Tourmalines from Appalachian-Caledonian massive sulfide deposits; textural, chemical, and isotopic relationships

Abstract

Tourmaline is a common gangue mineral in several strata-bound massive sulfide deposits of the Appalachian-Caledonian orogen. Most of the tourmaline-rich deposits occur in dominantly metasedimentary terranes such as Bleikvassli, Norway; Black Hawk, Maine; Ore Knob, North Carolina; and Elizabeth, Vermont. Minor amounts of tourmaline are also known in association with volcanic-hosted deposits in the Appalachians and in the Trondheim district of Norway. The tourmaline forms isolated euhedral crystals or crystal aggregates in massive sulfide and adjacent wall rocks; in a few areas, it is found in conformable tourmaline-rich layers (tourmalinites). Associated minerals may include quartz, plagioclase, phlogopite, and dolomite, with pyrrhotite, chalcopyrite, galena, and sphalerite. Many of the tourmaline crystals are optically zoned, in which brown zones concentrically alternate with pale yellow or green zones. A few specimens of tourmaline intergrown with massive sulfide (e.g., Ore Knob) display fine, delicate growth lamellae similar to those commonly observed in hydrothermal (vein) sphalerites, skarn garnets, and sphalerites from the massive sulfide deposits of the Kuroko district, Japan.Detailed geochemical studies indicate that the tourmalines have characteristic major and trace element compositions. Electron microprobe analyses show that, with rare exception, the tourmalines are Mg-rich dravites (avg. FeO/(FeO + MgO) = 0.21). Abrupt transitions between color zones within some tourmaline are accompanied by relative variations in FeO/(FeO + MgO) of as much as 30 percent; yellow-brown zones are more Mg rich than blue or green zones. In contrast, the fine growth lamellae are not characterized by such variations in chemistry. Spectrographic analyses of pure mineral separates indicate that the tourmalines also contain high average contents of Cr (78 ppm), Cu (77 ppm), Pb (114 ppm), Sr (98 ppm), and V (195 ppm), while uniformly low in Li (7 ppm) and Mn (382 ppm). In all analyzed samples, fluorine is much more abundant than chlorine; the average F/Cl ratio is 33.Tourmalines from tourmalinites and massive sulfide deposits are typically characterized by heavier delta 18 O values (9.5-15.5ppm) and less variable, heavier delta D values (generally -45 to -60ppm) than tourmalines from unmetamorphosed igneous pegmatites. The latter typically have delta 18 O < 9.5 per mil and delta D < or = -60 per mil. Oxygen isotope fractionations between quartz and tourmaline (delta = 1.27-4.01) do not correlate directly with inferred grade of subsequent metamorphism, although in areas of high metamorphic grade the delta values approach those of igneous quartz-tourmaline pairs. The delta 18 O values of quartz and tourmaline are believed to reflect variation in the oxygen isotope compositions of the host lithologies. The similar, narrow range in hydrogen isotope composition for tourmaline in all studied deposits is compatible with a modified seawater origin for the tourmaline- and sulfide- forming fluids.The distinctive textural and chemical characteristics of the tourmalines reported here have both exploration and genetic significance. Though it may have undergone recrystallization and some accompanying chemical and isotopic exchange during regional metamorphism, tourmaline in the massive sulfide deposits is an early formed, perhaps diagenetic, mineral in which fine lameliar zoning may be a preserved premetamorphic feature. In any case, the tourmalines from the massive sulfide deposits are Mg-rich dravites with trace element and stable isotope compositions substantially different from those of the Fe-rich schorls typical of most granitic rocks. These geochemical and isotopic signatures are attributed to the original environment of formation of the associated sulfide deposits and suggest that Mg-rich tourmaline may be a valuable prospecting guide in the search for massive sulfide deposits.