Gold mineralization at the Blue Rock Deposit, Gadzema Greenstone Belt: Implications on genesis and exploration for orogenic gold mineralization within Archaean Greenstone Belts of Zimbabwe
- Authors: Mavuwa, Tavashavira
- Date: 2024-10-11
- Subjects: Felsite , Quartz porphyry , Orogenic gold deposit , Shear zones (Geology) , Prospecting
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464944 , vital:76559
- Description: The Blue Rock gold deposit was re-discovered by African Consolidated Resources (ACR) in 2007, over a defunct historical gold mine at Blue Rock, during a regional geochemical soil sampling program, within the Gadzema Greenstone Belt (GGB), in Central Zimbabwe. Most significant orogenic gold deposits within this belt occur as BIF- and quartz vein hosted orebodies. But unlike them, gold mineralization at Blue Rock is associated with felsite and quartz porphyry rocks. The GGB is a northern extension of the Midlands Greenstone Belt (MGB), where the common occurrence of mineralized felsites, in close association with major gold reefs within numerous gold mines is well documented. But no significant effort was directed towards their understanding or exploration in the past. They were never considered viable exploration targets for significant economic gold deposits, until recently. More attention was instead focused on high-grade BIF and quartz vein hosted gold, that dominate most orebodies exploited by numerous mines within the belt. At Blue Rock, ACR defined a significant JORC compliant felsite-hosted gold resource of close to a million ounces, which represents a brand new attractive open-pit mining opportunity. But the successful exploration for such type of mineralization, whose footprint is so different from the common ones previously mined within the GGB, no doubt, calls for a good understanding of this type of mineralization. Which makes felsite-hosted gold mineralization a prime candidate for research, based on a deposit whose discovery and development, I was fortunate to be part of during the past few years. In this contribution, the genesis, localization and economic significance of felsite-hosted gold mineralization is investigated, using the gold deposit at Blue Rock as a case study. The deposit could be understood best through the Mineral Systems Approach, used in this study to interrogate alternative ideas about its genesis using published information and deposit-scale exploratory data. Results from the synthesis of published information on the evolution of Archaean Greenstone Belts and genesis of their host orogenic gold deposits, are consistent with models that view orogenic terrains as having formed through horizontal accretion in modern-day like subduction-accretion systems, at continental margins, where orogenic gold deposition occurred via processes that could be explained quite simply, through a universal orogenic gold mineral systems model. According to this model, orogenic gold deposits are believed to have formed from near neutral fluids containing dissolved gold, generated directly from the devolatilization of a subducted oceanic slab together with its overlying gold-bearing sulphide-rich sedimentary package, or indirectly through fluid released from a mantle lithosphere that was originally metasomatized and fertilized during an earlier subduction event. The fluid migrated up-dip from the mantle to crustal levels, through advection or seismic pumping along lithosphere- to crustal-scale fault zones, to form orogenic gold deposits within lower order structures. If these models are all accurate, then the GGB formed through subduction-related east-directed horizontal accretion at the continental margins of the Sebakwe Proto-Craton (SPC), and the genesis of felsite-hosted gold mineralization at Blue Rock could be explained eloquently through a universal orogenic gold model, in which mantle derived auriferous fluids were localised within lower order structures associated with felsites during the late stages of terrain accretion. Evidence from surface mapping and 3D modelling of exploratory drilling data, conducted during this research, strongly support the argument that the felsite hosted gold mineralization at Blue Rock, is neither unique nor accidental. It is just but, a simple product of the conjunction of favourable geological factors, no different to those that birthed typical GGB orogenic gold mineralization hosted within sheared sulphidic BIFs and quartz vein reefs. They all share the same geodynamic setting, fertility, preservation and regional architectural factors reminiscent of accretionary orogenic settings, albeit with differences in local architecture, variably controlled by geochemical and rheological properties of the different local host rocks. At deposit scale, the felsites occur as small dykes and sills emplaced along pre-existing structural zones of weakness. Gold mineralization is structurally controlled and associated mostly with brittle-ductile shears. During deformation, rheological contrast played a significant role in the selective failure of the more competent felsite rocks, resulting in the creation of permeability channels that allowed fluid migration. The more brittle and