Abstract

The cobalt mineralization in the Blackbird district of Lemhi County, Idaho, probably the most extensive mineralization of its kind in the United States, is confined to irregular and discontinuous veins and lodes along zones of fractured and schistose quartzite belonging to the Belt series (pre-Cambrian). These lodes are represented by (1) zones of cobaltite-impregnated tourmalinized quartzite (cobalt-tourmaline lodes); (2) zones of cobaltite-impregnated biotitized quartzite (cobalt-biotite lodes); (3) bodies of cobaltite-containing quartz in zones of cobaltite-impregnated biotitized quartzite (cobalt-quartz lodes); and (4) stringers and lenses of quartz and such ore minerals as cobaltite, native silver, chalcopyrite, gold, and electrum in zones of cobaltite-impregnated biotitized quartzite (gold-copper-cobalt lodes).These lodes are the products of three stages of mineralization: (1) an early cobalt stage during which biotite, tourmaline, and cobaltite were formed, (2) a copper-cobalt stage during which biotite, quartz, cobaltite, arsenopyrite, pyrite, pyrrhotite, native silver, chalcopyrite, siderite and other minerals were deposited, and (3) a gold-electrum stage during which scant amounts of these minerals and quartz were deposited. The differences that exist among the lodes depend largely on the variations in the amounts and kinds of minerals added during each stage of mineralization. All lodes show marked effects of the first stage of mineralization; the cobalt-quartz and the gold-copper-cobalt, also the effects of the second stage; and the gold-copper-cobalt lodes, mild effects of the third.The mineralizing solutions apparently had their source in early Tertiary magma which also provided the substance for accompanying bodies of gabbro and lamprophyre. Lode schistosity, which developed concomitantly with the mineralization, indicates deposition along zones of deepseated shearing under intense stress conditions. Mineral associations suggest relatively high temperatures, particularly when cobaltite was deposited. The early-stage solutions apparently carried the cobalt direct from its magmatic source, but the cobalt contained in the second-stage solutions was probably obtained by solution of the earlier-deposited cobaltite.

First Page Preview

First page PDF preview
You do not currently have access to this article.