Perspectives on premetamorphic stratabound tourmalinites

[摘要] Stratabound tourmalinites are metallogenically important rocks that locally show a close spatial association with diversetypes of mineralization, especially volcanogenic massive sulfides (VMS) and clastic-dominated (CD) Zn–Pb deposits.These tourmalinite occurrences span the geologic record from Eoarchean to Jurassic. Host lithologies are dominated byclastic metasedimentary rocks but in some areas include metavolcanic rocks, marble, or metaevaporites. Strataboundand stratiform (conformable) tourmalinites commonly display sedimentary structures such as graded beds, cross-beds,and rip-up clasts. In most cases, field and microtextural relationships are consistent with a synsedimentary to earlydiagenetic introduction of boron as a precursor to tourmaline formation. Whole-rock geochemical data for major, trace,and rare earth elements (REE) provide valuable insights into tourmalinite origins. Al-normalized values relative to thosefor least-altered host metasedimentary rocks suggest that tourmalinites in proximal settings at or near hydrothermal ventsites characterized by high fluid/rock regimes (e.g., Sullivan Pb–Zn–Ag deposit, Canada) have very different signaturesthan those in low fluid/rock, distal settings (e.g., Broken Hill Pb–Zn–Ag deposit, Australia). The high fluid/rock regimesat Sullivan show large mass changes of +60 % for Mg and +180 % for Mn, as well as large variations in abundancesof light and middle REE. In contrast, tourmalinite formation in low fluid/rock regimes yields minimal Al-normalizedchanges in major elements, trace elements, and REE. Boron isotope values of tourmalinite-hosted tourmaline varywidely from –26.1 to +27.5 ‰, and are attributed mainly to boron sources (e.g., sediments, evaporites) with generallyminor influence from processes such as formational temperature, fluid/rock ratio, and secular variation in seawaterδ11B values. Laterally extensive stratiform tourmalinites formed mainly by syngenetic or early diagenetic processes onor beneath the seafloor. The syngenetic process is attributed to the interaction of vented B-rich brines with aluminousminerals in sediments, whereas the diagenetic process involves the selective replacement of aluminous sediments byB-rich fluids. Modern examples of tourmalinites, as yet undiscovered, may exist in metalliferous sediments of the RedSea and the eastern Pacific Ocean, in altered volcaniclastic sediments within active seafloor-hydrothermal systems ofthe South Pacific, and in hydrothermal mounds and vents associated with mafic sill complexes in extensional basins asin the North Sea and South China Sea. Stratabound tourmalinites that contain base-metal sulfides, high Mn concentrations (>1 wt. % MnO), or positive Eu anomalies can be valuable exploration guides for base-metal sulfide deposits insedimentary and volcanic terranes.

[学科分类] 自动化工程

[关键词] Tourmalinite;Metallogeny;Syngenetic;Diagenetic;Boron Isotopes;Genetic Models