Contents
- Introduction
- Gold Needs a Chemical Trap
- Iron Sulfides and Invisible Gold
- Pyrite, Arsenopyrite, and Gold Capture
- Iron Oxides and Weathered Gold Systems
- Iron-Rich Host Rocks and Wall-Rock Reaction
- Banded Iron Formations and Gold Deposits
- The Great Lakes Iron Ranges Clarification
- What Prospectors Should Look For
- Conclusion
- Citation
The Full Gold Deposits Category
https://bigrivergold.com/category/gold-deposits/
1. Introduction
Iron-rich rocks can influence gold deposition because iron is one of the most reactive elements encountered by gold-bearing hydrothermal fluids. Gold does not usually move through bedrock as visible flakes or nuggets. In most lode systems, it travels dissolved in hot water as chemical complexes, commonly attached to sulfur or chloride. The gold is deposited only when the fluid loses its ability to keep gold dissolved. Iron-rich rocks can help cause that change. Rocks containing pyrite, pyrrhotite, magnetite, hematite, siderite, iron-rich carbonates, mafic minerals, or banded iron formation can react with mineralizing fluids and create chemical traps. Those traps may produce visible native gold, but more often they produce microscopic or “invisible” gold locked in sulfide minerals. This is why iron should not be treated only as a background color or stain in gold geology. In many deposits, iron-bearing minerals are part of the mechanism that makes gold stop moving, especially where faults, quartz veins, carbonate alteration, sulfides, and reducing wall rocks occur together. [1][2][3]
2. Gold Needs a Chemical Trap
A gold-bearing fluid can move through a large volume of rock without making an ore deposit. For gold to accumulate, the fluid must encounter a trap that changes temperature, pressure, acidity, oxidation state, sulfur activity, or wall-rock chemistry. Iron-rich rocks are important because they can change several of those conditions at once. If a hydrothermal fluid carries gold as a sulfur complex, reactive iron in the wall rock may combine with sulfur and form iron sulfides. When sulfide minerals form, the chemistry of the fluid changes, and gold may be removed from solution. This process is one reason sulfidation is so important in many gold deposits. Iron-rich minerals can also buffer oxygen conditions, consume sulfur, alter pH, or provide mineral surfaces where gold can adsorb or nucleate. The trap is not simply “iron equals gold.” The better rule is that iron-rich rock becomes important when it is crossed by the right gold-bearing fluid in the right structural setting. A massive iron formation with no mineralizing fluid may contain no gold. A smaller iron-rich shear zone cut by quartz veins and sulfides may be much more important. [1][2][4]
3. Iron Sulfides and Invisible Gold
Iron sulfides are among the most important minerals in gold deposition. Pyrite, pyrrhotite, arsenopyrite, and marcasite can all occur in gold-bearing systems, but pyrite and arsenopyrite are especially important because they can contain gold that is too small to see. In Carlin-type deposits, gold is commonly microscopic or invisible and associated with arsenic-rich pyrite and arsenopyrite. In orogenic gold systems, sulfides may make up only a small percentage of a vein or altered rock, but they can still control much of the gold grade. Gold may occur as tiny inclusions, nanoparticles, lattice-bound gold, or native gold grains along sulfide boundaries and fractures. This matters because a rock can look unimpressive and still carry gold if the gold is locked in fine sulfides. It also means that rusty rock at the surface may be the weathered remains of a sulfide-bearing zone below. The visible iron oxide is not the original ore mineral in many cases. It is often the oxidized product of pyrite, pyrrhotite, or arsenopyrite that once formed under reducing hydrothermal conditions. [2][3][5]
4. Pyrite, Arsenopyrite, and Gold Capture
Pyrite and arsenopyrite can influence gold deposition in two ways. First, their formation can force gold out of the fluid by changing sulfur chemistry. Second, once formed, they can physically and chemically host gold. Arsenian pyrite is especially important in sediment-hosted gold systems because gold can be incorporated into the pyrite structure or occur as extremely small particles within it. Arsenopyrite is also a common gold-associated mineral in many metamorphic and vein systems. In practical field terms, this means that gold may not be where the quartz looks cleanest. It may be in altered wall rock, sulfide-rich selvages, graphitic shear zones, iron-carbonate alteration zones, or fine pyritic replacement zones next to the quartz. The presence of pyrite alone does not prove gold, because pyrite is common in many barren rocks. But pyrite with arsenic, deformation, quartz-carbonate veining, carbonate alteration, iron staining, and gold pathfinder elements is more meaningful. Iron sulfides matter because they are not merely associated minerals. In many systems, they are part of the gold deposition process. [3][5][6]
