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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