Origin of Silver-Copper-Lead Deposits in Red-Bed Sequences of Trans-Pecos Texas

RI0145. Origin of Silver-Copper-Lead Deposits in Red-Bed Sequences of Trans-Pecos Texas: Tertiary Mineralization in Precambrian, Permian, and Cretaceous Sandstones, by J. G. Price, C. D. Henry, A. R. Standen, and J. S. Posey. 65 p., 37 figs., 15 tables, 1 appendix, 1985. ISSN: 0082335X: Print.


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ABSTRACT
Silver deposits occur in Precambrian, Permian, and Cretaceous red-bed sequences near Van Horn, Texas. These deposits are geochemically similar and contain economically important quantities of silver, copper, and lead, as well as anomalously high amounts of arsenic, zinc, cadmium, and molybdenum. Gold is not enriched. Primary minerals include chalcopyrite, tennantite-tetrahedrite, bornite, galena, sphalerite, acanthite, pyrite, marcasite, barite, and calcite.

The deposits are dominantly steeply dipping veins. Strata-bound occurrences are near veins or closely spaced fractures. Structural evidence, including orientations of veins, relative ages of fractures, and relationships to major tectonic events in the region, suggests that the most likely time of mineralization was during late Basin and Range extensional deformation. Ore deposition probably occurred at least 18 m.y. after the period of voluminous silicic volcanism (38 to 28 m.y.a.) in the Trans-Pecos region.

Other features indicate that, in contradiction to a hypothesis suggested by previous workers, the deposits did not form as a result of middle Tertiary magmatism. (1) Centers of igneous activity were distant from the sites of mineralization. (2) Potentially reactive limestones above and below the ore zones in red beds are generally unmineralized. (3) Zones of argillic, phyllic, and propylitic alteration typical of igneous-hydrothermal veins are absent. (4) Characteristic igneous hydrothermal gangue minerals such as quartz and fluorite are rare or absent. (5) Homogenization temperatures of fluid inclusions in barite and calcite suggest formation temperatures in the range of 120°C to 170°C, that is, lower than temperatures typical in copper-lead-zinc-bearing igneous-hydrothermal veins. These temperatures are higher than those usually attributed to strata-bound, red-bed copper deposits. The elevated formation temperatures are the result of high heat flow in the Basin and Range province of Texas at the time of mineralization; they are not the result of igneous activity.

The hypothesis developed in this study on the origin of the veins involves a rise of moderate-temperature, moderately saline hydrothermal fluids along Basin and Range fractures and precipitation of metal sulfides in response to mixing with shallow ground water. This hypothesis has implications for exploration of additional deposits in Trans-Pecos Texas and elsewhere.

Keywords: Texas, Trans-Pecos, economic geology, metals, silver, copper, lead, red beds



CONTENTS

ABSTRACT

INTRODUCTION

Purpose of study

Scope of study

Previous work

History of mining

Methods of investigation

REGIONAL GEOLOGY

Stratigraphy

Precambrian rocks

Paleozoic rocks

Mesozoic rocks

Cenozoic rocks

Limited lithologic control of mineralizationand tectonics

Precambrian deformation

Paleozoic deformation

Mesozoic· Cenozoic activity

Laramide deformation

Middle Tertiary magmatism

Basin and Range extension

STRUCTURAL CONTROLS OF VEIN MINERALIZATION

East-northeast and northeast veins in Precambrian rocks

East-northeast and northeast veins in Cretaceous rocks

Structures in Permian host rocks

Changing stress orientations with time and probable age of vein mineralization

Other mineralization along Basin and Range faults

MINERALOGY AND GEOCHEMISTRY OF ORES AND HOST ROCKS

Primary ore minerals

Secondary minerals

Gangue minerals

Wall-rock alteration

Geochemical character of the ores

Comparison with igneous-hydrothermal ores

Comparison with other ores in red beds

Fluid-inclusion studies

Samples

Data from heating

Data from freezing

Comparison with other areas of known hydrothermal activity associated with Basin and Range faulting

Carbon and sulfur isotope studies

ORIGIN OF DEPOSITS

Review of observations and conclusions

Unanswered questions

EXPLORATION POTENTIAL

ACKNOWLEDGMENTS

REFERENCES

APPENDIX: STRUCTURAL AND CHEMICAL DATA

Figures

1. Locations of silver-copper-lead deposits in sandstones near Van Horn, Texas

2. Geology and locations of silver-copper-lead deposits in Precambrian rocks of the Van Horn - Allamoore district

3. Glory hole east of the East shaft, Hazel mine

4. Stratiform ore at the Sancho Panza mine

5. Nearly vertical vein cutting Cretaceous Yucca Formation conglomerate at the Rossman prospect, Indio Mountains

6. Headframe at the East shaft, Hazel mine

7. Simplified geologic map of the Plata Verde mine, Trans-Pecos Texas

8. Geologic map of the central Indio Mountains

9. Stratigraphic column, Van Horn area

10. Yucca Formation, 'Central Indio Mountains

11. Eastward-dipping Cretaceous sedimentary rocks east of the Indio Fault, Indio Mountains

12. Cross section A-A', illustrating major structural features in the Indio Mountains

13. Orientations of faults, slickenlines, and corresponding principal stress axes, Laramide deformation, Indio Mountains

14. Strike rosettes of veins and dikes that were emplaced during the major period of igneous activity, 32 to 41 m.y.a., in the Trans-Pecos region

