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1.
Two drill cores of the UG2 chromitite from the eastern and western Bushveld Complex were studied by whole-rock analysis, ore microscopy, SEM/Mineral Liberation Analysis (MLA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. The top and base of the UG2 main seam have the highest bulk-rock Pd and Pt concentrations. Sulfides mostly occur as aggregates of pentlandite, chalcopyrite, and rare pyrrhotite and pyrite or as individual grains associated mostly with chromite grains. In situ LA-ICP-MS analyses reveal that pentlandite carries distinctly elevated platinum-group element (PGE) contents. In contrast, pyrrhotite and chalcopyrite contain very low PGE concentrations. Pentlandite shows average maximum values of 350–1,000 ppm Pd, 200 ppm Rh, 130–175 ppm Ru, 20 ppm Os, and 150 ppm Ir, and is the principal host of Pd and Rh in the studied ores of the UG2. Mass balance calculations were conducted for samples representing the UG2 main seam of the drill core DT46, eastern Bushveld. Pentlandite consistently hosts elevated contents of the whole-rock Pd (up to 55 %) and Rh (up to 46 %), and erratic contents of Os (up to 50 %), Ir (2 to 17 %), and Ru (1–39 %). Platinum-group mineral (PGM) investigations support these mass balance results; most of the PGM are Pt-dominant such as braggite/cooperite and Pt-Fe alloys or laurite (carrying elevated concentrations of Os and Ir). Palladium and Rh-bearing PGM are rare. Both PGE concentrations and their distribution in base-metal sulfides (BMS) in the UG2 largely resemble that of the Merensky Reef, as most of the Pd and Rh are incorporated in pentlandite, whereas pyrrhotite, chalcopyrite, and pyrite are almost devoid of PGE. 相似文献
2.
Noble Metal Enrichment Processes in the Merensky Reef, Bushveld Complex 总被引：21，自引：7，他引：14
We have analysed sulphides, silicates, and chromites of theMerensky Reef for platinum-group elements (PGEs), Re and Auusing laser ablation-inductively coupled plasma mass spectrometryand synthetic pyrrhotite standards annealed with known quantitiesof noble metals. Os, Ir and Ru reside in solid solution in pyrrhotiteand pentlandite, Rh and part of the Reef’s Pd in pentlandite,whereas Pt, Au, Re and some Pd form discrete phases. Olivineand chromite, often suspected to carry Os, Ir and Ru, are PGEfree. All phases analysed contain noble metals as discrete micro-inclusionswith diameters typically <100 nm. Inclusions in sulphidescommonly have the element combinations Os–Ir–Ptand Pt–Pd–Au. Inclusions in olivine and chromiteare dominated by Pt ± Au–Pd. Few inclusion spectracan be related to discrete noble metal phases, and few inclusionshave formed by sub-solidus exsolution. Rather, some PGE inclusions,notably those in olivine and chromite, are early-magmatic nuggetstrapped when their host phases crystallized. We suggest thatthe silicate melt layer that preceded the Merensky Reef wasPGE oversaturated at early cumulus times. Experiments combinedwith available sulphide–silicate partition coefficientssuggest that a silicate melt in equilibrium with a sulphidemelt containing the PGE spectrum of the Merensky ore would indeedbe oversaturated with respect to the least soluble noble metals.Sulphide melt apparently played little role in enriching thenoble metals in the Merensky Reef; rather, its role was to immobilizea pre-existing in situ stratiform PGE anomaly in the liquid-stratifiedmagma chamber. KEY WORDS: Bushveld Complex; Merensky Reef; laser-ablation ICP-MS; platinum-group mineralization 相似文献
3.
The petrology of base metal sulfides and associated accessory minerals in rocks away from economically significant ore zones such as the Merensky Reef of the Bushveld Complex has previously received only scant attention, yet this information is critical in the evaluation of models for the formation of Bushveld-type platinum-group element (PGE) deposits. Trace sulfide minerals, primarily pyrite, pyrrhotite, pentlandite, and chalcopyrite are generally less than 100 microns in size, and occur as disseminated interstitial individual grains, as polyphase assemblages, and less commonly as inclusions in pyroxene, plagioclase, and olivine. Pyrite after pyrrhotite is commonly associated with low temperature greenschist alteration haloes around sulfide grains. Pyrrhotite hosted by Cr- and Ti-poor magnetite (Fe3O4) occurs in several samples from the Marginal to Lower Critical Zones below the platiniferous Merensky Reef. These grains occur with calcite that is in textural equilibrium with the igneous silicate minerals, occur with Cl-rich apatite, and are interpreted as resulting from high temperature sulfur loss during degassing of interstitial liquid. A quantitative model demonstrates how many of the first-order features of the Bushveld ore metal distribution could have developed by vapor refining of the crystal pile by chloride–carbonate-rich fluids during which sulfur and sulfide are continuously recycled, with sulfur moving from the interior of the crystal pile to the top during vapor degassing. 相似文献
4.
