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

1993; Volume: 88; Issue: 6 Linguagem: Inglês

10.2113/gsecongeo.88.6.1376

1554-0774

Rui Feng, Jianzhong Fan, Robert Kerrich,

Geochemistry and Elemental Analysis

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