5. Iron Oxides and Weathered Gold Systems
Iron oxides such as hematite, goethite, limonite, and magnetite can also matter, but they must be interpreted differently from iron sulfides. Hematite and magnetite may be primary minerals in some iron-rich rocks, including banded iron formations and certain altered mafic rocks. Goethite and limonite, however, are commonly weathering products formed when iron sulfides oxidize near the surface. This is why rusty gossans, iron-stained quartz, red-brown fractures, and yellow-brown clay can attract prospectors. They may mark the surface expression of a sulfide-bearing zone. However, iron staining by itself is not proof of gold. Many rocks rust because they contain ordinary iron minerals with no gold at all. The useful question is what the iron oxide is replacing. If the rust is replacing pyrite, pyrrhotite, or arsenopyrite in a shear zone, quartz vein, breccia, carbonate replacement body, or altered mafic rock, it becomes more interesting. If it is only general staining from weathered basalt, shale, or ironstone with no structure or sulfides, it is less meaningful. Iron oxides are best treated as surface clues pointing back to earlier iron-rich minerals and hydrothermal reactions. [1][2][6]
6. Iron-Rich Host Rocks and Wall-Rock Reaction
Iron-rich host rocks can influence gold deposition through wall-rock reaction. When a gold-bearing fluid enters a rock rich in iron-bearing minerals, the fluid and rock exchange chemicals. Mafic volcanic rocks, iron-rich sediments, iron carbonate rocks, chloritic schists, magnetite-bearing rocks, and banded iron formations can all react with hydrothermal fluids. These reactions may form pyrite, pyrrhotite, ankerite, siderite, chlorite, sericite, quartz, or carbonate alteration. In orogenic gold deposits, quartz-carbonate veins commonly occur with carbonate alteration, and the surrounding rock can be just as important as the vein itself. Iron-rich wall rocks can help consume sulfur, form sulfides, and localize gold along vein margins. This is one reason the best gold is not always in the center of a milky quartz vein. It may occur along the contact between quartz and altered wall rock, in sulfide bands, in iron-carbonate alteration, or in sheared rock next to the vein. The field lesson is simple: iron-rich rock becomes important when it reacts with mineralizing fluid, not merely because it contains iron. Structure, fluid flow, and alteration are still required. [1][2][4]
7. Banded Iron Formations and Gold Deposits
Banded iron formations can be associated with gold in some major deposits because they contain iron-rich layers that react strongly with hydrothermal fluids. Banded iron formations are layered chemical sedimentary rocks commonly made of iron minerals and silica-rich layers such as chert or jasper. In some gold districts, deformation and metamorphism create folds, faults, and shear zones through these iron-rich layers. Later hydrothermal fluids may move through the same structures and react with the iron formation. The Homestake deposit in South Dakota is a well-known example of an iron-formation-hosted gold deposit. Its gold is associated with the Homestake Formation, an Early Proterozoic iron-rich unit containing iron carbonate and iron silicate, later deformed and metamorphosed. This kind of deposit shows why iron-rich rocks can matter: the iron formation provides reactive chemistry, while folding and shearing provide the plumbing. However, this does not mean every banded iron formation contains gold. Most do not. The important combination is iron-rich rock plus deformation plus hydrothermal fluid plus sulfide formation plus gold-bearing chemistry. Without those added controls, iron formation may remain only an iron formation. [7][8]
8. The Great Lakes Iron Ranges Clarification
This does not mean that the huge Great Lakes iron region should be described as a giant gold province just because it contains iron-rich rocks. The Lake Superior iron ranges, including the Mesabi, Vermilion, Gunflint, Cuyuna, Gogebic, Marquette, and Menominee ranges, are famous mainly for iron ore, taconite, cherty iron formation, hematite, magnetite, and related Precambrian iron deposits. These are not simply gold deposits waiting to be mined. There was a small Lake Vermilion gold rush in Minnesota after prospectors found small specks of gold in quartz in the 1860s, but the gold effort was not profitable and the more important long-term discovery was the iron. Gold does occur locally in the broader Precambrian shield region around Lake Superior, especially in greenstone belts, quartz veins, shear zones, sulfide-bearing structures, and Canadian gold camps such as Hemlo. But the giant commercial iron ranges themselves are primarily iron systems. The correct statement is that iron-rich rocks can influence gold deposition where they are cut by the right gold-bearing fluids and structures. It is not correct to imply that every large iron range, or the Great Lakes iron region as a whole, should automatically be treated as a major gold deposit. [8][9][10]