competent felsites acted as rigid bodies, that localised strain along their contacts with the surrounding less competent ductile mafic schists which acted as a relatively less permeable fluid cap rock. The irregular felsite contact zones with surrounding mafic schist caused a significant variation in the orientation of local principal maximum stress relative to the internally imposed regional stress, causing anomalously low minimum stress zones at deposit scale. Gold deposition occurred within low minimum stress structural traps dominated by sheared felsite contacts and their fractured interiors as well as triple junctions formed by complex structural geometries created by multiple felsite intrusions. The felsite hosted gold at Blue Rock can therefore be recognized as an orogenic gold mineral system archetype, for which an occurrence model is proposed. Understanding this type of mineralization is key for developing a robust exploration strategy - one that could be applied in a predictive capacity in exploration, to locate new economic gold deposits especially within well-endowed mature orogenic terrains, where exploration risk could be minimized by leveraging on new forward-thinking initiatives like Artificial Intelligence (AI) to re-analyze data from previous mining and exploration, allowing for a faster route to a return on investment. In a world of diminishing natural resources, the potential for previously ignored gold mineralization like the one at Blue Rock, becomes very significant. The prophetic words of Foster (1984), writing in Gold ‘82, therefore remain true and relevant to our time, that; “…. the way ahead for successful gold exploration is to search for new deposits not commonly recognized – in auriferous muds, disseminations in carbonate rocks, porphyries, and in felsic intrusive and extrusive volcanics…”. , Thesis (MSc) -- Faculty of Science, Geology, 2024
- Full Text:
- Authors: Mavuwa, Tavashavira
- Date: 2024-10-11
- Subjects: Felsite , Quartz porphyry , Orogenic gold deposit , Shear zones (Geology) , Prospecting
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464944 , vital:76559
- Description: The Blue Rock gold deposit was re-discovered by African Consolidated Resources (ACR) in 2007, over a defunct historical gold mine at Blue Rock, during a regional geochemical soil sampling program, within the Gadzema Greenstone Belt (GGB), in Central Zimbabwe. Most significant orogenic gold deposits within this belt occur as BIF- and quartz vein hosted orebodies. But unlike them, gold mineralization at Blue Rock is associated with felsite and quartz porphyry rocks. The GGB is a northern extension of the Midlands Greenstone Belt (MGB), where the common occurrence of mineralized felsites, in close association with major gold reefs within numerous gold mines is well documented. But no significant effort was directed towards their understanding or exploration in the past. They were never considered viable exploration targets for significant economic gold deposits, until recently. More attention was instead focused on high-grade BIF and quartz vein hosted gold, that dominate most orebodies exploited by numerous mines within the belt. At Blue Rock, ACR defined a significant JORC compliant felsite-hosted gold resource of close to a million ounces, which represents a brand new attractive open-pit mining opportunity. But the successful exploration for such type of mineralization, whose footprint is so different from the common ones previously mined within the GGB, no doubt, calls for a good understanding of this type of mineralization. Which makes felsite-hosted gold mineralization a prime candidate for research, based on a deposit whose discovery and development, I was fortunate to be part of during the past few years. In this contribution, the genesis, localization and economic significance of felsite-hosted gold mineralization is investigated, using the gold deposit at Blue Rock as a case study. The deposit could be understood best through the Mineral Systems Approach, used in this study to interrogate alternative ideas about its genesis using published information and deposit-scale exploratory data. Results from the synthesis of published information on the evolution of Archaean Greenstone Belts and genesis of their host orogenic gold deposits, are consistent with models that view orogenic terrains as having formed through horizontal accretion in modern-day like subduction-accretion systems, at continental margins, where orogenic gold deposition occurred via processes that could be explained quite simply, through a universal orogenic gold mineral systems model. According to this model, orogenic gold deposits are believed to have formed from near neutral fluids containing dissolved gold, generated directly from the devolatilization of a subducted oceanic slab together with its overlying gold-bearing sulphide-rich sedimentary package, or indirectly through fluid released from a mantle lithosphere that was originally metasomatized and fertilized during an earlier subduction event. The fluid migrated up-dip from the mantle to crustal levels, through advection or seismic pumping along lithosphere- to crustal-scale fault zones, to form