Isotopic exchange of oxygen, sulfur, hydrogen and copper between aqueous phase and the copper minerals brochantite, libethenite and olivenite

Juraj Majzlan,Stanislava Milovská,Rastislav Milovský,Ryan Mathur

doi:10.1180/mgm.2021.77

Abstract

AbstractFractionation factors for the isotopes of O, H, S, or Cu (as appropriate) were determined for the minerals brochantite [Cu4(SO4)(OH)6], libethenite [Cu2(PO4)(OH)] and olivenite [Cu2(AsO4)(OH)] and corresponding aqueous solutions at temperatures between 30 and 70°C. All samples used for this determination were synthetic and the degree of fractionation was expressed as 1000 ln α = (A × 106/T2) + B, where A and B are empirical parameters. A few natural libethenite samples from its type locality Ľubietová-Podlipa were also analysed and compared to the prediction based on the isotopic composition of meteoric water and our fractionation factors. The hydrogen fractionation factors agreed with the prediction well, whereas those for oxygen did not. A possible explanation is the disequilibrium of aqueous phosphate (and also arsenate) species and the solution in our experiments or the interaction of meteoric fluids with the isotopically heavy (in terms of oxygen) country rocks. Because the effects of isotopic disequilibrium in our experiments cannot be ruled out, the oxygen fractionation factors should be used with caution. The determined fractionation factors can be used as an isotope geothermometer, given that it can be proven that the phases of interest precipitated from the same fluid in equilibrium. Libethenite is predicted to have slightly lower δ65Cu values than its parental solution, but brochantite slightly higher δ65Cu values than its parental solution. Simple forward models, simulating neutralisation or reduction of mine drainage, show that precipitation of these minerals and removal of the co-existing fluid, could cause isotopic variations (in δ65Cu) on the order of 1‰ or more.

Highlights

Copper sulfate, phosphate, and arsenate minerals are common constituents of many oxidation zones of ore deposits (e.g., Williams 1990), mine wastes and tailings (Sracek et al 2010, Portales et al 2015), or oxidation products of copper artifacts (e.g., Livingston 1991, Karlén et al 2002)
The distribution of substituting ions between the aqueous phase and solid solution is controlled by mixing parameters (e.g., Glynn 1991, 2000), graphically expressed by the Lippmann diagrams (Lippmann 1980)
Oxygen isotopes were measured on isotope ratio mass spectrometer (IRMS) MAT253 at Slovak Academy of Sciences, Banská Bystrica, Slovakia, using an automated carbonate preparation system (KIEL IV) coupled to IRMS in dual-inlet mode

Summary

Introduction

Phosphate, and arsenate minerals are common constituents of many oxidation zones of ore deposits (e.g., Williams 1990), mine wastes and tailings (Sracek et al 2010, Portales et al 2015), or oxidation products of copper artifacts (e.g., Livingston 1991, Karlén et al 2002) Their formation and fate are to a certain extent controlled by equilibrium thermodynamics. A special type of distribution is the one that involves isotopes of one element, partitioned among co-existing phases (Sharp 2017) This distribution is associated with significantly smaller energies than chemical equilibria between aqueous and pure solid phases or equilibria that involve solid solutions (Sharp 2017, page 3-3). Despite such tiny energy changes, the isotopic equilibria are instrumental in many areas of geosciences as they can reveal details about the geological history of various samples, their temperature of formation, or the origin of the parental solutions

Methods

Results

Conclusion