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Current climate change may induce positive carbon cycle feedbacks that amplify anthropogenic warming on time scales of centuries to millennia. Similar feedbacks might have been active during a phase of carbon cycle perturbation and global warming, termed the Paleocene‐Eocene Thermal Maximum (PETM, 56 million years ago). The PETM may help constrain these feedbacks and their sensitivity to warming. We present new high‐resolution carbon isotope and sea surface temperature data from Ocean Drilling Program Site 959 in the Equatorial Atlantic. With these and existing data from the New Jersey Shelf and Maud Rise, Southern Ocean, we quantify the lead‐lag relation between PETM warming and the carbon input that caused the carbon isotope excursion (CIE). We show ~2 °C of global warming preceded the CIE by millennia, strongly implicating CO2‐driven warming triggered a positive carbon cycle feedback. We further compile new and published barium (Ba) records encompassing continental shelf, slope, and deep ocean settings. Based on this compilation, we calculate that average Ba burial rates approximately tripled during the PETM, which may require an additional source of Ba to the ocean. Although the precipitation pathway is not well constrained, dissolved Ba stored in sulfate‐depleted pore waters below methane hydrates could represent an additional source. We speculate the most complete explanation for early warming and rise in Ba supply is that hydrate dissociation acted as a positive feedback and caused the CIE. These results imply hydrates are more temperature sensitive than previously considered, and may warrant reconsideration of the political assignment of 2 °C warming as a safe future scenario. more...

Back to topThe early Eocene (56 to 48 million years ago) is inferred to have been the most recent time that Earth's atmospheric CO2 concentrations exceeded 1000 ppm. Global mean temperatures were also substantially warmer than those of the present day. As such, the study of early Eocene climate provides insight into how a super-warm Earth system behaves and offers an opportunity to evaluate climate models under conditions of high greenhouse gas forcing. The Deep Time Model Intercomparison Project (DeepMIP) is a systematic model–model and model–data intercomparison of three early Paleogene time slices: latest Paleocene, Paleocene–Eocene thermal maximum (PETM) and early Eocene climatic optimum (EECO). A previous article outlined the model experimental design for climate model simulations. In this article, we outline the methodologies to be used for the compilation and analysis of climate proxy data, primarily proxies for temperature and CO2. This paper establishes the protocols for a concerted and coordinated effort to compile the climate proxy records across a wide geographic range. The resulting climate “atlas” will be used to constrain and evaluate climate models for the three selected time intervals and provide insights into the mechanisms that control these warm climate states. We provide version 0.1 of this database, in anticipation that this will be expanded in subsequent publications. more...

Climate plays a significant role in determining the styles of depositional processes at different latitudes, which in turn influence the locations of hydrocarbon systems. Results of climate modeling may therefore provide important information for predicting the presence or absence of suitable hydrocarbon plays. To determine whether the models provide realistic results, the critical step is to validate the model results against proxy data where they are available. Paleoclimate proxy data are most often derived from more accessible low- to midlatitude regions and are biased towards warm climate states. However, general circulation models (GCMs) have traditionally been biased to colder temperatures, in particular at high-latitudes, struggling to maintain the high-latitude regions warm enough to sustain forests that were present during greenhouse periods, such as the mid-Cretaceous (~130–89 Ma), without exaggerated warming of the equatorial regions. To improve this approach, the HadCM3L coupled atmosphere–ocean GCM, a state-of-the-art model for the long simulations required to reach an equilibrium climate, was run for each stage of the Cretaceous using new paleogeographic base maps. Here, we compare the results for the Aptian (118.5 Ma) and Albian (105.8 Ma) with paleoclimate proxy data from the high northern latitudes in order to determine if the model produces viable results for this region. Paleoclimate analysis of fossil wood from conifer forests from Svalbard of Aptian–Albian age suggests that they grew in moist cool upland areas adjacent to warmer temperate lowland regions, probably with rivers and/or swamps present. Studies of conifers from the Canadian Arctic islands indicate that they grew under slightly cooler conditions than on Svalbard, similar to northern Canada today. The HadCM3L GCM results for Svalbard show that the dominant biome was evergreen taiga/montane forest with lowland temperate vegetation present during the Albian Stage, possibly with an elemen more...

