Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

• DOI: 10.2113/gsecongeo.88.6.1376
• Publication Date: 2009-04-03T13:35:40Z
• Authors: Rui Feng; Jianzhong Fan; Robert Kerrich
• Keywords: Metallogeny; Overprinting; Greenstone belt; Mesothermal

Six granitic magma series were generated sequentially during evolution of the Archean Abitibi greenstone belt and adjacent high-grade metasedimentary Pontiac subprovince. Minor synvolcanic tonalite-trondhjemite granodiorite intrusions (the TTG series) emplaced at approximately 2700 Ma; fresh samples have whole-rock delta 18 O values 5.6 + or - 0.5 per mil (1Sigma ), retain magmatic fractionations between quartz feldspar (1.5 0.5), are characterized by Au abundances 2.1 1.6 ppb ). Altered counterparts variably depleted high-temperature interaction with seawater generation volcanic-hosted massive base metal sulfide deposits. Compositionally, these plutons similar to low Al-type tonalite-trondhjemite-granodiorite, extremely K 2 ( , Fe 3 Nb, Y, Sc, unfractionated rare earth elements (REE), normalized Ta Nb depletions.Batholithic tonalite-granodiorite-granite-quartz monzodiorite TGGM senries), such as Lac Round Lake batholiths, developed in green-stone 2690 Ma. They average primary (sub whole rock) 7.2 ) 8.0 respectively, mean (0.74 1.03 ppb, 1Sigma Cu, Pb, Zn, Sn contents. The hosts minor stockwork Cu-Mo-Au, Cu-Zn mineralization. resembles high (1-3 wt %), Rb, Ba, but Sr abundances, strongly fractionated REE patterns, Ta, Ti depletions; batholiths from partial melting wet, subducted mafic-ultramafic slabs a compressional setting.Late tectonic syenite-quartz monzonite-granite stocks SMG 2685 2676 Ma belt. Watabeag batholith this has = 8.2 0.3 Na O) ratios more abundant Th, U contents.Post-tectonic alkali syenite-alkali syenite shoshonitic lamprophyres SS intruded along regional structures 2680 2670 relatively enriched (delta 8.8 0.7 ppm, also possess close crustal concentration 1.73 3.98 These primitive magmas feature conjunction elevated large ion lithophile element light enhanced Mg no., Cr, Co, Ni concentrations. host P, Zn mineralization.In subprovince Lacorne block belt, monzodiorite-monzonite-granodiorite-syenite (MMGS) syntectonically Ma, followed garnet-muscovite granite (GMG) over 2665 2630 Average abundance 6.8 1.22 1.46 for MMGS 0.6 0.12 0.11 GMG series, respectively. granitoids resemble except lower values, Ba/Th ratios, MgO, Li, Cs contents some phases. which other K-rich granitoids, concentrations Cs, U, Mo, Be, U-bearing pegmatites. six granitoid indicate evolving source regions, determined systematic secular trends CaO/(K concentrations.Mafic phases depleted, platinum-group (PGE)-normalized where Ir 0.26 Pd 0.49 2.86 Pt 0.46 3.84 Ir/Pd 0.06 0.24, Pt/(Pt Pd) 0.43 0.57. There is smooth slope Ru, through Pt, Au, indicating rocks that not relative crust. This contrast analyses lithologies near gold deposits, show secondary enrichment an overprint deposit halos. PGE behaved conservatively alteration deposits sphalerite gold-bearing veins them.Four major episodes involved time plutonic formed, namely: (1) pre-arc suprasubduction spreading (similar back arc) arc stage tonalite-trondhjemite-granodiorite 2730-2700 Ma), (2) syntectonic synmetamorphic collisional all volcanic-arc-type TGGM, SMG, series)were produced 2695-2680 (3) postcollisional strike-slip faulting local extensional accommodated volcanic rocks, (4) late when underthrust southern zone (2670-2630 Ma) thickened crust yield S-type granitoids. giant mesothermal restricted zone; they associated space TTG, nor there evidence intrusions, their extrusive counterparts, intrinsically rich source. Gold known zone. shoshonites both formed translithospheric structures, albeit different processes. former ore fluids H 5-11 ppm) dehydration subcreted oceanic sediments, whereas latter degrees metasomatized mantle wedge uplift decompression. postdate deposits: last thermal event probably reset several isotopic systems, generating younger apparent ages mineralization locally, 80 after deposition.