Crystal chemical control on intra-structural copper isotope fractionation in natural copper-iron-sulfur minerals

Persistent Link:
http://hdl.handle.net/10150/289914
Title:
Crystal chemical control on intra-structural copper isotope fractionation in natural copper-iron-sulfur minerals
Author:
Young, Steven E.
Issue Date:
2003
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
The nature of Cu isotope fractionation in natural Cu-Fe-S minerals was investigated through acid ferric sulfate leaching of copper ore from Morenci, Arizona. Copper isotope composition of the derived solutions varies from δ⁶⁵Cu = 0.47‰ to 5.21‰ over the course of progressive copper extraction. High δ⁶⁵Cu values characterize solutions collected in the first half of the leach, while the solutions collected between 35% and 45% copper recovery exhibit lower δ⁶⁵Cu values. This general pattern was observed for both bacterially-mediated and abiotic leaching. Sulfate solutions derived from dissolving pure djurleite show variable Cu isotope compositions as well, although the range is protracted from δ⁶⁵Cu = 0.01‰ to 1.21‰. As the Cu:S ratio of the remaining sulfide decreases, crystal structure parameters change as mineralogy passes through a series of nonstoichiometric copper sulfides. Mineralogy converges to yarrowite near 44% copper dissolution. Crystal chemical studies show that distribution of the two copper-sulfur bond coordination geometries, triangular planar and tetrahedral, in the copper sulfides, approximately corresponds to changes in δ⁶⁵Cu of the leachates. In particular, the proportion of CuS3 relative to CuS4 groups decreases from Cu/S = 2.00 (chalcocite) to 1.40 (geerite). Between Cu/S = 1.40 to 1.00 (covellite), the relative proportion of CuS3 groups increases slightly. Connection between coordination number and Cu isotope fractionation implies affinity of CuS₃ groups for the heavier, ⁶⁵Cu, isotope. This can be justified through bond length-bond strength arguments. Solutions from bornite dissolution vary from δ⁶⁵Cu = -0.79‰ to 1.14‰, with the largest values associated with solutions from early stage of reaction (up to 15% copper removal). Around 25% dissolution, δ⁶⁵Cu of the solution approaches that of the original bornite (δ⁶⁵Cu = 0.02‰). This is explained by disappearance of all remaining CuS₃ groups. Sulfur isotope compositions of solutions and sulfides derived from djurleite leaching were determined to investigate the possibility of intra-mineral fractionation. Very soon after reaction initiation, δ³⁴S of both sulfur reservoirs reach a steady-state with sulfate solutions about 2‰ enriched in ³⁴S relative to residual sulfide. Unlike the case of Cu isotopes, the main partitioning affecting S isotopes is exchange between sulfate and sulfide.
Type:
text; Dissertation-Reproduction (electronic)
Keywords:
Engineering, Metallurgy.; Geochemistry.
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Geosciences
Degree Grantor:
University of Arizona
Advisor:
Ruiz, Joaquin

Full metadata record

DC FieldValue Language
dc.language.isoen_USen_US
dc.titleCrystal chemical control on intra-structural copper isotope fractionation in natural copper-iron-sulfur mineralsen_US
dc.creatorYoung, Steven E.en_US
dc.contributor.authorYoung, Steven E.en_US
dc.date.issued2003en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractThe nature of Cu isotope fractionation in natural Cu-Fe-S minerals was investigated through acid ferric sulfate leaching of copper ore from Morenci, Arizona. Copper isotope composition of the derived solutions varies from δ⁶⁵Cu = 0.47‰ to 5.21‰ over the course of progressive copper extraction. High δ⁶⁵Cu values characterize solutions collected in the first half of the leach, while the solutions collected between 35% and 45% copper recovery exhibit lower δ⁶⁵Cu values. This general pattern was observed for both bacterially-mediated and abiotic leaching. Sulfate solutions derived from dissolving pure djurleite show variable Cu isotope compositions as well, although the range is protracted from δ⁶⁵Cu = 0.01‰ to 1.21‰. As the Cu:S ratio of the remaining sulfide decreases, crystal structure parameters change as mineralogy passes through a series of nonstoichiometric copper sulfides. Mineralogy converges to yarrowite near 44% copper dissolution. Crystal chemical studies show that distribution of the two copper-sulfur bond coordination geometries, triangular planar and tetrahedral, in the copper sulfides, approximately corresponds to changes in δ⁶⁵Cu of the leachates. In particular, the proportion of CuS3 relative to CuS4 groups decreases from Cu/S = 2.00 (chalcocite) to 1.40 (geerite). Between Cu/S = 1.40 to 1.00 (covellite), the relative proportion of CuS3 groups increases slightly. Connection between coordination number and Cu isotope fractionation implies affinity of CuS₃ groups for the heavier, ⁶⁵Cu, isotope. This can be justified through bond length-bond strength arguments. Solutions from bornite dissolution vary from δ⁶⁵Cu = -0.79‰ to 1.14‰, with the largest values associated with solutions from early stage of reaction (up to 15% copper removal). Around 25% dissolution, δ⁶⁵Cu of the solution approaches that of the original bornite (δ⁶⁵Cu = 0.02‰). This is explained by disappearance of all remaining CuS₃ groups. Sulfur isotope compositions of solutions and sulfides derived from djurleite leaching were determined to investigate the possibility of intra-mineral fractionation. Very soon after reaction initiation, δ³⁴S of both sulfur reservoirs reach a steady-state with sulfate solutions about 2‰ enriched in ³⁴S relative to residual sulfide. Unlike the case of Cu isotopes, the main partitioning affecting S isotopes is exchange between sulfate and sulfide.en_US
dc.typetexten_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.subjectEngineering, Metallurgy.en_US
dc.subjectGeochemistry.en_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineGeosciencesen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorRuiz, Joaquinen_US
dc.identifier.proquest3090030en_US
dc.identifier.bibrecord.b44427049en_US
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