Low Platinum-group Element Contents Research Articles

Platinum-group element mineralization in the critical zone of the western Bushveld Complex; I, Sulfide poor-chromitites below the UG-2

Primary stratiform platinum-group element (PGE) mineralizations associated with chromitites in the Critical zone of the Bushveld Complex, South Africa, are confined to cumulate layers that are regionally persistent over tens of kilometers. Four categories ofchromitite are recognized: layers at the bases of cycles in the Lower Critical zone (type I), layers at the bases of cycles in the Upper Critical zone (type II), thin layers in the intermediate parts of cycles (type III), and stringers associated with orthopyroxene pegmatoids (type IV). These latter may not be of primary magmatic origin, nevertheless, probably each type of chromitite contains anomalous concentrations of PGE.Chromitites at the bases of cycles, comprising the Lower Group (LG), Middle Group (MG), and Upper Group (UG), are further categorized into types Ia (LG-1/LG-4), Ib (LG-5/MG-1), IIa (MG-2/UG-1), and IIb (UG-2 and above) on the basis of lithostratigraphy, chromite and PGE chemistry, and sulfide content. Type IIb chromitites contain significant quantities of sulfides and are not investigated here. New whole-rock and electron microprobe chemical analyses of the sulfide-poor type Ia, Ib, and IIa chromitites (Cu contents 1.8, low PGE contents ( 2. Type Ib chromitites occur in pyroxenite cycles and form an intermediate group.These regular variations with increased height are interpreted as a differentiation trend. PGE are concentrated in sulfide-poor chromitites as a result of chromitite control, a process not fully understood but thought to involve a combination of direct nucleation of platinum-group minerals and localized S saturation. It is the crystallization of copious quantities of chromite from hybridized melts close to the crystal-liquid interface that triggers this process, but magma mixing, essential to drive the melt into the chromite field, is the overall cause. Replenishment of the magma chamber was from a single magma batch, because the chromitites below the UG-2 are not associated with major isotopic inflections. Huge volumes of dense hot magma were fed as basal flows along the crystal-liquid interface. The magma chamber was compartmentalized at an early stage, possibly in response to basement tectonics, and lateral mixing over distances of > 150 km is an alternative mechanism for achieving sufficiently high R values.Repeated replenishment of the magma chamber resulted in mixing with resident liquid that became increasingly differentiated with height. Type Ia chromitites are ascribed to replenishment and mixing with the relatively primitive Lower zone residue, type Ib crystallized as a result of mixing with Lower Critical zone resident liquid, and type IIa, due to mixing with more evolved Upper Critical zone resident liquid. Differentiation of PGE with height is a critical part of the process which culminates in the occurrence of higher grade PGE mineralization in sulfide-bearing chromitites (UG-2, Merensky reef) in the uppermost part of the Upper Critical zone.

Characteristics of Ni-Cu-PGE mineralization and genesis of the Jinchuan Deposit, Northwest China

The Jinchuan Ni-Cu sulfide deposit is hosted by a northwest-striking ultramafic body located at the southwest margin of the Sino-Korean platform, China. Sulfide mineralization occurs in the lower part of the intrusion and comprises three main orebodies, 24 and 1 in the west, and 2 in the east of the intrusion. Three types of sulfide ores are identified: net textured, disseminated, and a small amount of massive ore which occurs only in orebody 2. The principal ore minerals are pyrrhotite, pentlandite, violarite, chalcopyrite, and cubanite.A systematic study has been made of the S, Ni, Cu, Co, platinum-group elements (PGE), and Au of the ore samples. A lateral zonation is defined by these elements in the two western orebodies. The concentrations of Ni, Cu, and PGE increase from the margins toward the center of the orebodies. This trend is also present when the metal elements are normalized to the S content of the samples. A vertical variation of elements is present in the east orebody where the amounts of Ni, Cu, and PGE, as well as S, decrease upward. However, when normalized to the S content, PGE increase upward, but Ni and Cu remain more or less constant. A plot of PGE versus S clearly shows three sample sets: (1) samples from orebody 24 have lower S contents, mostly less than 5 wt percent, and their PGE contents roughly increase with that of sulfur; (2) orebody 1 has higher PGE contents, but the PGE plots are more scattered, suggesting multiple processes during PGE mineralization; and (3) orebody 2 has very low PGE concentrations.Ore samples studied show an average Ni/Cu ratio of 2, and (Pt + Pd)/(Ru + Ir + Os) ratios of 5 to 30, with most values falling in the range of 10 to 15. These are typical features of sulfide ores that relate to mafic magma. There is little difference in the (Pt + Pd)/Os + Ir + Ru) ratios and the chondrite-normalized PGE distribution patterns between the PGE-rich and PGE-poor ores, suggesting that they originated from magmas with similar major element compositions.The close association of PGE enrichment with sulfide mineralization and the strong correlations exhibited in all ore types between S and Ni, Co, and PGE indicate that the ores are the consequence of the segregation of sulfide from silicate magmas. The high PGE contents of ore-bodies 24 and 1 and the low PGE contents of orebody 2 suggest that two batches of sulfide liquid were involved. The PGE-rich sulfide liquid equilibrated with a relatively primitive silicate magma whereas the PGE-poor sulfide liquid equilibrated with a silicate magma from which PGE had been depleted by prior (probably fractional) sulfide segregation.The low values of Ni/Cu ratios (about 2) and high values of (Pt + Pd)/(Ru + Ir + Os) ratios (10-15) of the samples, together with the chemical composition and rare earth element data of the intrusive rocks provide convincing evidence that the initial magma of the intrusion was gabbroic rather than ultramafic.

Platinum-group minerals in various rock types of the Merensky Reef; genetic implications

In addition to what is usually termed Merensky reef, mining operations have revealed various other types of the Merensky reef, which are not only of geologic interest but also display specific mineralization characteristics. The platinum mineralogy of these reef conditions is described with regard to the following varieties: normal, wide, pothole, contact, rolling, lens, regional pothole, pegmatoid-replaced, and dunite-affected.Utilizing our current data on the platinum-group minerals, certain suites peculiar to normal and anomalous reef conditions have been recognized. A model for their genesis is proposed which invokes various kinds and periods of volatile activity.Potholes are characterized by a zonal distribution of platinum-group minerals, with Pt-Fe at their centers and Pt-Pd-Te at their edges. Similarly, dunite pegmatites contain Pt-Fe and Pt-Pd-Te as well as Pt-As.Monomineralic (anorthositic) footwall sintering releases volatiles that result in the overlying Merensky reef being Pt-Fe (and olivine) bearing, with large amounts of platinum-group elements in the base metal sulfides. Multiphase footwall cumulates do not generate excess volatiles, and in such localities the Merensky reef is characteristically Pt-Pd-S bearing with low platinum-group element contents in the base metal sulfides. Local deep-seated volatiles, perhaps associated with floor-rock upwarpings, are responsible for potholes of the Brakspruit type.Pothole and dunite platinum-group mineral characteristics are locally superimposed on the Merensky reef platinum-group minerals, which are in the first place characterized by footwall type and perhaps regionally by feeder proximity.