The platinum-group elements (PGE) in base metal sulfides (BMS) of the Merensky reef are mostly close to the detection limit
of the proton microprobe. The only phase that accommodates appreciable PGE is pentlandite. Total average PGE plus Au grades
of the sulfide fraction of the Merensky reef are about 500 ppm. We estimate the modal proportions of the major BMS to be around
53 percent pyrrhotite, 25 percent pentlandite, and 22 percent chalcopyrite (ignoring minor phases). Using this estimate, we
calculate by how much the sulfides are oversaturated with respect to individual PGE. With respect to Pt, the sulfides are
many times oversaturated, i.e., nearly all Pt occurs as discrete PGE phases. With regard to Pd the sulfides are oversaturated
by about a factor of two. The Ru and Rh levels are at and below saturation levels. Available experiments suggest that the
entire PGE content of the sulfide fraction can easily be accommodated in solid solution in BMS at temperatures as low as 500°C.
The fact that the BMS are oversaturated with most PGE thus indicates that the sulfides have continued to exsolve PGE below
that temperature. Calculated sulfur fugacities indicate that f
S2 is controlled by silica activity, as expected in high-temperature ores, suggesting that metal/sulfur ratios of the ore may
not have changed much since complete solidification of the intercumulus silicate melt of the Merensky reef. All sulfides investigated
have cooled below the maximum temperature of pentlandite-pyrite coexistence, which experiments place at 250±30°C. Final closure
temperatures of the sulfide-PGE mineral assemblages, approximated by extrapolating the pentlandite-pyrrhotite solvus beyond
its experimentally determined range, are possibly as low as 80 to 90°C.
Received: 25 April 1995/Accepted: 5 September 1995 相似文献
5.
铂族元素在地壳中的富集:以布什维尔德杂岩为例(英文) 总被引：1，自引：0，他引：1
地幔是地壳铂族元素富集的主要源库。铂族元素迁移主要有两个途径:(1)地幔部分熔融物质侵入地壳;(2)地幔板片就位于俯冲/碰撞带。前一途径比后一途径重要得多。地幔物质进入地壳造成铂族元素富集并成为可供开采的主矿产而非副产品,这一过程可包含许多成矿作用机制:(i)基性侵入体中Ni-Cu硫化物矿浆的发育,岩浆冷却与分离结晶作用导致富含Cu,Pt,Pd的硫化物矿浆的形成;(ii)层状侵入体一定层位形成高品位的铂族元素硫化物层,伴生或不伴生铬铁岩;(iii)富铂族元素及硫化物的岩浆沿着层状侵入体的边缘就位;(iv)直至层状侵入体结晶分异作用晚期的硫化物不混溶滞后分离;(v)不发育硫化物不混溶作用的铬铁矿结晶作用;(vi)低程度硫化物浸染带中的热液作用与铂族元素富集;(vii)乌拉尔-阿拉斯加型侵入体重结晶过程中的铂族元素与铬铁矿的次生富集作用,岩体在风化过程中形成砂矿床;(viii)黑色页岩形成过程中Pt的富集。南非布什维尔德火成杂岩蕴藏世界Pt资源的75%,Pd资源的54%,Rh资源的82%,并具有(ii)、(iii)、(iv)、(v)、(vi)成矿作用的实例。在这些作用中,作用(ii)形成的现有经济储量和资源量占90%,作用(iii)占9%。Merensky矿层(占总资源量30%)是一个铂族元素富集层位,它含1~3铬铁矿薄层,在可采宽度内硫化物平均含量为1%~3%(质量分数)。硫化物一般被认为是铂族元素的主要聚集体。该矿层由两个或两个以上含硫化物的基性热岩浆上升汇聚而成。这些岩浆的汇聚造成超镁铁质堆晶岩的厚度(主要是斜方辉石岩,某些地区包括橄榄岩)变化于50cm至数米之间。开采通常集中在厚度不到1m的地带。矿层的成因至今仍存在争议,一些观点认为铂族元素来自下部上升的热液流体,另一些观点认为铂族元素来自上部岩浆的硫化物沉降作用,并形成了Merensky辉石岩。已经知道矿层上覆的辉石岩、苏长岩和斜长岩中矿物来自两种岩浆类型:一种富含MgO(12%,质量分数)和Cr,而贫Al2O3(12%);另一种含典型的粒玄岩成分。UG-2铬铁岩含有全部经济资源量的58%,由一0.6~1m厚的铬铁岩层(有时见辉石岩夹层)和上覆的1~3层由铬铁矿所构成的薄层。虽然硫化物被认为至少是某些情况下对铂族元素的富集起作用,但UG-2的硫化物含量(0.5%~1.5%)显著低于Merensky矿层。UG-2层之下共有13个铬铁岩层位,所有的都含铂族元素,虽然铂族元素总含量和(Pt+Pd)/(Ru+Ir+Os)比值远低于UG-2。UG-2内所含的辉石岩"夹层"具高的87Sr/86Sr比值,说明与顶部熔融岩石的混合促进了铬铁岩和硫化物的形成。作用(iii)的主要实例是Platreef。目前它占总资源量的9%。不过,沿该带正积极开展找矿勘探工作,这一比例将来还会提高。这一矿层的厚度比Merensky和UG-2都要大,目前开采厚度达50多米。Platreef呈带状,上部为斜方辉石岩的堆晶岩;下部为辉石岩、长石辉石岩和苏长岩,它们与页岩、铁矿层和白云岩强烈相互作用,直接形成了底盘岩石。笔者认为Platreef是不同期次岩浆作用的结果,这些作用形成了不同的单元产物,包括布什维尔德主岩浆房的UG-2和Merensky矿层。新的岩浆进入主岩浆房会造成先存岩浆移位、岩浆错动并会冲破岩浆房的壁。圆筒状、带状岩管中的超镁铁岩含极高的Pt品位,在布什维尔德杂岩的下部切穿堆晶层,被认为是热液再活化的产物。它们现在未被开采,只是构成存封的铂族元素资源,对整个杂岩体资源没有重要的贡献。 相似文献
6.