During an interval of the Late Pliocene, referred to here as the mid-Pliocene Warm Period (mPWP; 3.264 to 3.025 million years ago), global mean temperature was similar to that predicted for the end of this century, and atmospheric carbon dioxide concentrations were higher than pre-industrial levels. Sea level was also higher than today, implying a significant reduction in the extent of the ice sheets. Thus, the mPWP provides a natural laboratory in which to investigate the long-term response of the Earth's ice sheets and sea level in a warmer-than-present-day world. At present, our understanding of the Greenland ice sheet during the mPWP is generally based upon predictions using single climate and ice sheet models. Therefore, it is essential that the model dependency of these results is assessed. The Pliocene Model Intercomparison Project (PlioMIP) has brought together nine international modelling groups to simulate the warm climate of the Pliocene. Here we use the climatological fields derived from the results of the 15 PlioMIP climate models to force an offline ice sheet model. We show that mPWP ice sheet reconstructions are highly dependent upon the forcing climatology used, with Greenland reconstructions ranging from an ice-free state to a near-modern ice sheet. An analysis of the surface albedo variability between the climate models over Greenland offers insights into the drivers of inter-model differences. As we demonstrate that the climate model dependency of our results is high, we highlight the necessity of data-based constraints of ice extent in developing our understanding of the mPWP Greenland ice sheet. more...

The understanding of the nature and behavior of ice sheets in past warm periods is important for constraining the potential impacts of future climate change. The Pliocene warm period (between 3.264 and 3.025 Ma) saw global temperatures similar to those projected for future climates; nevertheless, Pliocene ice locations and extents are still poorly constrained. We present results from the efforts to simulate mid-Pliocene Greenland Ice Sheets by means of the international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP). We compare the performance of existing numerical ice sheet models in simulating modern control and mid-Pliocene ice sheets with a suite of sensitivity experiments guided by available proxy records. We quantify equilibrated ice sheet volume on Greenland, identifying a potential range in sea level contributions from warm Pliocene scenarios. A series of statistical measures are performed to quantify the confidence of simulations with focus on inter-model and inter-scenario differences. We find that Pliocene Greenland Ice Sheets are less sensitive to differences in ice sheet model configurations and internal physical quantities than to changes in imposed climate forcing. We conclude that Pliocene ice was most likely to be limited to the highest elevations in eastern and southern Greenland as simulated with the highest confidence and by synthesizing available regional proxies; however, the extent of those ice caps needs to be further constrained by using a range of general circulation model (GCM) climate forcings. more...

The Pliocene Model Intercomparison Project (PlioMIP) is the first coordinated climate model comparison for a warmer palaeoclimate with atmospheric CO2 significantly higher than pre-industrial concentrations. The simulations of the mid-Pliocene warm period show global warming of between 1.8 and 3.6 °C above pre-industrial surface air temperatures, with significant polar amplification. Here we perform energy balance calculations on all eight of the coupled ocean–atmosphere simulations within PlioMIP Experiment 2 to evaluate the causes of the increased temperatures and differences between the models. In the tropics simulated warming is dominated by greenhouse gas increases, with the cloud component of planetary albedo enhancing the warming in most of the models, but by widely varying amounts. The responses to mid-Pliocene climate forcing in the Northern Hemisphere midlatitudes are substantially different between the climate models, with the only consistent response being a warming due to increased greenhouse gases. In the high latitudes all the energy balance components become important, but the dominant warming influence comes from the clear sky albedo, only partially offset by the increases in the cooling impact of cloud albedo. This demonstrates the importance of specified ice sheet and high latitude vegetation boundary conditions and simulated sea ice and snow albedo feedbacks. The largest components in the overall uncertainty are associated with clouds in the tropics and polar clear sky albedo, particularly in sea ice regions. These simulations show that albedo feedbacks, particularly those of sea ice and ice sheets, provide the most significant enhancements to high latitude warming in the Pliocene. more...

