408 



UNITED STATES MINERAL RESOURCES 



New Guinea and along the Indonesia-Sumatra arc. 

 (See fig. 48). Although the deposits are in belts of 

 volcanic activity, many major ore bodies are remote 

 from any young volcanic rocks. 



The six most productive mines or districts, and 

 their approximate production, are: 



Flasks 



Almaden, Spain 7,500,000 



Idria, Yugoslavia 3,000,000 



Monte Amiata, Italy 2,000,000 



Santa Barbara, Peru 1,500,000 



New Almaden, Calif 1,100,000 



New Idria, Calif 600,000 



Very brief descriptions of the geology of these 

 most important deposits will serve to show the di- 

 versity of occurrence that characterizes mercury 

 deposits. 



The famous Almaden mine, which has yielded 

 about one-third of the world production, is in an 

 area of folded and faulted shale, limestone, and 

 quartzite of Ordovician to Devonian age (Mamen, 

 1971). Igneous rocks are represented in the mine 

 area by narrow diabase dikes; the nearest major 

 intrusive is a granite batholith 25 kilometers away. 

 The ore bodies occur in three parallel vertical 

 quartzite beds that from the surface to a depth of 

 at least 750 meters are extensively replaced by cin- 

 nabar and locally impregnated with native mercury. 

 The most productive bed is 30 feet thick, and a 

 central zone about 10 feet thick contains in some 

 places more than 10 percent mercury. The minable 

 strike length is over 1,000 feet on the 17th level. 

 Average grade of ore treated is about 2 percent. 



The Idria mine in northwestern Yugoslavia lies 

 in a thick sequence of sedimentary rocks ranging 

 in age from Carboniferous to Eocene and occurring 

 in four superposed thrust sheets (Mlakar, 1964). 

 Most of the ore is formed by cinnabar that has re- 

 placed Permian and Triassic dolomite, but some 

 occurs in shale or sandstone. Individual ore bodies 

 are large, many having a plan area of over 20,000 

 square feet and extending through a vertical inter- 

 val of 200 feet. Ore has been mined to a depth of 

 1,300 feet. In recent years, an ingenious solution of 

 a fault problem led to the discovery of a wholly new 

 major ore zone offset 2 miles from that which has 

 been mined for the past 450 years. Because only 

 sparse native mercury occurred in the uppermost 

 thrust plate and rich cinnabar occurred in the plate 

 below, it has been suggested that the ore was formed 

 in Triassic time, before the thrust faulting, and 

 thereafter was protected from erosion by being cov- 

 ered by the overthrust plate. The near-surface na- 

 tive mercury is believed to be the result of postore 

 and postthrust upward migration of vapor from the 



main ore body. The grade of ore processed ranges 

 from 0.1 to 0.2 percent. 



The major mine of the Monte Amiata district, 

 central Italy, is the San Salvatore, which is on the 

 lower slopes of Monte Amiata volcano (Eckel, 1948). 

 The rocks below the lava are much deformed and 

 broken beds of Eocene limestone and shale. Ore 

 bodies of detrital cinnabar occur in curious solu- 

 tion caves in limestone, which are widest just be- 

 neath the lava and taper downward to narrow roots 

 several hundred feet below the contact. The ore 

 bodies were formed by rising hot solutions that first 

 deposited cinnabar just below the contact and later 

 dissolved the limestone, releasing the cinnabar to 

 accumulate in the caves. Other, deeper primary ore 

 bodies consist of cinnabar coatings on broken lime- 

 stone and shale. The grade of ore treated is about 

 0.4 percent. 



The Santa Barbara mine, high in the Andes near 

 Huancavelica, Peru, once rivaled Almaden, Spain, 

 in production but has had only minor production in 

 the past century. Mesozoic limestone, sandstone, and 

 shale are overlain by a great pile of Tertiary vol- 

 canic rocks and intruded by volcanic breccia. Tabu- 

 lar ore bodies occurred through a depth range of 

 about 1,000 feet in places where cinnabar filled pore 

 spaces or fractures in sandstone or voids in vol- 

 canic breccia. The average grade was about 2 per- 

 cent, though ore mined in recent years is much 

 leaner. 



The New Almaden mine in California (Bailey and 

 Everhart, 1964) is the greatest mercury producer 

 in North America, but most of its yield came prior 

 to World War I. In the mine area, folded and faulted 

 Mesozoic eugeosynclinal sedimentary and volcanic 

 rocks are intruded by serpentine, which in Tertiary 

 time was altered along its margins to silica- 

 carbonate rock. Rich ore occurs where silica- 

 carbonate rock was replaced by cinnabar along steep 

 parallel fractures, and large ore bodies were found 

 from the surface to depths of about 2,000 feet. The 

 grade of all ore recovered through the life of the 

 mine is about 4 percent. 



The New Idria mine in central California (Linn, 

 1968) is the second most productive mercury mine 

 in North America and was a major producer until 

 the spring of 1972. The mine is near the northern 

 margin of a pluglike mass of serpentine that has 

 arched upward and pierced through a thick Cre- 

 taceous shale-sandstone sequence. Steeply dipping 

 shale near the serpentine has been rendered brittle 

 through induration, and subsequently shattered. 

 Most ore bodies occur where cinnabar alone fills the 

 fractures or coats their walls, but some cinnabar and 



