INVESTIGATION INTO POTENTIAL MAGMATIC CONTRIBUTIONS TO VOLCANOGENIC MASSIVE SULFIDE DEPOSITS: METHOD DEVELOPMENT FOR LA-ICP-MS ANALYSIS OF FLUID INCLUSIONS AND CRITICAL ASSESMENT OF THE WINDY CRAGGY DEPOSIT, NORTHWESTERN BRITISH COLUMBIA, CANADA

Abstract

In recent years, the existence of variable magmatic contributions to the mineralizing fluids in the formation of volcanogenic massive sulfide (VMS) deposits has been gaining acceptance in the scientific community. The world-class Windy Craggy Cu-Co-Au deposit (>300 MT @ 2.12 wt.% Cu) located in northwestern British Columbia is the ideal location to investigate this potential for multiple reasons: 1) a previously completed study documented fluid inclusions with anomalously high salinities relative to other VMS deposits; 2) Windy Craggy has an atypically large size and Cu grade compared to other VMS deposits. In this study a method was developed using an excimer (193 nm) laser ablation system interfaced to a quadrupole inductively coupled plasma mass spectrometer, which proved effective in quantifying key metals and metalloids in the inclusion fluids considered by many to be indicative of magmatic contributions to hydrothermal ore deposits. Despite the highly transient nature of the signal from the low salinity (most 6 – 16 wt.% eq. NaCl) inclusions, , of the total 34 elements that were monitored, Na, Mg, K, Ca, Mn, Fe, Co, Cu, Zn, Sr, Sn, Ba, Ce, Pb and Bi were consistently detected and quantified. Furthermore, Cl, Sb, Cd, Mo, Rb, Br, and As were also detected in a significant number of inclusions. Positive correlations between Cu, Mn, Zn, Sb, Sn and Bi were observed. The Ca/Na values in the fluid inclusions are greater than seawater and generally similar to values reported from other maficdominated VMS systems. The host lithologies and mineralization (footwall argillite, relatively fresh to highly altered footwall mafic volcanic rocks and stringer and massive sulfide) from Windy Craggy were also bulk geochemically analysed. The fluid inclusions show similar trends and overlap the host rocks in Fe vs Mn and Cu vs Zn, but show clear excesses over the host rocks, and in particular the sulfide-rich samples, in Sb, Sn and Bi that are commonly ascribed as indicative of magmatic contributions in other deposit types. These excess values strongly indicate a direct magmatic contribution to the Windy Craggy ore-forming fluids, as opposed to simply reflecting leaching of metals from the footwall sedimentary and igneous rocks.