David McMillan

Support: BHP, University of Utah

Abstract

Disseminated tennantite, (Cu,Ag)10(Fe,Zn,Cu)2(As,Sb,Bi)4S13, accompanies chalcopyrite mineralization in pyritic shales of the Newland Formation, Belt Basin sediments, Meagher County, Montana. Copper mineralization within the lower Newland Fm. occurs in two distinct zones. The upper sulfide zone, USZ) is characterized by high Cu, Co, Ni, Ba, As, Pb and Ag, and the presence of tennantite, whereas the lower zone lacks cobalt, nickel and tennantite. Tennantite from the USZ varies systematically in chemical composition with stratigraphic elevation. Vertical chemical zoning, reflected in the substitution of copper and zinc for iron, and antimony and bismuth for arsenic, occurs in all mineralized intervals. A systematic change in tennantite composition occurs at a sharp inflection point approximately midway through the mineralized Cu-i interval, where 100*(XZn) = 75 in the upper USZ, and 1OO*(XZn) = 25 in the lower portion of the mineralized interval. Thus, the USZ can be divided into two distinct zones, an upper zinc-rich zone and a lower iron-rich zone. The zinc-rich interval is characterized by relatively deficient oxygen and sulfur conditions, and lower temperatures. A noticeable peak in the copper grade, and in the copper content in tennantite, is observed at the interface between these two chemically distinct zones. Additionally, XCu varies in concert with copper in whole rock assays, implying a close chemical relationship between tennantite and chalcopyrite.

Mineralization occurred under oxidizing conditions, fO2 = l0-30±1, at a pH of about 5.5, and a temperature of 300oC. Thermo-chemical modelling suggest that mineralizing fluids migrated up along the aneestral Voleano Valley Fault, and then laterally within highly permeable barite breccia zones that acted as precursors to copper mineralization. Sulfide deposition within the main ore stage occured during waning stages of the hydrothermal system, and was accompanied by barite dissolution from copper-rich zones. A fractional crystallization model, with initial hydrothermal fluid conditions 0.15 < Kcu < 0.6, 0.10 < Kas < 0.3, 0.75 < XCu < 0.8, and 0.8 < Xas < 0.95, was developed to explain elemental variations in tennantite. Observations that proximal tennantite is relatively Sb-, and (Zn+Fe)-rich, whereas distal tennantite is enriched in As and Cu, support this model.

Fluid inclusion data record the evolution of mineralizing fluids within the USZ, and show that three distinct pulses were responsible for mineralization within the USZ. An early, low temperature, low salinity, diagenetic fluid reflects conditions under which syndepositional precipitation of pyrite occurred. Quartz and dolomite precipitated from a low temperature, moderate salinity fluid, during the waxing and waning stages of the hydrothermal system. Barite precipitated under near-isothermal conditions from 2.3m NaCl brines at temperatures around 300oC; these latter conditions likely represent the conditions of economic copper mineralization at Sheep Creek.

Cut and polished core of Sheep Creek ore samples.

Photomicrographs and SEM image (scale bar is 2 microns) of Sheep Creek ore samples.

Tetrahedrite and chalcopyrite in fine grained pyrite.

Variation in terahedrite composition (afpu, unless noted otherwise) with depth.

Variation in tetrahedrite mineral chemistry indicated variations in ambient ƒS₂.

McMillan, D., 1995, Compositional variations in tennantite: Implications for mineral genesis: Sheep Creek, Meagher County, Montana. MS Thesis, The University of Utah, 161 p.

McMillan, D. and Petersen, E.U., 1991, Vertical compositional zoning in tennantite: The Sheep Creek Deposit, Meagher County, Montana. Geological Society of America Abstracts, 23, 5, A-464.

Petersen, E.U., McMillan, D. and Himes, M.D., 1991, Routine mineral composition characterization by electron microprobe applied to district scale base metal exploratio. 120th A.I.M.E. Program Abstracts, 70 , Denver (invited paper). pdf

Himes, M.D. and Petersen, E.U., 1990, Geologic features of the Sheep Creek ores, A Proterozoic Cu-Co sediment-hosted massive sulfide deposit, Meagher County, Montana: in D.M. Hausen et al., Eds., Gold '90, 533-546.