15. Strike rosettes of early Miocene Rim Rock dikes

16. Orientations of faults and slickenlines, Basin and Range deformation, Indio Mountains

17. Veins at the Hazel mine in 1891

18. Orientations of mineralized fractures at the Mohawk mine.

19. Calcite-filled fractures in a streambed at the Hazel mine

20. Typical fracture intersections in the Hazel Formation

21. Orientations of fractures at mines in the Hazel Formation and at nearby exposures in the Hueco Formation

22. Fracture patterns at and near silver-copper-lead deposits in the Precambrian Hazel Formation and the Cretaceous Yucca Formation

23. Proposed orientations of stress during Late Cretaceous through Recent(?) time, Trans-Pecos Texas

24. Open cut of manganese-barite vein along a Basin and Range normal fault at the Mayfield prospect

25. Photomicrograph of Hazel ore from glory hole, East shaft area

26. Photomicrograph of sample collected from the dump at the Pecos mine

27. Photomicrograph of clusters of radiating hemimorphite blades, Purple Sage mine, Indio Mountains

28. Photomicrograph of quartz overgrowths, Finlay prospect, Indio Mountains

29. Photomicrographs of gray, weakly altered sandstone, Pecos mine

30. Photomicrograph of disseminated copper mineralization at the Rossman prospect, Indio Mountains

31. Geochemical characteristics of ores and unmineralized rocks in the Precambrian Hazel Formation

32. Geochemical characteristics of Plata Verde ores and unmineralized rocks in the Powwow Member of the Permian Hueco Limestone

33. Geochemical characteristics of mineralized and unmineralized rocks in the Cretaceous Yucca Formation

34. Two-phase (liquid-vapor) fluid inclusion in barite, Pecos mine

35. Homogenization temperatures of fluid inclusions from the Pecos mine in the Precambrian Hazel Formation and from the Finlay prospect in the Cretaceous Yucca Formation

36. Freezing temperatures of fluid inclusions from the Pecos mine in the Precambrian Hazel Formation and from the Finlay prospectin the Cretaceous Yucca Formation

37. Homogenization temperatures of fluid inclusions from the Mayfield manganese prospect

Tables

1. Estimated total production, silver grade, copper-to-silver ratio, and silver-to-gold ratio of silver-copper-lead deposits in red-bed sequences in Trans-Pecos Texas

2. Estimated ore, copper, and silver production from mines in the Van Horn - Allamoore district

3. Average copper grade, silver grade, and copper-to-silver ratio, based on production records from the Van Horn - Allamoore district

4. Tectonic history of the Van Horn area, Trans-Pecos Texas

5. Slickenline data that correspond to only one of four probable phases of deformation, Indio Mountains

6. Percentages of fractures of the four major trends in the Hazel Formation that exhibit extensional features

7. Relative ages of fractures determined from fracture intersections in the Hazel Formation

8. Vein mineralogy, Precambrian Hazel Formation, Van Horn area

9. Ore mineralogy, Powwow Member of the Permian Hueco Limestone, Plata Verde mine

10. Mineralogy, hosts, and controls on silver-copper-lead deposits, Cretaceous Yucca Formation, Indio Mountains

11. X-ray powder diffraction data on a sulfur-rich beudantite-group mineral, Finlay prospect, Indio Mountains

12. Electron-microprobe analysis of a sulfur-rich beudantite-group mineral, Finlay prospect, Indio Mountains

13. Silver grades and silver-to-gold ratios in middle Tertiary, igneous hydrothermal ores in the Trans-Pecos region

14. Carbon isotope data from the Eureka prospect and Precambrian carbonates

15. Sulfur isotope data on vein barite, Van Horn area

Appendix Tables

A1. Minor fault orientations in the Indio Mountains

B1. Geochemical data on ores in the Hazel Formation

B2. Geochemical data on weakly mineralized sandstone and siltstone wall rock of the Hazel Formation

B3. Geochemical data on unmineralized sandstone and siltstone of the Hazel Formation

B4. Geochemical data on other types of copper ore from the Hazel and Allamoore Formations

B5. Geochemical data on mineralized Yucca sandstones and conglomerates, Indio Mountains

B6. Geochemical data on weakly mineralized Yucca sandstones, Indio Mountains

B7. Geochemical data on unmineralized Yucca sandstones, Indio Mountains, and on average sandstone

B8. Geochemical data on limestone and calcite in the Yucca Formation, Indio Mountains



Citation
Price, J. G., Henry, C. D., Standen, A. R., and Posey, J. S., 1985,  Origin of Silver-Copper-Lead Deposits in Red-Bed Sequences of Trans-Pecos Texas: Tertiary Mineralization in Precambrian, Permian, and Cretaceous Sandstones: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 145, 65 p.