Copper Isotope Constraints on the Genesis of the Keweenaw Peninsula Native Copper District, Michigan, USA: A Comment

Alex Brown

doi:10.3390/min8110506

Abstract

Well-grouped δ65Cu values (−0.3 to +0.8‰) from120 samples of native copper fromthe Keweenaw native copper district have been attributed [...]

Highlights

IntroductionWell-grouped δ65 Cu values (−0.3 to +0.8h) from 120 samples of native copper from the Keweenaw native copper district have been attributed, by Bornhorst and Mathur [1], to isotopic fractionations toward heavy δ65 Cu values during oxidative leaching of copper (Cu) from deeply buried Portage
Well-grouped δ65 Cu values (−0.3 to +0.8h) from 120 samples of native copper from the Keweenaw native copper district have been attributed, by Bornhorst and Mathur [1], to isotopic fractionations toward heavy δ65 Cu values during oxidative leaching of copper (Cu) from deeply buried PortageLake Volcanics (PLV) basalts, followed by isotopic fractionations toward lighter δ65 Cu values during reductive deposition of the leached copper in up-dip aquifers of the same basalt-dominant Keweenaw strata
Because all source basalts experienced advanced leaching, the isotopic compositions of the leach solutions would have converged toward a narrow range, which is represented by the well-grouped array of δ65Cu values found in Keweenaw native copper deposits of +1.0h

Summary

Introduction

Well-grouped δ65 Cu values (−0.3 to +0.8h) from 120 samples of native copper from the Keweenaw native copper district have been attributed, by Bornhorst and Mathur [1], to isotopic fractionations toward heavy δ65 Cu values during oxidative leaching of copper (Cu) from deeply buried Portage. Lake Volcanics (PLV) basalts, followed by isotopic fractionations toward lighter δ65 Cu values during reductive deposition of the leached copper in up-dip aquifers of the same basalt-dominant Keweenaw strata. The sum of these two independent and largely uncontrolled fractionations is unlikely to have produced such a well-grouped array of δ65 Cu values. An alternative copper isotopic fractionation history may be offered, as explained below. It is doubtful that the copper isotopic fractionations can be explained solely within the context of classic metamorphogenic models (e.g., [2,3]), as implied by the authors. Topography-driven evolved meteoric water could provide other complementary explanations [4,5]

Copper Isotopic Fractionations

Representation

Conclusions