| Summary: | A study of the mineralogy of a suite of specimens, collected by Dr.C.Riley from the Bonanza silver deposit, has been made. Particular attention is paid to the silver mineralization and the origin of the dendritic structure. A brief examination of the wall rock alteration is included. The mineralogy of the deposit is relatively simple, consisting of the following metallic minerals in their order of abundance: native silver, magnetite, hematite, tetrahedrite, argentite, chalcopyrite, and an unknown mineral. Pitchblende and cobalt-nickel minerals are absent. Magnetite and hematite are restricted to the wall rock and are not associated with the other metallic minerals. The magnetite is believed to be of pyrometasomatic origin and related to a granodiorite intrusion, while the other metallic and gangue minerals are considered to be of hydrothermal origin. The gangue minerals consist of quartz, sericite, and carbonate. Ninety-five percent of the native silver occurs as dendrites and the other five percent as replacement of tetrahedrite and chalcopyrite. Core replacement by the silver is well developed. The dendritic structure of the silver is inherited from quartz through replacement. In a quartz gangue this structure appears to be controlled by rows of specially oriented, doubly terminated, quartz prisms, while in a sericitic gangue the euhedral quartz grains, arranged in a rude dendritic pattern, are the controlling factor. The mineral deposits of the Echo Bay area are compared with similar deposits throughout the world. Science, Faculty of Earth, Ocean and Atmospheric Sciences, Department of Graduate
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