Diamond, rutile and zircon placers have formed over a thousand kilometers from their sources. Transport distance is increased by high-energy flow in constricted fluvial channels. An understanding of how placers form, combined with the study of gold particle morphology and composition, assists in the search for primary deposits.
Shibboleth Sign In. OpenAthens Sign In. Institutional Sign In. From the first issue in onward, Economic Geology has been the main publication for those who study mineral deposits; indeed, it is now difficult to imagine economic geology without Economic Geology. It is interesting to ask, therefore, Who were the farsighted people who founded the journal, and Why did they think a specialized publication devoted to mineral deposits was needed?
Let us first address the question, Who were the founders? They were the 12 men who collectivelydecided a new publication was needed, who then planned the financial structure to support the venture, and who served as the original editorial group.
All were employed by, or associated with, the U. Geological Survey. Josiah Edward Spurr suggested the need for a journal sometime in November or December After informal discussions, nine of the founders met in the office of Waldemar Lindgren in the headquarters of the U.
Geological Survey in Washington, D. Eight of the men at the founding meeting formed the first board of directors; Spurr was president, Frederick L. Ransome, secretary, and George O. Smith, treasurer. Other members were Arthur H.
Brooks, Marius R. Campbell, Walter H. Theninth man at the meeting was H. Foster Bain. Irving was appointed editor. Lindgren, Ransome, and Campbell from the U. Geological Survey, together with three academics, James F. Leith of the University of Wisconsin, were appointed associate editors.
The initial board members, the editor, and associate editors are the people we now recognize as the founders of Economic Geology. Two others, Frank D. The source of the mechanical energy required for the sifting includes streams or rivers, ocean waves, wind or glaciers. The placer deposit may form very near the bedrock from which it originated, or it could be at a great distance.
The deposit may either be buried or exposed, or it could even be fossilized, if it was formed in the geologic past. Stream placers usually originate at a mineral-containing exposure of bedrock on a hillside overlooking a river valley. The bedrock disintegrates by weathering, which initially forms placers adjacent to the bedrock called residual placers; these may creep further downhill and get trapped in a ravine part way down the hill to form eluvial placers.
Finally, the placers reach the stream at the bottom of the valley to form stream placers, also known as alluvial placers; these are the most important type of placers. In arid regions where there is no water, placers known as eolian placers form by wind action.
Material broken from the bedrock disintegrates and the wind blows away the lighter rock matrix, leaving behind the placer containing the mineral s of economic value. Shore currents and wave action concentrate placers along beaches, from ore-bearing material that has either fallen from nearby cliffs or brought in by streams discharging into the ocean.
The sifting and concentration by gravity is possible only when the mineral is heavier than the surrounding geologic medium, resilient and hard to break down as well as chemically inert in its current environment.
Diamonds and gold satisfy these requirements and are the best-known placer minerals; others include monazite, platinum and ilmenite and lesser gems such as garnets. Thorium -- a possible alternate to uranium for fueling nuclear reactors -- occurs in monazite, whereas tin is produced from ilmenite. This is a preview of subscription content, log in to check access. Cronan, D. Handbook of Marine Mineral Deposits. Google Scholar.
Komar, P. Physical processes of waves and currents and the formation of marine placers. Reviews in Aquatic Sciences , 1 : — Roy, P. Heavy mineral beach placers in southeastern Australia. Their nature and genesis. Economic Geology , 94 : —
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