Lead isotopic evidence on the origin of hydrothermal veins, Lake City, Colorado

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6420394

Sanford, R F; Ludwig, K R

The 23-m.y. Lake City caldera (LCC) and enclosing 29-m.y. Uncompahgre caldera (UC) and associated Eureka graben (EG) are host to Ag-Pb-Zn epithermal veins. Lead isotopic data on 32 new galena samples and 12 previously measured galenas from these areas show that 206Pb/204Pb values for veins within the LCC are sharply lower than for veins outside the LCC. Thus, fluids that formed the two sets of veins derived their Pb from distinctly different sources. Veins in the UC-EG typically have 206Pb/204Pb values in the range 18.6-19.0, but four small areas around the LCC have higher values (up to 19.6). These hot spots, which include the two largest and most productive veins, may represent locally deeper circulation of fluids or locally shallower Precambrian basement. There are no systematic differences among the different areas in the UC-EG. Instead, the largest variations are within areas or districts. Likewise, the Pb isotopic values from different paragenetic assemblages are not distinctly different but show considerable overlap. Many 207Pb/204Pb values are higher relative to their 206Pb/204Pb than those from veins else wherein the San Juan Mountains and are also higher than these values in likely source rocks, which suggests that source rocks having higher 207Pb/206Pb values than are currently known in this area exist at depth. Thorogenic Pb (208Pb/204Pb) values are closely correlated positively with 206Pb/204Pb values and indicate a source that is relatively homogeneous with respect to its Th/U ratio. Thus, the data suggest at least two major hydrothermal systems corresponding to veins in the UC-EG and LCC, respectively. Local differences in source rocks, fluid circulation, and perhaps heat flow, contributed to locally different Pb signatures in closely spaced but separate hydrothermal cells within these areas.

• Research Organization: Geological Survey, Denver, CO (USA)
• OSTI ID: 6420394
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

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CHALCOGENIDES
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
CHEMISTRY
COLORADO
ENERGY SYSTEMS
FEDERAL REGION VIII
FLUIDS
GALENA
GEOCHEMISTRY
GEOTHERMAL FLUIDS
GEOTHERMAL SYSTEMS
HYDROTHERMAL ALTERATION
HYDROTHERMAL SYNTHESIS
HYDROTHERMAL SYSTEMS
ISOTOPE RATIO
ISOTOPES
LEAD COMPOUNDS
LEAD ISOTOPES
LEAD ORES
LEAD SULFIDES
MINERALIZATION
MINERALS
NORTH AMERICA
ORES
PETROGENESIS
QUANTITATIVE CHEMICAL ANALYSIS
ROCK-FLUID INTERACTIONS
SILVER ORES
SULFIDE MINERALS
SULFIDES
SULFUR COMPOUNDS
SYNTHESIS
USA
ZINC ORES