The last interglaciation (130 to 116 ka) is a time period with a strong astronomically induced seasonal forcing of insolation compared to the present. Proxy records indicate a significantly different climate to that of the modern, in particular Arctic summer warming and higher eustatic sea level. Because the forcings are relatively well constrained, it provides an opportunity to test numerical models which are used for future climate prediction. In this paper we compile a set of climate model simulations of the early last interglaciation (130 to 125 ka), encompassing a range of model complexities. We compare the simulations to each other and to a recently published compilation of last interglacial temperature estimates.We show that the annual mean response of the models is rather small, with no clear signal in many regions. However, the seasonal response is more robust, and there is significant agreement amongst models as to the regions of warming vs cooling. However, the quantitative agreement of the model simulations with data is poor, with the models in general underestimating the magnitude of response seen in the proxies. Taking possible seasonal biases in the proxies into account improves the agreement, but only marginally. However, a lack of uncertainty estimates in the data does not allow us to draw firm conclusions. Instead, this paper points to several ways in which both modelling and data could be improved, to allow a more robust model–data comparison. more...

The last interglaciation (-130 to 116 ka) is a time period with a strong astronomically induced seasonal forcing of insolation compared to the present. Proxy records indicate a significantly different climate to that of the modern, in particular Arctic summer warming and higher eustatic sea level. Because the forcings are relatively well constrained, it provides an opportunity to test numerical models which are used for future climate prediction. In this paper we compile a set of climate model simulations of the early last interglaciation (130 to 125 ka), encompassing a range of model complexities. We compare the simulations to each other and to a recently published compilation of last interglacial temperature estimates.We show that the annual mean response of the models is rather small, with no clear signal in many regions. However, the seasonal response is more robust, and there is significant agreement amongst models as to the regions of warming vs cooling. However, the quantitative agreement of the model simulations with data is poor, with the models in general underestimating the magnitude of response seen in the proxies. Taking possible seasonal biases in the proxies into account improves the agreement, but only marginally. However, a lack of uncertainty estimates in the data does not allow us to draw firm conclusions. Instead, this paper points to several ways in which both modelling and data could be improved, to allow a more robust model-data comparison. © Author(s) 2013. more...

There is a growing number of proxy-based reconstructions detailing the climatic changes that occurred during the last interglacial period (LIG). This period is of special interest, because large parts of the globe were characterized by a warmer-than-present-day climate, making this period an interesting test bed for climate models in light of projected global warming. However, mainly because synchronizing the different palaeoclimatic records is difficult, there is no consensus on a global picture of LIG temperature changes. Here we present the first model inter-comparison of transient simulations covering the LIG period. By comparing the different simulations, we aim at investigating the common signal in the LIG temperature evolution, investigating the main driving forces behind it and at listing the climate feedbacks which cause the most apparent inter-model differences. The model inter-comparison shows a robust Northern Hemisphere July temperature evolution characterized by a maximum between 130-125 ka BP with temperatures 0.3 to 5.3K above present day. A Southern Hemisphere July temperature maximum, ?1.3 to 2.5K at around 128 ka BP, is only found when changes in the greenhouse gas concentrations are included. The robustness of simulated January temperatures is large in the Southern Hemisphere and the mid-latitudes of the Northern Hemisphere. For these regions maximum January temperature anomalies of respectively ?1 to 1.2K and ?0.8 to 2.1K are simulated for the period after 121 ka BP. In both hemispheres these temperature maxima are in line with the maximum in local summer insolation. In a number of specific regions, a common temperature evolution is not found amongst the models. We show that this is related to feedbacks within the climate system which largely determine the simulated LIG temperature evolution in these regions. Firstly, in the Arctic region, changes in the summer sea-ice cover control the evolution of LIG winter temperatures. Secondly, for the Atl more...

