HYDROTHERMAL ALTERATION OF A BRECCIA PIPE DEPOSIT, TRIBAG MINE, BATCHAWANA BAY, ONTARIO

The Tribag mine is situated approximately 45 miles north-northwest of Sault Ste. Marie, Ontario. Precambrian rocks exposed in the area consist of: silici to basic metavolcanics, metasediments and iron formations; granite; diabase dikes and sills; and a series of olivine basalts interlayered with conglomerate and sandstone. The metavolcanics, metasediments, and iron formations represent an accumulation of 30,000 feet of material that was later intruded by granitic batholiths. All of these rocks are faulted and intruded by diabase dikes and sills. A thick series of olivine basalts was extruded into the Lake Superior basin. Conglomerate and sandstone are interlayered with the basalts, which dip southwest at 20 to 40 degrees. Felsite bodies intrude the volcanic and sedimentary layers. The Tribag mine is in the Breton breccia, one of five breccia pipes near the eastern margin of Township 28, Range 13. The pipe has surface dimensions of 1,400 by 350 feet, and the size increases slightly with depth. The breccia contains angular to subrounded fragments of trondhjemite or sodic granite, basic volcanics, felsite, and diabase. These are set in a matrix of quartz, pyrrhotite, chalcopyrite, sphalerite, marcasite and galena. Minor amounts of molybdenite, scheelite, fluorite, laumontite, and barite also occur in the matrix. Secondary effects on the rock fragments include hematitic alteration, seritization, silicification, kaolinitization, chloritization, and minor carbonatization. A small amount of pyrite and leucoxene is associated with sericitized biotite. Hematitic alteration of the feldspars preceded brecciation. Quartz, sericite, and kaolinite are abundant near highly mineralized areas. Hydrothermal alteration and the extrusion of basaltic magma nearby have both been dated at near 1,050 million years by the K-Ar method. An amygdaloidal dike having a similar mineralogic composition to the basaltic lavas cuts the breccia at the Tribag mine. This indicates a possible genetic relation between the Middle Keweenawan extrusives and the Tribag copper deposit. The presence of abundant sericite and quartz in and near the mineralized zone at Tribag, along with the mineralogy and general geologic setting, suggest deposition in a moderate to high intensity environment. The large amount of open space between breccia fragments suggests deposition in a relatively low pressure environment. © 1969 Society of Economic Geologists, Inc.