from The American Heritage® Dictionary of the English Language, 4th Edition
- n. An exposed, oxidized portion of a mineral vein, especially a rust-colored outcrop of iron ore.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. decomposed rock, usually reddish or ferruginous (owing to oxidized pyrites), forming the upper part of a metallic vein
from the GNU version of the Collaborative International Dictionary of English
- n. Decomposed rock, usually reddish or ferruginous (owing to oxidized pyrites), forming the upper part of a metallic vein. Called also iron hat.
from The Century Dictionary and Cyclopedia
- n. In mining, the ferruginous quartzose material which often forms a large part of the outcrop of a lode in which the metallic contents at depths exist chiefly in the form of sulphids, among which pyrites, a combination of sulphur and iron, is rarely wanting, and is often present in large quantity.
A few years since quite a fever of speculation raged in regard to copper mines, and in the pursuit of the mineral gossan, which is supposed to indicate the locality of veins of ores.
In others, again, are found considerable quantities of soft powdery iron oxide or "gossan," and compounds such as limonite, aluminous clay, etc., which, under the action of the crushing mill become finely divided and float off in water as "slimes," carrying with them atoms of gold, often microscopically small.
Australia reef gold is almost invariably associated with iron, either as oxide, as "gossan;" or ferruginous calcite, "limonite;" or granular silica, conglomerated by iron, the "ironstone" which forms the capping or outcrop of many of our reefs, and which is often rich in gold.
What is left is either iron oxide, "gossan," or the oxides of the other metals.
Under weathering conditions pyrite oxidizes, the sulphur forming sulphuric acid, -- an important agent in the secondary enrichment of copper and other sulphides, -- and the iron forming the minerals hematite and limonite in the shape of a "gossan" or "iron-cap."
Wilkinson found that when the solution of gold chloride was as strong as, say, four grains to the ounce of water, that the pyrites or other base began to decompose, and the iron sulphide changed to yellow oxide, the "gossan" of our lodes, and that though the gold was deposited, this occurred in an irregular way, and it was coated with a dark brown powdery film something like the "black gold," often found in drifts containing much ferruginous matter.
I have tried a further adaptation of this process when treating ores containing a large percentage of iron oxide, where the bulk of the gold is impalpably fine, and contained in the "gossan."
The gossan does not often carry much value, though it may show traces of minerals which suggest what may be found below.
The gossan is likely to resist erosion and to be conspicuous at the surface, -- though this depends largely on the relative resistance of the wall rocks, and on whether the gangue is a hard material like quartz, or some material which weathers more rapidly like limestone or igneous rock.
In the extreme eastern portion of Ashe county the gossan ores (brown hematite) of the Ore Knob copper lode, bear mention.