orogenic gold deposits within lower order structures. If these models are all accurate, then the GGB formed through subduction-related east-directed horizontal accretion at the continental margins of the Sebakwe Proto-Craton (SPC), and the genesis of felsite-hosted gold mineralization at Blue Rock could be explained eloquently through a universal orogenic gold model, in which mantle derived auriferous fluids were localised within lower order structures associated with felsites during the late stages of terrain accretion. Evidence from surface mapping and 3D modelling of exploratory drilling data, conducted during this research, strongly support the argument that the felsite hosted gold mineralization at Blue Rock, is neither unique nor accidental. It is just but, a simple product of the conjunction of favourable geological factors, no different to those that birthed typical GGB orogenic gold mineralization hosted within sheared sulphidic BIFs and quartz vein reefs. They all share the same geodynamic setting, fertility, preservation and regional architectural factors reminiscent of accretionary orogenic settings, albeit with differences in local architecture, variably controlled by geochemical and rheological properties of the different local host rocks. At deposit scale, the felsites occur as small dykes and sills emplaced along pre-existing structural zones of weakness. Gold mineralization is structurally controlled and associated mostly with brittle-ductile shears. During deformation, rheological contrast played a significant role in the selective failure of the more competent felsite rocks, resulting in the creation of permeability channels that allowed fluid migration. The more brittle and competent felsites acted as rigid bodies, that localised strain along their contacts with the surrounding less competent ductile mafic schists which acted as a relatively less permeable fluid cap rock. The irregular felsite contact zones with surrounding mafic schist caused a significant variation in the orientation of local principal maximum stress relative to the internally imposed regional stress, causing anomalously low minimum stress zones at deposit scale. Gold deposition occurred within low minimum stress structural traps dominated by sheared felsite contacts and their fractured interiors as well as triple junctions formed by complex structural geometries created by multiple felsite intrusions. The felsite hosted gold at Blue Rock can therefore be recognized as an orogenic gold mineral system archetype, for which an occurrence model is proposed. Understanding this type of mineralization is key for developing a robust exploration strategy - one that could be applied in a predictive capacity in exploration, to locate new economic gold deposits especially within well-endowed mature orogenic terrains, where exploration risk could be minimized by leveraging on new forward-thinking initiatives like Artificial Intelligence (AI) to re-analyze data from previous mining and exploration, allowing for a faster route to a return on investment. In a world of diminishing natural resources, the potential for previously ignored gold mineralization like the one at Blue Rock, becomes very significant. The prophetic words of Foster (1984), writing in Gold ‘82, therefore remain true and relevant to our time, that; “…. the way ahead for successful gold exploration is to search for new deposits not commonly recognized – in auriferous muds, disseminations in carbonate rocks, porphyries, and in felsic intrusive and extrusive volcanics…”. , Thesis (MSc) -- Faculty of Science, Geology, 2024
- Full Text:
The depositional history and evaluation of two late quaternary, diamondiferous pocket beaches, south-western Namibia
- Authors: Milad, Micael George
- Date: 2004-03
- Subjects: Pocket beach , Geology, Stratigraphic Holocene , Diamond deposits Namibia Sperrgebiet , Prospecting
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/420934 , vital:71795
- Description: The two Late Quaternary, diamondiferous pocket beach deposits studied here are situated along a 10 km stretch of the storm-dominated, Atlantic coastline of the Sperrgebiet, south-western Namibia. The pocket beaches are approximately 130 km north of the Orange River mouth, which is widely accepted as a long-lived point source for diamonds sourced from the interior of southern Africa. A total of fourteen pocket beach deposits were recently evaluated in this area, but only two of these, namely Site 2 (to the south) and Site 3 (to the north), are considered here. The main diamondbearing horizons are beach gravels, which occur within, and form part of, the pocket beach sequences. The beach gravels are mostly blanketed by sand overburden, meaning that exposures available for study were limited, and much reliance was placed on borehole logging and observations of evaluation sample tailings. The main aims are to unravel the depositional history of the pocket beach sequences, identify the controls on diamond mineralisation in the beach gravels, and critically examine two different methods of estimating average diamond size for the deposits. In pursuit of these aims, sedimentological characteristics of the unconsolidated