The Kaalamo massif is located in the Northern Ladoga region, Karelia, on the extension of the Kotalahti Belt of Ni-bearing ultramafic intrusions in Finland. The massif, 1.89 Ga in age, is differentiated from pyroxenite to diorite. Nickel–copper sulfide mineralization with platinoids is related to the pyroxenite phase. The ore consists of two mineral types: (i) pentlandite–chalcopyrite–pyrrhotite and (ii) chalcopyrite, both enriched in PGE. Pd and Pt bismuthotellurides, as well as Pd and Pt tellurobismuthides, are represented by the following mineral species: kotulskite, sobolevskite, merenskyite, michenerite, moncheite, keithconnite, telluropalladinite; Pt and Pd sulfides comprise vysotskite, cooperite, braggite, palladium pentlandite, and some other rare phases. High-palladium minerals are contained in pentlandite–chalcopyrite–pyrrhotite ore. Native gold intergrown with kotulskite commonly contains microinclusions (1–3 μm) of Pd stannides: paolovite and atokite. Ore with 20–60% copper sulfides (0.2–6.0% Cu) contains 5.1–6.6 gpt PGE and up to 0.13–2.3 gpt Au. Pd minerals, arsenides and sulfoarsenides of Pt, Rh, Ir, Os, and Ru are identified as well. These are sperrylite, ruthenium platarsite, hollingworthite, and irarsite; silvery gold and paolovite have also been noted. All these minerals have been revealed in the massif for the first time. The paper also presents data on the compositions of 25 PGE minerals (PGM) from Kaalamo ores. 相似文献
7.
Platinum-group Elements and Microstructures of Normal Merensky Reef from Impala Platinum Mines, Bushveld Complex 总被引：11，自引：4，他引：11
The Merensky Reef of the Bushveld Complex contains one of theworld’s largest concentrations of platinum-group elements(PGE). We have investigated ‘normal’ reef, its footwalland its hanging wall at Impala Platinum Mines. The Reef is 46cm thick and consists from bottom to top of leuconorite, anorthosite,chromitite and a very coarse-grained melanorite. The footwallis leuconorite and the hanging wall is melanorite. The onlyhydrous mineral present is biotite, which amounts to 1%, orless, of the rock. All of the rocks contain 0·1–5%interstitial sulphides (pyrrhotite, pentlandite and chalcopyrite),with the Reef rocks containing the most sulphides (1–5%).Lithophile inter-element ratios suggest that the magma fromwhich the rocks formed was a mixture of the two parental magmasof the Bushveld Complex (a high-Mg basaltic andesite and a tholeiiticbasalt). The Reef rocks have low incompatible element contentsindicating that they contain 10% or less melt fraction. Nickel,Cu, Se, Ag, Au and the PGE show good correlations with S inthe silicate rocks, suggesting control of the abundance of thesemetals by sulphides. The concentration of the chalcophile elementsand PGE in the silicate rocks may be modelled by assuming thatthe rocks contain sulphide liquid formed in equilibrium withthe evolving silicate magma. It is, however, difficult to modelthe Os, Ir, Ru, Rh and Pt concentrations in the chromititesby sulphide liquid collection alone, as the rocks contain 3–4times more Os, Ir, Ru, Rh and Pt than the sulphide-collectionmodel would predict. Two possible solutions to this are: (1)platinum-group minerals (PGM) crystallize from the sulphideliquid in the chromitites; (2) PGM crystallize directly fromthe silicate magma. To model the concentrations of Os, Ir, Ru,Rh and Pt in the chromitites it is necessary to postulate thatin addition to the 1% sulphides in the chromitites there isa small quantity (0·005%) of cumulus PGM (laurite, cooperiteand malanite) present. Sulphide liquids do crystallize PGM atlow fS2. Possibly the sulphide liquid that was trapped betweenthe chromite grains lost some Fe and S by reaction with thechromite and this provoked the crystallization of PGM from thesulphide liquid. Alternatively, the PGM could have crystallizeddirectly from the silicate magma when it became saturated inchromite. A weakness of this model is that at present the exactmechanism of how and why the magma becomes saturated in PGMand chromite synchronously is not understood. A third modelfor the concentration of PGE in the Reef is that the PGE arecollected from the underlying cumulus pile by Cl-rich hydrousfluids and concentrated in the Reef at a reaction front. Althoughthere is ample evidence of compaction and intercumulus meltmigration in the Impala rocks, we do not think that the PGEwere introduced into the Reef from below, because the rocksunderlying the Reef are not depleted in PGE, whereas those overlyingthe Reef are depleted. This distribution pattern is inconsistentwith a model that requires introduction of PGE by intercumulusfluid percolation from below. KEY WORDS: Merensky Reef; platinum-group elements; chalcophile elements; microstructures 相似文献
