Paper No. 247-8
Presentation Time: 3:15 PM
EPITHERMAL PRECIOUS METAL MINERALIZATION OF THE SUMMIT MINE
Ores from the underground Summit mine possess classic characteristics of epithermal precious metal mineralization. The mine is located in the Steeple Rock mining district in Grant County, New Mexico. Host rocks consist of Oligocene to Miocene intermediate volcanic rocks overlain by rhyolitic ash flow tuffs. The intermediate rocks are cut by fine grained, locally flow banded, rhyolite dikes and plugs. The gold-silver ore consists of fine-grained pyrite and argentite with minor electrum, chalcopyrite and very fine gold occurring in black bands and patches within colloform banded white quartz. The vein is typically 85 to 90 % silica, and the sulfide content is less than 1%. Other common gangue minerals include calcite, siderite, manganiferous calcite and locally occurring illite and chlorite. Upper portions of the mineralized vein are characterized by additional minerals related to oxidation including hematite, manganese oxides and clays. Silicified breccia fragments of adjacent wall rock are common in mineralized ore shoots. Brecciation and recementation of mineralized vein indicate contemporaneous fault movement during the mineralizing event. The Summit vein structure strikes northwest to nearly east-west, and dips steeply to the northeast. The Summit mine occurs in the north central portion of the district within the Summit-East Camp fault-vein structure that trends northwest. The vein expression at the surface varies from a zone of kaolinite-illite-quartz alteration a few feet wide, to a zone of quartz and calcite veining with adjacent clay-chlorite alteration over 115’ wide. Mineralization occurs in 1’ to 30’ wide shoots within a wider quartz-calcite vein. The Summit mineralization generally is related to a broad bend in the Summit structure, and individual ore shoots are associated with flexures and other variations in the vein strike and dip.