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The sulphidic tailings dumps at Matchless (Namibia) and Selebi-Phikwe (Botswana) are located in a similar semiarid environment
but have a contrasting mineralogical composition. The Matchless tailings are pyrite-rich, whereas the Selebi-Phikwe tailings
are dominated by pyrrhotite. Hydrochemical models are established with computer codes for water-balance, sulphide oxidation
rate and hydrochemical equilibrium calculations. The data input is based on detailed mineralogical, chemical and kinetic investigations
carried out on the core of boreholes drilled in 2000 and 2003. The oxidation of pyrrhotite proceeds at a much faster rate
than the oxidation of pyrite. The PYROX code, which is used for kinetic calculations, can take these differences into account
by applying different oxide-coating diffusion coefficients (D2) for pyrrhotite and pyrite. Humidity-cell testing is widely used to predict the post-mining composition of drainage water
in humid climates. However, the semiarid conditions at Matchless and Selebi-Phikwe only allow a minimal water flux within
the dump. Under such conditions, humidity-cell testing is likely to overestimate the seepage-water pH. This is suggested by
the hydrochemical equilibrium calculations for the post-mining period at Selebi-Phikwe, which predict a seepage-water pH about
one unit lower than the pH at the end of the 26-weeks humidity-cell testing period. The acidity of the seepage water can be
reduced by about half a pH unit, if an oxygen barrier below the evaporation zone is installed. A clay layer 0.5 m thick covered
by >1.5 m tailings represents the optimal design for a wet barrier. All three computer codes used for water-balance calculations
(HELP3, UNSAT-H and HYDRUS-1D), predict >85% average water saturation for such a layer, which diminishes the diffusion of
oxygen into the pile and production of SO4−2 and H+. The alternative design for a dry barrier consists of a vegetated silt layer 1 m thick on top of the tailings. This barrier
does not significantly influence the diffusion of oxygen although it reduces the net infiltration to ≤11 mm/year. 相似文献
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塞莱比-皮奎镍铜矿床是博茨瓦纳最大的镍铜矿床,位于博茨瓦纳东部。在收集整理、系统总结区域成矿地质背景、镍铜矿床的地质特征的基础上,探讨了矿床的成因。认为在岩浆演化的早期,当岩浆中的硫达到饱和,便结晶形成富镍的块状硫化物,而在岩浆演化的后期,结晶出富铜的浸染状硫化物。在重力作用的影响下,这两种硫化物堆积于岩浆房,后来,由于构造作用,含有这两种硫化物的岩浆被挤压出岩浆房,侵入塞莱比-皮奎序列地层,形成了塞莱比-皮奎矿床。最后,文章认为该矿床属于与基性超基性岩有关的构造变质改造的岩浆Ni-Cu硫化物矿床。 相似文献
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W. D. Maier S.-J. Barnes G. Chinyepi J. M. BartonJr B. Eglington I. Setshedi 《Mineralium Deposita》2008,43(1):37-60
We studied a number of magmatic Ni–Cu–(PGE) sulfide deposits in two distinct belts in eastern Botswana. The Tati belt contains
several relatively small deposits (up to 4.5 Mt of ore at 2.05% Ni and 0.85% Cu) at Phoenix, Selkirk and Tekwane. The deposits
are hosted by ca 2.7 Ga, low- to medium-grade metamorphosed gabbroic–troctolitic intrusions situated within or at the periphery
of a greenstone belt. The deposits of the Selebi-Phikwe belt are larger in size (up to 31 Mt of ore grade). They are hosted
by high-grade metamorphosed gabbronorites, pyroxenites and peridotites believed to be older than ca 2.0 Ga that intruded gneisses
of the Central Zone of the Limpopo metamorphic belt. The composition of the sulfide mineralisation in the two belts shows
systematic variation. Most of the mineralisation in the Tati belt contains 2–9% Ni and 0.05–4% Cu (Cu/Cu + Ni = 0.4–0.7),
whereas most of the mineralisation in the Selebi-Phikwe belt contains 1–3% Ni and 0.1–4% Cu (Cu/Cu + Ni = 0.4–0.9). The Cu–Ni
tenors of the ores in both belts are consistent with crystallization from a basaltic magma. The Tati ores contain mostly >3 ppm
Pt + Pd (Pt/Pd 0.1–1), with Pd/Ir = 100–1,000, indicative of a differentiated basaltic magma that remained S-undersaturated
before emplacement. Most of the Selebi-Phikwe ores have <0.5 ppm Pt + Pd (Pt/Pd < 0.1–1), with Pd/Ir = 10–500. This suggests
a relatively less differentiated magma that reached S saturation before emplacement. The Tati rocks show flat mantle-normalised
incompatible trace element patterns (average Th/YbN = 1.57), except for strong enrichments in large ion lithophile elements (Cs, Rb, Ba, U, K). Such patterns are characteristic
of relatively uncontaminated oceanic arc magmas and suggest that the Tati intrusions were emplaced in a destructive plate
margin setting. Most of the Selebi-Phikwe rocks (notably Dikoloti) have more fractionated trace element signatures (average
Th/YbN = 4.22), possibly indicating digestion of upper crustal material during magma emplacement. However, as there are also samples
that have oceanic arc-like signatures, an alternative possibility is that the composition of most Selebi-Phikwe rocks reflects
tectonic mingling of the intrusive rocks with the country rocks. The implication is that orogenic belts may have a higher
prospectivity for magmatic Ni–Cu ores than presently recognised. The trigger mechanism for sulfide saturation and segregation
in all intrusions remains unclear. Whereas the host rocks to the intrusions appear to be relatively sulfur poor, addition
of crustal S to the magmas is suggested by low Se/S ratios in some of the ores (notably at Selebi-Phikwe). External S sources
may thus remain unidentified due to poor exposure and/or S mobility in response to metamorphism. 相似文献
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