Brian J. Skinner

High Crystallization Temperatures Indicated for Igneous Rocks from Tranquillity Base

Complex intergrowths of troilite (FeS) and iron in the igneous rocks from Tranquillity Base contain 8.4 percent native iron by volume. The intergrowths were derived from an initially homogeneous sulfide liquid that separated immiscibly from the magma at 1140�C or above. Textures show that the sulfide liquid formed late in the crystallization and cooling history of the igneous rocks and after the major ilmenite and pyroxene had formed.

Evidence for movement of ore materials during high-grade metamorphism

Abstract The effects of regional metamorphism on mineral deposits, the massive ores in particular, have been the subject of renewed research in the last 25 years. Documentation of movement of ore materials and identification of the mechanisms of transport have proven easier for deposits metamorphosed at low and intermediate grades than for those metamorphosed at high grades. This paper focusses mainly on evidence for mobilization at high grades of metamorphism; genetic conclusions regarding the mechanisms of transport are offered somewhat more tentatively. Synmetamorphic brecciation, a consistent relationship of metal ratios to position in folds in deformed ores, and the occurrence of mineralization in tension gash veins and pressure shadows, indicate that ore materials can be redistributed within ore-masses. Mechanisms which may explain these observations are brittle behaviour in response to stress, ductile flow of sulfide minerals and pressure solution. Movement of material into or out of ore-masses is indicated by the gradients in fO2 and fS2 between ore and country rock which have been documented at a number of localities. Uranium can also be mobilized and transported over significant distances during high-grade metamorphism. The supporting evidence is, however, indirect in that high-grade rocks are consistently depleted in U relative to similar rocks metamorphosed to lower grades. The development and migration of a sulfide melt during metamorphism is a possible mechanism for moving materials into or out of ore-masses; however, there is no good evidence that this process occurs in nature. Finally, some ore-masses may have been displaced physically during high-grade metamorphism. There is some suggestion that massive sulfide and oxide ores in high-grade terranes occur preferentially in synformal structures. If this is the case, it may be due to a sinking process initiated by the large density contrast between massive ores and country rocks. A simple dynamic model for the Sterling Hill deposit, New Jersey is consistent with this hypothesis and gives a sinking rate of 102–104 cm Ma−.

Submarine Volcanic Exhalations That form Mineral Deposits; An Old Idea Now Proven Correct

Submarine volcanism, the circulation of seawater through rocks of the seafloor and the formation of mineral deposits on and in rocks on the seafloor are all processes that were realized during the 19th Century. The possibility that the three processes might be connected and that formation of many kinds of mineral deposit might result, is a realization of the 20th Century, with the principal ideas coming from Japan and Norway. Observations of modern ore-forming systems in operation in the Mediterranean, the Red Sea and the Pacific have proven the process but leave open the question of equivalence in tectonic settings between modern deposits and older ores in the geological record.

Acceptance of the Society of Economic Geologists Ralph W. Marsden Award for 2002

President Price, members of the Society, ladies and gentlemen: I remember the sad day in 1986 when Ralph Marsden died. It happened during the annual meeting of the Society in San Antonio and Ralph, a key figure in the affairs of the Society, was greatly missed. He was treasurer to the Society, the Foundation, and the Economic Geology Publishing Company. He was conservative, hard working, efficient, and modest. I am the first to admit that I am not cast in the same mold as Ralph, but I am greatly honored to have been selected to receive the medal and for it to bear the name of Ralph Marsden. It was …