Field, isotopic, and chemical studies of tourmaline-bearing rocks in the Belt–Purcell Supergroup: genetic constraints and exploration significance for Sullivan type ore deposits

Abstract

The Sullivan Pb–Zn–Ag massive sulphide deposit (located in the mid-Proterozoic Belt–Purcell Supergroup near Kimberley, British Columbia) is associated with a large tourmalinite alteration zone. A petrologically similar, but barren, alteration zone has recently been discovered at Trestle Creek, Idaho. Because of the importance of the Sullivan deposit, several methods of exploration involving tourmaline have recently been proposed in the literature. To test these proposals, we compared the alteration zone at Sullivan (using literature data) with that at Trestle Creek. The two areas can clearly be distinguished using Mg content in tourmaline and δ18O in albitite, can probably be distinguished using δ18O in tourmaline-bearing rocks, but cannot be distinguished using alteration mineralogy alone. In addition we found that tourmaline occurs in at least eight different geologic settings in the Belt–Purcell Supergroup (one of which includes the alteration zones at Sullivan and Trestle Creek).Assuming a high water/rock ratio, a moderate to very high degree of oxygen-isotope exchange, and a hydrothermal fluid with δ18O ≥ 0, a series of hydrothermal models can be calculated for the alteration zones at Sullivan and Trestle Creek. The most realistic models at both localities involve temperatures of about 200–250 °C and fluids of marine derivation with a variable 18O shift. The isotopic differences between Sullivan and Trestle Creek are probably the result of a greater degree of fluid evolution and higher temperatures at Sullivan. Oxygen isotopes, hydrogen isotopes, and mineral chemistry of tourmaline from other geologic settings in the Belt–Purcell Supergroup suggest deposition from fluids of marine origin under various temperature conditions.