pocket beach deposits were recorded using small diameter drill holes, hydraulic grab bulk samples, trench exposures and surface outcrops. The surface geology, geomorphology and modern wave patterns were mapped using high-resolution, Airborne Laser Survey imagery coupled with extensive field checking. Three-dimensional geological modeling software was used to gain insight into the subsurface morphology of the deposits. Fossil shell samples were used to aid interpretation of ancient depositional environments and to date parts of the pocket beach sequences. Variations in diamond concentration and the size of diamonds were recorded using bulk samples, some of which were taken from a trench, but most of which were excavated using a hydraulic grab tool called the GB50. Finally, by using diamond size data from Site 3, sample data from diamondiferous beach gravels to the south of the study area and sample campaign simulations, two alternative methods of evaluating average diamond size in marine gravel deposits were appraised.The pocket beach sequences occur within north-south trending valleys of a major deflation basin and are separated from one another by rocky headlands. The ridge-and-valley topography of the deflation basin has resulted from differential erosion of Late Proterozoic basement rock units, alternating layers of which differ greatly in their resistance to the long-lived, local denudationalprocesses. On the basis of the stratigraphic information collected from the unconsolidated pocket beach valley fills, interpreted within the context of global, Late Pleistocene sea level records, the following depositional history is deduced : a) Deposition of sheetflood gravels by ephemeral streams, activated during a regressive phase. b) Transgression, culminating in the deposition of a gravel beach, representing a sea level highstand of +4 metres above mean sea level (mamsl) at between 120 000 and 130 000 BP. c)A regressive phase, resulting in deflation of former valley fills to the bedrock valley floor and accompanied by re-activation of ephemeral stream activity to form sheetflood deposits; this represents a protracted period of subaerial exposure of the +4 m gravel beach deposit. d) Deposition of a great volume of sediment in the valleys during the latter stages of the transgression from the Last Glacial Maximum (LGM). The sequence generated during this phase, which started at ca. 9 000 BP, contains : i) pan/coastal sabkha sediments, ii) shallow, sheltered bay sediments, iii) back-barrier lagoonal sediments, iv) a gravel beach deposit representing a sea level stillstand at -5 mamsl, laid down between 7 600 and 5 600 BP, v) another gravel beach deposit representing the well-known Middle Holocene sea level highstand at +2 to +3 mamsl, laid down at ca. 5 000 BP, and which terminated the transgression from the LGM. e) A minor regression to the current sea level, accompanied by progradation of the shoreline to its current position. This progradational marine unit consists almost entirely of sand and grit, reflecting the lack of gravel supply to this part of the coastline in the most recent past. f) Deposition of modern coastal dunes, which cap the pocket beach sequence and are the youngest sediments in the study area. Using trench and hydraulic grab evaluation sample results, in combination with analysis of wave patterns and field observations, the following local controls on the density distribution (ie. concentration) and size distribution of diamonds in the gravel beach deposits (+4, -5 and +2 to +3 mamsl stands) are recognised: a) Gravel beach depositional processes, which are responsible for clast sorting on the beach, have influenced the density and size distribution of diamonds. The infill zone, or beach toe, favours maximum diamond concentration while diamond size decreases from the imbricate zone (intertidal) to the infill zone (subtidal). b) Wave energy is identified as the dominant local control on diamond size distribution, but has also influenced diamond concentration to a limited degree. Larger diamonds are intimately associated with coarser beach gravels, both of which are a reflection of increased wave energy. Higher concentrations of diamonds are sometimes associated with zones of coarser gravel and therefore greater wave energy. c) The time of deposition of the host gravel beach is seen to be the dominant controlling factor with respect to diamond concentration. This is seen as evidence of significant temporal variation in the availability of diamonds in the littoral evironment. A significant reduction (20%) in average diamond size from Site 2 to Site 3, over a distance of only 6 km, is evident. The following were identified as reasons for this reduction in diamond size : a) Longshore sorting processes, of which the long-lived northerly littoral drift is a key part, are known to have played a role in the diminution of diamond size northwards from the Orange River mouth point source. However, it is believed that this can only partly account for the observed 20% reduction in diamond size. b) Input of sediment and smaller diamonds at Site 3, reworked out of an older, Eocene-aged marine succession in the hinterland, is recognised as