The formation of the secondary silicate zone in contact-metamorphic ore deposits
Economic Geology and The Bulletin of the Society of Economic Geologists
Enrichment in this type of vein is caused by (x) the leaching of the calcite, which causes a removal of a valueless element, leaving a smaller mass of richer ore, and (2) l>y mechanical concentration of fine gold along channels caused by fracturing and by the removal .of the calcite by solution. F. T. EDDINGFIELD. THE FORMATION OF THE SECONDARY SILICATE ZONE IN CONTACT-METAMORPHIC ORE-DEPOSITS. Sir: That recryatallization of the impurities originally preseni in a limestone plays some part in
... ays some part in the formation of the secondary silicates of contact-metamorphic zones will hardly be denied even by those who believe that magmatic emanations have added their quota of silica, iron and alumina to aid in forming the new minerals. It has .been admitted by Bartell for the Marysville deposits x and by Knopf for those on the Seward Peninsula," even though in both cases it is shown .that magmatic additions are also important, and the present writer some years ago maintained' that careful study would probably show in most contact metamorphic ores two series of silicates--one due to recrystallization, and the other the result of magmatic additions? I believe, however, that the latter process is the predominating one, and that the facts do not bear out the contention of Mr. Uglow that "the balance of the evidence seems to be quite in favor of the origin of the secondary silicate zones by the elimination of the excess of CaCO, and the recrystallization of the impurities into lime silicates. "4 According to his definition Mr. Uglow includes in "secondary silicates" all silicates of the contact zones not containing fluorine, boron, or other mineralizers, and that means for the districts cited practically all of the contact minerals, for tourmaline, fluorine, etc., are notably scarce or entirely lacking.