Late Devonian conodont biostratigraphy of the Earn Group with age constraints for stratiform mineral deposits, Selwyn and Kechika Basins, Northern British Columbia and Yukon
Creators: name: Irwin, Steven Edward Bruce
Publisher: name: University of British Columbia
Year: 2010

description: Devonian and Early Carboniferous marine clastic rocks of the Earn Group host several economically important stratiform massive sulphide and bedded barite deposits. Due to the chaotic sedimentation, considerable regional metamorphic overprint and, relative inaccessibility, little was known about the stratigraphy, the Late Devonian conodont fauna, or the age of the stratiform mineral deposits within the Earn Group. Conodont microfossils, however, are an excellent fauna for an Earn Group biostratigraphy program because of their ability to withstand both temperatures in excess of 400° C, and significant physical stress. With standard laboratory techniques conodonts were readily extracted from fine grain calcareous elastics and carbonate lenses within the Earn Group. The conodonts are described from three specific areas where the Earn Group is known to host stratiform barite and barite-lead-zinc mineral deposits: Macmillan Pass, Midway, and Gataga. As the majority of conodonts were diverse and well preserved platform elements of the genus Palmatolepis, the taxonomic studies focused on this genus; other genera including Ancyrodella, Icriodus, Klapperina, Mesotaxis, and Polygnathus were examined as part of the biostratigraphic/taxonomic studies. Previous to this study the widespread stratiform mineralization was dated as only Late Devonian. The conodont taxonomy and biostratigraphy in the Earn Group provide age constraints for duration and formation of the stratiform mineralization. The ability to tightly constrain the age of the stratiform mineralization adds to the knowledge of Earn Group deposition, the paleogeography of the Selwyn and Kechika Basins, and has implications for stratiform mineral exploration strategies in the Earn Group. On the basis of conodont faunal ages barite mineralization at MACMILLAN PASS apparently occurs as three different levels: 1) CATHY property - Eifelian to early Frasnian; 2) PETE, JEFF, GARY, and GHMS properties - middle to late Frasnian; 3) TEA property -Early Carboniferous. In addition, barite-lead-zinc mineralization at TOM and JASON properties likely occurs during middle to late Frasnian. In the GATAGA area barite and barite-lead-zinc mineralization have been recognized at several temporally distinct levels in the early to middle Famennian: 1) Lower rhomboidea Zone; 2) Lower marginifera Zone; 3) Upper marginifera Zone. Several other mineralized horizons are loosely constrained within the same interval. Within the MIDWAY area the stratiform barite mineralization at the EWEN and PERRY properties is of Early Carboniferous, Tournaisian age, and correlates broadly with the TEA barite in the Macmillan Pass area. In summary, events that produced stratiform barite-lead-zinc and barite mineralization in the Selwyn and Kechika Basins were not coeval. The Late Givetian and early Frasnian barite mineralization took place in the Macmillan Pass and southernmost Gataga areas. During the middle Frasnian barite and barite-lead zinc mineralization events occurred at Macmillan Pass. Several episodes of barite and/or barite-lead-zinc mineralization occurred in the Gataga area during the middle Famennian. The youngest barite mineralization events in the Earn Group took place in the Early Carboniferous, Tournaisian time at Macmillan Pass and Midway.