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Download PDF Poster 4.6 MDRU New U-Pb age constraints and lithogeochemical classication for Late Cretaceous volcanics in the TREK project area, central British Columbia R.S. Kim1, J.J. Angen1, C.J.R. Hart1 1 Mineral Deposit Research Unit (MDRU), The University of British Columbia 1. Introduction & Regional Geology 2. Northwestern TREK Geology 3. Lithogeochemistry 4. U-Pb Dates Immobile Element Discrimination for Volcanic Rocks (Nb/Y vs. Zr/Ti) data-point error symbols are 2σ data-point error ellipses are 2σ data-point error symbols are 2σ o o o 87 data-point error ellipses are 2σ 96 127 40’W Burns Lake 125 00’W Vanderhoof 122 50’W 88 Terrane 127°0'0"W 126°0'0"W 125°0'0"W [5] Sample: 98 [1] Sample: 220 Sample: Mean= 74.6 ±1.9 [2.5%] 95% conf. 10’N Fraser Lake [2] Phonolite 0.0150 92 o 0.0136 SQ EN [4] Alkali 15RK-022 15RK-184 Wtd by data -pt errs only, 0 of 8 rej. 54 Coast Plutonic Complex Rhyolite Phonolite 15RK-022 N 86 85 MSWD = 1.8, probability = 0.082 88 Tephriphonolite Trachyte 0.0140 Ootsa Lake Trachydacite Evolved Rhyolite Dacite Trachyte Stikine 0.0132 100 Sample: HC 84 84 HB Phonotephrite Tephriphonolite Foidite 83 15RK-184 U Trachyandesite U 8 8 3 Kemano O (%) 0.0130 23 82 2 B Trachy 2 80 0.1000 0.1400 0.1800 0.2200 0.2600 0.3000 Cache Creek Basaltic Andesite b/ 207Pb/235U Tephrite/ Pb/ Rhyolite P 82 6 Trachyandesite 6 0.0128 YM 0 20 O + K Basanite Andesite Basaltic 2 2 81 76 Trachybasalt Zr/Ti - Andesite 0.0120 Na 00’N o CH Quesnel Intermediate 54°0'0"N 54 CP Dacite 80 72 Alkali Foidite Basalt 0.0124 Basaltic Andesite Basalt 79 0.0110 BW Picrobasalt Basalt Mean = 82.2 ± 1.3 [1.6%] 95% conf. 14RK048 54°0'0"N andesite 68 132°W 124°W 116°W Mineral Deposits/Showings ( ) 78 Wtd by data-pt errs only, 0 of 6 rej. 66 C Basic MSWD = 1.7, probability = 0.13 BW - Blackwater (LS/IS, Au-Ag) 0.0100 64 15JL226 0.0120 77 Nazko SiO2 (%) 0.0400 0.0600 0.0800 0.1000 0.1200 0.1400 58°N 15RK147 0.076 0.080 0.084 0.088 0.092 0.096 207Pb/235U 58°N CH - Chu (Porph, Mo-Cu) 207Pb/235U CP - Capoose (Porph-Vein, Au-Ag-Zn) Ootsa Lake Group Sub-Alkaline Alkaline Ultra-Alkaline σ σ σ 15RK022 Hazelton Group data-point error ellipses are 2 data-point error symbols are 2 data-point error ellipses are 2σ data-point error symbols are 2 SQ - Silver Queen (Vein, Zn-Ag-Au) 84 75 82 Sample: Sample: 0.0130 Sample: 54°N A TREK Project Area EN - Endako (Porph, Mo) 74 Sample: 54°N Nb/Y 0.0115 15JL-226 15JL-226 14RK-048 73 14RK-048 80 Anahim Lake HB - Huckleberry (Porph, Cu-Ag) 0.0126 Legend 80 71 HC - Holy Cross (LS, Au-Ag) Overlap Assemblage Post-Accretionary 70 78 0 50 100 0.0122 50°N Weakly Peraluminous o YM - Yellow Moose (LS, Au) Quanchus Suite 69 Chilcotin Group [6] 50°N 125 00’W Columnar basalt & basalt ows Feldspar porphyry 76 Samples are subdivided into intermedi- U km U 8 76 8 0.0105 Bulkley Plutonic Suite 23 0.0118 Nechako Plateau Group Quartz-hornblende-biotite diorite, Metaluminous 23 66 67 (Endako Formation) 74 feldspar porphyry Pb/ Pb/ 132°W 124°W 116°W Andesite & basalt ows 6 ate and felsic groups [4]. Hazelton Group 6 Capoose Pluton 65 20 20 0.0114 Ootsa Lake Group Biotite granodiorite Massive to ow 72 72 laminated rhyolites 62 Laidman Batholith 63 Kasalka Group Quartz monzonite-monzogranite rocks lie largely in the sub-alkaline eld 0.0095 0.0110 Andesitic to felsic ows, 70 fragmental volcanics Endako Batholith Granite, quartz diorite 61 Development of the Blackwater deposit has generated interest in the volcanic host rocks, 68 Skeena Group 15RK184 58 Mean= 68.64±0.77 [1.1%] 95% conf. 0.0106 Mean= 73.76±0.90 [1.2%] 95% conf. Sandstone, siltstone, TREK Project boundary N (dominantly basalts, andesitic basalts); 68 Wtd by data-pt errs only, 0 of 20 rej. argillite, conglomerate Kilometers 59 Wtd by data-pt errs only, 0 of 20 rej. D MSWD = 1.7, probability = 0.027 Stikine Terrane MSWD = 1.9, probability = 0.012 which belong to the Late Cretaceous Kasalka Group [1]. Characterization of the Kasalka 0 5 10 20 30 40 0.0102 66 Hazelton Group Kasalka & Ootsa Lake Group rocks show 0.0085 57 0.0550 0.0650 0.0750 0.0850 0.0950 Andesite ows & breccias 126°0'0"W 125°0'0"W 0.0400 0.0500 0.0600 0.0700 0.0800 207Pb/235U Group in this study is focused on 3 main objectives: 207Pb/235U Strongly Peraluminous more felsic trends toward intermediate to A selection of U-Pb age results on zircon obtained by LA-ICP-MS. Samples collected are from across the study area [2, 3]. Two graphs are shown [3] A B C D Peralkaline [8]. • Identication of lithogic units and regional stratigraphic correlations 15JL226 evolved alkaline species [5]. for each sample: one showing a concordia plot for each sample, the second with a weighted average for each analysis. 