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Contains fulltext : 122807.pdf (preprint version ) (Open Access)

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Contains fulltext : 92015.pdf (publisher's version ) (Open Access) Contains fulltext : 92015.pdf (preprint version ) (Open Access)

Two drill core samples of haematite/goethite from the Robe River channel iron ore deposits (CID) were dated using (U-Th)/He methods in order to constrain the timing of iron oxide formation and thereby provide a temporal context for CID genesis. (U-Th)/He ages (He ages) from these samples range from late Oligocene to Late Miocene and, despite a high degree of scatter, corroborate relationships expected from the internal ooidal stratigraphy. For individual ooids, the ages from haematitic core are older than or indistinguishable from the ages of the surrounding goethitic cortex. The goethitic cortices are, in turn, older than the ferruginised wood fragments recovered from the cementing goethitic matrix. The data suggest the following succession of ore formation phases: haematitic cores in ooids of both samples formed in the Early to Middle Miocene; goethitic cortices of both samples formed in the late Middle to early Late Miocene, the age of goethite cortex formation regardless of depth in the core, which does not support the top-down infill genetic model proposed by Heim et al (2006); and wood fragments forming a prominent component of the matrix were ferruginised during the Late Miocene. Thus the data suggest that the unique environmental conditions for the CID formation existed during the Miocene. A methodological implication is that the temperature utilised for He extraction from iron oxides has a critical impact on the mobility of parent nuclides; the typical 1 100 degree C laser heating used for crystalline minerals like apatite or zircon induces loss of U and Th and results in erroneously old ages., Two drill core samples of haematite/goethite from the Robe River channel iron ore deposits (CID) were dated using (U-Th)/He methods in order to constrain the timing of iron oxide formation and thereby provide a temporal context for CID genesis. (U-Th)/He ages (He ages) from these samples range from late Oligocene to Late Miocene and, despite a high degree of scatter, corroborate relationships expected from the internal ooidal stratigraphy. For individual ooids, the ages from haematitic core are older than or indistinguishable from the ages of the surrounding goethitic cortex. The goethitic cortices are, in turn, older than the ferruginised wood fragments recovered from the cementing goethitic matrix. The data suggest the following succession of ore formation phases: haematitic cores in ooids of both samples formed in the Early to Middle Miocene; goethitic cortices of both samples formed in the late Middle to early Late Miocene, the age of goethite cortex formation regardless of depth in the core, which does not support the top-down infill genetic model proposed by Heim et al (2006); and wood fragments forming a prominent component of the matrix were ferruginised during the Late Miocene. Thus the data suggest that the unique environmental conditions for the CID formation existed during the Miocene. A methodological implication is that the temperature utilised for He extraction from iron oxides has a critical impact on the mobility of parent nuclides; the typical 1 100 degree C laser heating used for crystalline minerals like apatite or zircon induces loss of U and Th and results in erroneously old ages.

The Zn-Cu-Pb vanadate ore deposits (descloizite, mottramite and vanadinite) in the Otavi Mountainland of Namibia are now mostly worked out. They were contained in collapse breccias and solution cavities related to a karstic network associated with the post-Gondwana land surfaces in the carbonate rocks of the Neoproterozoic Otavi group. These deposits represent a special low-temperature, weathering-related non- sulphide ore type, found elsewhere in Southern Africa. The geological settings and ore mineralogy of the main deposits are described, together with comprehensive major and trace-element data for the vanadate ore minerals and gangue carbonates, as well as fluid inclusion micrometry. The age of most deposits appears to be confined to the Tertiary, with a distinct period of descloizite formation dated at 24 to 33 Ma., The Zn-Cu-Pb vanadate ore deposits (descloizite, mottramite and vanadinite) in the Otavi Mountainland of Namibia are now mostly worked out. They were contained in collapse breccias and solution cavities related to a karstic network associated with the post-Gondwana land surfaces in the carbonate rocks of the Neoproterozoic Otavi group. These deposits represent a special low-temperature, weathering-related non- sulphide ore type, found elsewhere in Southern Africa. The geological settings and ore mineralogy of the main deposits are described, together with comprehensive major and trace-element data for the vanadate ore minerals and gangue carbonates, as well as fluid inclusion micrometry. The age of most deposits appears to be confined to the Tertiary, with a distinct period of descloizite formation dated at 24 to 33 Ma.

