John F. Ziegler

Thesis: Genesis of the Silver Chert Jasperoid, Mercur, Utah

Support: University of Utah

Abstract

The Silver Chert Jasperoid, Mercur District, Utah replaced the upper portion of the Topliff Member and lower portion of the Mercur Member of the Mississippian Great Blue Formation. High angle normal faults delineated by brecciated jasperoid provided the conduits by which silicifying solutions migrated. Silicification can be divided into three temporally associated stages. Stage I jasperoid is dominated by "moth-eaten" barite with quartz being relatively minor. Native silver was precipitated during Stage I and is often encapsulated by barite. Stage II jasperoid comprises the greatest volume and is dominated by quartz. Secondary minerals include pyrite and a TiO2 polymorph. Stage III jasperoid is a late stage quartz veining event that crosscuts Stage II quartz. Secondary minerals include barite, pyrite, and a TiO2 polymorph.

Silver grades are typically higher than gold grades in the Silver Chert jasperoid with ratios of Ag:Au typically between 0.1:1 and 17:1. Brecciated jasperoid has the highest gold and silver values; brecciated jasperoid may contain up to 0.02 oz/ton silver and 0.102 oz/ton gold. Stratiform jasperoid contain up to 0.05 oz/ton silver and 0.02 oz/ton gold.

Microthermometric analysis of Stage III quartz fluid inclusions yields an average of 3.2 equivalent weight percent NaCl and a homogenization temperature of 207oC +/-43oC. Maximum pressure and temperature for the hydrothermal fluid are estimated at 800 bars and 270oC, respectively, from the presence of kaolinite and quartz and absence of pyrophyllite and fluid inclusion data. Estimates of the geochemical parameters of the hydrothermal fluid indicate that a maximum pH is 3.4 to 6.63 depending on aCa2+ and aK+ and logfO2 = -35 to -30.

Oxygen istotopic composition of Stage II and Stage III quartz indicates that two fluids were responsible for Silver Chert jasperoid formation. The d18O values of Stage II quartz relative to SMOW range from +13.0 permil to +24.3 permil. Stage III quartz d18O values range from +2.8 permil to 25.1 permil. The d18O fluid values for Stage II and Stage III quartz at 210oC range from +1.8 permil to +13.1 permil and -8.4 to +13.9 permil, respectively. The isotopically light fluid could represent an unevolved meteoric water that migrated in an aquifer. The isotopically heavy fluid could represent a formation water or magmatic water that has exchanged with a carbonate reservoir. The large range in the d18O data can be explained as a result of mixing of the isotopically light fluid with the isotopically heavy fluid.

Mass transfer calcualtions suggest that mixing of towe thermally contrasting waters formed the Silver Chert jasperoid. The hydrothermal fluid was modelled at 270oC, 800 bars, and an average salinity of 3.2 equivalent weight percent NaCl. The "cold" groundwater was modelled as pure water at 25oC and 75oC. Mixing required a larger volume of groundwater than hydrothermal fluid, approximately 70 volume percent, for calcite dissolution to occur.

A pressure of 800 bars indicates that mixing may have occurred at a depth of 3.3 km under lithostatic conditions. Faulting may have intersected an aquifer present in the Magazine Sandstone and Lower Great Blue Limestone allowing hydrothermal fluids to mix with cold groundwater. This is consistent with the thickest zones of jasperoid occurring along faults where mixing is likely. Precipitation of native silver transproted as a chloride complex occurred as the hydrothermal fluids mixed with groundwater.

Silver Chert Jasperoid outcrop, Mercur Mine, Utah

Silicified fossil in Silver Chert Jasperoid (left- PL, right- cross polarized light)

Ziegler, J. and Petersen, E.U., 1990, Textures and mineralogy of the Silver Chert Jasperoid, Mercur, Utah, in D.M. Hausen et al., Eds., Gold '90, 25-31. pdf

Ziegler, J. and Petersen, E.U., 1990, Textures and mineralogy of the Silver Chert Jasperoid, Mercur District, Utah: Salt Lake City, UT, 119TH A.I.M.E. Pocket Program Abstracts, 88.

Ziegler, J.F. and Petersen, E.U., 1991, Genesis of the Silver Chert Jasperoid, Mercur District, Utah: Geological Society of America Abstracts, 23, 5, A464.

Petersen, E.U. and Ziegler, J., 1995, Jasperoid Alteration: in Atlas of Alteration, Thompson, A.J.B. and Tompson, J.F.H., Eds., 100.