METALLOGENY
in Modern Basins

Much of what we recover today from orebodies must have been concentrated in ancient basins similar to the present day ones. An understanding of metallogeny in modern basins is therefore important in order to decipher the geological setting and details of ore forming processes.

Deposition in Basins having Ventilated Waters: In restricted basins where the thickness of outflow is less than the depth of the threshold, a two-way flow is established in the portal. This permits outflow of fresh water at the surface and inflow of salt water below. In these basins light gray to greenish gray sediments are being deposited, which are apparently devoid of any significant metallic content. Examples of such modern basins are the fjords on the coast of British Columbia, Canada and Norway.

Deposition in Basins Having Euxinic Conditions: In these basins, the thickness of outflowing water equals the depth of threshold so that only one-way flow is possible and no sea water can enter the basin. The level of pycnocline (i.e. a zone in a stratified body of water in which density varies rapidly with depth) lies deeper than the threshold. No oxygen is thus supplied and euxinic conditions are established promoting anaerobic bacteria which generate H2S. Such conditions are conducive for the acumulation of black silt and clays containing 25-35% organic matter (in oxidizing conditions organic matter is 1-2.5%), iron
sulphides (pyrite and hydrotroilite) and manganese oxides. Such basins are potential oilfields of the future. Examples - Dead Sea, parts of Baltic SEa, Kaoe Bay (on the coast of Holmahera, Indonesia) and some Norwegian fjords.

Deposition in Basins with occassional Ventilation: When the supply of fresh water to a basin having euxinic conditions becomes less (eg during winter or drought), the outward flow becomes less than required to cover the entire portal. Large amounts of sea water thus flow into the basin at such times. The dense sea water flows along the bottom of the basin displacing upwards the lighter basin water from the lower levels containing large amounts of H2S. This results in large scale mortality of organisms in ventilated waters above the pycnocline, resulting in deposition of sediments rich in organic matter. These constitute potential source rocks for hydrocarbons. Example -periodic influxes of aerated saltwater from the Mediterranean into the Black Sea.

Deposition in basins losing water by evaporation: Some enclosed basins, particularly in hot and dry climatic zones lose a lot of water by evaporation. If such a basin is connected to the sea and the depth of its threshold is great enough, a two way flow is established (in at the surface, out at depth), leading to the deposition of evaporites along the margins of the basin. Examples - the Mediterranean Sea and the Red Sea.
 

METALLOGENY ALONG OCEANIC RIDGES AND RISES:

There is a considerable concentration of a variety of metals in brines and sediments along the oceanic ridges, some of which are of economic potential. All examples are of great importance since they establish a relationship between sea-floor spreading and ore genesis.

Geothermal Brines: The metalliferous brines and muds associated with spreading centres are enriched in metallic sulphides viz., sphalerite with subordinate amounts of pyrite, galena, chalcopyrite, and marcasite; iron minerals such as goethite, hematite and siderite; and manganese minerals like rhodochrosite. The Salton Sea (California) brines are noted for their high metal content , in particular Cu, Ag, Au, Fe, Mn, B, Zn and Pb, and it may be envisaged that there is a syngenetic deposition of Pb-Zn minerals in progress along the oceanic ridges.

Sediments near Ridges: Sediments presently being deposited on the flanks of ridges, particularly the East-Pacific Rise, contain large quantities of Fe, Mn, Pb, Zn, Ni, Cu and Ba. Other elements in which these sediments are enriched are U, V, Ag, Sn, and Ti. The colse relationship of these sediments with the volcanics of the rift zones indicates that the metals are being precipitated from hydrothermal exhalations of volcanic origin.

Ridge Basalts: Dredged basalts from the oceanic ridges have been found to contain copper-bearing sulphides as spherules in vesicles and in veins.

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