Climate and environments of the mid-Pliocene warm period (3.264 to 3.025 Ma) have been extensively studied. Whilst numerical models have shed light on the nature of climate at the time, uncertainties in their predictions have not been systematically examined. The Pliocene Model Intercomparison Project quantifies uncertainties in model outputs through a coordinated multi-model and multi-model/data intercomparison. Whilst commonalities in model outputs for the Pliocene are clearly evident, we show substantial variation in the sensitivity of models to the implementation of Pliocene boundary conditions. Models appear able to reproduce many regional changes in temperature reconstructed from geological proxies. However, data/model comparison highlights that models potentially underestimate polar amplification. To assert this conclusion with greater confidence, limitations in the time-averaged proxy data currently available must be addressed. Furthermore, sensitivity tests exploring the known unknowns in modelling Pliocene climate specifically relevant to the high latitudes are essential (e.g. palaeogeography, gateways, orbital forcing and trace gasses). Estimates of longer-term sensitivity to CO2 (also known as Earth System Sensitivity; ESS), support previous work suggesting that ESS is greater than Climate Sensitivity (CS), and suggest that the ratio of ESS to CS is between 1 and 2, with a "best" estimate of 1.5. more...

The Miocene (~24 to ~5 million years ago) was a period of relative global warmth compared to the Quaternary (~2 million years ago to present; e.g. Zachos et al., 2001) and was characterised by the intermittent glaciation of Antarctica only. Paradoxically, the majority of available proxy data suggest that during the Miocene, pCO2 was similar, or even lower, than the pre-industrial levels (280 ppmv; Pagani et al., 1999; Pearson and Palmer, 2000; Kürschner et al., 1996, 2008) and at times probably crossed the modelled threshold value required for sustained glaciation in the Northern Hemisphere (DeConto et al., 2008). Records of ice rafted debris and the oxygen isotope composition of benthic foraminifera suggest that at several times over the last 25 million years substantial amounts of continental ice did build up in the Northern Hemisphere but none of these led to prolonged glaciation. In this contribution, we review evidence that suggests that in the Miocene the North American Cordillera was, at least in parts, considerably lower than today. We present new GCM simulations that imply that small amounts of uplift of the North American Cordillera result in significant cooling of the northern North American Continent. Offline ice sheet modelling, driven by these GCM outputs, suggests that with a reduced topography, inception of the Cordilleran ice sheet is prohibited. This suggests that uplift of the North American Cordillera in the Late Miocene may have played an important role in priming the climate for the intensification of Northern Hemisphere glaciation in the Late Pliocene. more...

In 2008 the temporal focus of the Palaeoclimate Modelling Intercomparison Project was expanded to include a model intercomparison for the mid-Pliocene warm period (3.29–2.97 million years ago). This project is referred to as PlioMIP (Pliocene Model Intercomparison Project). Two experiments have been agreed upon and comprise phase 1 of PlioMIP. The first (Experiment 1) will be performed with atmosphere-only climate models. The second (Experiment 2) will utilise fully coupled ocean-atmosphere climate models. The aim of this paper is to provide a detailed model intercomparison project description which documents the experimental design in a more detailed way than has previously been done in the literature. Specifically, this paper describes the experimental design and boundary conditions that will be utilised for Experiment 1 of PlioMIP. more...

During the Middle Pliocene, the Earth experienced greater global warmth compared with today, coupled with higher atmospheric CO2 concentrations. To determine the extent to which the Middle Pliocene can be used as a ‘test bed’ for future warming, we compare data and model-based Middle Pliocene vegetation with simulated global biome distributions for the mid- and late twenty-first century. The best agreement is found when a Middle Pliocene biome reconstruction is compared with a future scenario using 560ppmv atmospheric CO2. In accordance with palaeobotanical data, all model simulations indicate a generally warmer and wetter climate, resulting in a northward shift of the taiga–tundra boundary and a spread of tropical savannahs and woodland in Africa and Australia at the expense of deserts. Our data–model comparison reveals differences in the distribution of polar vegetation, which indicate that the high latitudes during the Middle Pliocene were still warmer than its predicted modern analogue by several degrees. However, our future scenarios do not consider multipliers associated with ‘long-term’ climate sensitivity. Changes in global temperature, and thus biome distributions, at higher atmospheric CO2 levels will not have reached an equilibrium state (as is the case for the Middle Pliocene) by the end of this century. more...

