Slab Breaks and Early Mesozoic Paired Alkaline Porphyry Deposit Belts (Cu-Au Ag-Pt-Pd-Mo) in the Canadian Cordillera James Logan Mitchell Mihalynuk Cordilleran terranes Stikine arc Quesnel arc Cache Creek Slide Mountain Initial Sr Pericratonic Yukon-Tanana/Okanagan 0.704 0.705 Kootenay/Arctic-Alaska
0.707 North American displaced & autochthonous Canadian Cordilleran Porphyry Deposits 0.70 7 0.70 5 0.70 4 (Armstrong, 1988) Po Ages - Magmatic Belts St Qn Why are these mineralizing epochs
restricted to short intervals of arc growth along >2000 km of the arc and what is the ultimate cause of these mineralizing events Slab breakoff Model Relate L>-EJ magmatism and porphyry mineralization to stalled subduction of low (Davies and von Blanckenburg, 1995; van de Zedde and Wortel, 2001) density crustal welt (KutchoSitlika-Venables) Shallow slab detachment generates a thermal spike (50o00C+) at mid lithospheric depths Picrite lavas produced via large degree partial melt - indicate a direct route from mantle to surface ~5-10 Ma later alkalic melts ( Cu, Au, Ag, PtPd-Mo) from low degree partial melt of a broad region of metasomatized mantle wedge Rapid uplift of the orogen L>-EJ unconformity Kutcho-Sitlika-WineglassFragments of a
Venables: Tethyan Permo Triassic intra-arc LP - M> basalt, rhyolite, volcaniclastic rocks and tonalite 263 -237 Ma Nd, Pb & geochem data suggest tholeiitic affinity (Childe and Thompson, 1997; Barrett, 1996) Kutcho assemblage Schiarizza ( 2012) Low-density crustal welt within Cache
Chemistry L> Iron Mask Picrite volcanic rocks Sample/Primitive Mantle high T. high degree partial melt Lherzolite (40%, reflects rapid ascent w/out crustal contamination) Brown and Mussett, 1993 N a2O +K 2O ( wt %) 20 16 12
8 Alkalic Flow, intrusion Silicaundersat . Silicasat. Galore Creek Mount Polley Iron Mask Batholith 4 0 40 Lang et al.(1994) Alkalic Chemistry 50
60 SiO 2 Subalkali c 70 80 (wt %) A CIPW Norms Q S L Sugarloaf dike Ajax West
C C S L PH + Hy P Foid I-4 Galore Creek Bootjack stock Mount Polley Low-degree partial melts metasomatised
Initiation of orogen Unconformitie uplift sNonconformiti es NW Stikinia U U U EJ U EJ L > Angular unconformity - subvertical Stuhini Group mudstones and Jack Formation at base of Hazelton Group (Henderson et al., 1992) Hazelton Group
L>-EJ U L > Arc Exhumation Whitehorse trough Granitic clast content increases up-section older units comprised primarily of volcanic clasts (Hart et al., 1995) Late Triassic collision of the Kutcho-Sitlika-Venables arc with the Quesnel-Stikine arc Cross sectional representation of
slab break and resultant thermal spike Isotherm contours 0 to 1800 0C (after van de Zedde and Wortel, 2001) Schematic Stratigraphy IMA cmplx Stikinia - S N Cache Creek Quesnellia - Kutcho L K P K G
B G S F M P NS W H A B C IA B
Harper Ranch Stikine Assemblage IA T MOR B IA B Magmatic Episodes Late Triassic arc initiation, mafic volcanism, and emplacement of calc-alkaline plutons of the Stikine (236221 Ma) and Guichon (212208 Ma) suites Arc magmatism changed abruptly along the entire length of the arc in response to mantle fusion initiated by slab tear and the ensuing thermal spike. Triassic magmatism culminated with intrusion of the silica-saturated
and silica-undersaturated Copper Mountain suite (210200 Ma; 206200 Ma) With resumption of subduction, a second calc-alkaline to alkaline cycle began after an approximate 5- to 10-m.y. hiatus Calcalkaline magmatism (199 - 190 Ma) comprises intrusions of the Texas Creek suite in Stikinia and Takomkane -Wildhorse plutons in Quesnellia Conclusion A buoyancy-stalled subduction and slab-break model is a viable explanation that embraces the: Mesozoic stratigraphy magmatic episodes and calc-alkaline and alkaline mineral deposits of the Intermontane arc complex Thank you
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