9. What Prospectors Should Look For
Prospectors should look for iron-rich rocks in context. The strongest targets are not just red rocks, black magnetite layers, or rusty quartz. The stronger targets are iron-rich rocks that are also sheared, folded, veined, brecciated, silicified, sulfidized, carbonatized, or associated with known gold districts. Useful signs include pyrite, arsenopyrite, pyrrhotite, iron carbonate, quartz-carbonate veins, magnetite destruction, hematite flooding, jasperoid, graphitic shear zones, and pathfinder elements such as arsenic, antimony, mercury, bismuth, tellurium, or tungsten, depending on deposit type. In oxidized ground, rusty gossan may mark old sulfides, but it should be followed by sampling and geological mapping rather than assumption. A clean banded iron formation with no veining or sulfides may be less interesting than a narrow, ugly, rusty shear zone cutting iron-rich rock. The best question is always: what caused fluid to move here, and what caused gold to drop out? Iron-rich rocks can answer the second question only when the first question is also answered by faults, fractures, folds, contacts, permeability, or replacement zones. [1][2][6]
10. Conclusion
Iron-rich rocks can influence gold deposition because they react with gold-bearing fluids and help create chemical traps. Iron sulfides such as pyrite, pyrrhotite, and arsenopyrite can form during mineralization, change the sulfur chemistry of the fluid, and host microscopic or invisible gold. Iron oxides can mark the weathered surface expression of earlier sulfide zones, although rust alone is never proof of gold. Iron-rich wall rocks can react with hydrothermal fluids and localize gold along vein margins, shear zones, carbonate alteration zones, and sulfide-rich replacements. Banded iron formations can host important gold deposits where deformation and fluid flow create the right plumbing, but most iron formations are not gold ore. The Great Lakes iron ranges are the best caution. They are immense iron systems with some local gold history and nearby regional gold belts, but they are not automatically gold deposits because they contain iron. The accurate rule is this: iron-rich rocks can help precipitate gold when they intersect gold-bearing fluids, sulfur chemistry, alteration, and structure. Iron matters most when it is part of an active hydrothermal trap, not when it is merely present as a rock color or iron ore body. [1][2][7][8]
Related Reading
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] U.S. Geological Survey. “Low-Sulfide Quartz Gold Deposit Model.” USGS Open-File Report 03-077.
[2] Goldfarb, R. J., Groves, D. I., and Gardoll, S. “Orogenic Gold and Geologic Time: A Global Synthesis.” Ore Geology Reviews, 2001.
[3] Reich, M., Kesler, S. E., Utsunomiya, S., Palenik, C. S., Chryssoulis, S. L., and Ewing, R. C. “Solubility of Gold in Arsenian Pyrite.” Geochimica et Cosmochimica Acta, 2005.
[4] Phillips, G. N., and Powell, R. “Formation of Gold Deposits: A Metamorphic Devolatilization Model.” Journal of Metamorphic Geology, 2010.
[5] Cline, J. S., Hofstra, A. H., Muntean, J. L., Tosdal, R. M., and Hickey, K. A. “Carlin-Type Gold Deposits in Nevada: Critical Geologic Characteristics and Viable Models.” Economic Geology 100th Anniversary Volume, 2005.
[6] Groves, D. I., Goldfarb, R. J., Gebre-Mariam, M., Hagemann, S. G., and Robert, F. “Orogenic Gold Deposits: A Proposed Classification in the Context of Their Crustal Distribution and Relationship to Other Gold Deposit Types.” Ore Geology Reviews, 1998.
[7] Caddey, S. W., Bachman, R. L., Campbell, T. J., Reid, R. R., and Otto, R. P. “The Homestake Gold Mine, an Early Proterozoic Iron-Formation-Hosted Gold Deposit, Lawrence County, South Dakota.” In Geology and Resources of Gold in the United States, U.S. Geological Survey Bulletin 1857, 1991.
[8] Ojakangas, R. W. “Minnesota’s Geology.” University of Minnesota Press.
[9] Minnesota Department of Natural Resources. “Minnesota Mining History.”
[10] Muir, T. L., Schnieders, B. R., and Smyk, M. C. “Geology and Gold Deposits of the Hemlo Area.” Geological Association of Canada, 1995.