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Geochemistry and morphology of gold in lateritic profiles in savanna and semi-arid climates
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.
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2nd I N T E R N A T I O N A L
G E O C H E M I S T R Y AND M O R P H O L O G Y OF G O L D IN L A T E R I T I C PROFILES IN SAVANNA AND SEMI-ARID C L I M A T E S
FREYSSINET Ph. *, LAWRANCE L.M. ** and BUT1~ C.R.M. ** * B.R.G.M., BP 6009, Orl6ans Cedex, France. ** CSIRO - Division of Exploration Geoscience, Wembley, Western Australia.
The nature of many of the processes of weathering and erosion that are important in geochemical dispersion in the regolith are determined by climate. Lateritic weathering is typically associated with seasonal tropical (savanna) climates but, in many terrains, climates have changed quite profoundly since lateritization. The dominant weathering processes have changed accordingly and this is reflected in the mineralogy and geochemistry of the regolith. Three supergene gold deposits characteristic of lateritic terrain, in contrasting climatic and geomorphological environments, have been compared to demonstrate how these changes are reflected in the distribution, composition and morphology of particulate gold. The sites are : 1- Kangaba, Mali, West Africa, in a strongly seasonal tropical climate with an annual rainfall of 1300 ram. 2- Bardoc, Western Australia, in a warm, semi-add climate with an annual rainfall of approximately 200 mm. 3 - Hannan South, Western Australia has a similar climate to Bardoc, but lies beneath a saline playa. At Kangaba and Bardoc, particulate gold was systematically panned from several horizons in the profile. At Hannan South, gold grains were examined in situ in enriched samples. The characteristics of individual grains were determined by optical and electron-optical techniques.
Kangaba, S.W. Mall Gold mineralization is associated with quartz veins, silicified breccias and sulphides in a shear zone; the
gold-bearing quartz has a sugary texture and contains minor pyrite, arsenopyrite and molybdenite Kangaba is situated in a lateritic plateau and, over the schistes, the regolith is 50-80 m thick. The weathering profile consists of saprolite overlain by a mottled zone (2-4 m thick) and an outcropping cuirasse (5-10 m thick). The quartz veins become disaggregated in the mottled zone and destroyed in the cuirasse. In the upper saprolite (8 - 20 m depth), gold grains are mainly associated with the veins of sugary quartz, with some included in gossanous veins derived from sulphides. Gold in the quartz tends to be xenomorphic, whereas that dervied from sulphides is highly irregular in shape. Most of the grains are pristine, with only a few (15%) exhibiting any corrosion features such as etching, pitting and rounded edges. Grains in the mottled zone have similar morphologies but over 40% are etched partly rounded. In the cuirasse, 90% of the grains show corrosion features; over half have lost their primary morphology and are rounded. The mottled zone and cuirasse also contain very fine (<10mm) gold grains, predominantly micro-spherules less than 1 mm in diameter, intimately associated with ferruginous material. Gold is also present as arborescent microdendrites or as secondary rims around primary grains. Silver is the only major alloy in the primary grains. Silver contents of strongly etched grains are lower (2 - 4% Ag) than in pristine, uncorroded grains (3 - 10% Ag), whereas grains having specific secondary morphologies, such as dendrites, spherules and rims on primary grains, have no detectable silver.
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GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en P r o v e n c e , F r a n c e .
2nd
A well-developed secondary dispersion halo of gold is present in the cuirasse and mottled zones, extending for up to 100 m from the mineralization. This corresponds to the occurrence of the micro-spherular grains. The halo appears to have developed by the dissolution of the primary grains originally in the cuirasse, and the reprecipitation of gold laterally in the same horizon and in the underlying mottled zone.
