Naturally occurring borates are the major economic source of boron. Borates were first used over 4,000 years ago in precious-metal working and are now essential components of modern industry. Although borates have been exploited from other sources, three minerals from non-marine evaporites now form the major commercial sources of borate – borax, colemanite and ulexite. These major commercial deposits are associated with Neogene volcanism in tectonically active extensional regions at plate boundaries. The most important continental borate provinces are located in the USA, Argentina, Chile, Peru, and China, with the largest borate reserves in the world being found in western Anatolia (Turkey).
The lower portion of the Paleoproterozoic Northern Liaohe Group, Liaoning, northeast China, comprises metavolcanics (tuff-dominated) and metasediments (mostly arkoses and dolostones) that host stratiform-strata bound Fe(Cu) sulfides and Ca(Ba) sulfates. The ore-bearing sequence consists of, in ascending order, pyroclastic rocks, terrigenous clastics, and carbonates, all of which have been metamorphosed to greenschist amphibolite grade. Extensive albite-rich breccias are present at the base of the succession and as pipes crosscutting a variety of lithologic units in the Lieryu Formation. The geometry, geochemistry, and associated structures of these albite-rich rocks suggest that they are metamorphosed salt domes.The sulfide-sulfate Fe(Cu) ores occur in a transitional zone between tuffs and dolomitic carbonates, with those in the upper cycle being economically more important. The sulfides were precipitated at two sites: around venting centers as breccias, lenses, and veins; and as stratiform pyrite-chalcopyrite associated with anhydrite that is distal to the salt domes. Within each mining district the proximal ores are associated with brecciated, albite- and tourmaline-rich rocks. The sulfides in the proximal ores have delta 34 S values of 8.9 to 12.7 per mil, whereas the distal ores have delta 34 S values of 2.6 to 8.8 per mil for the sulfides and 7.9 to 19.0 per mil for the anhydrite; suggesting a sulfate-dominated sulfur source, with the pyrite delta 34 S variations arising mainly from decreasing temperatures away from the centers of the deposits.The origin of the deposits is best described by a model in which sulfate-carbonate rocks were deposited in peripheral sinks during salt diapirism. The salt domes then acted as focuses for subsequent hydrothermal venting and were the centers of sulfide mineralization. The sulfides precipitated during interaction of the reduced, metal-bearing hydrothermal fluids with the in situ sulfates that were the major source of the sulfur.
Abstract Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass‐wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (∼930 to ∼900 ka, ∼810 to ∼760 ka, and ∼190 to ∼120 ka) that coincide with periods of increased volcano instability and mass‐wasting. The youngest of these periods marks the peak in activity at the Soufrière Hills volcano. The largest flank collapse of this volcano (∼130 ka) occurred toward the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km 3 ) flank collapses of the Soufrière Hills edifice, and their timing also coincides with periods of rapid sea level rise (>5 m/ka). Available age data from other island arc volcanoes suggest a general correlation between the timing of large landslides and periods of rapid sea level rise, but this is not observed for volcanoes in intraplate ocean settings. We thus infer that rapid sea level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean‐island settings.
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