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The Rattling Brook deposit is a low-grade, disseminated to stockwork-style gold deposit hosted by Precambrian granodiorite and adjacent Cambrian sedimentary rocks. Alteration and gold mineralization also occur in foliated and metamorphosed mafic dykes, likely of late Precambrian age. The auriferous granodiorite is in turn cut by relatively fresh, unaltered, and locally chilled diabase dykes, interpreted as Paleozoic post-mineralization intrusions. A fresh post-mineralization diabase gave a 40Ar/39Ar amphibole plateau age of 412.9 ± 4.3 Ma, which is interpreted as the time of its crystallization, and which provides a younger limit for the timing of gold mineralization. An altered, metamorphosed dyke of pre-mineralization timing gave an identical 40Ar/39Ar biotite plateau age of 412.6 ± 2.3 Ma, which is more difficult to interpret. It could represent post-metamorphic cooling, or alternatively, resetting of metamorphic biotite during alteration related to gold mineralization. In the first case, the age provides a reasonable upper limit for the timing of gold mineralization, provided that the ambient temperature during mineralization was not significantly above the closure temperature for Ar in biotite (~ 300 oC). On this basis, gold mineralization at Rattling Brook occurred during the latest Silurian or earliest Devonian, between 415 and 409 Ma. The possibility that mineralization occurred at temperatures above 300 oC, prior to 415 Ma, cannot be completely excluded, but it must be younger than ca. 430 Ma, the time of peak metamorphism in adjacent areas. In conjunction with sparse data on the ages of gold deposits elsewhere in Newfoundland, the results support two discrete episodes of mineralization corresponding to the Silurian-Devonian boundary (420–410 Ma) and middle to late Devonian (380–370 Ma). These age groupings resemble those defined by recent Re-Os isotopic studies of sulphides from vein-style gold deposits in the Meguma terrane of Nova Scotia and may in part correspond to the timing of intrusion-related gold in New Brunswick. Given the small amount of data from all of these areas, further interpretation is speculative. However, such hints of discrete orogen-scale episodes of gold mineralization, perhaps correlative with regional tectonothermal events, provide a powerful incentive for further geochronological studies of gold mineralization in the Appalachian orogen.  Resume  Le gite du ruisseau Rattling est un gite aurifere a faible teneur qui, de dissemine, devient un stockwerk inclus dans de la granodiorite precambrienne et des roches sedimentaires cambriennes adjacentes. Une alteration et une mineralisation aurifere se manifestent egalement dans des dykes mafiques metamorphises et feuilletes, remontant probablement au Precambrien tardif. La granodiorite aurifere est a son tour recoupee par des dykes de diabase figes par endroits, non alteres et relativement sains, interpretes en tant qu’intrusions paleozoiques ulterieures a la mineralisation. Une diabase post-mineralisation inalteree a accuse un âge plateau sur amphibole 40Ar/39Ar de 412,9 ± 4,3 Ma, ce qui est interprete comme le moment de sa cristallisation et rapproche la limite du moment de la mineralisation aurifere. Un dyke metamorphise altere, anterieur a la mineralisation, a livre un âge plateau sur biotite 40Ar/39Ar identique de 412,6 ± 2,3 Ma, qui est plus difficile a interpreter. Celui-ci pourrait representer un refroidissement post-metamorphique ou, subsidiairement, une remise en place de la biotite metamorphique au cours de l’alteration apparentee a la mineralisation aurifere. Dans le premier cas, la datation fournit une limite superieure raisonnable quant au moment de la mineralisation aurifere, a condition que la temperature ambiante pendant la mineralisation n’ait pas ete substantiellement superieure a la temperature de convergence de l’AR dans la biotite (environ 300 oC). Le cas echeant, la mineralisation d’or du ruisseau Rattling est survenue au cours du Silurien tardif ou du Devonien precoce, entre 415 et 409 Ma. La possibilite que la mineralisation se soit produite a des temperatures superieures a 300 oC, avant 415 Ma, ne peut pas etre entierement exclue, mais la mineralisation doit etre anterieure a 430 Ma environ, point culminant du metamorphisme dans les secteurs voisins. Outre les rares donnees sur les âges des gites d’or ailleurs a Terre-Neuve, les resultats appuient la manifestation de deux episodes distincts de mineralisation correspondant a la limite siluro-devonienne (420–410 Ma) et a la periode du Devonien moyen a tardif (380–370 Ma). Ces groupes d’âges ressemblent a ceux definis lors de recentes etudes isotopiques Re-Os de sulfures provenant de gites auriferes filoniens dans le terrane de Meguma en Nouvelle-Ecosse et ils pourraient en partie correspondre au moment de l’apparition de l’or lie a une intrusion au Nouveau-Brunswick. Vu la somme restreinte de donnees tirees de l’ensemble de ces secteurs, effectuer une interpretation plus detaillee serait se livrer a des conjectures. Les indices comme les episodes distincts de mineralisation aurifere a l’echelle orogenique, qui pourraient etre correlatifs avec des evenements tectonothermiques regionaux, fournissent toutefois un puissant incitatif pour realiser des etudes chronologiques plus poussees de la mineralisation aurifere dans l’orogene des Appalaches.

atlantic geology

The timing of gold mineralization in White Bay, western Newfoundland: Evidence from 40Ar/39Ar studies of mafic dykes that predate and postdate mineralization