Date of Award

12-2015

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dr. Joyashish Thakurta

Second Advisor

Dr. R.V. Krishnamurthy

Third Advisor

Dr. Heather Petcovic

Access Setting

Masters Thesis-Open Access

Abstract

The Back Forty Zn-Au deposit is the eastern-most Volcanogenic Massive Sulfide (VMS) deposit in the Penokean Volcanic Belt (PVB). It is the only VMS deposit found in Michigan, and is located along the east side of the Menominee River in Menominee County (45°26'57.5"N, 87°49'43.2"W). The Back Forty is the most zinc-enriched, copper depleted deposit among the major VMS deposits within the PVB that include Flambeau, Crandon, Bend, and Lynne. This work constitutes the first sulfur isotope study on the Back Forty VMS deposit.

The Back Forty is characterized by massive, semi-massive, disseminated and stringer sulfide mineralization hosted within felsic-dominated volcanic rocks, consistent with the bimodal-felsic, or Kuroko-type, VMS model. The host rocks include coarse-ash and fine-ash crystal tuffs, as well as lapilli lithic tuffs. δ34S values obtained from pyrite, sphalerite, chalcopyrite, galena, and bornite range from 0.76 to 5.06‰-VCDT, with an average value of 3.07 ± 1.08‰ (1σ). These values are similar to the other major deposits of the PVB and are consistent to a mantle origin with minor intermixing with surficial sulfur-rich sediments. The sulfur in the ore-forming system is interpreted to have been derived by hydrothermal leaching of seafloor volcanic rocks.

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