Ore Minerals under the Microscope

THE student of geology in the course of his training usually passes through three stages of achievement: A first in which he learns to identify the materials with which he works; a second in which he learns to recognize the distribution of these materials in space; and a third in which, from this distribution, he interprets the geologic history of the area of the assemblage of materials. Paraphrasing this generalization into words the student himself might use, he eVentualy must answer these three questions: "What is it?," "Where is it?," and "How did it get that way?" Certainly not all geologic study can be compressed into this simple formula, but the generalization fits several branches of applied geology. In particular, it adapts itself to the study of polished sections of ore minerals as a means of interpreting processes of ore deposition. Though a student may have progressed through the stage of identifying minerals and rocks in hand specimens and with the aid of a microscope, though he may have acquiJ;ed proficiency in field mapping, and though he may have acquired some skill in the interpretation of the space relations, he is back at the beginning, in so far as the study of ore deposits is concerned, when he sees his first set of polished sections, for he has to learn' to identify the minerals all over again. This change in appearance of even the most familiar minerals when seen on the polished section results from the deO'ree of polish which is needed in the study of opaque'" minerals. Because most of the ore minerals are essentially opaque in any thickness that can be conveniently handled, it is impossible to study them in the familiar thin section used for;,th,e study of rock specimens. As a result, therefpre,a::;,mooth surface of the material instead of a thln~seb:ion is prepared, and all the technique available is utilized to make that surface a true plane devoid of relief. The treatment of the ore sample consists in cutting the specimen and grinding and polishing the cut surface. The grinding and polishing processes call for experimentation with abrasives and for many refinements of method. Accom-