Ore deposits : a discussion re-published by Unknown

Author:Unknown
Language: eng
Format: epub
Tags: Ore deposits
Publisher: New York ; London : The Journal
Published: 1905-03-25T05:00:00+00:00

or vapor when the magma is erupted near the surface, it F. L. RANSOME, is a debatable question whether the magma is so saturated with these substances as to give them off in abundance under great pressure, and above the critical temperature. The fact that granite when heated to i,ooo° C. was found by Gautier to give off more than loo times its own volume of gases and vapors shows that under the conditions of original solidification these gases were retained in the rock.

Part of the argument in favor of a pneumatolytic origin for certain ore-deposits is drawn from the well-known effect of mineralizing agents in the artificial production of minerals by synthesis. The mineralizing agent in most cases, however, either does not enter into the composition of the mineral formed, or does so in very small amounts. An ore-body, on the other hand, implies a local concentration of materials elsewhere very sparingly present in the rocks. It cannot be explained as the result of crystallization of material already provided, in the presence of some mineralizing agent given off from a contiguous body of molten magma. It implies extensive transfers of material, and it is this considerable concentration and transfer which I find most difficult to ascribe entirely to pneumatolytic processes, particularly when it is remembered that one of the characteristics of ordinary contact metamorphism is a surprisingly slight change in the chemical composition of the metamorphosed rock. The case of the garnetized limestone of San Jose in Mexico, cited in a recent paper by Prof. Kemp, is not clearly an exception to this statement. His argument for the original purity of the limestone, and the consequent introduction of material to form the garnet, is based upon smelter records. But it is not unusual to find that the metamorphism of an impure limestone is a clarifying process, resulting in the formation of garnet and pure crystalline calcite. Moreover, smelters as a rule seek the purest limestone for fluxing purposes. At Cananea, for example, where extensive garnetization of the limestone has taken place, the limestone for smelting purposes is brought from a distance

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