a possible additional reason for the large reduction in diamond size from Site 2 to Site 3. It is also speculated that the large size of the pocket beach at Site 3, relative to Site 2, may have resulted in lower average wave energy at Site 3, with consequent reduced average diamond size. Diamond size in the beach gravels of Site 3, as well as in beach gravels elsewhere in the Sperrgebiet, is seen to be lognormally-distributed within geologically homogeneous zones. In theory, lognormal mean estimators represent the best method of estimating average diamond size in such cases, whereas the arithmetic mean estimator has the tendency to overestimate when large outlier values occur. Lognormal mean estimators have the added benefit of providing for the calculation of confidence limits, which are becoming increasingly more important as financial lending institutions insist on better quantification of the risk involved in resource estimates. Sample campaign simulations demonstrate, for the kinds of diamond size-frequency distributions typical of beach gravel deposits at Site 3, that there is no significant improvement in the accuracy of average diamond size estimates when lognormal mean estimators are used instead of the arithmetic mean estimator. This is because the variance (a ) of the diamond populations is low, and large outlier values are extremely unlikely to occur. However, simulation of a diamond population with high variance, drawn from a sample of beach gravels near the Orange River mouth, shows that lognormal estimators produce significantly more accurate results when a is large. Since individual diamond weights were not recorded during evaluation sampling of Site 3, numerical solution of lognormal estimators is not possible, and these would need to be solved using a less accurate graphical method. It is therefore recommended that individual diamond weights are recorded in future sampling campaigns, allowing for the use of lognormal mean estimators, and the calculation of confidence limits for average diamond size estimates. , Thesis (MSc) -- Science, Geology, 2004
- Full Text:
- Authors: Milad, Micael George
- Date: 2004-03
- Subjects: Pocket beach , Geology, Stratigraphic Holocene , Diamond deposits Namibia Sperrgebiet , Prospecting
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/420934 , vital:71795
- Description: The two Late Quaternary, diamondiferous pocket beach deposits studied here are situated along a 10 km stretch of the storm-dominated, Atlantic coastline of the Sperrgebiet, south-western Namibia. The pocket beaches are approximately 130 km north of the Orange River mouth, which is widely accepted as a long-lived point source for diamonds sourced from the interior of southern Africa. A total of fourteen pocket beach deposits were recently evaluated in this area, but only two of these, namely Site 2 (to the south) and Site 3 (to the north), are considered here. The main diamondbearing horizons are beach gravels, which occur within, and form part of, the pocket beach sequences. The beach gravels are mostly blanketed by sand overburden, meaning that exposures available for study were limited, and much reliance was placed on borehole logging and observations of evaluation sample tailings. The main aims are to unravel the depositional history of the pocket beach sequences, identify the controls on diamond mineralisation in the beach gravels, and critically examine two different methods of estimating average diamond size for the deposits. In pursuit of these aims, sedimentological characteristics of the unconsolidated pocket beach deposits were recorded using small diameter drill holes, hydraulic grab bulk samples, trench exposures and surface outcrops. The surface geology, geomorphology and modern wave patterns were mapped using high-resolution, Airborne Laser Survey imagery coupled with extensive field checking. Three-dimensional geological modeling software was used to gain insight into the subsurface morphology of the deposits. Fossil shell samples were used to aid interpretation of ancient depositional environments and to date parts of the pocket beach sequences. Variations in diamond concentration and the size of diamonds were recorded using bulk samples, some of which were taken from a trench, but most of which were excavated using a hydraulic grab tool called the GB50. Finally, by using diamond size data from Site 3, sample data from diamondiferous beach gravels to the south of the study area and sample campaign simulations, two alternative methods of evaluating average diamond size in marine gravel deposits were appraised.The pocket beach sequences occur within north-south trending valleys of a major deflation basin and are separated from one another by rocky headlands. The ridge-and-valley topography of the deflation basin has resulted from differential erosion of Late Proterozoic basement rock units, alternating layers of which differ greatly in their resistance to the long-lived, local denudationalprocesses. On the basis of the stratigraphic information collected from the unconsolidated pocket beach valley fills, interpreted within the context of global, Late Pleistocene sea level records, the following depositional