ORIGIN OF THE BLACK BUTTE STRATIFORM CU-CO-AG MASSIVE SULFIDE DEPOSIT, CENTRAL MONTANA: PRELIMINARY SR ISOTOPE EVIDENCE

Black Butte is located approximately 25 km north of White Sulphur Springs, in central Montana. The Johnny Lee deposit, one of several deposits at Black Butte, is a stratiform Cu-Co-Ag massive sulfide deposit within the Newland Formation of the mid-Proterozoic Belt Supergroup. The geology and mineralogy of this deposit have recently been described in detail (Graham et al.,2012, Econ Geol. 107, 1115-1141; White et al., Miner. Deposita, in press, DOI: 10.1007/s00126-013-0492-1). Two lateral zones of massive sulfide mineralization in the Johnny Lee deposit, referred to as the Upper and Lower Sulfide Zones (USZ and LSZ), are dominated by pyrite + chalcopyrite with subordinate tennantite, bornite, siegenite, and Co-Ni-arsenides. The USZ is rich in barite, whereas gangue in the LSZ is mainly quartz and dolomite. Besides barite, the USZ contains local accumulations (multi-percent) of Sr-rich gangue minerals, including strontianite and celestine, which focus along the bottom of the NNE-trending axis of thickest sulfide accumulation.

In the USZ, ⁸⁷Sr/⁸⁶Sr ratios of barite, strontianite, and celestine cluster near 0.7047-0.7048. In contrast, dolomite from the LSZ has ⁸⁷Sr/⁸⁶Sr ratios in the range of 0.7052 to 0.7064. Together, the data fall on a possible mixing line, with the more radiogenic source consistent with1.5 Ga (i.e., Belt-aged) sea water. The less radiogenic end member is less well constrained, but may have involved leaching of Sr from the Archean basement and/or the Neihart Formation, a thick, hematitic quartzite that forms an intervening layer between the Archean basement and the lower Belt Supergroup. Dolomite in the Newland Fm. away from mineralization has d¹³C near 0 ‰ (VPDB) and d¹⁸O in the range of +23 to +25‰ (VSMOW), consistent with deposition from sea water. In contrast, coarse dolomite grains from the LSZ have d¹³C near -5‰ and d¹⁸O near 19 to 20‰. This trend in carbonate isotopes is very similar to what has been observed from the Proterozoic McArthur River deposit in Australia. Sr-rich carbonate minerals from the USZ are highly depleted in d¹³C and d¹⁸O compared to the other dolomites investigated, suggesting a higher temperature of deposition and the possible incorporation of isotopically light C from the diagenesis of organic carbon buried in the Newland and/or Chamberlain Formations.