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TitreUsing biotite composition of the Devonian Lake George granodiorite, New Brunswick, as a proxy for magma fertility and differentiation in W-Mo-Au-Sb mineralized magmatic hydrothermal systems
AuteurAzadbakht, Z; Lentz, D; McFarlane, C
SourceL'Association géologique du Canada-L'Association minéralogique du Canada, Réunion annuelle conjointe, Recueil des résumés vol. 37, 2014 p. 15
Séries alt.Secteur des sciences de la Terre, Contribution externe 20130614
RéunionGAC-MAC Joint annual meeting; Fredericton, NB; CA; mai 21-23, 2014
Documentpublication en série
Mediaen ligne; numérique
SNRC21G/14; 21G/15
Lat/Long OENS -67.5000 -66.5000 46.0000 45.7500
Sujetsroches granitiques; roches intrusives; roches ignées; or; molybdène; tungstène; antimoine; minéralisation; étain; magmatisme; biotite; differentiation magmatique; gîtes magmatiques; pétrologie ignée et métamorphique; géochimie; Dévonien
ProgrammeÉtude des gîtes porphyriques, Initiative géoscientifique ciblée (IGC-4)
LiensOnline - En ligne (PDF 8.75 MB)
Résumé(disponible en anglais seulement)
The Early Devonian (412+5/-4 Ma, zircon U-Pb) granodioritic Lake George polymetallic deposit, New Brunswick, Canada, is a metaluminous to weakly peraluminous, calc-alkaline body that shows volcanic arc affinity. The stock is cut by stibnite-quartz veins and quartz-scheelitemolybdenite veinlets. Fresh biotite from this intrusion was analyzed at both the core and rim by electron microprobe, and along rim-to-rim transects by laser ablation-ICP-MS at the University of New Brunswick to test whether biotite preserves a record of magma evolution (in terms of halogens and trace-elements) that may help identify: a) W-Mo-Au-Sb fertility of the magma; b) processes responsible for enrichment of these elements.
Subhedral biotite phenocyrsts (0.5 to 1.5 mm) are reddish brown in colour (indicative of reduced I-type source) and locally altered to chlorite. Crystallization temperature for biotite phenocrysts were estimated on the basis of Ti-in-biotite thermometry and averaged 716°C. However, altered grains recorded lower temperature around 645°C.
Al-in-hornblende and biotite geobarometry revealed two populations of pressures around 4.3 and 2 Kb respectively, indicating of two crystallization depths for this porphyritic intrusion. Crystallization of amphibole at higher pressure clearly indicates a high water content of the source magma. Hydroxyl is also the most dominant component of the hydroxyl site (Average 1.89 wt%) in biotites. The limited range of IV(F/Cl) values of the Lake George biotite suggests that they equilibrated with one fluid.
Despite homogeneous intragranular major element chemistry, evidence of magma evolution is recorded by core-to-rim trace element variations. For example, Cu, Rb, Cr, K, Mo, Sn, Cs and W increase from cores to rims, whereas Ba, Ni, Mn, and Li act inversely. Sb distribution is homogeneous at an average value of 0.15 ppm. Trace element zoning may result from changes in pressure, temperature, or fluid conditions; this characteristic along with increasing F/Cl ratio from core to rim of biotites in the Lake George intrusion indicates fractional crystallization in the body. Furthermore, estimation of biotite/liquid partition coefficients may help in understanding magma fertility, as it can provide important modelling constraints on metal behaviour toward the late stage fluid. For example, KSb is 0.06 in fresh biotites, increasing to 0.77 in more altered biotites reflecting enrichment of these elements towards the late stage fluid.
Based on these observations, the concept of using mica composition to help identify fertile Acadian magma systems has been established; this method may be a useful tool to indicate a difference between barren and mineralized granophile-element rich systems.