MINERALOGICAL, GEOCHEMICAL, AND ISOTOPIC ANALYSIS OF MASSIVE PYRITE, TINTIC MINING DISTRICT, UTAH

Recent geochemical and isotopic work in the Tintic mining district, Utah, has elucidated the age and mineralogy of fresh, unaltered, massive pyrite collected from a mine dump near (from?) the historic Tintic Standard silver mine.

We studied this massive pyrite to determine 1) if it represents historically-mined ore, 2) if it contains anomalous concentrations of ore elements, 3) its age relative to documented ages of regional igneous activity, and 3) its genetic origin.

 The sample material consists mostly (>99%) of pyrite with minor amounts of other minerals: pyrrhotite > quartz > galena (≤ 500 µm) > sphalerite (≤ 300 µm) > unidentified sulfate/phosphate mineral. The sphalerite contains no Fe, the galena contains no Ag, the sphalerite occurs as inclusions within the galena, and both sphalerite and galena occur as inclusions within the pyrite. The pyrite, pyrrhotite, and quartz are intergrown. Sulfur stable isotope data of the pyrite (δ³⁴S_CDT = -2.9‰) may infer a magmatic source. Fluid inclusion and O stable isotope data from the quartz within the massive pyrite are forthcoming.

Assay analysis reveals the following concentrations of base- and precious-metals: Au, 0.575 ppm; Ag, 171 ppm; Cu, 731 ppm; Zn, 2.83%; Pb, 2.45%.

Re-Os analyses of the pyrite-rich material yield an isochron age of 0.66 +/- 0.21 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os of 1.27, consistent with mineralizing fluids from the upper continental crust. However, concentration data indicate this age may represent a binary mixing line between older, Os-rich sulfide (TMA model age of 36 Ma) derived from magmatic fluids and a younger, Os-poor sulfide with more upper continental crust affinities.