8.
Sarah A. S. Dare Sarah-Jane Barnes Hazel M. Prichard Peter C. Fisher 《Mineralium Deposita》2011,46(4):381-407
Magmatic sulfide deposits consist of pyrrhotite, pentlandite, chalcopyrite (± pyrite), and platinum-group minerals (PGM).
Understanding the distribution of the chalcophile and platinum-group element (PGE) concentrations among the base metal sulfide
phases and PGM is important both for the petrogenetic models of the ores and for the efficient extraction of the PGE. Typically,
pyrrhotite and pentlandite host much of the PGE, except Pt which forms Pt minerals. Chalcopyrite does not host PGE and the
role of pyrite has not been closely investigated. The Ni–Cu–PGE ores from the South Range of Sudbury are unusual in that sulfarsenide
PGM, rather than pyrrhotite and pentlandite, are the main carrier of PGE, probably as the result of arsenic contribution to
the sulfide liquid by the As-bearing metasedimentary footwall rocks. In comparison, the North Range deposits of Sudbury, such
as the McCreedy East deposit, have As-poor granites in the footwall, and the ores commonly contain pyrite. Our results show
that in the pyrrhotite-rich ores of the McCreedy East deposit Os, Ir, Ru, Rh (IPGE), and Re are concentrated in pyrrhotite,
pentlandite, and surprisingly in pyrite. This indicates that sulfarsenides, which are not present in the ores, were not important
in concentrating PGE in the North Range of Sudbury. Palladium is present in pentlandite and, together with Pt, form PGM such
as (PtPd)(TeBi)2. Platinum is also found in pyrite. Two generations of pyrite are present. One pyrite is primary and locally exsolved from
monosulfide solid solution (MSS) in small amounts (<2 wt.%) together with pyrrhotite and pentlandite. This pyrite is unexpectedly
enriched in IPGE, As (± Pt) and the concentrations of these elements are oscillatory zoned. The other pyrite is secondary
and formed by alteration of the MSS cumulates by late magmatic/hydrothermal fluids. This pyrite is unzoned and has inherited
the low concentrations of IPGE and Re from the pyrrhotite and pentlandite that it has replaced. 相似文献
9.
The paper presents concentrations of the platinum-group and chalcophile elements in the base metal sulfides (BMS) from the Jinchuan Ni–Cu sulfide deposit determined by laser ablation-inductively coupled plasma-mass spectrometry. Mass balance calculations reveal that pentlandite hosts a large proportion of Co, Ni and Pd (> 65%), and that pentlandite and pyrrhotite accommodate significant proportions of Re, Os, Ru, Rh, and Ag (~ 35–90%), whereas chalcopyrite contains a small amount of Ag (~ 10%) but negligible platinum-group elements. Iridium and Pt are not concentrated in the BMS and mostly occur in As-rich platinum-group minerals. The enrichments of Co, Ni, Re, Os, Ru, and Rh in pentlandite and pyrrhotite, and Cu in chalcopyrite are consistent with the fractionation of sulfide liquid and exsolution of pentlandite and pyrrhotite from the mono-sulfide solid solution (MSS). The Ir-bearing minerals exsolved from the MSS, depleting pentlandite and pyrrhotite in Ir, whereas sperrylite exsolved from the residual sulfide liquid on cooling. Diffusion of Pd from residual sulfide liquid into pentlandite during its exsolution from the MSS and crystallization of Pt-bearing minerals in the residual sulfide liquid resulted in the enrichment of Pd in pentlandite and decoupling between Pd and Pt in the Jinchuan net-textured and massive ores. 相似文献
10.
