Lode deposits

Gold veins in rocks are known as lode deposits. The principal vein is the mother lode , now a figure of speech as well as the name of one of the most famous deposits of the Californian Gold Rush. When these lodes form at relatively low temperatures and low pressures near the surface of the Earth s crust, they can become exceptionally rich in gold. Such deposits can be found in Colorado and Nevada, and became known in the nineteenth century by the Spanish word for prosperity bonanza, a term now redolent with the mythology of the Wild West. 

Tin is a relatively scarce element with an average abundance in the Earth s crust of about two parts per million (ppm) compared with 94 ppm for zinc, 63 ppm for copper, and 12 ppm for lead. Tin is produced from lode (hard-rock) deposits and placer deposits derived from the lodes. The tin mineral cassiterite (Sn02) is the source of most tin production. A notable exception is the complex tin sulfide minerals in the subvolcanic or tin-silver lode deposits in Bolivia. Cassiterite has a high specific gravity (6.8 to 7.1), a Moh s scale hardness of 6 to 7, and is usually a dark brown or black color with an adamantine luster. 

Giles, D.L. and Nelson, C.E. (1983) Principal features of epithermal lode gold deposits of the Circum-Pacific energy and minerals resource conference, Circum-Pacific series, AAPG. 

Coats, R. 1940. Propylitization and related types of alteration on the Comstock Lode. Economic Geology, 35, 1-16 Creasey, S. C. 1959. Some phase relations in hydrothermally altered rocks of porphyry copper deposits. Economic Geology, 54, 351-373. 

Kerrich, R. Hodder R.W. 1982. Archean Lode Gold and Base Metal Deposits Evidence for Metal Separation into Independent Hydrothermal Systems, Geology of Canadian Gold Deposits Canadian Institute of Mining and Metallurgy, 144-160. 

This lode gold is the stuff worked by legendary dwellers below the earth, like the dwarf who forged the treasure of the Nibelungs. Recall, however, how the Rhinegold was first found, like a placer deposit, at the bottom of a river. 

ENRICHMENT. I, Also secondary enrichment. The term applied by students of ore deposits to the natural processes by which the lower levels of an ore deposit are enriched at the expense of the upper levels, or the original protore. Particularly applied to lodes in which the sulfide ores have been concentrated by the leaching of the upper levels of the vein and redeposition below the groundwater table. Important ore minerals belonging to this type are chalcocite and argentite. 

Tin-mining methods depend on the character of the deposit. Primary deposits are embedded in undeiground granitic rock and recovery methods are complex. The more important secondary deposits are in the form of an alluvial mud in the stream beds and placers and the recovery is simpler than lode mining. Cassiterite is recovered from alluvial deposits by dredging, hydraulicking where a head of water permits it, jets and gravel pumps on level... 

The presence of pyrites in mineral veins is frequently betrayed by the brown, rusty appearance of the upper parts of the deposits. This is well exemplified in the case of certain copper lodes, the iron of the contained copper pyrites having become converted into hydrated ferric oxide or rust. Such weathered zones are known as gozzans, and frequently present a cavernous or honeycombed appearance, due to the removal of pyritic material. In France the gozzan is termed the chapeau de fer, and in Germany der eiserne Hut, and an old adage states that4... 

These deposits were found to deflagrate on heating, to e lode on impact, and to ignite when brought in contact with coned sulfuric, nitric or mixed nitric-sulfuric acids. These substances seemed to be similar to those found during WWI Ref A. Kovache H. Thibon, MP 34, 369-78 0952)... 

Lode gold deposits Carlin, Nevada Fluid convection of meteoric water deep in the crust Leaching of source rocks, transport and deposition in fractured rocks due to declining temperature... 

Guthrie, G. M., and lusher, C. M., 1989, Geologic setting of lode gold deposits in the northern Piedmont and Brevard Zone, Alabama U.S. Geological Survey Bulletin 136. 

---