Bardoc Mine, Western Australia Primary gold mineralization is associated with pyrite and arsenopyrite in quartz veins in a meta-dolerite host rock. The weathering profile, 50-60 m thick, consists of a thick, dark red to brown saprolite overlain by a paler mottled clay zone. The former lateritic cuirasse has been degraded and the surface horizon is a ferruginous soil, 1-3 m thick, containing Fe oxide nodules and pisoliths, and some pedogenic calcrete. Primary and secondary gold particles were found in all weathering horizons : Primary gold grains contain 2-11% Ag and occur either as unweathered xenomorphic forms, with smooth and bright faces showing the imprint of adjacent crystals, or are dull, rounded and etched. All grains containing < 6% Ag are etched and many exhibit progressive Ag depletion on grain boundaries and along cracks. Secondary gold grains contains < 0.07% Ag and have several morphological types, including xenomorphic forms, individual or intergrown subhedral to euhedral prismatic and flat hexagonal crystals, and complex irregular aggregates. The grains may be pristine or corroded, and commonly have a later generation of fine Au crystals or spherules adhering to them. In the lower saprolite, about 20-40% of the grains are primary. A much lower proportion (4-10%) has survived the intense leaching of the upper saprolite and the mottled zone and most of the surviving grains are strongly etched. In comparison, more than 40% of the grains in the ferruginous soil are primary. They were presumably preserved within vein quartz or protected by Fe oxides. Corrosion of the secondary grains is greatest in the lower saprolite and mottled zone, whereas those in the upper saprolite are predominantly pristine. In the mottled zone and upper saprolite, secondary gold occurs mainly as irregular aggregates. These are strongly corroded in the mottled zone but commonly have numerous uncorroded crystals developed on them. Apart from silver, there appears to be no significant difference in the compositions of primary
and secondary grams.
Hannan South Mine, Western Australia Primary g.old mineralization is associated with arsenopynte and bismuth sulphides in pyrite in a siliceous epidote-ankerite host rock. The original lateritic profile has been truncated in the upper saprolite by the migration of a salt lake drainage system and, in the present profile, 50-75 m thick, saprolite is overlain by 2-5 m of lacustrine and evaporitic sediments. The base of a zone of ferruginization at 25-35 m depth marks a redox interface, possibly associated with a former water-table. The present water-table is at 3 m and the groundwater is hyper-saline. Gold is present in three distinct modes in the saprolite : 1 - as primary ovoid inclusions, < 1-100 /.tm in diameter, in the outer zones of pyrite grains below the redox interface. 2- as secondary grains and clusters, dispersed around partly weathered pyrite in the lower saprolite. Most of these grains are extremely small (<20gm), with smooth, rounded faces. More rarely, irregular, isolated grains to 100 ~tm are found in iron oxide pseudomorphs of pyrite in oxidized environments in faults. 3 - as secondary grains in a lateral dispersion halo above the redox interface extending for up to 80 m (at 1 ppm) in saprolite from the host unit into the wallrocks. Gold occurs most commonly as thin hexagonal and trigonal plates, 0.015-3 mm in diameter and octahedral crystals (5-20 ~tm). Rare partially rounded and fused octahedra (0.05-0.1 mm) occur, having smaller platelets and octahedra on their surfaces. Less commonly, irregular fine semi-crystalline aggregates of gold to 40 mm occur in voids with plates and octahedral crystals. Dendritic, mammillary, wire, "paint" and "mustard" gold are also found, particularly on fracture surfaces coated with iron oxides or carbonate. Most of the crystals show at least some corrosion but nevertheless may be associated with grains that are pristine. None of the secondary gold grains contains detectable silver or other elements. DISCUSSION AND C O N C L U S I O N The difference in the nature and distribution of gold at the three sites can best be explained by gold having been mobilized by different processes at different times.
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd I N T E R N A T I O N A L SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.
In West Africa, the dissolution of gold is related to the lateritization process and occured essentially at the top of the profiles in ferruginous horizons. Gold was probably mobilized as organic complexes in surface horizons. Dissolved and colloidal gold became dispersed in this horizon to create a secondary halo, probably augmented by physical movement of primary gold grains and of Fe oxides segregations such as pisoliths that contain gold. Dissolution processes during lateritisation are of minor importance and don't lead to economic concentrations within ferruginous horizons. In Australia, gold was also mobilized by
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lateritizations like in West Africa, but further mobilization probably occured under arid conditions, when groundwaters were saline. Gold solubilization was greater than during lateritization and took place mainly in the mottled zone or in saprolite. Most of the primary gold in this zones was dissolved and reprecipitated as secondary gold of high fineness within the original mineralized zone and, especially at Hannan South, within saprolitic wallrocks, as sub-horizontal enrichment zones that mark past redox boundaries or water-tables. The hexagonal and octahedral crystal habits appear to be characteristic of precipitation from saline solutions.