This paper reviews North Atlantic shelf seas palaeoclimate during the interval 4–3Ma, prior to and incorporating the ‘Mid-Pliocene warm period’ (ca 3.29–2.97Ma). Fossil assemblages and stable isotope data demonstrate northwards extension of subtropical faunas along the coast of the Carolinas–Virginia (Yorktown and Duplin Formations) relative to the present day, suggesting a more vigorous Florida Current, with reduced seasonality and warm water extending north of Cape Hatteras (reconstructed annual range for Virginia 12–30°C). This interpretation supports conceptual models of increased meridional heat transport for the Pliocene. Sea temperatures for Florida (Lower Pinecrest Beds) were similar to or slightly cooler than (summers 25–27°C) today, and were probably influenced by seasonal upwelling of cold deep water. Reduced seasonality is also apparent in the Coralline Crag Formation of the southern North Sea, with ostracods suggesting winter sea temperatures of 10°C (modern 4°C). However, estimates from Pliocene bivalves (3.6–16.6°C) are similar to or cooler than the present day. This ‘mixed’ signal is problematic given warmer seas in the Carolinas–Virginia, and climate model and oceanographic data that show warmer seas in the ‘Mid-Pliocene’ eastern North Atlantic. This may be because the Coralline Crag Formation was deposited prior to peak Mid-Pliocene warmth. more...

We examine several aspects of the oceanatmosphere system over the last 30 000 years, by carrying out simulations with prescribed ice sheets, atmospheric CO2 concentration, and orbital parameters. We use the GENIE-1 model with a frictional geostrophic ocean, dynamic sea ice, an energy balance atmosphere, and a land-surface scheme with fixed vegetation. A transient simulation, with boundary conditions derived from ice-core records and ice sheet reconstructions, is compared with equilibrium snapshot simulations, including the Last Glacial Maximum (21 000 years before present; 21 kyrBP), mid-Holocene (6 kyrBP) and preindustrial. The equilibrium snapshot simulations are all very similar to their corresponding time period in the transient simulation, indicating that over the last 30 000 years, the model’s ocean-atmosphere system is close to equilibrium with its boundary conditions. However, our simulations neglect the transfer of fresh water from and to the ocean, resulting from the growth and decay of ice sheets, which would, in reality, lead to greater disequilibrium. Additionally, the GENIE-1 model exhibits a rather limited response in terms of its Atlantic Meridional Overturning Circulation (AMOC) over the 30 000 years; a more sensitive AMOC would also be likely to lead to greater disequilibrium. We investigate the method of accelerating the boundary conditions of a transient simulation and find that the Southern Ocean is the region most affected by the acceleration. The Northern Hemisphere, even with a factor of 10 acceleration, is relatively unaffected. The results are robust to changes to several tunable parameters in the model. They also hold when a higher vertical resolution is used in the ocean. more...

We use the GENIE Earth system modelling framework to examine how the stability / hysteresis diagram of the thermohaline circulation (THC) depends on the use of complex (GCM) or simple (energy-moisture balance) atmosphere models and how it varies with ocean resolution. The model versions all use the GOLDSTEIN frictional geostrophic ocean, but with 3 different horizontal resolutions (and 8 depth layers in each case): (i) 36x36 longitudesine (latitude), (ii) 72x72 longitude-sine(latitude), (iii) 64x32 longitude-latitude. To these we have coupled the Reading Intermediate General Circulation Model (IGCM) at T21 resolution with 7 vertical levels. We contrast this with earlier work using an energy-moisture balance model (EMBM) and ocean resolution (i). For each model version, we construct an ensemble of runs in which we vary atmospheric freshwater transport from the Atlantic to Pacific. In some cases we also vary a parameter controlling equator to pole freshwater transport. The resulting ensembles are run toward equilibrium and then restarts are used to search parameter space for regions of THC bi-stability. The resulting hundreds of thousands of years of 3D ocean-atmosphere model integration were achieved by using UK Grid computing resources, including 6 nodes of the National Grid Service, and additional clusters in Norwich, Southampton and Bristol. A specially developed database system was used to execute and manage the runs. The results are expected to shed light on whether a dynamical atmosphere alters or removes the bi-stability of the THC, and whether THC stability is sensitive to ocean resolution. more...

