v3.19.1

PROPERTY, PLANT, EQUIPMENT AND MINERAL PROPERTIES (Schedule of Property, Plant, Equipment and Mineral Properties) (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Property, Plant and Equipment [Line Items] |  |
Total depreciable assets | $ 401,815 | $ 388,284
Accumulated depreciation | (167,168) | (141,818)
Total depreciable assets, net | 234,647 | 246,466
Mineral properties and development costs | 139,418 | 138,841
Accumulated Depletion | (31,197) | (26,840)
Total depletable assets, net | 108,221 | 112,001
Total property, plant, equipment, and mineral properties, net | 346,209 | 364,542
Buildings and Plant [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Total depreciable assets | 81,429 | 79,757
Machinery and equipment [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Total depreciable assets | 241,977 | 234,861
Vehicles [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Total depreciable assets | 5,669 | 4,835
Office equipment and improvements [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Total depreciable assets | 13,779 | 12,637
Ponds and land improvements [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Total depreciable assets | 58,961 | 56,194
Land [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Total depreciable assets | 519 | 519
Construction in progress [Member] |  |
Property, Plant and Equipment [Line Items] |  |
Total depreciable assets | $ 2,822 | $ 5,556

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