Abstract

The ore of the Ashbrook district occurs as irregular patchy replacement deposits in fractured and folded limestone, whose structure has strongly influenced distribution of the orebodies. The distribution is controlled by small crenulations within the limestone, the apical portions of which are usually marked by a strong development of secondary calcite, which is generally more susceptible to replacement by mineralizing solutions. Due to two episodes of minor fracturing, the sequence of primary mineralization can be devided into three periods of deposition: (1) rhodochrosite to the exclusion of other minerals, (2) quartz, pyrite, arsenopyrite, sphalerite, and minor amounts of chalcopyrite, and (3) galena, argyrodite, pyrargyrite, pearceite, tennantite, and an unidentified mineral. The second phase seems to be responsible for the minor gold values in the ores. Processes of enrichment and oxidation have produced a large group of secondary minerals. Alteration of the primary silver minerals has been mainly to argentite and native silver which together account for the main values found in the ores.

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