Sources of fluids and metals in orogenic deposits : the Otago Schists, New Zealand
Sources of fluids and metals in orogenic deposits : the Otago Schists, New Zealand
The theme of this thesis is to investigate the source of ore forming elements enriched at the ore deposits in Otago, and the fluids responsible for their formation. A large suite of samples collected across the Otago and Alpine Schists has been analysed to investigate whether any specific rock types, metamorphic grades or structural settings has been depleted in ore farming elements relative to un-metamorphosed samples, and could consequently be the source of enrichments in the Otago ore deposits. A new method to analyse gold at ultra low concentrations (< 10 ppt) in rocks has been developed, because the determination of Au in un-mineralised or leached rocks requires very sensitive analytical methods. As, Se, Sb and Hg have also been analysed by methods with ultra low detection limits.
Many of the elements analysed show localised mobility, particularly in greenschist facies rocks, but most elements show no systematic mobility with lithological variation or crustal depth. However, As, Se, Mo, Ag, Cd, Sb, W, Au and Hg are all systematically depleted in metamorphosed rocks relative to un-metamorphosed rocks. Depletion often begins at different crustal levels; Hg and Cd are depleted from low-grade rocks, whereas depletion of As only occurs in high-grade rocks. This suite of elements is identical to that enriched in mineralised samples from Macraes gold mine. Mass balance calculations show that during metamorphism from protolith to amphibolite facies, 1 km3 of rock could yield over 23000 T of As, 50 T of Se, 1350 T of Mo, 100 T of Ag, 1100 T of Sb, 440 T of W, 3 T of Au, and 90 T of Hg. The enrichments observed at Macraes could have been sourced from a cube of amphibolite facies rock with side length 5 km. Elemental leaching at depth throughout the 30000 km2 exposure of the Otago schist provided sufficient ore forming elements to form more than 3000 Macraes-sized deposits.
University of Southampton
2004
Pitcairn, Iain Kerr
(2004)
Sources of fluids and metals in orogenic deposits : the Otago Schists, New Zealand.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
The theme of this thesis is to investigate the source of ore forming elements enriched at the ore deposits in Otago, and the fluids responsible for their formation. A large suite of samples collected across the Otago and Alpine Schists has been analysed to investigate whether any specific rock types, metamorphic grades or structural settings has been depleted in ore farming elements relative to un-metamorphosed samples, and could consequently be the source of enrichments in the Otago ore deposits. A new method to analyse gold at ultra low concentrations (< 10 ppt) in rocks has been developed, because the determination of Au in un-mineralised or leached rocks requires very sensitive analytical methods. As, Se, Sb and Hg have also been analysed by methods with ultra low detection limits.
Many of the elements analysed show localised mobility, particularly in greenschist facies rocks, but most elements show no systematic mobility with lithological variation or crustal depth. However, As, Se, Mo, Ag, Cd, Sb, W, Au and Hg are all systematically depleted in metamorphosed rocks relative to un-metamorphosed rocks. Depletion often begins at different crustal levels; Hg and Cd are depleted from low-grade rocks, whereas depletion of As only occurs in high-grade rocks. This suite of elements is identical to that enriched in mineralised samples from Macraes gold mine. Mass balance calculations show that during metamorphism from protolith to amphibolite facies, 1 km3 of rock could yield over 23000 T of As, 50 T of Se, 1350 T of Mo, 100 T of Ag, 1100 T of Sb, 440 T of W, 3 T of Au, and 90 T of Hg. The enrichments observed at Macraes could have been sourced from a cube of amphibolite facies rock with side length 5 km. Elemental leaching at depth throughout the 30000 km2 exposure of the Otago schist provided sufficient ore forming elements to form more than 3000 Macraes-sized deposits.
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Published date: 2004
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Local EPrints ID: 465472
URI: http://eprints.soton.ac.uk/id/eprint/465472
PURE UUID: 4e0ec5c1-ae2c-4277-a860-d2495a077880
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Date deposited: 05 Jul 2022 01:15
Last modified: 05 Jul 2022 01:15
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Author:
Iain Kerr Pitcairn
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