28 Geochronology of Igneous Processes (Posters)

Thursday, 19 May 2016
8:00 AM - 5:00 PM
Clearwater Room

REGIONAL GEOCHRONOLOGY OF DIKES ASSOCIATED WITH THE IDAHO PORPHYRY BELT AND TRANS-CHALLIS FAULT SYSTEM

Paper number: 28-3
Program: Theme Sessions
Time: 8:00 AM – 5:00 PM

Authors

Abstract

The Idaho Porphyry Belt (IPB) and Trans-Challis Fault System (TCFS) extend northeast-southwest ~270 km across central Idaho and are associated with epithermal gold-silver deposits. The timing of mineralization associated with these systems is poorly constrained. We analyzed U/Pb zircon ages of five dikes from the IPB near Horseshoe Bend, Idaho to constrain the timing of intrusive magmatic activity during regional Paleogene transtensional tectonics. Sample lithologies include dacite porphyry, rhyolite, andesite, and two phases of diorite. The ages reported here provide additional constraints on the timing and emplacement of IPB and TCFS intrusives and associated mineralization.

U-Pb zircon geochronology demonstrates that the diorite, dacite porphyry, and rhyolite were intruded within a narrow window between 49 and 47 Ma. These dike ages are thus coeval with the ages for a variety of diorite, monzodiorite, granodiorite and syenite plutons of the Challis intrusives [1] and correlative volcanics [2]. Cross-cutting relationships demonstrate that fracturing, dilation and vein filling leading to mineralization post-date these intrusions [3]. By contrast, we measured an Early Oligocene (~31 Ma) zircon age for the andesite phase of intrusion - an episode of magmatism previously unreported for the area. Andesite dikes of similar age and orientation are reported in the Salmon Creek Volcanics of Owyhee County [4], and in the Kamiah Volcanics of Lewis County [5]. The orientation and degree of alteration of the Oligocene andesite dikes are coherent with the orientation and alteration of Eocene dikes, which suggests that IPB emplacement, TCFS extension, and associated mineralization persisted through the Oligocene, lasting longer than previous studies have suggested [3].

[1] Gaschnig et al. (2010) CONTRIB MINERAL PETR 159(6), 863-883. [2] Mcdonald et al. (1998) EARTH PLANET SCI LETT 156, 225-237. [3] Anderson (1947) USGS BULL 944C, 119-319. [4] Norman (1987) Dissertation- Rice University. [5] Jones (1982) Bulletin – Idaho Bureau of Mines and Geology 26, 43-52.

Geological Society of America Abstracts with Programs. Vol. 48, No. 6

doi: 10.1130/abs/2016RM-276013

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