The Black Mountain porphyry Cu-Au deposit in the Baguio district, Northern Luzon, is associated with late Miocene to Pliocene intrusive rocks that retain primary crystallisation signatures, providing an excellent natural laboratory to study the details of magma chamber processes. Each igneous phase contains disequilibrium textures, including resorptive textures in plagioclase and distinct core rim textures in amphibole. High crystallisation temperatures and pressures in felsic (ca. 880 degrees C, 2.8 kbar) and mafic (ca. 1 030 degrees C, 7.7 kbar) rocks recorded by amphibole phenocrysts reflect crystallisation in the magma chamber. Amphibole in felsic rocks aged 6.39 ± 0.49 to 2.83 ± 0.23 Ma have similar crystallisation temperatures and pressures, suggesting a long-lived and hot felsic magma chamber under the Black Mountain deposit. The common occurrence of rounded or elongated sulphide inclusions in the megacrystic amphibole from the mafic rocks (2.81 ± 0.15 Ma) suggests a greater enrichment of Cu in mafic rocks relative to the ore-forming porphyries (2.91-2.83 Ma). This is also consistent with higher Cu contents (3.4 ± 0.9 ppm) in the megacrystic amphibole from the mafic rocks relative to amphibole in felsic rocks (1.0 ± 0.2 ppm) and may indicate up to 3.4 times higher Cu in the mafic magma relative to felsic magma. Therefore, large-scale mafic magma recharge (particularly at 2.8 Ma), likely introduced ore-forming metals to the long-lived felsic magma chamber, further contributing to the fertility of the felsic rocks and ultimately to Cu-Au mineralisation at the Black Mountain deposit., The Black Mountain porphyry Cu-Au deposit in the Baguio district, Northern Luzon, is associated with late Miocene to Pliocene intrusive rocks that retain primary crystallisation signatures, providing an excellent natural laboratory to study the details of magma chamber processes. Each igneous phase contains disequilibrium textures, including resorptive textures in plagioclase and distinct core rim textures in amphibole. High crystallisation temperatures and pressures in felsic (ca. 880 degrees C, 2.8 kbar) and mafic (ca. 1 030 degrees C, 7.7 kbar) rocks recorded by amphibole phenocrysts reflect crystallisation in the magma chamber. Amphibole in felsic rocks aged 6.39 ± 0.49 to 2.83 ± 0.23 Ma have similar crystallisation temperatures and pressures, suggesting a long-lived and hot felsic magma chamber under the Black Mountain deposit. The common occurrence of rounded or elongated sulphide inclusions in the megacrystic amphibole from the mafic rocks (2.81 ± 0.15 Ma) suggests a greater enrichment of Cu in mafic rocks relative to the ore-forming porphyries (2.91-2.83 Ma). This is also consistent with higher Cu contents (3.4 ± 0.9 ppm) in the megacrystic amphibole from the mafic rocks relative to amphibole in felsic rocks (1.0 ± 0.2 ppm) and may indicate up to 3.4 times higher Cu in the mafic magma relative to felsic magma. Therefore, large-scale mafic magma recharge (particularly at 2.8 Ma), likely introduced ore-forming metals to the long-lived felsic magma chamber, further contributing to the fertility of the felsic rocks and ultimately to Cu-Au mineralisation at the Black Mountain deposit.

Karouni is anomalous among orogenic gold deposits for its dominant sodic alteration and distinct lack of potassic alteration. Gold is located within inclusions in coarse, disseminated pyrite associated with the proximal alteration zones and as coarse native gold within the quartz-carbonate veins. Minor gold is also located within Au-bearing telluride minerals. The high TiO2 dolerites formed a favourable chemical trap due to their high magnetite content, suggesting sulphidation via redox reaction as a possible mechanism of gold deposition. Mass balance modelling of the hydrothermal alteration indicates a wall rock-dominated system with limited addition or subtraction of major elements with the exception of C, S, and Na. Modelling of the proximal alteration has also shown strong trace element enrichment of W-Bi-Ag-Te-Mo-Pb, all of which are correlative with gold. In situ laser ablation-inductively coupled plasma-mass spectrometry trace element geochemistry and secondary ion mass spectrometry S isotope analysis of pyrite from the gold-bearing hydrothermal system within the deposit indicate a geochemically and isotopically homogeneous system with only minor trace element variation due to differences in host rock, suggesting a single hydrothermal pulse correlative with the late stages of the Trans-Amazonian orogeny., Karouni is anomalous among orogenic gold deposits for its dominant sodic alteration and distinct lack of potassic alteration. Gold is located within inclusions in coarse, disseminated pyrite associated with the proximal alteration zones and as coarse native gold within the quartz-carbonate veins. Minor gold is also located within Au-bearing telluride minerals. The high TiO2 dolerites formed a favourable chemical trap due to their high magnetite content, suggesting sulphidation via redox reaction as a possible mechanism of gold deposition. Mass balance modelling of the hydrothermal alteration indicates a wall rock-dominated system with limited addition or subtraction of major elements with the exception of C, S, and Na. Modelling of the proximal alteration has also shown strong trace element enrichment of W-Bi-Ag-Te-Mo-Pb, all of which are correlative with gold. In situ laser ablation-inductively coupled plasma-mass spectrometry trace element geochemistry and secondary ion mass spectrometry S isotope analysis of pyrite from the gold-bearing hydrothermal system within the deposit indicate a geochemically and isotopically homogeneous system with only minor trace element variation due to differences in host rock, suggesting a single hydrothermal pulse correlative with the late stages of the Trans-Amazonian orogeny.

