Abstract

The Tribag breccia pipes, located near the eastern extremity of Lake Superior, are associated with Keweenawan felsic intrusions and have yielded a K-Ar age of 1,055 + or - 35 m.y. The pipes cut Archean diabase and greenstones but in terms of regional setting exhibit a relationship to Keweenawan rifting.Four major breccia pipes occur in the district. Three contain large tonnages of disseminated subeconomic Cu and Cu-Mo mineralization, but only two have supported mining. That production amounted to 16,000 metric tons of copper from relatively high grade ore associated with domal fractures within the two pipes.The pipes contain mainly angular fragments of the surrounding metavolcanic and granitic country rocks and exhibit sharp contacts. Hydrothermal alteration within the pipes is restricted to the vicinity of fractures and orebodies but becomes more pervasive downward. Mineralization of the pipes can be divided into four periods, each marked by somewhat distinctive mineral assemblages and ore fluid characteristics. Period 1 resulted in formation of disseminated chalcopyrite-molybdenite sulfides associated with magnetite, pyrite, epidote, and quartz. Fluid temperatures were up to 469 degrees C and salinities ranged up to 48.5 equiv. wt percent NaCl. Period 2 resulted in deposition of coarsely crystalline quartz and carbonates in the breccia matrices accompanied by lesser amounts of zeolites, pyrite, and chalcopyrite. Fluid temperatures were approximately 420 degrees to 330 degrees C and salinities ranged from 12 to 1.5 equiv. wt percent NaCl. Period 3 and 4 mineralization resulted in deposition of orebodies in fracture-controlled sites. Principal sulfides in period 3 ores are chalcopyrite and pyrite; these sulfides are massive in character and were deposited from fluids ranging from approximately 412 degrees to 291 degrees C and 14 to 5 equiv. wt percent NaCl salinity. Period 4 ores have a mineralogy similar to that of period 3 but, in addition, contain sphalerite and galena, are of a disseminated nature, and were deposited from fluids of approximately 363 degrees to approximately 250 degrees C and of 17 to 6 equiv. wt percent NaCl salinity. Estimates of pressure during the entire mineralization episode range from about 248 to 550 bars, with pressure up to 1 kb at times during period 1. These estimates suggest a regime that mostly fluctuated between hydrostatic and lithostatic conditions at a depth close to 2,500 m, but during period 1, pressures attained nearly twice lithostatic pressure.Analyses of the fluids in inclusions indicate mineralizing waters of two types: period 1, 3, and 4 waters were Ca-Na brines, some of which contained copper in concentrations of thousands of ppm; period 2 fluids were Na-Ca brines containing about 90 ppm Cu. Analysis of fluid inclusion gases detected CO 2 , H 2 , H 2 S, He, and Ar. From these data and thermodynamic considerations it can be demonstrated that H 2 S concentrations decreased through the paragenesis, the f (sub O 2 ) varied from below to above H 2 S-HSO (super -) 4 equilibria in period 1, 2, and 3 solutions which boiled, and the pH remained between 4.2 to 5.1. Limited stable isotope measurements suggest fluids of primarily magmatic origin.Three of the breccias are thought to be the result of collapse resulting from the creation of open space by alteration and solution of rock at depths of about 3 km by fluids streaming upward along faults. The remaining breccia has features indicative of fracturing by fluid under high pressure. Tribag breccias are postulated to be the result of identical processes which have formed Cu-Mo breccias in cordilleran settings. The difference between subeconomic disseminated mineralization and the viable, more concentrated mineralization in domal fractures appears to be related to the creation of open space and the higher Cu content of the ore-depositing fluids.

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