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The isotopic composition of ore lead of the Creede mining district and vicinity, San Juan Mountains, Colorado: Text of a talk presented at the San Juan Mountains symposium to honor Thomas A. Steven; Rocky Mountain Section meeting of the Geological Society of America, May 2, 1987, Boulder, Colorado

Open-File Report 88-510

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Abstract

Galenas from the major Creede veins and their northern extensions are remarkably homogeneous in Pb-isotopic composition and are too radiogenic to have been derived from any magma comparable in composition to the principal volcanic rocks. This pattern was identified by Doe et al. in 1979 who proposed that the lead was derived from the Precambrian basement. The homogeneity of the ore leads, however, requires a uniform reservoir; an unlikely prospect for lead from the Precambrian basement. We report on 16 new analyses of geographically and paragenetically dispersed galenas from the Creede district and other areas as far as 11 km to the north. The lead values range from 18.972 to 19.060 for 206Pb/204Pb, from 15.591 to 15.671 for 207Pb/204Pb, and from 37.781 to 37.921 for 208Pb/204Pb. These ranges overlap those previously reported for the main ore zone.

Recent work allows us to extend the results of Doe et al. and to consider alternative processes to explain the widespread homogeneity and radiogenic nature of the ore lead: 1) David Matty (pers. commun., 1986) has shown that some minor volcanic units in the area have unusually radiogneic lead values; magmas comparable in composition to the units are a possible, though improbable, source of the ore lead. 2) The uniformity of the isotopic values of galenas may have resulted from homogenization during an extensive potassium-metasomatic event that predated the ores; this possibility is being tested in an on-going study of feldspars from metasomatized and unmetasomatized rocks. 3) Recent regional studies suggest the possibility of a prevolcanic, NNW-trending graben system filled by clastic sediments derived from the Precambrian basement, a process that would have an homogenizing effect on the lead isotopes. This interpretation implies importation, deep within the Creede hydrologic system, of fluids from remote sources. These alternatives show that the Pbisotope systematics may have a profound impact on the interpretation of the Creede hydrothermal system, and that further study is warranted.

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Additional publication details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
The isotopic composition of ore lead of the Creede mining district and vicinity, San Juan Mountains, Colorado: Text of a talk presented at the San Juan Mountains symposium to honor Thomas A. Steven; Rocky Mountain Section meeting of the Geological Society of America, May 2, 1987, Boulder, Colorado
Series title:
Open-File Report
Series number:
88-510
Year Published:
1988
Language:
English
Publisher:
U.S. Geological Survey
Description:
24 p.
Country:
United States
State:
Colorado
Online Only (Y/N):
N
Additional Online Files (Y/N):
N