history is deduced : a) Deposition of sheetflood gravels by ephemeral streams, activated during a regressive phase. b) Transgression, culminating in the deposition of a gravel beach, representing a sea level highstand of +4 metres above mean sea level (mamsl) at between 120 000 and 130 000 BP. c)A regressive phase, resulting in deflation of former valley fills to the bedrock valley floor and accompanied by re-activation of ephemeral stream activity to form sheetflood deposits; this represents a protracted period of subaerial exposure of the +4 m gravel beach deposit. d) Deposition of a great volume of sediment in the valleys during the latter stages of the transgression from the Last Glacial Maximum (LGM). The sequence generated during this phase, which started at ca. 9 000 BP, contains : i) pan/coastal sabkha sediments, ii) shallow, sheltered bay sediments, iii) back-barrier lagoonal sediments, iv) a gravel beach deposit representing a sea level stillstand at -5 mamsl, laid down between 7 600 and 5 600 BP, v) another gravel beach deposit representing the well-known Middle Holocene sea level highstand at +2 to +3 mamsl, laid down at ca. 5 000 BP, and which terminated the transgression from the LGM. e) A minor regression to the current sea level, accompanied by progradation of the shoreline to its current position. This progradational marine unit consists almost entirely of sand and grit, reflecting the lack of gravel supply to this part of the coastline in the most recent past. f) Deposition of modern coastal dunes, which cap the pocket beach sequence and are the youngest sediments in the study area. Using trench and hydraulic grab evaluation sample results, in combination with analysis of wave patterns and field observations, the following local controls on the density distribution (ie. concentration) and size distribution of diamonds in the gravel beach deposits (+4, -5 and +2 to +3 mamsl stands) are recognised: a) Gravel beach depositional processes, which are responsible for clast sorting on the beach, have influenced the density and size distribution of diamonds. The infill zone, or beach toe, favours maximum diamond concentration while diamond size decreases from the imbricate zone (intertidal) to the infill zone (subtidal). b) Wave energy is identified as the dominant local control on diamond size distribution, but has also influenced diamond concentration to a limited degree. Larger diamonds are intimately associated with coarser beach gravels, both of which are a reflection of increased wave energy. Higher concentrations of diamonds are sometimes associated with zones of coarser gravel and therefore greater wave energy. c) The time of deposition of the host gravel beach is seen to be the dominant controlling factor with respect to diamond concentration. This is seen as evidence of significant temporal variation in the availability of diamonds in the littoral evironment. A significant reduction (20%) in average diamond size from Site 2 to Site 3, over a distance of only 6 km, is evident. The following were identified as reasons for this reduction in diamond size : a) Longshore sorting processes, of which the long-lived northerly littoral drift is a key part, are known to have played a role in the diminution of diamond size northwards from the Orange River mouth point source. However, it is believed that this can only partly account for the observed 20% reduction in diamond size. b) Input of sediment and smaller diamonds at Site 3, reworked out of an older, Eocene-aged marine succession in the hinterland, is recognised as a possible additional reason for the large reduction in diamond size from Site 2 to Site 3. It is also speculated that the large size of the pocket beach at Site 3, relative to Site 2, may have resulted in lower average wave energy at Site 3, with consequent reduced average diamond size. Diamond size in the beach gravels of Site 3, as well as in beach gravels elsewhere in the Sperrgebiet, is seen to be lognormally-distributed within geologically homogeneous zones. In theory, lognormal mean estimators represent the best method of estimating average diamond size in such cases, whereas the arithmetic mean estimator has the tendency to overestimate when large outlier values occur. Lognormal mean estimators have the added benefit of providing for the calculation of confidence limits, which are becoming increasingly more important as financial lending institutions insist on better quantification of the risk involved in resource estimates. Sample campaign simulations demonstrate, for the kinds of diamond size-frequency distributions typical of beach gravel deposits at Site 3, that there is no significant improvement in the accuracy of average diamond size estimates when lognormal mean estimators are used instead of the arithmetic mean estimator. This is because the variance (a ) of the diamond populations is low, and large outlier values are extremely unlikely to occur. However, simulation of a diamond population with high variance, drawn from a sample of beach gravels near the Orange River mouth, shows that lognormal estimators produce significantly more accurate results when a is large. Since individual diamond weights were not recorded during evaluation sampling of Site 3, numerical solution of lognormal estimators is not possible, and these would need to be solved using a less accurate graphical method. It is therefore recommended that individual diamond weights are recorded in future sampling campaigns, allowing for the use of lognormal mean estimators, and the calculation of confidence limits for average diamond size estimates. , Thesis (MSc) -- Science, Geology, 2004