Kaveh Pazand Firuz Aliniya Yousef Ghanbari Hossein Hassani Nasrin Aghavali 《Arabian Journal of Geosciences》2011,5(5):1021-1029
Platinum group elements (PGE) enrichment occurs in Zn–Cu and Ni-rich ophiolities in a number of geological settings. Platinum group elements (PGE) mineralization in Pyroxenite from the Faryab ophiolities of Zagros belt in south Iran was studied. The ophiolite rocks represent blocks of Tethyan oceanic crust that were emplaced on the continental margin during the late Cretaceous period. Much of lower ophiolitic section is composed of homogeneous harzburgite, while upper sections harzburgite interlayer with dunite and pyroxenite are included. This study focused on pyroxenite that includes most of sulfide mineralization in Faryab. More than 500 samples were investigated from polished thin sections; that cover all area of Faryab. The sulfide phases include pyrrhotite, pentlandite, millerite, violarite, smythite, and heazlewoodite. The results show that in almost all the samples Os is below the 2 ppb detection limit, Platinum values vary from <5 to 91 ppb and the light PGE (Ru, Rh, and Pd) relative to the heavy PGE (Os, Ir, and Pt) are more concentrated. Calculation showed that in pyroxenites Pd–Pt is occurring with orthopyroxenite and Rh–Os is occurring in clinopyroxenite. Ni/Pd ratios in Faryab vary between 7 and 356 and Pd/Ir ratio is 0.1–27. This indicates that in Faryab area partial melt of mantle occurred. Pd/Rh ratio in Faryab is 0.1–11, and Pd/Pt varies between 0.2 and 1.5. Pd/Ir ratio in Faryab decreases and shows that PGE in Faryab occurred. 相似文献
11.
The Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a mineralized, layered package of mafic cumulates developed to the south of the town of Mokopane, at a similar stratigraphic position to the Platreef. The concentration of platinum-group elements (PGE) in base metal sulfides (BMS) has been determined by laser ablation inductively coupled plasma–mass spectrometry. These data, coupled with whole-rock PGE concentrations and a detailed account of the platinum-group mineralogy (PGM), provide an insight into the distribution of PGE and chalcophile elements within the GNPA member, during both primary magmatic and secondary hydrothermal alteration processes. Within the most unaltered sulfides (containing pyrrhotite, pentlandite, and chalcopyrite only), the majority of IPGE, Rh, and some Pd occur in solid solution within pyrrhotite and pentlandite, with an associated Pt–As and Pd–Bi–Te dominated PGM assemblage. These observations in conjunction with the presence of good correlations between all bulk PGE and base metals throughout the GNPA member indicate the presence and subsequent fractionation of a single PGE-rich sulfide liquid, which has not been significantly altered. In places, the primary sulfides have been replaced to varying degrees by a low-temperature assemblage of pyrite, millerite, and chalcopyrite. These sulfides are associated with a PGM assemblage characterized by the presence of Pd antimonides and Pd arsenides, which are indicative of hydrothermal assemblages. The presence of appreciable quantities of IPGE, Pd and Rh within pyrite, and, to a lesser, extent millerite suggests these phases directly inherited PGE contents from the pyrrhotite and pentlandite that they replaced. The replacement of both the sulfides and PGM occurred in situ, thus preserving the originally strong spatial association between PGM and BMS, but altering the mineralogy. Precious metal geochemistry indicates that fluid redistribution of PGE is minimal with only Pd, Au, and Cu being partially remobilized and decoupled from BMS. This is also indicated by the lower concentrations of Pd evident in both pyrite and millerite compared with the pentlandite being replaced. The observations that the GNPA member was mineralized prior to intrusion of the Main Zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal S. We therefore believe that our observations are more compatible with a multistage emplacement model, where preformed PGE-rich sulfides were emplaced into the GNPA member. Such a model explains the development and distribution of a single sulfide liquid throughout the entire 400–800 m thick succession. It is therefore envisaged that the GNPA member formed in a similar manner to its nearest analogue the Platreef. Notable differences however in PGE tenors indicate that the ore-forming process may have differed slightly within the staging chambers that supplied the Platreef and GNPA member. 相似文献
12.
