Abstract

Optical properties and chemical analyses are presented for 15 pyroxenes and amphiboles from zinc mines in Mexico and New Mexico. These analyses indicate that the characteristic pyroxenes in the pyrometasomatic zinc deposits examined by the writers in Mexico and New Mexico are manganoan hedenbergite and manganoan magnesian hedenbergite. At some localities johannsenite and ferroan johannsenite were formed, and later these were replaced by rhodonite coming from surges of manganese that were supplied from the invading magma of stocks, visible or concealed. Zoned birefringent garnets (andradite) were formed early in the silicate stage. On the basis of the temperature at which andradite from the Pewabic mine loses its birefringence on heating, the early stage of silicate deposition began at a temperature below 860°C. Later, at a lower temperature, possibly about 550°C., uralitization of the pyroxenes took place with a great loss of CaO and a gain in SiQ2, FeO, Al2O3, and MgO. The variety of uralite formed at the three mines where samples were analyzed is too low in CaO to be actinolite, and its chemical composition suggests cummingtonite. This is the first reported occurrence of cummingtonite formed by the uralitization of a calcium pyroxene. The sulfide ore minerals, sphalerite and pyrite, and also some magnetite are later than the silicate minerals and were formed by hydrothermal solutions at a somewhat lower temperature than that of the cummingtonite.

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