Graptolite-based evidence for a revised stratigraphic and structural setting of the Szechuan, Hunan and Xingchang antimony prospects, Exploits Subzone, central Newfoundland

Abstract

Early Ordovician (late Arenig) graptolites are identified from strata near the Northwest Gander River of the Exploits Subzone which were previously considered to be of Silurian and/or middle Ordovician age. A second locality within a graphitic shale yields a Middle Ordovician (Caradoc) graptolite assemblage, suggesting that it is equivalent to the Lawrence Harbour Formation. The fossiliferous units are associated with antimony prospects and have fundamental implications regarding structural interpretation and age of mineralization. The early Or dovician occurrence is similar both in terms of age and biogeographic affinity to that from a fossiliferous greywacke unit which overlies ophiolites at Coy Pond to the south. This apparently confirms that early Ordovician rocks within the Exploits Subzone were formed in high latitudes along the southeast margin of Iapetus and suggests the possibility of widespread turbidite deposition at that time, possibly related to a major tectonic event. On a repere des graptolites de l'Ordovicien inferieur (Arenig tardif) dans des couches gdologiques proches de la riviere Northwest Gander de la sous-zone d'Exploits, qui etaient auparavant considerees comme un secteur de la periode du Silurien ou de l'Ordovicien moyen. Un deuxieme emplacement a l'interieur de schiste graphitique a donne une zone d'assemblage de graptolites de l'Ordovicien moyen (Caradoc), ce qui laisse supposer qu'elle equivaut a la Formation Lawrence Harbour. Les unites fossiliferes sont associees a des zones d'interet d'antimoine et elles ont des repercussions fondamentales en ce qui touche ['interpretation structural et Page de la mineralisation. La venue de l'Ordovicien inferieur est semblable, tant du point de vue de Page que de l'affinite biogeographique, a celle d'une unite de grauwacke fossiliere qui recouvre des ophiolites a Coy Pond, au sud. Ce fait semble confirmer que les roches de l'Ordovicien inferieur a l'interieur de la sous-zone d'Exploits ont ete formees a de hautes latitudes le long de sud-ouest de la rive d'lapetus et il permet de supposer la possibility d'une sedimentation repandue de turbidite a cette epoque, possiblement reliee a un evenement tectonique marquant.