A stream sediment geochemical re-investigation of the discovery of the platiniferous Merensky Reef, Bushveld Complex 总被引：1，自引：0，他引：1
The platiniferous Merensky Reef was discovered on the farm Maandagshoek in the Bushveld Complex in August 1924. A historical review of these events is presented, based partly on an unpublished report and a map by Dr Hans Merensky. It has been supposed that Merensky first traced the platinum to discordant ultramafic pipes. However, a re-examination of the topography and river systems in the area shows that this was not possible. A stream sediment study from this area has been undertaken to determine the source of the platinum originally panned at the discovery site. Determination of the Pt:Pd:Au ratio in these samples indicates a strong similarity with ratios in the Merensky Reef, and is completely different from the ratio found in the pipes, which are deficient in Pd and Au. These ratios also suggest that weathering and transportation has occurred predominantly due to physical rather than chemical processes. Two platinum-group element anomalies in the stream sediments are found upstream from the Merensky Reef outcrops. This results from a change in the fluvial system due to Pliocene to recent up warping with a reversal in flow direction. 相似文献
13.
The concentrations of platinum-group elements (PGE), Co, Re,Au and Ag have been determined in the base-metal sulphide (BMS)of a section of the Merensky Reef. In addition we performeddetailed image analysis of the platinum-group minerals (PGM).The aims of the study were to establish: (1) whether the BMSare the principal host of these elements; (2) whether individualelements preferentially partition into a specific BMS; (3) whetherthe concentration of the elements varies with stratigraphy orlithology; (4) what is the proportion of PGE hosted by PGM;(5) whether the PGM and the PGE found in BMS could account forthe complete PGE budget of the whole-rocks. In all lithologies,most of the PGE (65 up to 85%) are hosted by PGM (essentiallyPt–Fe alloy, Pt–Pd sulphide, Pt–Pd bismuthotelluride).Lesser amounts of PGE occur in solid solution within the BMS.In most cases, the PGM occur at the contact between the BMSand silicates or oxides, or are included within the BMS. Pentlanditeis the principal BMS host of all of the PGE, except Pt, andcontains up to 600 ppm combined PGE. It is preferentially enrichedin Pd, Rh and Co. Pyrrhotite contains, Rh, Os, Ir and Ru, butexcludes both Pt and Pd. Chalcopyrite contains very little ofthe PGE, but does concentrate Ag and Cd. Platinum and Au donot partition into any of the BMS. Instead, they occur in theform of PGM and electrum. In the chromitite layers the whole-rockconcentrations of all the PGE except Pd are enriched by a factorof five relative to S, Ni, Cu and Au. This enrichment couldbe attributed to BMS in these layers being richer in PGE thanthe BMS in the silicate layers. However, the PGE content inthe BMS varies only slightly as a function of the stratigraphy.The BMS in the chromitites contain twice as much PGE as theBMS in the silicate rocks, but this is not sufficient to explainthe strong enrichment of PGE in the chromitites. In the lightof our results, we propose that the collection of the PGE occurredin two steps in the chromitites: some PGM formed before sulphidesaturation during chromitite layer formation. The remainingPGE were collected by an immiscible sulphide liquid that percolateddownward until it encountered the chromitite layers. In thesilicate rocks, PGE were collected by only the sulphide liquid. KEY WORDS: Merensky Reef; Rustenburg Platinum Mine; sulphide; platinum-group elements; image analysis; laser ablation ICP-MS 相似文献
14.
Trace Element and Platinum Group Element Distributions and the Genesis of the Merensky Reef, Western Bushveld Complex, South Africa 总被引：5，自引：0，他引：5
The Merensky Reef of the Bushveld Complex is one of the world'slargest resources of platinum group elements (PGE); however,mechanisms for its formation remain poorly understood, and manycontradictory theories have been proposed. We present precisecompositional data [major elements, trace elements, and platinumgroup elements (PGE)] for 370 samples from four borehole coresections of the Merensky Reef in one area of the western BushveldComplex. Trace element patterns (incompatible elements and rareearth elements) exhibit systematic variations, including small-scalecyclic changes indicative of the presence of cumulus crystalsand intercumulus liquid derived from different magmas. Ratiosof highly incompatible elements for the different sections areintermediate to those of the proposed parental magmas (CriticalZone and Main Zone types) that gave rise to the Bushveld Complex.Mingling, but not complete mixing of different magmas is suggestedto have occurred during the formation of the Merensky Reef.The trace element patterns are indicative of transient associationsbetween distinct magma layers. The porosity of the cumulatesis shown to affect significantly the distribution of sulphidesand PGE. A genetic link is made between the thickness of theMerensky pyroxenite, the total PGE and sulphide content, petrologicaland textural features, and the trace element signatures in thesections studied. The rare earth elements reveal the importantrole of plagioclase in the formation of the Merensky pyroxenite,and the distribution of sulphide. KEY WORDS: Merensky Reef; platinum group elements; trace elements 相似文献
15.
