The origin of mineralizing hydrothermal fluids recorded in apatite chemistry at the Cantung W–Cu skarn deposit, NWT, Canada

The distribution and abundance of major, minor and trace elements in hydrothermal and magmatic apatite associated with the Cantung W–Cu skarn deposit, Northwest Territories, Canada was studied to elucidate the origin of mineralizing fluids. Skarn-hosted apatite of hydrothermal origin occurs in carbonate-hosted garnet-clinopyroxene, amphibole and biotite skarn, and precedes scheelite and silicate crystallization. Most skarn-hosted apatites are compositionally similar to magmatic apatite with respect to Mn, Fe, and Pb contents, but show very different Sr contents, as well as chondrite-normalized rare-earth element (REE) and Y abundance patterns. Skarn-hosted apatite exhibits four different patterns: (i) negatively sloped (average La_N/Yb_N range: 3.9–10) patterns with negative Eu anomalies (average Eu_N/Eu* ranging from 0.3 to 0.4; whereby Eu* = √[Sm_N × Gd_N]) are most common and reflect equilibrium with fluids derived from a felsic magma, (ii) flat patterns (average La_N/Yb_N = 1.3) and strong negative Eu anomalies (average Eu_N/Eu* = 0.09), record the evolution of the felsic magma during fractional crystallization of feldspar and light REE (LREE)-rich phases, (iii) steeply dipping (average La_N/Yb_N = 127) patterns with negative Eu anomalies (average Eu_N/Eu* = 0.6) indicate diffusion of heavy REEs (HREEs) into surrounding garnet, or crystallization of apatite from metamorphic fluids, and (iv) concave up patterns with positive Eu anomalies (Eu_N/Eu* ranging from 1.5 to 8.8) suggest equilibration with fluid derived from an amphibole-saturated mafic magma. Mafic magmas in the system are evidenced by the presence of high-Mg mafic melt inclusions in xenocrystic magmatic apatite in the nearby, early (with respect to skarn formation) Mine Stock monzogranite pluton, and the presence of lamprophyre dykes near sites of mineralization. The data indicate that high W contents in fluids may ultimately be sourced from mafic magma through mixing with a felsic melt in a deep crustal magma chamber. Skarn-associated apatite can be differentiated from magmatic apatite by relative abundances of Sr and REE.