The Triassic Pulang porphyry Cu-Au deposit, located in the South Yidun terrane, is the oldest and one of the largest porphyry deposits in the southeastern Tibetan Plateau. Mineralisation occurs mostly in the potassic alteration zone of the Pulang intrusive complex. U-Pb-He triple dating, namely apatite (U-Th)/He, zircon U-Pb, and zircon (U-Th)/He dating, together with inverse thermal modelling, reveals that the Pulang complex was emplaced at a palaeodepth of 5.0-6.5 km at 215 ± 2 Ma. The deep-level emplacement of the complex, coupled with the episodic replenishment of the magma chamber, gave rise to the establishment of a prolonged magmatic-hydrothermal system at Pulang. Although a range of single-grain zircon and apatite (U-Th)/He ages were obtained on each sample, the weighted mean zircon and apatite (U-Th)/He ages vary systematically with elevation, defining a multistage cooling/denudation history at Pulang. Specifically, three phases of cooling were recognised from inverse thermal modelling. Overall, results indicate that the Pulang complex experienced two stages of exhumation at 33 to 45 m/m.y. and 5 to 17 m/m.y. Based on these data, it is estimated that approximately 558-1 099-m thickness of materials have been removed from the Pulang complex during uplift and erosion, including a large volume of ore. The long time span (more than 50 m.y.) of extremely slow cooling and erosion at Pulang could be related to the formation and preservation of a peneplain on the southeastern Tibetan Plateau since the Late Cretaceous. A relict peneplain thus signifies a favourable tectonic environment for the preservation of ancient porphyry systems worldwide., The Triassic Pulang porphyry Cu-Au deposit, located in the South Yidun terrane, is the oldest and one of the largest porphyry deposits in the southeastern Tibetan Plateau. Mineralisation occurs mostly in the potassic alteration zone of the Pulang intrusive complex. U-Pb-He triple dating, namely apatite (U-Th)/He, zircon U-Pb, and zircon (U-Th)/He dating, together with inverse thermal modelling, reveals that the Pulang complex was emplaced at a palaeodepth of 5.0-6.5 km at 215 ± 2 Ma. The deep-level emplacement of the complex, coupled with the episodic replenishment of the magma chamber, gave rise to the establishment of a prolonged magmatic-hydrothermal system at Pulang. Although a range of single-grain zircon and apatite (U-Th)/He ages were obtained on each sample, the weighted mean zircon and apatite (U-Th)/He ages vary systematically with elevation, defining a multistage cooling/denudation history at Pulang. Specifically, three phases of cooling were recognised from inverse thermal modelling. Overall, results indicate that the Pulang complex experienced two stages of exhumation at 33 to 45 m/m.y. and 5 to 17 m/m.y. Based on these data, it is estimated that approximately 558-1 099-m thickness of materials have been removed from the Pulang complex during uplift and erosion, including a large volume of ore. The long time span (more than 50 m.y.) of extremely slow cooling and erosion at Pulang could be related to the formation and preservation of a peneplain on the southeastern Tibetan Plateau since the Late Cretaceous. A relict peneplain thus signifies a favourable tectonic environment for the preservation of ancient porphyry systems worldwide.

