GEOL3051A L8-13 Orthomagmatic and Hydrothermal Ore Deposits PDF

GEOL3051A L8 Orthomagmatic ore deposits: kimberlites Dr Linda Iaccheri School of Geosciences [email protected]  5 Processes of ore formation in magmatic systems: 1. Concentration of ore elements as a result of very low degrees of partial melting; 2. Accumulation and conce...

GEOL3051A L8 Orthomagmatic ore deposits: kimberlites Dr Linda Iaccheri School of Geosciences [email protected]  5 Processes of ore formation in magmatic systems: 1. Concentration of ore elements as a result of very low degrees of partial melting; 2. Accumulation and concentration of ore minerals in magma chambers during progressive crystallization and fractionation of mafic and ultramafic magmas;  Chromitites deposits 3. Separation of two immiscible melts in a magma mafic and ultramafic magma chamber;  Ni-Cu-PGE sulphides deposits 4. Extreme fractionation during progressive crystallisation of a magma; 5. Incorporation of a mineral that occurs at a specific depth in the Earth in a ultramafic melt. Ores deposits formed through incorporation of a mineral that occurs at a specific depth in the Earth into an ultramafic magma:  Primary magmatic diamond deposits in kimberlites Ores deposits formed through incorporation of a mineral that occurs at a specific depth in the Earth into an ultramafic magma:  Primary magmatic diamond deposits in kimberlites What is a kimberlite? What is a Kimberlite?  Kimberlites are alkaline, volatile- rich potassic, low density, ultramafic rocks  They formed as small degree of partial melts of carbonate-bearing and hydrous mantle peridotite (larger degrees of partial melting would result in a basaltic magma)  are important as they may host diamonds Kimberlites in hand specimen: Inequigranular texture fragmented minerals and clasts in a fine- grained ground mass Main minerals: - olivine, often altered in serpentine; - diopside - pyrope garnet - enstatite - ilmenite - chromite The predominant mineral in kimberlite is olivine. On the surface of the Earth, olivine is easily altered to a mineral called serpentine. Kimberlites are susceptible to alteration during intrusion and cooling and at the Earth’s surface olivine is serpentinised.  This alteration of olivine promotes the liberation of diamonds from the kimberlite host-rock Diamond in kimberlite Kimberlites and diamonds Kimberlite melts bring diamonds to Earth’s surface, and represent primary magmatic diamond deposits. Diamond is an accidental passenger. It does not form during partial melting and/or fractional crystallisation of a kimberlite melt. Prior to the discovery of kimberlites, diamonds were all mined from secondary alluvial sedimentary deposits: river environments where diamonds had been eroded from their primary source and accumulated as detritus along river beds  Global Distribution of Primary magmatic diamond deposits Dominantly found in the Archean (>2.5 Ga) crustal blocks of Precambrian terranes/cratons. Carat = unit of weight for precious stones equal to 200 milligrams  Estimated Diamond Production Country 2008 2009 Botswana 25,000 1 32,000 Russia 23,300 2 21,900 Australia 230 260 Canada 18,000 3 12,000 South Africa 6,100 6 5,200 Congo 5,400 5 5,400 Angola 10,000 4 8,000 Namibia 2,200 2,000 Ghana 720 500 Brazil 200 200 Cent. African Rep. 470 350 Guinea 1,100 2,000 Sierra Leone 600 300 China 100 100 Tanzania 230 190 Guyana 350 269 Data in thousands of carats of gem diamond. Kimberlite plumbing system Kimberlite melts originated from low amount of mantle partial melting at considerable depth (>150km) They come to the surface as a volcanic eruption, and via a volcanic pipe called diatremes They occur as diatremes, pipes, dykes and sills, less commonly as volcanic tuffs Diatremes form to about 1 km depth from the root system Kimberlite magmatic plumbing system The high volatile content of the magma in the root system at about 1 km depth causes an eruptive “blowout”  The melts+ gases find their way toward the surface forming the diatreme and the volcanic crater As the hot kimberlite magma >900°C interacts near-surface with groundwaters, water flashes to steam and results in explosive activity  crater facies at the surface Kimberlite magmatic plumbing system Diamonds are transported toward the surface via the melt The kimberlite (+ diamond) is only in the diatreme and not in the surrounding rocks The emplacement of the diatreme is discordant with the local geology  It emplaces as a vertical pipe/dyke and sills  Example of Cullinam diatreme discordant with the local geology Morphology of Kimberlite Bodies Kimberlites worldwide form small ( to HERE The physical and chemical characteristics of hydrothermal fluids are controlling factors in the formation of hydrothermal mineral deposits…  Key Principals for Metals in Fluids: Fluids need to be undersaturated in a certain element(s) to allow the dissolution or leaching of metals from rocks. The fluid needs to become oversaturated of a certain element(s) to induce the precipitation of metals. Undersaturated fluids can carry metals in solution. These fluids collect/scavenge metals in many different ways: Metals (incompatible and/or soluble elements) get concentrated into the late-stage fluids of a crystallising magmatic system Metals get leached from the rocks and minerals that Alteration commonly the fluids pass through = hydrothermal alteration – causes the “remobilisation” dissolution of minerals of metals  Key Principals for dropping of Metals from hydrothermal fluids: Oversaturation of a metal/element in the fluid and dropping of metals from the fluids can be achieved in many ways: Increase in the metal/element content in the fluid Change in Temperature Change in Pressure Change in eH or pH Change in the ligand content/species in the fluid Any change in the equilibrium conditions of the fluid Hydrothermal system components summary:  Sources and sinks/traps of the fluids  Sources and sinks/traps of the chemical components (metals and ligands) transported  Pathways  Processes and events that caused fluid migration  Processes that cause metals to drop from fluids  Geological time  Mechanism of preservation There are 3 main types of hydrothermal ore deposits: 1. Deposits that have a close spatial and temporal association to magmatic activity 2. Deposits which form during periods of regional magmatism and tectonism, but are not clustered close to or around magmatic centres 3. Deposits that lack widespread or temporally related magmatic activity 1. Deposits that have a close spatial and temporal association to magmatic activity Where can you find magmatic fluids on the Earth’s crust? Different types of magmatism on Earth’s crust: Where do you have mafic magmatism and where felsic magmatism? We are dealing with hydrothermal ore deposits……… ……..Where do you think there is the interaction of magmatic activity and fluids? Magmatism in the continental crust and major ore deposits: Which of these ore deposits is formed by magmatic processes? Which of these ore deposits is formed by hydrothermal processes? Magmatic ore deposits in the continental crust Hydrothermal ore deposits in the continental crust spatially and temporally associated with magmatic activity Magmatic-hydrothermal fluids:  Felsic silicate melts can contain up to a few % by weight of volatile components (water + CO2, F, HCl, CH4, H2S) in solution  Most of the volatile components in solution in the magma cannot be incorporated into crystallising minerals  They accumulate in the residual melt during fractional crystallization  At some stage in the rise and crystallization of the felsic magma, there is saturation in volatile and they exsolve from the felsic residual melt to form a separate, immiscible, fluid phase (= a hydrothermal magmatic brine)  This immiscible exsolved hydrothermal magmatic fluid then escapes the residual met, migrates into the country rock away from the magmatic intrusion (taking away the metals) Metal content in Magmatic-hydrothermal fluids The elements/metals that will concentrate in the escaping magmatic- hydrothermal fluids are both: 1. incompatible in crystallising phases and 2. highly soluble in brines. Factors that control the concentration of trace ore metals in an exsolved fluid from a felsic magma are:  Concentration of the metals in the magma;  Concentration of the ligands in the magma;  Degree of fractionation of the magma, and which mineral already started crystallising before reaching fluids saturation in the melt;  Timing of exsolution, which depends on depth of magma emplacement, concentration of water and other volatiles in the magma. The solubility of volatiles in a melt decreases with a decrease in pressure:  With the rising of the felsic melt toward the surface there is a decrease in pressure, as there is less rock load  When the solubity of volatiles decrease, they exsolve from the felsic residual melt to form a separate, immiscible, fluid phase (= a hydrothermal magmatic brine) The exsolution of volatile from rising felsic melts can happen in two processes, called BOILING: First Boiling: exsolution occurs by decompression of the magma by rising at higher levels in the crust ` Second Boiling: Exsolution can occur due ` to increase concentration of the volatile elements in the residual melt as magma crystallise.  GEOL3051A L13 Hydrothermal- Magmatic ore deposits – Porphyry copper Dr Linda Iaccheri School of Geosciences [email protected] There are 3 main types of hydrothermal ore deposits: 1. Deposits that have a close spatial and temporal association to magmatic activity 2. Deposits which form during periods of regional magmatism and tectonism, but are not clustered close to or around magmatic centres 3. Deposits that lack widespread or temporally related magmatic activity To form Hydrothermal ore deposits that have a close spatial and temporal association with magmatic activity, we need: - A magma (often close to volcanic activity); - Exsolution of magmatic fluids from the residual felsic melt with ligands (volatiles) and metals (=hydrothermal magmatic fluids); - A change in the hydrothermal-magmatic system that causes the metals to drop from the hydrothermal magmatic fluids and cause the crystallization of ore minerals  Why are Porphyry Copper Deposits important? Bingham Canyon Mine (Utah) – the world’s largest hole  Importance of Porphyry Copper Deposits World’s primary source of Cu & Mo (~70% of world’s copper from 100 mines and 95% world’s molybdenum)  Au, Ag, Sn, W as by-products Typical grades 0.4 – 1% Cu + Au + Mo extremely large in size (50-500 million tonnes) ore minerals are disseminated, and distributed at low grades throughout large volumes of rock main minerals are chalcopyrite (CuFeS2) and bornite (Cu5FeS4)  What is a porphyry deposit? Large tonnage and low hypogene grade (