GEOLOGICAL OCCURRENCE
Deposits of magnesite are widely distributed throughout the world and occur in two distinct forms, amorphous and crystalline. Amorphous magnesite, the most common form, is fine-grained, and compact; it is usually found in veins or masses in serpentine resulting from the alteration of magnesia-rich rocks of the peridotite family. To this group belong the Grecian deposits, nearly all the California deposits, and those in Mexico, Venezuela, and other parts of the world. Crystalline magnesite is medium to coarse grained, and occurs as masses in limestone, dolomite or associated sediments which have been metamorphosed. The principal deposits of this class are those in Austria, Hungary, Quebec, and Washington.
Deposits of magnesite are regarded as having originated in three ways. The massive non-crystalline variety, such as that in California and Greece, is believed to have been formed by the decomposition of serpentine. Magnesite deposits near Bissel, California, and on Muddy River, near St. Thomas, Nevada, are said to be of sedimentary origin. The Austro-Hungarian, Washington, and Quebec deposits are regarded as resulting from the replacement of calcareous sedimentary rocks by magnesian-bearing solutions.
Magnesite deposits that occur as veins in connection with serpentinized magnesian rocks probably are formed both from the breaking down of the serpentine-making minerals and from the serpentine itself. It seems probable that usually both serpentine and magnesite are formed in the process of decay of the original minerals in peridotite and the allied basic rocks, and that during the decay of the serpentine the formation of magnesite continues. In any case the magnesia or magnesian mineral is changed to carbonate, dissolved by percolating water charged with carbon dioxide, and precipitated in cracks and crevices as veins. When formed in this way the magnesite occurs in large and small veins, lenses, and stockwork, and its distribution and extent are erratic. It seems fair to assume that these deposits may extend to the limit of depth of easily circulating surface waters, which in favorable conditions may be several hundred feet. Faulting, on the other hand, is as likely to cut the veins off in depth as in length. Any estimate of available tonnage of magnesite in deposits of this type therefore is unwarranted in advance of development work.
Sedimentary deposits such as those of Bissel, California, and near St. Thomas, Nevada, are by their nature more regular in occurrence, and their tonnage can be estimated from the outcrop in natural exposures and prospects.
Replacement deposits like those in Washington and Quebec are not so regular as the sedimentary deposits, but are more regular than the veins, and tonnage estimates may be based on the surface exposure and an assumed depth of 50 to 100 feet.