| 11. | The inner crater is under high and shows pervasive hydrothermal alteration and abundant fractures.
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| 12. | It typically occurs as a hydrothermal alteration product of primary phosphate minerals in granite pegmatites.
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| 13. | The mineral tends to form in granite pegmatites by hydrothermal alteration at a late stage.
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| 14. | Hydrothermal alteration has occurred on these rocks.
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| 15. | Hydrothermal alteration occurred later, 20.2 ?0.35 million years ago.
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| 16. | This process is called hydrothermal alteration.
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| 17. | Hydrothermal alterations have generated fault systems.
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| 18. | The northern mine is known as Azufrera Esperanto and associated with an area of hydrothermal alteration.
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| 19. | Changes in composition can be caused by processes such as weathering or metasomatism ( hydrothermal alteration ).
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| 20. | This process, called hydrothermal alteration, is a major way in which ore deposits are produced.
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