Trace and platinum-group element geochemistry and the development of the Merensky unit of the Western Bushveld complex 总被引：2，自引：0，他引：2
Whole-rock samples of the Merensky Unit were analysed for major elements, Cu, Ni, Cr, Co, Eu, Th, U, Cs, and the platinum-group elements Pt, Pd, Rh, Ir and Au. There is a sympathetic variation of Cu, Ni, Co, and S throughout the sequence. Th and U abundances are highest around the Merensky Reef, and their distribution in the rest of the unit reflects the amount of mesostasis. Eu distribution is governed by primary plagioclase. Pt, Pd, Rh, Ir and Au (PM) are concentrated in the Merensky Reef. The relative proportions throughout the unit are Ir2 enrichment trends; interaction of these trends resulted in the Merensky Reef and its attendant mineralization. 相似文献
16.
Diamond drill core traverses across the Platreef were carried out at Tweefontein, Sandsloot, and Overysel in order to establish
the relationship between crustal contamination and platinum group element (PGE) mineralization. The footwall rocks are significantly
different at each of these sites and consist of banded iron formation and sulfidic shales at Tweefontein, of carbonates at
Sandsloot, and of granites and granite gneisses at Overysel. As demonstrated in this study, Platreef rocks are characterized
by two stages of crustal contamination. The first contamination event occurred prior to emplacement of the magma and is present in Platreef rocks at all three sites, as well as in the Merensky
Reef. This event is readily identified on trace element spidergrams and trace element ratio scattergrams. The second contamination event was induced by interaction of the Platreef magma with the local footwall rocks. It is most easily identified at Tweefontein,
where there is a large increase in the FeO content of the Platreef rocks, and at Sandsloot, where there is a large increase
in their CaO and MgO contents, relative to Bushveld rocks that are uncontaminated by the local footwall rocks. At Overysel,
the second contamination event did not result in pronounced changes in the major element composition of the Platreef rocks,
but can be detected in their trace element chemistry. A strong inverse relationship between PGE tenors and S/Se ratios is
interpreted to suggest that the PGE-rich sulfides were formed prior to emplacement of the Platreef magmas through assimilation
of crustal S and became progressively enriched in the PGE during transport. Rather than promoting S-saturation, interaction
of the Platreef magma with the footwall rocks diluted the metal tenors of the sulfides. Although both the Platreef and the
Merensky Reef magmas were contaminated by the same crustal contaminant and were probably PGE-rich, they have radically different
Pd/Pt ratios. Their Pd/Pt ratios suggest that whereas the Merensky Reef magma became PGE-rich due to dissolution of PGE-rich
sulfides segregated from a pre-Merensky magma that had undergone relatively little fractionation prior to reaching S-saturation,
the pre-Platreef magma had undergone greater fractionation prior to the sulfide saturation event, thereby increasing its Pd/Pt
ratio. We suggest that the magmas that formed the Platreef and Merensky Reef may have simply been carrier magmas for sulfides
that had formed elsewhere in the plumbing system of the Bushveld Complex by the interaction of earlier generations of magmas
with the crustal rocks that underlie the Complex. 相似文献
17.
M. L. Fiorentini S. W. Beresford B. Grguric S. J. Barnes W. E. Stone 《Australian Journal of Earth Sciences》2013,60(6):801-824
A new style of komatiite-associated sulfide-poor platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) mineralisation has been identified at Wiluna in the strongly nickel sulfide (NiS) mineralised Agnew – Wiluna Greenstone Belt, Western Australia. The komatiite sequence at Wiluna is ～200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate-textured peridotite core, which is overlain by rhythmically layered wehrlite, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic (harrisitic) domains are common features, whereas spinifex-textured horizons and flow-top breccias are absent. The presence of anomalous PGE-enriched horizons (ΣPt – Pd = 200 – 500 ppb) in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained (1 – 10 μm) platinum-group minerals (PGMs). More than 70 PGM grains were identified, and a considerable mineralogical variability was constrained. However, only Pd – Pt-bearing phases were identified, whereas no Ir – Ru-bearing PGMs were found in any of the sections examined. Interestingly, all PGMs are not in paragenetic association with sulfides, and only sulfide-poor/free intervals contain significant PGM concentrations. The whole-rock PGE sequence largely reflects the PGM distribution. It is hypothesised that the Pd – Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir – Ru depletion in the upper mafic section of the sequence are the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulfide-undersaturated environment. The crystallisation of clinopyroxene in the oikocrystic pyroxenite horizon may have triggered the formation of Pt – Pd-bearing alloys and arsenides, which were the first PGMs to form. Stratiform sulfide-poor PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich sulfide-poor mineralisation zones in thick differentiated intrusions, rather than to other PGE-enriched zones in komatiite-hosted systems, where PGE enrichment is directly associated with accumulations of magmatic sulfides. 相似文献
18.