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2nd I N T E R N A T I O N A L
G E O C H E M I S T R Y AND M O R P H O L O G Y OF G O L D IN L A T E R I T I C PROFILES IN SAVANNA AND SEMI-ARID C L I M A T E S
FREYSSINET Ph. *, LAWRANCE L.M. ** and BUT1~ C.R.M. ** * B.R.G.M., BP 6009, Orl6ans Cedex, France. ** CSIRO - Division of Exploration Geoscience, Wembley, Western Australia.
The nature of many of the processes of weathering and erosion that are important in geochemical dispersion in the regolith are determined by climate. Lateritic weathering is typically associated with seasonal tropical (savanna) climates but, in many terrains, climates have changed quite profoundly since lateritization. The dominant weathering processes have changed accordingly and this is reflected in the mineralogy and geochemistry of the regolith. Three supergene gold deposits characteristic of lateritic terrain, in contrasting climatic and geomorphological environments, have been compared to demonstrate how these changes are reflected in the distribution, composition and morphology of particulate gold. The sites are : 1- Kangaba, Mali, West Africa, in a strongly seasonal tropical climate with an annual rainfall of 1300 ram. 2- Bardoc, Western Australia, in a warm, semi-add climate with an annual rainfall of approximately 200 mm. 3 - Hannan South, Western Australia has a similar climate to Bardoc, but lies beneath a saline playa. At Kangaba and Bardoc, particulate gold was systematically panned from several horizons in the profile. At Hannan South, gold grains were examined in situ in enriched samples. The characteristics of individual grains were determined by optical and electron-optical techniques.
Kangaba, S.W. Mall Gold mineralization is associated with quartz veins, silicified breccias and sulphides in a shear zone; the
gold-bearing quartz has a sugary texture and contains minor pyrite, arsenopyrite and molybdenite Kangaba is situated in a lateritic plateau and, over the schistes, the regolith is 50-80 m thick. The weathering profile consists of saprolite overlain by a mottled zone (2-4 m thick) and an outcropping cuirasse (5-10 m thick). The quartz veins become disaggregated in the mottled zone and destroyed in the cuirasse. In the upper saprolite (8 - 20 m depth), gold grains are mainly associated with the veins of sugary quartz, with some included in gossanous veins derived from sulphides. Gold in the quartz tends to be xenomorphic, whereas that dervied from sulphides is highly irregular in shape. Most of the grains are pristine, with only a few (15%) exhibiting any corrosion features such as etching, pitting and rounded edges. Grains in the mottled zone have similar morphologies but over 40% are etched partly rounded. In the cuirasse, 90% of the grains show corrosion features; over half have lost their primary morphology and are rounded. The mottled zone and cuirasse also contain very fine (<10mm) gold grains, predominantly micro-spherules less than 1 mm in diameter, intimately associated with ferruginous material. Gold is also present as arborescent microdendrites or as secondary rims around primary grains. Silver is the only major alloy in the primary grains. Silver contents of strongly etched grains are lower (2 - 4% Ag) than in pristine, uncorroded grains (3 - 10% Ag), whereas grains having specific secondary morphologies, such as dendrites, spherules and rims on primary grains, have no detectable silver.
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GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en P r o v e n c e , F r a n c e .
2nd
A well-developed secondary dispersion halo of gold is present in the cuirasse and mottled zones, extending for up to 100 m from the mineralization. This corresponds to the occurrence of the micro-spherular grains. The halo appears to have developed by the dissolution of the primary grains originally in the cuirasse, and the reprecipitation of gold laterally in the same horizon and in the underlying mottled zone.
