Saline minewaters of the Keweenaw Peninsula, northern Michigan: their nature, origin and relation to similar deep waters in Precambrian crystalline rocks of the Canadian Shield ( USA).

Abstract

Stagnant pockets of Ca-Na-Cl brine occur at depth in many, if not all, native copper mines of the Keweenaw Peninsula. Typically occurring below approx 300 m depth, the brines are characterized by high salinities (200 g/l or more), high Ca/Na ratios (5:1-7:1), low K and Mg contents, and delta D (-17 to -21 per mil) and delta 18O (-7.3 to -7.6 per mil) values which plot well above the global meteoric water line. Advocates a basinal water model involving several stages: 1) initial evolution of the waters as Paleozoic-hosted formation waters, 2) deep infiltration, reaction, and 'self-sealing' of these brines in basement rocks under conditions of relatively high temperature and W/R ratios, 3) removal of Paleozoic cover and back-reaction of the waters, chemically and isotopically, under new conditions of low temperature and W/R ratios, and 4) subsequent and variable mixing with fresh meteoric waters. -from Authors