100m • Distinguishing lithogeochemical signature 15RK184 Concordia plots and weighted averages for several U-Pb zircon samples [8]. Sample 15RK-022 is a plagioclase-hornblende por- • Rening timing constraints phyritic andesite, providing the oldest Kasalka age in the suite of 82.2 Ma. Sample 15RK-184 is the southernmost of the suite, [7] from ow laminated rhyolite that outcrops just north of the Blackwater camp returns age of 74.6 Ma. In the Cabin Lake area, an 14RK048 Blackwater Capoose Felsic Kasalka samples are distinctly metaluminous to andesite-dacite tu (14RK-048) provides an age of 73.6 Ma; in the same locality a white crystal-lithic rhyolite tu (15JL-226) re- The Upper Cretaceous Kasalka Group and volcanic rocks of the Eocene Ootsa Lake Group peraluminous [6] compared to Ootsa Lake samples sults in the youngest age of 68.6 Ma, interpreted to be part of the uppermost felsic sequence for the Kasalka Group. are part of the overlap assemblage that extensively cover the Interior Plateau. Originally 15RK022 (metaluminous to weakly peraluminous), Hazelton identied in the Tahtsa-Whitesail-Kasalka Ranges, the Kasalka Group is documented to Crystal-lithic tu, vitric tu Brecciated rhyolite, latite lapilli samples are metaluminous. High alumina felsic units Pumiceous to vitreous clasts, 5. Summary & Conclusions & lithic-ash tus quartz-crystals, minor welded textures Pumice, rhyolite bearing lithic-ash tus extend eastward into the Interior Plateau region. Thickness Approximate are also reported at garnet-bearing rhyolite dykes at Andesite ows & breccias Andesite ows & breccias Plagioclase-hornblende phyric Autobreccias, plagioclase phyric Capoose (Andrew, 1988) and rhyodacites at the Kasal- The Kasalka Group is a calc-alkaline to alkaline Late Cretaceous volcanic package in BC’s Interior Plateau region. It includes a Flow banded rhyolite Maroon pebble-cobble White-grey to mauve-grey, conglomerate The Kasalka Group identied in the northern TREK area bears strong resemblance to the Ju- nely laminated. Interbedded with Polymict, dominantly andesitic ka type section (MacIntyre, 1977). trachyandesite dominated intermediate and a high-silica rhyolite dominated component. A two-stage fractionation process is brecciated felsic sequence clasts, red sand matrix rassic Hazelton and Eocene Ootsa Lake Group volcanic rocks in the eld. All three are made suggested: initial fractionation interpreted to occur at the base of thickened lithosphere where hornblende was a fractionat- up of both competent and fragmental intermediate to felsic volcanic rocks. The Kasalka Stratigraphic schematics from regional mapping indicate a Normalized C1 chondrite spider plots show the inter- ing phase, forming trachyandesites with listric REE proles. The high-silica rhyolites are interpreted to reect signicant upper mediate Kasalka samples have a listric trace, suggest- crustal fractionation where plagioclase and K-feldspar are stable, leading to Eu anomalies and peraluminous compositions Group represents a number of volcanic centres, resulting in a heterogeneous stratigraphy. transition from andesite-dominated units to increasingly ing early hornblende fractionation [7] (Davidson et al., (Gualda & Ghiorso, 2013). Lateral continuity is also truncated by late block faulting associated with Eocene extension felsic members to the east and south [2, 3]. Moving upwards 2007; Richards, 2009). Comparatively, the felsic sam- in stratigraphy, the increase in felsic trends is also dominant & volcanism. Widespread cover in the form of younger volcanics and till hinders bedrock ples exhibit a strong negative Eu anomaly, suggesting New igneous zircon ages provide updated age brackets of 68-82Ma across the study area. This range in ages may reect long exposure in the area. especially when preserved in regional graben structures. plagioclase-stable fractionation in the upper crust. lived, localized and sporadic magmatic activity. Acknowledgements Contact The authors would like to thank J. Lipske, R. Whiteaker, and others from New Gold, Rachel Kim Inc. for thoughtful geological discussions, insights in the eld, and access to core MSc Student Mineral Deposit Research Unit and data. R. Kim graciously thanks Geocience BC for providing generous nancial University of British Columbia © 2017 MDRU and The University of British Columbia. All rights reserved. support. Vancouver, BC V6T 1Z4 No part of this document may be reproduced or transmitted in any form or by any means electronic, [email protected] mechanical, photocopying, recording, or otherwise, without prior written permission of the authors..
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