China
Africa
Explore chapters and articles related to this topic
Elvan and ongonite magmas with associated rare metal mineralization
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
V. Antipin, C. Halls, R. Seltmann
The granite cupolas of the Cornubian batholith, located in the SW peninsula of the UK, have a distinctive belt of porphyry dykes trending WSW-ENE along the main intrusive axis. The dykes cross the five major plutons of the batholith and the Devonian and Carboniferous country rocks. These porphyry dykes of granitic composition have been given the name elvan, a term deriving from the Cornish language. Typical elvans are quartz-porphyries with total alkalis similar to those of associated granites, but with values of K2O ≫ Na2O (Hall 1973). The elvans and granites were formed during closely related intervals of time between 269-282 Ma and 268-290 Ma (Darbyshire & Shepherd 1994). Granites and elvans both have peraluminous compositions with SiO2 = 70-75%, K2O+Na2O = 7.0-8.5% and low contents of CaO, MgO and K/Na>1. Elvans also have relatively high contents of Rb, Cs, Ba and Sn (Table 1), which are similar to those of the granites in the Cornubian batholith. New analyses of the total contents and distribution patterns of REE in elvans are very similar to those of the associated Cornubian granites as given by Darbyshire & Shepherd (1987). Elvans from Praa Sands and Wherry Rocks have high REE contents of 186 and 169 ppm respectively, and also have La/Yb ratios of 21.8 and 17.3. These values are close to those characteristic for the granites of the Lands End pluton which has total REE=208 ppm and La/Yb=26.6.
Use of DRI/HBI and SR/MBF Hot Metal in Iron- and Steelmaking
Published in Amit Chatterjee, Beyond the Blast Furnace, 2017
Trials have been conducted at a number of sites to establish the fact that sponge iron can replace pig iron/steel scrap as a feed stock for cupolas. The amount of sponge iron in the charge depends on the type of cupola operation. With acid cupola operation, a charge with as much as 70% sponge iron can be used, since the gangue in the sponge iron is mainly acidic; with basic cupola operation, the sponge addition must be correspondingly lower, 15 to 30%, to avoid any additional lining wear problems. In both cases, cupola operation with sponge iron leads to an increase in the coke rate, an increase in the slag volume, and a decrease in the carbon content of cast iron, the exact amount of each depending on the percentage of sponge iron in the charge and the gangue content in sponge iron. At the Rheinstahl Foundry, sponge iron briquettes, making up 40% of the charge, were used, and regular grade cast iron was produced without any change in its metallurgical structure or strength. Based on some trials conducted at Tata Steel, the following was reported:19Up to 25% sponge iron can be used easily in a cold blast cupola (to replace pig iron) without causing any significant operational difficulties. With higher percentages of sponge iron in the charge, however, the permeability of the charge tends to decrease; as a result, the gas flow experiences greater resistance. This calls for increased air pressure in the air supply system and decreased air flow. With 25% sponge iron, the air pressure can increase by about 30%. In such cases, care must be taken to consider this aspect before using significant percentages of sponge iron in cupolas.Sponge iron should not be included in the first six to eight charges. If added earlier, it causes difficulty in tapping out the liquid iron. Optimal charging ensures that when the sponge iron reaches the melting area, its temperature is sufficiently high.The slag volume of the cupola increases with an increasing amount of sponge iron in the charge, so care should be taken in slagging. With 25% sponge iron, the slag volume can increase by about 15 to 20%.The liquid iron temperature is not affected by sponge iron addition.The smelting capacity of the furnace is expected to go down with increasing amounts of sponge iron in the charge. The exact effect depends on its gangue content and the degree of metalization. It is, therefore, necessary to choose sponge iron with a low gangue content and as high a degree of metalization as possible for use in cupolas.
A unifying model for tin mineralisation in granites–pegmatites–greisens and veins: an African perspective
Published in Applied Earth Science, 2019
Tin-bearing magmas generally have monzo- to syenogranite compositions. Primary cassiterite-hosting deposits require a combination of factors: (i) the generation of magma, (ii) ascent of the magma through structurally-controlled passageways, (iii) crystallisation of the magma with or without fluid separation, (iv) generation of a hydrothermal fluid phase, and (v) escape of a hydrothermal fluid. The style of tin deposits can be grouped into different categories based on whether, and when, a fluid phase separated from the magma: Zoned and unzoned pegmatites generated as small melt fractions without a parent granite at depth. Magmatic cassiterite may have crystallised at the pegmatitic stage or/and from later hydrothermal alteration of the pegmatite, including the formation of a greisen (Fuchsloch 2018).Disseminated mineralisation in granite cupolas where fluid was retained.Irregular greisenised or tourmalinised zones of a granite cupola where hydrothermal fluids separated from the magma before the ductile–brittle transition.Pipes, stockworks, sheeted veins and greisens within a granite cupola after the ductile–brittle transition.Exogranitic greisens and sheeted veins within the country rock with associated alteration of the country rock and localised occurrence of skarns.Later endo- or exogranitic quartz-rich cassiterite-wolframite bearing veins representing the last stage in fluid evolution.Metamorphic re-distribution of cassiterite into zones of altered host rock.Alluvial and eluvial deposits derived from any of the above cassiterite-bearing deposits.