Numerical hydrologic modeling of the Creede epithermal ore-forming system, Colorado
Hayba, Daniel Owen
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https://hdl.handle.net/2142/18944
Description
Title
Numerical hydrologic modeling of the Creede epithermal ore-forming system, Colorado
Author(s)
Hayba, Daniel Owen
Issue Date
1993
Doctoral Committee Chair(s)
Bethke, Craig M.
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Hydrology
Language
eng
Abstract
One of the fundamental objectives of the extensive research on Creede district, Colorado, has been to determine the sources, pathways, and interactions of fluids involved in the genesis of the epithermal ores. From the large volume of data on the main period of mineralization, a well-constrained conceptual flow model has evolved. In this model, an intrusion at depth drove hydrothermal convection, and topography drove shallow groundwater flow. Fluids from at least three sources fed the system, and boiling and/or mixing promoted ore deposition. In this study, I examine the qualitative constraints on this model, derive new controls, and then use numerical methods to evaluate the hydrology.
To establish the boundaries of the paleo-system, I determined that the average topographic slope across the district was approximately 10%, and that the depth of hydrothermal circulation was 1.6 to $\sim$3 km. A detailed fluid inclusion study of growth-banded sphalerite provides another important constraint by conclusively demonstrating that fluid mixing, rather than boiling, was the primary mechanism of ore deposition. Temperature and salinity variations indicate that a 285$\sp\circ$C hydrothermal brine ($\sim$11.5 wt% NaCl eq.) mixed with dilute, 160$\sp\circ$C groundwater. I also estimate that the mass flux through the system was approximately 50 kg/sec, and that the size of the granitic heat source was at least 7.5 km$\sp3$ ($\sim$2.1 $\times$ 10$\sp{13}$ kg). The overall contribution of magmatic fluids to the hydrothermal solution was about 6 wt.%.
Two-dimensional numerical modeling demonstrates that the conceptual flow model for Creede is a viable representation of the ore-forming system. Critical hydrologic elements in this model are a low-permeability ($\sim$10$\sp{-12}$ cm$\sp2)$ horizon overlying a permeable ($\sim$10$\sp{-9}$ cm$\sp2)$ fracture system. This configuration promotes mineralization by focusing and prolonging mixing between topographically-driven groundwater and buoyancy-driven hydrothermal brines. Sensitivity analyses indicate that the conductive heat flux from the pluton was 500 $\pm$ 200 h.f.u., and that the maximum vertical flow velocity was $\sim$10$\sp{-4}$ cm/sec. Modeling results confirm that hydrothermal brines could not have transported the sulfate sulfur found in the ores because of the long residence times within the hydrothermal plume. The model also illustrates problems with determining mineralization depths from fluid inclusion data, and suggests alternative interpretations of such data.
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