Abstract
Authigenic potassium feldspar (K-feldspar) occurs throughout the North American midcontinent in late Proterozoic crystalline basement and in the overlying sequence of Paleozoic sedimentary strata often in proximity to the Great Unconformity. Age determinations for this authigenic K-feldspar range from Paleoproterozoic (ca. 1750 Ma) to Mississippian (ca. 360 Ma), commonly interpreted as emplaced by orogeny-driven brines. The accuracy of these formation ages is often complicated by occurrences of authigenic K-feldspar on primary igneous or detrital K-feldspar, where mixing of these components during analysis is unavoidable. In this study, we examined an occurrence of authigenic K-feldspar associated with a late fault that crosscuts Proterozoic crystalline basement rocks and the Reef Deposit, a Paleoproterozoic gold and copper occurrence in northern Wisconsin. The major fault that cuts the Reef Deposit was previously interpreted as associated with the ca. 1100 Ma Midcontinent Rift. However, analysis of K-feldspar microveins hosted within Precambrian quartz veins, devoid of primary K-feldspar, using in situ 40Ar/39Ar methods yielded an age consistent with faulting and fluid circulation at 499.7 ± 0.8 Ma. This age predates known Paleozoic Laurentian collisional tectonic events. We propose that these K-feldspar microveins reflect a continuation of rifting and related isostatic adjustments along the Iapetus margin, prolonging the duration of extension by ∼25 m.y. If correct, this occurrence of authigenic K-feldspar would represent the most distal observation of early Paleozoic deformation and fluid migration in the midcontinent of North America. Moreover, this work highlights the potential role of extensional tectonics in the formation and distribution of authigenic K-feldspar in midcontinent North America.