Approaches to the piloting and scale-up of oxidative and pressure leaching are presented, with particular emphasis on scale-up ratios, using examples from many projects. For pressure leach flowsheets, the duration of the pilot runs is critically important in proving the integrated process. For well-proven processes applied to typical feed materials, such as pressure oxidation of auriferous sulphide concentrates, around 100 hours of piloting may be sufficient. For novel flowsheets or atypical feed mineralogies or process waters, pilot runs of 1 000 to 2 000 hours may be necessary. Piloting of a process may be limited by the facilities of testing laboratories, therefore demonstration-scale testing of innovative flowsheets may be necessary, and specific large-scale testing of units for the scale-up of equipment is routine., Approaches to the piloting and scale-up of oxidative and pressure leaching are presented, with particular emphasis on scale-up ratios, using examples from many projects. For pressure leach flowsheets, the duration of the pilot runs is critically important in proving the integrated process. For well-proven processes applied to typical feed materials, such as pressure oxidation of auriferous sulphide concentrates, around 100 hours of piloting may be sufficient. For novel flowsheets or atypical feed mineralogies or process waters, pilot runs of 1 000 to 2 000 hours may be necessary. Piloting of a process may be limited by the facilities of testing laboratories, therefore demonstration-scale testing of innovative flowsheets may be necessary, and specific large-scale testing of units for the scale-up of equipment is routine. more...

Over an 8 year period between 1986 and 1994, Minproc Engineers Ltd. has undertaken the design, construction and commissioning of 6 grass roots gold projects in Ghana. The plants range from tailings retreatment to free milling and refractory ore processing. Treatment route selection was dependent on the lithology and mineralogy of the gold ore from the two major systems: Birimian and Tarkwaian. The Tarkwaian system resembles Witwatersrand gold reefs, whilst the Birimian deposits, notably Ashanti and Bogosu are characterised by highly complex mineralogy in which gold occurs in association with iron sulphide species (pyrite, arsenopyrite and pyrrhotite). This refractory ore requires an oxidation process. Sulphide ores are overlain by oxide and transitional ores which, in early operation, may be fed as mixtures to the plant. The design of the projects is discussed and the key issues affecting the selection of grinding equipment, flotation circuit and the 2-stage roasting process at Bogosu and the bioleaching at Ashanti are outlined., Over an 8 year period between 1986 and 1994, Minproc Engineers Ltd. has undertaken the design, construction and commissioning of 6 grass roots gold projects in Ghana. The plants range from tailings retreatment to free milling and refractory ore processing. Treatment route selection was dependent on the lithology and mineralogy of the gold ore from the two major systems: Birimian and Tarkwaian. The Tarkwaian system resembles Witwatersrand gold reefs, whilst the Birimian deposits, notably Ashanti and Bogosu are characterised by highly complex mineralogy in which gold occurs in association with iron sulphide species (pyrite, arsenopyrite and pyrrhotite). This refractory ore requires an oxidation process. Sulphide ores are overlain by oxide and transitional ores which, in early operation, may be fed as mixtures to the plant. The design of the projects is discussed and the key issues affecting the selection of grinding equipment, flotation circuit and the 2-stage roasting process at Bogosu and the bioleaching at Ashanti are outlined. more...

