Characterization of apatite within the Mactung W (Cu,Au) skarn deposit, Northwest Territories: implication for the evolution of skarn fluidsThursday, November 21, 2019 - 11:00am to 11:20am Theatre One
The Mactung W (Cu,Au) deposit, Northwest Territories, is a scheelite-rich, calc-silicate exoskarn located at MacMillan Pass, Selwyn Basin. Cretaceous magmatism and associated granitic intrusions led to ore development in two distinct packages of Cambrian to Silurian-aged limestone interbeded with pelite, referred to as the upper (units 3D-F) and lower (unit 2B) ore zones. Apatite is an accessory mineral occurring at Mactung, which accommodates a variety of trace elements within its crystal structure thereby recording the chemical evolution of fluids in geological systems. Multiple generations of skarn-hosted apatite from Mactung were characterized in order to understand the skarn paragenesis, chemical evolution and signatures of mineralizing fluids. Petrography and rare-earth element (REE) abundances constrain four generations of fluorapatite, that each recorded distinct chemical stages in the evolution of the skarn system. Type-i apatite occurs with detrital phosphate nodules. This apatite contains variable ΣREE+Y (1314 ± 821 ppm, 1σ, n = 78) and has negatively-sloping chondrite-normalized REE+Y (REE+YN) patterns with variable LREEN/HREEN (LaN/LuN = 27 ± 30) and weak Eu anomalies (EuN/EuN* = 0.6 ± 0.2; Eu* = √SmN*GdN). The REE abundance in type-i apatite is similar to that of the phosphate nodules, and both have similar REE abundances as in phosphate nodules reported in unaltered Devonian-aged calcareous mudstone of Howards Pass, Selwyn Basin. The similarities in REE suggest that type-i apatite is a recrystallization product of phosphate nodules, likely during isochemical contact metamorphism. Type-ii apatite occurs in anhydrous prograde skarns and show high abundances of REE, and flat lying REE+YN patterns (e.g., ΣREE+Y = 17194 ppm; LaN/LuN= 3.5; n = 31) with negative Eu anomalies (e.g., EuN/EuN* = 0.1). Hydrothermal type-iii apatite is associated with quartz-scheelite veins, which cross-cut prograde skarn. This apatite contains very high ΣREE+Y (7752 ± 496 ppm, n = 3) and exhibit bowl-shaped REE+YN patterns, corresponding to low MREE (LaN/SmN = 8 ± 0.3; SmN/LuN = 0.3), and no Eu anomaly. Lastly, type-iv apatite is hosted in hydrous retrograde skarns and shows bowl-shaped to negatively sloped REE+YN patterns, characterized by low MREE content (e.g., LaN/SmN = 10.2; SmN/LuN = 0.3; n = 17) and positive Eu anomalies (e.g., EuN/EuN* = 14.3). The paragenesis and distinct REE+YN patterns of hydrothermal type-ii, type-iii and type-iv apatite are explained by the presence of two chemically distinct ore fluids during the different stages of mineralization. The first fluid formed type-ii apatite, prograde skarn and early, fine-grained scheelite. The second fluid was associated with late-stage quartz veining, retrograde skarn, and coarse-grained scheelite. The REE+YN patterns of type-iii and type-iv apatite record a second fluid that underwent MREE fractionation either through crystallization of these apatites, or during the partial dissolution and remobilization of earlier scheelite. Preliminary cathodoluminescence images and trace element data from coeval scheelite show evidence for two generations of scheelite with distinct REE signatures, supporting the interpretation of two ore fluids. The trace element compositions of scheelite, and radiometric ages of scheelite and apatite, are currently being investigated to further constrain the chemical evolution and paragenesis of these two ore fluids.