Fluorapatite fenitization and gold enrichment in sheeted trondhjemites within the Destor–Porcupine fault zone, Taylor Township, Ontario
Authors: King R. W. , Kerrich R. 
Published: April 2011

Pronounced fluorapatite mineralization is present in sheeted trondhjemites near Timmins. The trondhjemites intrude a deformed ultramafic to Fe-tholeiite volcanic sequence disposed in the structural hanging wall of the Destor–Porcupine Fault, and they bound a domain of high-grade gold ore. Spinifex-textured ultramafic flows record variable intensities of secondary alteration to a carbonate–Cr-muscovite assemblage. They are characterized by chondritic Al2O3/TiO2 ratios (19 ± 1.7), enhanced Cr (1570–2090 ppm) and Ni (470–1070 ppm), together with low levels of the incompatible elements Ta, Nb, and Th.Fresh quartz–oligoclase trondhjemitic intrusions are compositionally coherent with respect to SiO2 (average 63 wt.%), Al2O3 (16.3 wt. %), and Na2O (6.7 wt. %), as well as to the alteration-insensitive elements V, Sc, Zr, and Hf. Whole-rock δ18O values of 10.0–10.6‰, combined with "magmatic" quartz–feldspar (0.8–0.9‰), signify that these are primary high-18O magmas, likely derived by partial melting of 18O-enriched mafic granulites. Albitic trondhjemites are characterized by enhanced Na2O (10.8 wt. %) and are systematically enriched by ~1‰ relative to fresh counterparts, due to a preferential positive 18O shift of albite, resulting in negative quartz–feldspar. Sodium enrichment coupled with isotopic disturbance is attributed to low-temperature spilitization in the presence of marine water.A domain of high-grade gold ore is focussed in an intensely silicified unit. The ore is composed of quartz, feldspar, muscovite, carbonate, graphite, pyrite, molybdenite, and gold, with corresponding pronounced enrichments of Si, K, Rb, Ba, Cs, S, C, As, Sb, Pb, and Mo. The lithology is compositionally diverse but uniform with respect to Al2O3/TiO2 (17–27) and Ti/Zr (60–73). These features, combined with variably elevated Cr (50–955 ppm) and Ni (60–570), low contents of the incompatible elements, and a flat REE distribution, collectively indicate Fe-tholeiite and basaltic komatiite precursors. The silicified unit is bounded by trondhjemites overprinted by a secondary muscovite–Fe-dolomite alteration assemblage, with associated anomalies of CO2, S, As, Sb, Mo, and Pb. Quartz is isotopically uniform (δ18O = 14.1 ± 0.2‰ (1σ), n = 15) throughout the diverse lithologies overprinted by alteration linked to gold mineralization. Quartz is shifted ~+3‰ and albite ~−1‰ relative to fresh trondhjemites, such that Δquartz–feldspar = 1.8 to 2.6‰ in ore. Fluids implicated in gold mineralization were characterized by ambient temperatures of 330 ± 20 °C and by [Formula: see text]. Carbon-isotope compositions of hydrothermal ferroan dolomite are constrained at δ13C = −2.5 ± 0.8‰, but they are systematically heavier in graphitic rocks. δCgraphite averages −20.8‰.Fluorapatite occurs as megacrysts superimposed on trondhjemites and as microveins in a narrow domain cospatial with the Destor–Porcupine Fault. Its modal abundance correlates directly with P2O5 and F, which attain levels of 6.8 wt.% and 6800 ppm, respectively. Fluorapatite is close to end-member composition and possesses extremely enhanced REE abundances: 87Sr/86Sr = 0.7012, a value similar to that for contemporaneous mantle. Rocks have been extensively stripped of Si, K, Na, and Al in domains of fluorapatite, signifying a process allied to fenitization.