Northwest Territories Geological Survey

The Mactung W (Cu,Au) deposit, Northwest Territories, is a scheelite-rich, calc-silicate skarn (17 Mt total mineral resources grading 0.97% WO3 and 0.078% Cu for a cutoff grade of 0.5%WO3) hosted in two distinct packages of Cambrian to Silurian aged limestone with pelite, referred to as the upper (units 3D-F) and lower (unit 2B) ore zones, separated by a thick unit of hornfelsed pelite (unit 3C). Recent studies on the nearby Cantung deposit show that apatite in skarn record petrogenetic processes. In order to understand the evolution of the Mactung deposit, and constrain chemical signatures of mineralizing fluids, multiple generations of apatite were characterized using classical petrographic techniques, hot cathodoluminescence (CL), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).

Representative skarn samples were collected from each stratigraphic unit of the deposit and examined for apatite. Apatite was observed in skarn from all stratigraphic units, including the upper and lower ore zones and unit 3C. However, apatite was most commonly observed in skarn replaced, hornfelsed, phosphatic (collophane-rich) pelite, intercalated with limestone, in unit 3D. Apatite occurred in all skarn types studied, including pyroxene-pyrrhotite, garnet-pyroxene, pyroxene, pyrrhotite and amphibole skarn. Textural evidence suggests that at least some apatite formed through the recrystallization of detrital collophane, as apatite were commonly distributed around partially dissolved phosphate nodules. Apatite was the earliest skarn phase within all skarn types, with the exception of some garnet-pyroxene and pyrrhotite skarn, where apatite and pyroxene appeared coeval. Scheelite and titanite (a minor skarn phase), likely formed coeval or immediately after apatite based on textural relationships.

Apatite exhibits five distinct fluorescence colours under CL, also distinguished by texture: i) irregular masses at the cores of oscillatory zoned apatite fluoresced light to dark grey, ii) the interior of oscillatory zoned apatite fluoresced blue, iii) the interior and/or rims of oscillatory zoned apatite fluoresced green (becoming lighter towards the rim), iv) the rims of oscillatory zoned apatite and entire unzoned crystals fluoresced yellow, and v) small patches of altered apatite rims fluoresced orange. Preliminary LA-ICP-MS data show that the different coloured florescent apatite have distinct rare earth element (REE) abundances. Dark grey apatite showed relatively low total REE contents (~590 ppm SREE, La-Lu), with relatively high LaN/YbN = 33 and negative Eu anomaly (Eu/Eu* = 0.4; where Eu* = vSmN*GdN). Green apatite contained moderate REE contents (average 1170 ± 180 ppm, 1s; n = 14), with LaN/YbN = 7 (± 3, 1s) and Eu/Eu* = 0.6 ± 0.1. Yellow apatite contained high REE contents (average 2130 ± 640, 1s; n = 23), with LaN/YbN = 4 (± 2, 1s) and Eu/Eu* = 0.4 ± 0.1. Orange apatite showed the highest concentration of REE (average 4040 ± 130 ppm, 1s; n = 2), with LaN/YbN = 1.83 (± 1, 1s) and Eu/Eu* = 0.1 ± 0.0. These preliminary results indicate that the breakdown of collophane likely influenced the HREE abundance of green apatite in unit 3D. As more data is collected, apatite compositions will be used to describe the evolution of skarn fluids.