Concentrations of platinum-group elements in samples from the Boulder Bed at five localities in the western Bushveld Complex range between 50 ppb and 70 ppm. Boulders thus have much more variable, and sometimes highly enriched, PGE contents relative to the other lithologies in the immediate foot-wall sequence of the Merensky Reef. The PGE enrichment can largely be modelled as a result of primary magmatic processes including collection of PGE by segregating sulphide melt and fractionation of mss. Other features of the Boulder Bed, such as the selvages of pure anorthosite and the chromitite stringers surrounding some of the boulders, bear evidence of recrystallisation. A model is proposed by which the Boulder Bed formed as a result of a combination of early and late magmatic processes. The PGEs were collected by magmatic sulphide melt which accumulated in a pyroxenite layer. The host rock to the pyroxenite was a thick package of norites which recrystallised in response to upward-migrating magmatic fluids. The fluids caused partial hydration melting of the norites adjacent to the pyroxenite, producing anorthosite. The boulders represent the broken-up remnants of the pyroxenite layer. The selvages of chromite and pure anorthosite around some of the boulders remain poorly understood, but may represent the latest recrystallisation event, in response to localised late-magmatic fluid overpressure upon cooling. 相似文献
19.
《Geochimica et cosmochimica acta》2001,65(16):2789-2806
Fourteen peridotite xenoliths collected in the Massif Central neogene volcanic province (France) have been analyzed for platinum-group elements (PGE), Au, Cu, S, and Se. Their total PGE contents range between 3 and 30 ppb and their PGE relative abundances from 0.01 to 0.001 × CI-chondrites, respectively. Positive correlations between total PGE contents and Se suggest that all of the PGE are hosted mainly in base metal sulfides (monosulfide solid solution [Mss], pentlandite, and Cu-rich sulfides [chalcopyrite/isocubanite]). Laser ablation microprobe-inductively coupled plasma mass spectrometry analyses support this conclusion while suggesting that, as observed in experiments on the Cu-Fe-Ni-S system, the Mss preferentially accommodate refractory PGEs (Os, Ir, Ru, and Rh) and Cu-rich sulfides concentrate Pd and Au. Poikiloblastic peridotites pervasively percolated by large silicate melt fractions at high temperature (1200°C) display the lowest Se (<2.3 ppb) and the lowest PGE contents (0.001 × CI-chondrites). In these rocks, the total PGE budget inherited from the primitive mantle was reduced by 80%, probably because intergranular sulfides were completely removed by the silicate melt. In contrast, protogranular peridotites metasomatized by small fractions of volatile-rich melts are enriched in Pt, Pd, and Au and display suprachondritic Pd/Ir ratios (1.9). The palladium-group PGE (PPGE) enrichment is consistent with precipitation of Cu-Ni-rich sulfides from the metasomatic melts. In spite of strong light rare earth element (LREE) enrichments (Ce/YbN < 10), the three harzburgites analyzed still display chondrite-normalized PGE patterns typical of partial melting residues, i.e., depleted in Pd and Pt relative to Ir and Ru. Likewise, coarse-granular lherzolites, a common rock type in Massif Central xenoliths, display Pd/Ir, Ru/Ir, Rh/Ir, and Pt/Ir within the 15% uncertainty range of chondritic meteorites. These rocks do not contradict the late-veneer hypothesis that ascribes the PGE budget of the Earth to a late-accreting chondritic component; however, speculations about this component from the Pd/Ir and Pt/Ir ratios of basalt-borne xenoliths may be premature. 相似文献
20.
总结南非布什维尔德杂岩体中Merensky Reef(简称MR矿层)和Platreef(简称PR接触带)两类铂族元素矿床的矿床地质、矿化特征以及铂族元素的赋存状态。MR矿床是典型的层状铂族元素(PGE)矿床,在杂岩体东部和西部发育,PGE总含量稳定,赋存在堆晶间隙硫化物中,常以PGE硫化物的形式产出。PR接触带型矿化集中在杂岩体北段,整体上不连续,各个矿床的具体特征由于底盘岩性的多变而不同,PGE主要赋存在碲化物和砷化物等半金属化合物中,可以脱离硫化物产在硅酸盐矿物中。相关的实验研究显示,PGE在岩浆结晶过程中发生分异,Pd/Ir比值体现了硫化物的分异程度;Pd比Pt更容易被氧化以及在热液中迁移,Pt/Pd比值体现了混染和热液的作用,这些因素造成了PR接触带与MR矿层中PGE赋存状态的差异。岩浆可能在侵入之前已经达到了硫饱和,岩浆房的压力变化和岩浆通道对于PGE的富集有重要意义,热液流体可以对已经形成的PGE矿化进行改造。 相似文献