ground includes volcanic or volcano-sedimentary belts, especially greenstone belts, submarine volcanic sequences, felsic volcanic rocks, mafic volcanic rocks, chert, argillite, barite horizons, exhalite layers, and known copper-zinc-lead-silver occurrences. Field clues include massive or semi-massive pyrite, chalcopyrite, sphalerite, galena, rusty gossan, boxwork textures, quartz-chlorite-sericite alteration, silicified horizons, barite, jasper, chert, copper staining, and heavy sulfide float. Stream-sediment and soil geochemistry may show copper, zinc, lead, silver, barium, arsenic, antimony, gold, mercury, or other elements depending on the system. The practical target is not one random rusty rock. The stronger target is a mineralized volcanic horizon with sulfides, alteration, and a coherent metal pattern. If gold is the only element present, the target may not be VMS. If copper, zinc, lead, silver, barium, pyrite, volcanic rocks, and alteration occur with gold, the VMS possibility becomes more serious. Sampling must include both gossan and fresh sulfide material because near-surface gold enrichment can differ from primary ore. [1][2][5]
10. Conclusion
Volcanogenic massive sulfide deposits are ancient seafloor hydrothermal systems formed in volcanic and volcano-sedimentary environments. They are best known for copper, zinc, lead, and silver, but gold can be an important byproduct or co-product. The gold may occur in massive sulfide lenses, stringer zones, precious-metal-rich caps, copper-rich feeder zones, zinc-lead-silver zones, or weathered gossans. It is often fine grained or locked in sulfides, which means it may not appear as visible yellow metal. This makes VMS gold different from placer gold or simple quartz-vein gold. The ore system is recognized by volcanic setting, sulfide mineralogy, alteration, metal zoning, barite or chert association, and geochemical patterns. For prospectors, the main lesson is practical. Do not dismiss sulfide-rich volcanic rocks because they do not show visible gold, but do not assume every rusty pyrite zone is gold-bearing either. VMS gold becomes meaningful when gold occurs with the broader VMS signature: copper, zinc, lead, silver, pyrite, chalcopyrite, sphalerite, galena, barite, chert, alteration, gossan, and submarine volcanic host rocks. [1][2][3]
The Complete Guide to Gold Geology and Gold Deposit Types
https://bigrivergold.com/the-complete-guide-to-gold-geology-and-gold-deposit-types/
Why Gold Forms, Moves, and Concentrates
https://bigrivergold.com/why-gold-forms-moves-and-concentrates/
The Complete Guide to Gold Prospecting Clues: Minerals, Alteration, Veins, and Host Rocks
https://bigrivergold.com/the-complete-guide-to-gold-prospecting/
11. Citations
[1] Shanks, W. C. P., III, and Thurston, R., eds. Volcanogenic Massive Sulfide Occurrence Model. U.S. Geological Survey Scientific Investigations Report 2010-5070-C, 2012.
https://pubs.usgs.gov/sir/2010/5070/c/
[2] Franklin, J. M., Gibson, H. L., Jonasson, I. R., and Galley, A. G. Volcanogenic Massive Sulfide Deposits. Economic Geology 100th Anniversary Volume, 2005.
https://pubs.geoscienceworld.org/segweb/books/edited-volume/1223/chapter/107024661/Volcanogenic-Massive-Sulfide-Deposits
[3] Barrie, C. T., and Hannington, M. D., eds. Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings. Society of Economic Geologists Reviews in Economic Geology, Volume 8.
https://doi.org/10.5382/Rev.08
[4] Hannington, M. D., de Ronde, C. E. J., and Petersen, S. Sea-Floor Tectonics and Submarine Hydrothermal Systems. Economic Geology 100th Anniversary Volume, 2005.
https://pubs.geoscienceworld.org/segweb/books/edited-volume/1223/chapter/107024686/Sea-Floor-Tectonics-and-Submarine-Hydrothermal
[5] Galley, A. G., Hannington, M. D., and Jonasson, I. R. Volcanogenic Massive Sulphide Deposits. Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5.
https://www.researchgate.net/publication/285742834_Volcanogenic_massive_sulphide_deposits
[6] Taylor, C. D., Johnson, C. A., and Seal, R. R. Geology and Geochemistry of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Southeastern Alaska. U.S. Geological Survey Professional Paper 1763.
https://pubs.usgs.gov/pp/1763/