Bardoc Mine, Western Australia Primary gold mineralization is associated with pyrite and arsenopyrite in quartz veins in a meta-dolerite host rock. The weathering profile, 50-60 m thick, consists of a thick, dark red to brown saprolite overlain by a paler mottled clay zone. The former lateritic cuirasse has been degraded and the surface horizon is a ferruginous soil, 1-3 m thick, containing Fe oxide nodules and pisoliths, and some pedogenic calcrete. Primary and secondary gold particles were found in all weathering horizons : Primary gold grains contain 2-11% Ag and occur either as unweathered xenomorphic forms, with smooth and bright faces showing the imprint of adjacent crystals, or are dull, rounded and etched. All grains containing < 6% Ag are etched and many exhibit progressive Ag depletion on grain boundaries and along cracks. Secondary gold grains contains < 0.07% Ag and have several morphological types, including xenomorphic forms, individual or intergrown subhedral to euhedral prismatic and flat hexagonal crystals, and complex irregular aggregates. The grains may be pristine or corroded, and commonly have a later generation of fine Au crystals or spherules adhering to them. In the lower saprolite, about 20-40% of the grains are primary. A much lower proportion (4-10%) has survived the intense leaching of the upper saprolite and the mottled zone and most of the surviving grains are strongly etched. In comparison, more than 40% of the grains in the ferruginous soil are primary. They were presumably preserved within vein quartz or protected by Fe oxides. Corrosion of the secondary grains is greatest in the lower saprolite and mottled zone, whereas those in the upper saprolite are predominantly pristine. In the mottled zone and upper saprolite, secondary gold occurs mainly as irregular aggregates. These are strongly corroded in the mottled zone but commonly have numerous uncorroded crystals developed on them. Apart from silver, there appears to be no significant difference in the compositions of primary
and secondary grams.
Hannan South Mine, Western Australia Primary g.old mineralization is associated with arsenopynte and bismuth sulphides in pyrite in a siliceous epidote-ankerite host rock. The original lateritic profile has been truncated in the upper saprolite by the migration of a salt lake drainage system and, in the present profile, 50-75 m thick, saprolite is overlain by 2-5 m of lacustrine and evaporitic sediments. The base of a zone of ferruginization at 25-35 m depth marks a redox interface, possibly associated with a former water-table. The present water-table is at 3 m and the groundwater is hyper-saline. Gold is present in three distinct modes in the saprolite : 1 - as primary ovoid inclusions, < 1-100 /.tm in diameter, in the outer zones of pyrite grains below the redox interface. 2- as secondary grains and clusters, dispersed around partly weathered pyrite in the lower saprolite. Most of these grains are extremely small (<20gm), with smooth, rounded faces. More rarely, irregular, isolated grains to 100 ~tm are found in iron oxide pseudomorphs of pyrite in oxidized environments in faults. 3 - as secondary grains in a lateral dispersion halo above the redox interface extending for up to 80 m (at 1 ppm) in saprolite from the host unit into the wallrocks. Gold occurs most commonly as thin hexagonal and trigonal plates, 0.015-3 mm in diameter and octahedral crystals (5-20 ~tm). Rare partially rounded and fused octahedra (0.05-0.1 mm) occur, having smaller platelets and octahedra on their surfaces. Less commonly, irregular fine semi-crystalline aggregates of gold to 40 mm occur in voids with plates and octahedral crystals. Dendritic, mammillary, wire, "paint" and "mustard" gold are also found, particularly on fracture surfaces coated with iron oxides or carbonate. Most of the crystals show at least some corrosion but nevertheless may be associated with grains that are pristine. None of the secondary gold grains contains detectable silver or other elements. DISCUSSION AND C O N C L U S I O N The difference in the nature and distribution of gold at the three sites can best be explained by gold having been mobilized by different processes at different times.
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd I N T E R N A T I O N A L SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France.
In West Africa, the dissolution of gold is related to the lateritization process and occured essentially at the top of the profiles in ferruginous horizons. Gold was probably mobilized as organic complexes in surface horizons. Dissolved and colloidal gold became dispersed in this horizon to create a secondary halo, probably augmented by physical movement of primary gold grains and of Fe oxides segregations such as pisoliths that contain gold. Dissolution processes during lateritisation are of minor importance and don't lead to economic concentrations within ferruginous horizons. In Australia, gold was also mobilized by
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lateritizations like in West Africa, but further mobilization probably occured under arid conditions, when groundwaters were saline. Gold solubilization was greater than during lateritization and took place mainly in the mottled zone or in saprolite. Most of the primary gold in this zones was dissolved and reprecipitated as secondary gold of high fineness within the original mineralized zone and, especially at Hannan South, within saprolitic wallrocks, as sub-horizontal enrichment zones that mark past redox boundaries or water-tables. The hexagonal and octahedral crystal habits appear to be characteristic of precipitation from saline solutions.