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The Pamphlet Collection of Sir Robert Stout: Volume 75

The Precipitation of Gold in Marine Sediments

The Precipitation of Gold in Marine Sediments.

Since gold exists in sea-water, it seems reasonable to believe that it is precipitated at the present time by natural reducing page 53 agents. Those writers who trace the metallic contents of lodes to metals dissolved in sea-water assume, indeed, that such a precipitation is constantly going on; but experimental proof of this assumption is lacking. I have attempted to investigate the question in two ways:
1.By the examination of coast-sediments, now being deposited under conditions favorable to the reduction of gold from the sea-water.
2.By the introduction into sea-water of reducing-agents such as naturally occur along the coasts at the present time, and a subsequent examination for precipitated gold.

Examination of Coast-Sediments—To secure trustworthy results, the whole of the drainage-basin to the erosion of which; the sediments are due should consist of non-auriferous rocks, so that we may be sure that any gold detected did not come from the land. Otago Harbor, on the upper part of which Dunedin is located, satisfies this requirement. The Leith and other small streams entering the harbor flow wholly through basic Tertiary volcanic rocks which contain no gold. At the same time the shores are more or less covered with timber, so that organic matter is abundant in the sediments of the streams. The volcanic rocks contain much iron. Beds of hematite and limonite abound along the shores. The conditions for the reduction of gold from sea-water are therefore very favorable. Besides organic matter, there is sulphide of iron, produced by the action of the sulphates in sea-water upon iron salts, forming sulphate of iron, reducible to sulphide by organic matter. (As will be seen, some such action does in fact take place. In every ease the concentrates contained insoluble sulphides.)

Analyses were made of mud and silt from different parts of Otago harbor, where the circumstances seemed most favorable. In each case, from 1 to 2 hundredweight of the silt was carefully panned off, till a residue of about 1000 grains was left. This residue consisted chiefly of magnetite, augite and hornblende, derived from the volcanic rocks of the coast. The percentage of sulphur (insoluble sulphides) in the concentrate was determined upon a small portion, and the remainder was roasted (giving in every case a strong reaction of sulphur dioxide).* page 54 The roasted material was then assayed. The details of four assays are given in the following table:

* When the magnetite was removed by means of a magnet, the remainder did not show, under the microscope, recognizable crystals of pyrite; but the presence of pyrite was certainly proved by roasting, and by the sulphur-determination.