- Full Text:
The porphyry copper system and the precious metal-gold potential
- Authors: Gendall, Ian Richard
- Date: 1994
- Subjects: Copper ores , Porphyry , Gold ores -- Geology , Prospecting
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4992 , http://hdl.handle.net/10962/d1005604 , Copper ores , Porphyry , Gold ores -- Geology , Prospecting
- Description: It has been established that porphyry copper/copper-gold deposits have formed from I Ma to 2 Ga ago. Generally, they are related to the Mesozoic-Cenozoic interval with few reported occurrences from the Palaeozoic or Precambrian. A reason cited is the erosion of these deposits which are often related to convergent plate margins and orogenic belts. Observations of the alteration and mineralisation within and around porphyry copper/copper-gold systems have been included in numerous idealised models. These alteration and mineralisation patterns are dependent on the phases of intrusion, the tectonic setting and rock type, depth of emplacement and relationship to coeval volcanics, physiochemical conditions operative within and surrounding the intrusive and many other mechanical and geochemical conditions. Island arc and cratonic arc/margin deposits are generally considered to be richer in gold than their molybdenum-rich, intra-cratonic counterparts. Metal zonation may occur around these copper/copper-gold deposits, e.g. copper in the core moving out to silver, lead, zinc and gold. This zonation is not always present and gold may occur in the core, intermediate or distal zones. Examples of gold-rich porphyry deposits from British Columbia, Chile and the SW Pacific Island regions suggest gold is closely associated with the potassic-rich zones. Generally these gold-rich zones have greater than 2% magnetite and a high oxygen fugacity is considered to be an important control for gold deposition. High Cl contents within the magma are necessary for gold mobility within the host intrusive centres. Beyond this zone HS₂ becomes an important transporting ligand. Exploration for porphyry copper-gold deposits includes an integrated geological, geophysical and geochemical approach. Petrographic work through to Landsat imagery may be used to determine the chemical conditions of the system, ore association, favourable structural zones and alteration patterns, in order to focus exploration activities.
- Full Text:
- Authors: Gendall, Ian Richard
- Date: 1994
- Subjects: Copper ores , Porphyry , Gold ores -- Geology , Prospecting
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4992 , http://hdl.handle.net/10962/d1005604 , Copper ores , Porphyry , Gold ores -- Geology , Prospecting
- Description: It has been established that porphyry copper/copper-gold deposits have formed from I Ma to 2 Ga ago. Generally, they are related to the Mesozoic-Cenozoic interval with few reported occurrences from the Palaeozoic or Precambrian. A reason cited is the erosion of these deposits which are often related to convergent plate margins and orogenic belts. Observations of the alteration and mineralisation within and around porphyry copper/copper-gold systems have been included in numerous idealised models. These alteration and mineralisation patterns are dependent on the phases of intrusion, the tectonic setting and rock type, depth of emplacement and relationship to coeval volcanics, physiochemical conditions operative within and surrounding the intrusive and many other mechanical and geochemical conditions. Island arc and cratonic arc/margin deposits are generally considered to be richer in gold than their molybdenum-rich, intra-cratonic counterparts. Metal zonation may occur around these copper/copper-gold deposits, e.g. copper in the core moving out to silver, lead, zinc and gold. This zonation is not always present and gold may occur in the core, intermediate or distal zones. Examples of gold-rich porphyry deposits from British Columbia, Chile and the SW Pacific Island regions suggest gold is closely associated with the potassic-rich zones. Generally these gold-rich zones have greater than 2% magnetite and a high oxygen fugacity is considered to be an important control for gold deposition. High Cl contents within the magma are necessary for gold mobility within the host intrusive centres. Beyond this zone HS₂ becomes an important transporting ligand. Exploration for porphyry copper-gold deposits includes an integrated geological, geophysical and geochemical approach. Petrographic work through to Landsat imagery may be used to determine the chemical conditions of the system, ore association, favourable structural zones and alteration patterns, in order to focus exploration activities.
- Full Text:
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