Epigenetic Zonation and Fluid Flow History of Uranium-Bearing Fluvial Aquifer Systems, South Texas Uranium Province

RI0119. Epigenetic Zonation and Fluid Flow History of Uranium-Bearing Fluvial Aquifer Systems, South Texas Uranium Province, by W. E. Galloway. Assisted by Constance Childs, Allen Standen, and John Suter. 31 p., 22 figs., 4 tables, 1982. ISSN: 0082335X: Print. To purchase this publication as a downloadable PDF, please order RI0119D.

ABSTRACT
The Oligocene-Miocene fluvial uranium host aquifers of the South Texas uranium province were deposited principally as syndepositionally oxidized sands and muds. Early intrusion of reactive sulfide- enriched waters produced large intrastratal islands of epigenetic sulfidic alteration, which contain isotopically heavy pyrite exhibiting unique replacement textures. The only known reservoir containing such sulfidic waters is the deeply buried Mesozoic carbonate section beneath the thick, geopressured Tertiary basin fill. Thermobaric waters were expulsed upward along major fault zones into shallow aquifers in response to a pressure head generated by compaction and dehydration in the abyssal ground-water regime. Vertical migration of gaseous hydrogen sulfide was less important. Repeated flushing of the shallow aquifers by oxidizing meteoric waters containing anomalous amounts of uranium, selenium, and molybdenum alternating with sulfidic thermobaric waters caused cyclic precipitation and oxidation of iron disulfide. Uranium deposits formed along hydrologically active oxidation interfaces separating epigenetic sulfidic and epigenetic oxidation zones. Multiple epigenetic events are recorded in imperfectly superimposed, multiple mineralization fronts, in regional and local geometric relations between different alteration zones, and in the bulk matrix geochemistry and mineralogy of alteration zones. The dynamic mineralization model described in this report may reflect processes active in many large, depositionally active basins.

Keywords: George West, Live Oak County, Ray Point, South Texas, uranium, aquifers, geochemistry, mineralization

CONTENTS

ABSTRACT

INTRODUCTION

OBJECTIVE

METHODOLOGY

AQUIFER ALTERATION

REGIONAL ALTERATION PATTERNS

DETAILED ALTERATION GEOMETRY AND ZONATION

GEOCHEMICAL AND MINERALOGICAL CHARACTERIZATION

GENERALIZED ALTERATION MODEL

COMPLEX ZONAL STRUCTURE

IMPLICATIONS FOR INTERPRETATION OF HYDROLOGIC HISTORY

HYDROGEOLOGY OF A COMPACTING BASIN

HYDROCHEMISTRY OF GROUND-WATER REGIMES

Abyssal regime fluids

Elisian regime fluids

Meteoric regime fluids

REGIME INTERACTIONS

DYNAMICS OF ORE GENESIS

MODERN FLOW DYNAMICS AND HYDROCHEMISTRY

DYNAMICS OF EPIGENETIC SULFIDIZATION

FLOW DYNAMICS OF AQUIFER METALLOGENESIS

PERSPECTIVES

ACKNOWLEDGMENTS

REFERENCES

FIGURES

1. Location of the study area and position of the underlying Lower Cretaceous shelf margin

2. Geochemical zonations produced by epigenetic oxidation and reduction of a shallow, meteoric aquifer

3. Regional alteration zonation of lower Oakville sands, Live Oak County and adjacent areas

4. Geologic framework, mineralization-front geometry, interpreted cogenetic alteration (oxidation) pattern, and direction of modern ground-water flow in George West and Ray Point districts

5. Cross-sectional geometry of mineralization fronts showing surrounding alteration zones and location of sampled core transects

6. Characteristic diagenetic features of the epigenetic sulfidic zone

7. Geochemical zonation across the Ray Point front sample transect

8. Limonite nest produced by epigenetic oxidation of iron disulfide grain

9. Diagenetic features characteristic of the resulfidized altered zone

10. Pervasive spar cement locally found in and around faulted parts of the Oakville aquifer

11. Generalized alteration zonation typical of the Oakville aquifer in the study area

12. Cross section of a complex multiple mineralization front in the George West district

13. Distribution of copper within the drilkutting sample section

14. Distribution of iron phases across the complex front

15. Distribution of calcite across the complex front.

16. Ground-water regimes in a thick, compacting basin fill

17. Tertiary basin fill of the northwestern Gulf Coast Basin

18. Typical downflow geochemical evolution of meteoric ground water

19. Plateaus of Eh characteristic of the meteoric ground-water regime of the Oakville aquifer

20. Modern ground-water flow directions and boundaries of the Oakville aquifer in Live Oak and adjacent counties

21. Water well cross section through the George West district

22. Schematic ground-water flux and alteration history characteristic of mineralized parts of the Oakville aquifer

TABLES

1. Geochemical characteristics of alteration zones sampled in George West and Ray Point districts

2. Carbon isotope composition of selected Oakville samples

3. Geochemical characterization of principal ground-water regimes of the northwestern Gulf Coast Basin

4. Volumes of sulfide and associated water required for sulfidization of the lower Oakville aquifer in the George West district and environs


Citation
Galloway, W. E., 1982, Epigenetic Zonation and Fluid Flow History of Uranium-Bearing Fluvial Aquifer Systems, South Texas Uranium Province: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 119, 31 p.