Case studies of Grasberg, Batu Hijau and Ciemas in Indonesia used multiple geochronometry techniques to test the hypothesis that rapidly cooled porphyry deposits display higher copper-grade hypogene ore shells than their more slowly cooled counterparts. Grasberg, in Irian Jaya, cooled extremely rapidly at a rate of more than 1 000 degrees C/m.y., due to emplacement within 1 km of the surface. Batu Hijau, in Nusa Tengarra, cooled more slowly at around 450 degrees C/m.y. while the sub-economic Ciemas prospect, in Java, cooled at only 60 degrees C/m.y. and was only exhumed from depths of 5-8 km during the Miocene. At such depths it would have been hard for high Cu grades to have been generated by a thermally driven deposition process, as the original magmatic Cu would have been homogeneously distributed over a larger volume of rock., Case studies of Grasberg, Batu Hijau and Ciemas in Indonesia used multiple geochronometry techniques to test the hypothesis that rapidly cooled porphyry deposits display higher copper-grade hypogene ore shells than their more slowly cooled counterparts. Grasberg, in Irian Jaya, cooled extremely rapidly at a rate of more than 1 000 degrees C/m.y., due to emplacement within 1 km of the surface. Batu Hijau, in Nusa Tengarra, cooled more slowly at around 450 degrees C/m.y. while the sub-economic Ciemas prospect, in Java, cooled at only 60 degrees C/m.y. and was only exhumed from depths of 5-8 km during the Miocene. At such depths it would have been hard for high Cu grades to have been generated by a thermally driven deposition process, as the original magmatic Cu would have been homogeneously distributed over a larger volume of rock.

To determine the most contaminant-free sample processing procedures, pure quartz was crushed and ground in a variety of media and analysed by inductively coupled plasma mass spectrometry with Pt, Pd, Ru and Ir calibrated by isotope dilution and external calibration of Au and Rh. Initial desegregation was best achieved in a Mn steel jaw crusher which contributed less than 0.1 ppb Pt and Au, less than 0.05 ppb Pd and Ru, less than 0.01 ppb Rh and less than 0.006 ppb Ir. Final grinding must be in a ceramic or agate mill; an alpha-alumina mill was found to contribute up to 0.008 ppb Ir and 0.08 ppb Au but other elements were at or below the limits of detection. Tungsten carbide appeared to be a problem, with up to 0.19 ppb Au contribution and isobaric interference of tungsten oxide on 198-Pt, though the latter can be avoided by choosing alternative isotopes. For mineral separates such as magnetite, sulphide ore chromite it was concluded that a combination of steel crushers and mills can be used provided suitable acid washing is performed prior to sample dissolution., To determine the most contaminant-free sample processing procedures, pure quartz was crushed and ground in a variety of media and analysed by inductively coupled plasma mass spectrometry with Pt, Pd, Ru and Ir calibrated by isotope dilution and external calibration of Au and Rh. Initial desegregation was best achieved in a Mn steel jaw crusher which contributed less than 0.1 ppb Pt and Au, less than 0.05 ppb Pd and Ru, less than 0.01 ppb Rh and less than 0.006 ppb Ir. Final grinding must be in a ceramic or agate mill; an alpha-alumina mill was found to contribute up to 0.008 ppb Ir and 0.08 ppb Au but other elements were at or below the limits of detection. Tungsten carbide appeared to be a problem, with up to 0.19 ppb Au contribution and isobaric interference of tungsten oxide on 198-Pt, though the latter can be avoided by choosing alternative isotopes. For mineral separates such as magnetite, sulphide ore chromite it was concluded that a combination of steel crushers and mills can be used provided suitable acid washing is performed prior to sample dissolution.

There is considerable interest in whether the geochemical and isotopic compositions of zircon can be applied to metallogenic problems. Zircons are widely used because they yield precise ages, their trace element contents can be linked to processes of magma differentiation and mineralisation, and they contain inclusions of minerals that further constrain the composition of the parental magmas. In the abstract on p.57 'Contrasting Cu-Au and Sn-W granite metallogeny through the zircon record', the paired magmatic belts of Myanmar with these contrasting metallogenic affinities and interpreted to have formed on the accretionary margin of the subducting Neo-Tethys ocean present an opportunity to compare and contrast geochemically the zircon compositions in two end-member types of granite-hosted mineral deposits generated in collisional settings. The abstract on p.58 'Zircons and granite metallogeny' reports that a comparison of REE data in zircons from Myanmar and the Zaaiplaats Granite of the Bushveld Complex highlights that there is an increase in the inferred degree of fractionation from Cu-bearing magmas in Myanmar, via Sn-bearing granite of the Zaaiplaats to the Sn-bearing granites of Myanmar., There is considerable interest in whether the geochemical and isotopic compositions of zircon can be applied to metallogenic problems. Zircons are widely used because they yield precise ages, their trace element contents can be linked to processes of magma differentiation and mineralisation, and they contain inclusions of minerals that further constrain the composition of the parental magmas. In the abstract on p.57 'Contrasting Cu-Au and Sn-W granite metallogeny through the zircon record', the paired magmatic belts of Myanmar with these contrasting metallogenic affinities and interpreted to have formed on the accretionary margin of the subducting Neo-Tethys ocean present an opportunity to compare and contrast geochemically the zircon compositions in two end-member types of granite-hosted mineral deposits generated in collisional settings. The abstract on p.58 'Zircons and granite metallogeny' reports that a comparison of REE data in zircons from Myanmar and the Zaaiplaats Granite of the Bushveld Complex highlights that there is an increase in the inferred degree of fractionation from Cu-bearing magmas in Myanmar, via Sn-bearing granite of the Zaaiplaats to the Sn-bearing granites of Myanmar.

