Mineralogy and geochemistry of the Read magnetite deposit, southwestern Stevens County, Washington

Abstract

The Read magnetite deposit of contact metasomatic origin has been formed along the contact of probable Cambrian limestone with granite of Mesozoic age. Notable among the various minerals formed is ludwigite, which appears to occupy a hanging wall position, or is generally farther from the granite than other minerals in the zone, and which appears to be a selective replacement of forsterite. Other nonmetallic contact metasomatic minerals include abundant diopside; lesser amounts of almandite, andradite, and hedenbergite; and minor amounts of spinel, epidote, tremolite, quartz, sanidine, labradorite, orthoclase, apatite, humite group minerals, serpentine, calcite, and dolomite. Metallic minerals include abundant magnetite; lesser amounts of pyrite, chalcopyrite, and pyrrhotite; and very minor amounts of scheelite and spalerite. Accessory metallic elements include Sn, Ti, Mn and probably Mo. The minerals are believed to have formed mainly in response to a decline in temperature inward from the contact. Magnetite, sphalerite, and possibly other metallic minerals appear to have formed in the pneumatolytic phase. Greatest loss of material was in carbon dioxide and lime; greatest gain was in silica, iron oxides, magnesia, B, and alumina. Ultimate sources of B probably were in Cambrian and Precambrian metamorphic rocks that were intruded and assimilated by the granite.