Gossan

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gossan

[′gas·ən]
(geology)
A rusty, ferruginous deposit filling the upper regions of mineral veins and overlying a sulfide deposit; formed by oxidation of pyrites. Also known as capping; gozzan; iron hat.

Gossan

 

(iron hat), a residual formation that occurs in the surface parts of ore (primary sulfide) deposits of copper, lead, zinc, and other metals as a result of chemical weathering and oxidation of the primary minerals of the ore body. Gossans consist primarily of iron oxides and hydrous iron oxides (goethite, hydrogoethite, hydrohematite, turgite), and as a result they are usually dark or light red, ocherous, or brownish red in color. The formation of gossans is associated with the oxidizing action of surface waters and is accompanied by the secondary enrichment of ore deposits. In the process of weathering, the sulfides of copper, silver, zinc, and other metals oxidize to easily soluble sulfates, after which they are leached and carried to deeper parts of the ore body. At the same time the iron in sulfurous compounds—pyrite, chalcopyrite, and other sulfides—is only partially removed in the form of a soluble salt (sulfate of iron oxide); most of the iron is oxidized and hydrated and remains in place in the form of hydrates of iron oxide (brown iron ores). These secondary brown iron ores, occurring as a result of the transformation of original pyritic and other ores containing iron sulfides, are what form the gossans near the earth’s surface. The depth at which gossans are found beneath the earth’s surface is usually restricted to the groundwater level and may extend to dozens or even hundreds of meters. In comparison with initial sulfide ores gossans are richer in iron in their upper parts and in gold in their lower parts. Contrasting strongly with the enclosing rocks, gossans serve as an important indicator in explorations for sulfide-ore deposits and in identifying the primary ores concealed in the depths.

REFERENCES

Smirnov, S. S. Zona okisleniia sul’fidnykh mestorozhdenii, 3rd ed. Moscow-Leningrad, 1955.
Saukov, A. A. Geokhimiia, 3rd ed. Moscow, 1966.

G. A. SOKOLOV

References in periodicals archive ?
The Iron Cap Zone is a large area of well-exposed gossanous, intensely and pervasisve quartz-sericite-pyrite-altered intrusive and volcanic rock at the northeast corner of the Seabridge claim block.
Last year's drill program added a total of 431 million tonnes of inferred resources at Deep Kerr and the Iron Cap Lower Zone, collectively containing an estimated 5.
Drill grades are running higher than the average reserve grade for the Iron Cap zone found above.
The size of mineralized zones at Iron Cap, Mitchell and Main Copper (discussed below) were expanded into areas of recent glacier retreat which were not accessible by Placer Dome.
Drilling continues on the Deep Kerr expansion, as well as on building an initial resource for this year's discovery at the Iron Cap Lower Zone and testing additional higher grade core zone targets.
The discovery, which began to emerge in the 2013 drill program, is called the Iron Cap Lower Zone.
Drilling below the Iron Cap deposit in 2013 obtained promising results, particularly IC-13-49 which returned 207 meters of 1.
The second objective is to test several more identified and highly prospective core zone targets which have the potential for large, high grade deposits similar to Deep Kerr, including the Iron Cap deposit at depth.
TORONTO, April 9, 2014 /PRNewswire/ - Seabridge Gold reported today that this year's 19,000 meter core drill program at its 100%-owned KSM Project in Northwestern British Columbia, Canada, will focus on expanding last year's Deep Kerr core zone discovery and following up on the strong evidence supporting a potential second core zone discovery at Iron Cap.
Development on Descubridora I resulted in exposure of a strongly oxidized iron cap, developed above high grade material, extending to surface over approximately 7 metres.
The new exposure is interpreted to represent a potential iron cap to a high grade copper mineralized vein, similar to that documented on the Descubridora I and Veta Gruesa Centre occurrences.
diameter cast iron caps at the top of each shaft were removed.