Factors in selecting a comminution circuit include project type and size, ore characteristics and properties, circuit costs, testwork costs and impact on project schedule, and scale-up risks. Cadia Hill in New South Wales and Bronzewing in Western Australia, while the physical properties of their ores are very similar, are at opposite ends of the spectrum in terms of project size, ore variability and circuit complexity. They therefore exhibit significant differences in the possibilities for offsetting development costs against increased project costs. Cadia Hill opted for a gyratory crusher, semiautogenous grinding, pebble crushing and two ball mills, with 20 000 kW primary mill power and 17 500 kW secondary mill power, designed on the basis of bench tests and a pilot study. Bronzewing has two 3 400 semiautogenous grinding circuits, with a jaw crusher for oxide ores and two-stage crushing for primary ores. Design was based on bench tests only., Factors in selecting a comminution circuit include project type and size, ore characteristics and properties, circuit costs, testwork costs and impact on project schedule, and scale-up risks. Cadia Hill in New South Wales and Bronzewing in Western Australia, while the physical properties of their ores are very similar, are at opposite ends of the spectrum in terms of project size, ore variability and circuit complexity. They therefore exhibit significant differences in the possibilities for offsetting development costs against increased project costs. Cadia Hill opted for a gyratory crusher, semiautogenous grinding, pebble crushing and two ball mills, with 20 000 kW primary mill power and 17 500 kW secondary mill power, designed on the basis of bench tests and a pilot study. Bronzewing has two 3 400 semiautogenous grinding circuits, with a jaw crusher for oxide ores and two-stage crushing for primary ores. Design was based on bench tests only. more...

The sulphide plant is designed to increase production from the Ashanti mine to over 1 000 000 oz/a Au by 1995. An integral component of this expansion is the construction of a 6 500 t/d concentrator and a 790 t/d biooxidation facility. These facilities will treat a complex carbonaceous refractory ore consisting predominantly of arsenopyrite and pyrite. Design has followed extensive laboratory and pilot-plant testwork that examined a variety of process options. The final flowsheet includes semiautogenous grinding, flash flotation, column and conventional flotation, the Biox process and carbon in leach gold recovery., The sulphide plant is designed to increase production from the Ashanti mine to over 1 000 000 oz/a Au by 1995. An integral component of this expansion is the construction of a 6 500 t/d concentrator and a 790 t/d biooxidation facility. These facilities will treat a complex carbonaceous refractory ore consisting predominantly of arsenopyrite and pyrite. Design has followed extensive laboratory and pilot-plant testwork that examined a variety of process options. The final flowsheet includes semiautogenous grinding, flash flotation, column and conventional flotation, the Biox process and carbon in leach gold recovery. more...

The tailings result from the 2 500 000 tonnes of refractory copper silicate ore, also containing gold and some silver, which was reduction roasted by the TORCO process at Akjoujt, Mauritania, in the early 1970s. They assay 3.1 g/t Au and 0.8-1.0% Cu and have a cyanide consumption in the range 5.5-6.5 kg NaCN/t ore, making them uneconomical to treat conventionally. The use of an ammonia-cyanide mixture can reduce the cyanide consumption to 0.5-1.5 kg/t at the expense of 4 kg/t NH3 and yield gold recoveries in the range 80-90%. After four years of sampling, testing and project development, the medium-tonnage agitation leach plant is due to be commissioned in the first half of 1992. A summary is given of the testwork results which led to the flowsheet selection and the process and plant are briefly described., The tailings result from the 2 500 000 tonnes of refractory copper silicate ore, also containing gold and some silver, which was reduction roasted by the TORCO process at Akjoujt, Mauritania, in the early 1970s. They assay 3.1 g/t Au and 0.8-1.0% Cu and have a cyanide consumption in the range 5.5-6.5 kg NaCN/t ore, making them uneconomical to treat conventionally. The use of an ammonia-cyanide mixture can reduce the cyanide consumption to 0.5-1.5 kg/t at the expense of 4 kg/t NH3 and yield gold recoveries in the range 80-90%. After four years of sampling, testing and project development, the medium-tonnage agitation leach plant is due to be commissioned in the first half of 1992. A summary is given of the testwork results which led to the flowsheet selection and the process and plant are briefly described. more...