Characterization and dating of argillic alteration in the Mercur gold district, Utah—a reply

We would like to extend our appreciation to Morris and Tooker for their comments, discussion, and additional information that they provide pertaining to the geologic environment of the Mercur gold district, Utah. Their review of the characteristics of the Sevier orogenic belt are particularly relevant; however, such characteristics must be interpreted within the context of the additional geologic events of the region, which include the Jurassic compressional event that has been described from northern Utah and western Nevada. For this purpose, we offer the following reply.

Morris and Tooker have two main points of disagreement with our paper. First, they find the range of K-Ar ages we reported as disturbing and indicate that they date neither tectonic, hydrothermal, nor gold mineralization events; and second, they contend that all mineralized structures at Mercur must be younger than Late Cretaceous in age.

We respond to concerns about our K-Ar data with the following points. First, we wish to clarify that 11 of the K-Ar ages come from within the orebodies. In addition, two dates were obtained from highly altered rock within a few tenths of a kilometer of other known mineralized bodies (T. Shier and L. Stanger, mine geologists, pers. commun.), and nine dates were located within 3.5 km or less of these areas of known significant mineralization. These nine dates were obtained from highly altered rock with heavy metal signatures similar to that of the orebodies. Gold mineralization does extend from Ophir Canyon to Five Mile Pass (11 km, see fig. 1 of Wilson and Parry, 1995) and our data set is representative of this area. Second, as to the range in the ages within the data, we emphasize that this is a region that is known to have a complex thermal history that includes deep burial of these rocks in the Oquirrh basin, at least two compressional tectonic events, and Tertiary intrusion of small dikes and sills. In Wilson and Parry (1995), we discussed the problems with dating low- to moderate-temperature events in sedimentary rocks

Natural History, University of Utah and Utah Geological and Mineral Survey, Department of Natural Resources, 279 p.

Tooker, E.W., 1983, Variations in structural style and correlation of thrust plates in the Sevier foreland thrust belt, Great Salt Lake area, Utah: Geological Society of America Memoir 157, p. 61-73.

------1987, Preliminary geologic maps, cross sections, and explanation pamphlet for the Ophir and Mercur Quadrangles, Utah: U.S. Geological Survey Open-File Report 87-152, scale 1:24,000.

------1990, Gold in porphyry copper systems: U.S. Geological Survey Professional Paper 1857-E, p. E1-E16.

Tooker, E.W., and Roberts, R.J., 1970, Upper Paleozoic rocks in the Oquirrh Mountains and Bingham mining district, Utah: U.S. Geological Survey Professional Paper 629-A, 76 p.

Wilson, P.N., 1992, Geochemistry and clay mineralogy of hydrothermally altered organic-rich shales, north-central Utah: Implications for a Mesozoic age of gold mineralization in the Mercur district: Unpublished Ph.D. dissertation, Salt Lake City, University of Utah, 269 p.

Wilson, P.N., and Parry, W.T., 1995, Characterization and dating of argillic alteration in the Mercur gold district, Utah: Economic Geology, v. 90, p. 1197-1216.

and indicated that this frequently results in a data set which has a range of ages (Clauer and Chaudhuri, 1992). Given the thermal history of the study area, a range of ages in the data set is reasonable and should not be unexpected. We interpreted the 127-m.y. range in ages in our data as a reflection of the extensive time period between thermal events related to development of the Oquirrh basin, at around 260 Ma, and subsequent much younger thermal events.

Third, the major point we wished to make in Wilson and Parry (1995) is that none of the 22 ages, including the ten from within the deposits, are Tertiary in age. We find it difficult to explain the lack of younger ages if Mercur mineralization is considered to be Tertiary, specifically the 32 Ma age of the Eagle Hill Rhyolite. The intensity of hydrothermal alteration is too great not to have left an identifiable signature in the age data. Comparison of fluid inclusion homogenization temperatures measured on quartz, calcite, and barite from the orebodies at Mercur (Jewell, 1984; Jewell and Parry, 1987, 1988; Jewell, pers. commun.) with argon retention curves calculated for heating events of 1 Ma, 100 Ka, and 10 Ka duration are shown in Figure 1. The argon retention curves indicate that these temperatures would have resulted in partial to nearly complete resetting of K-Ar ages. If the fluid inclusions represent a Tertiary age event, then Tertiary or near Tertiary ages should be represented in the age data.