Abstract

Deeply buried, high-grade oreshoots in carbonate-quartz veins are found in the Silver Belt of the Coeur d'Alene district by the application of various geologic criteria. Exploration is costly, and additional indications of the proximity of ore are highly desirable. The purpose of this study is to examine significant mineralogical features of the district in an attempt to develop additional helpful criteria.A search is made for indicator minerals; i.e., minerals, the spatial distribution of which may be indicative of the position of ore. In this respect, spatial distribution patterns of most of the vein and country-rock minerals are discussed, and the genesis and time relationships of the minerals are interpreted as an aid in the evaluation of these patterns.Arsenopyrite is believed to be an indicator, apparently forming envelopes around oreshoots. Similarly, late hydrothermal chlorite is considered to be an indicator. Sericite and carbonates of pre-hydrothermal-vein origin have a negative significance as indicators; i.e., little ore has been found in areas where these minerals are concentrated. Since beds rich in detrital quartz are the best ore horizons, such quartz is considered to be an indicator of limited practicability.An attempt is made to clarify the genetic classification of the various minerals, and within genetic types a few minerals not previously described in the district are listed. Six genetic types of chlorite are defined.The hydrothermal-vein history of the district is divided into three stages. In chronological order these are: (1) the bleaching alteration stage, (2) the carbonate-quartz stage, and (3) the sulfide stage. The hydrothermal bleaching alteration of large areas of the country-rock is largely the destruction of the rock pigments, and no strong sericitization is involved as previously thought.

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