The Yao'an porphyry Au system, Machangqing porphyry Cu-Mo system and Beiya porphyry-skarn Au system, are spatially and temporally associated with potassic felsic intrusions emplaced during the Eocene to Oligocene (37-33 Ma) in a postcollisional intracontinental setting in western Yunnan. It is postulated that continental collision may have preferentially thickened the continental lithospheric mantle adjacent to the Jinsha suture, in which overthickened lower continental lithospheric mantle was subsequently removed, inducing melting of residual metasomatised lithospheric mantle as well as lower crust. The gold-rich Yao'an and Beiya intrusions are alkaline and potassic, characterised by high zircon 18-O values (higher than 6.5 per mille), consistent with supracrustal contributions. In contrast, the Cu-Mo-rich Machangqing intrusions are high-K calc-alkaline with mantle-like zircon 18-O values (lower than 6.5 per mille) and juvenile Hf isotope signatures, indicating negligible supracrustal recycling. Empirically, source compositions played an important role in determing the metal endowment among intrusions formed under the same tectonic setting with similar ages in western Yunnan, where Au tends to have been associated with alkaline and potassic melts with a supracrustal contribution, whereas Cu-Mo mineralisation seems to have been more related to juvenile crustal sources with little supracrustal influence., The Yao'an porphyry Au system, Machangqing porphyry Cu-Mo system and Beiya porphyry-skarn Au system, are spatially and temporally associated with potassic felsic intrusions emplaced during the Eocene to Oligocene (37-33 Ma) in a postcollisional intracontinental setting in western Yunnan. It is postulated that continental collision may have preferentially thickened the continental lithospheric mantle adjacent to the Jinsha suture, in which overthickened lower continental lithospheric mantle was subsequently removed, inducing melting of residual metasomatised lithospheric mantle as well as lower crust. The gold-rich Yao'an and Beiya intrusions are alkaline and potassic, characterised by high zircon 18-O values (higher than 6.5 per mille), consistent with supracrustal contributions. In contrast, the Cu-Mo-rich Machangqing intrusions are high-K calc-alkaline with mantle-like zircon 18-O values (lower than 6.5 per mille) and juvenile Hf isotope signatures, indicating negligible supracrustal recycling. Empirically, source compositions played an important role in determing the metal endowment among intrusions formed under the same tectonic setting with similar ages in western Yunnan, where Au tends to have been associated with alkaline and potassic melts with a supracrustal contribution, whereas Cu-Mo mineralisation seems to have been more related to juvenile crustal sources with little supracrustal influence.

The thermal and exhumation histories of magmatic ore deposits are quantified by combining numerical modelling techniques with (U-Th)/He thermochronometry and U-Pb geochronology data. By integrating magmatic and exhumation cooling models for igneous intrusions and constraining the time-temperature histories with radiometric age data, it is possible to construct complete geological histories for magmatic-hydrothermal systems. The model was tested using thermochronology and geochronology data from the Sar Cheshmeh porphyry Cu deposit in Iran., The thermal and exhumation histories of magmatic ore deposits are quantified by combining numerical modelling techniques with (U-Th)/He thermochronometry and U-Pb geochronology data. By integrating magmatic and exhumation cooling models for igneous intrusions and constraining the time-temperature histories with radiometric age data, it is possible to construct complete geological histories for magmatic-hydrothermal systems. The model was tested using thermochronology and geochronology data from the Sar Cheshmeh porphyry Cu deposit in Iran.