Fluid inclusions and stable isotope study of quartz-tourmaline veins associated with beryl and emerald mineralization, Kafubu area, Zambia

Abstract

The Kafubu Emerald Area in Zambia is an important producer of gemstone-quality emeralds. These are hosted mostly by metasomatic phlogopite-rich zones (max. 2 m wide) that rim numerous pegmatite dykes and epigenetic quartz-tourmaline veins in Mg-rich metabasites. Where these dykes/veins occur in other rock types, the phlogopite-rich zones are either missing, or are very narrow and free of emeralds. Three types of hydrothermal quartz-tourmaline mineralization were identified: banded fine-grained rock, discordant to semiconcordant veins and late veins. A fluid inclusion study of the discordant type of veins revealed the presence of aqueous-carbonic and aqueous fluids. The earliest fluids (mostly trapped in the quartz, rarely in beryl) represent complex H 2O–CO 2–N 2–CH 4–NaCl–CaCl 2 mixtures (X CO2: 0.25 to ∼0.04) with traces of N 2 and CH 4 only (generally < 3 mol.% in gaseous phase). Aqueous fluids are younger and represent primary inclusions in beryl and secondary inclusions in quartz. As for their salinity, low-, medium-, and high-salinity fluids were distinguished (0–10, 12–25, and > 34 wt.% NaCl eq., respectively). Each salinity class usually comprises both aqueous-carbonic and aqueous fluids. Solid phases present in inclusions were studied by Raman, and/or by SEM-EDX. Daughter phases include: halite, sylvite, and Fe–Mn-carbonate. Accidental phases are quartz, beryl, carbonate, nahcolite, pyrophyllite(?) and muscovite(?). The oxygen isotope composition (δ 18O) of quartz and tourmaline is quite homogeneous (10.9 ± 1‰ and 8.4 ± 0.7‰ SMOW respectively) and yield vein-formation temperatures of 350–450 °C and parent fluid composition + 5 to + 7‰ SMOW. Three evolution stages of quartz-tourmaline ± beryl mineralization are inferred from fluid isochores and isotope thermometry: early (620–400 MPa and 450–350 °C), main (400–150 MPa and 450–350 °C) and late (∼250–200 MPa and 350–280 °C). The early stage includes formation of banded quartz-tourmaline rocks and of some quartz-tourmaline veins. Most of the studied quartz-tourmaline veins and all studied beryls were formed within the main stage. Late stage corresponds to the formation of late quartz-tourmaline veins with lamellar tourmaline-quartz intergrowths.