Preliminary Study on the PGE Geochemistry of the Permian Basalts in the Jinping Area

INTRODUCTION

In comparison with studies on the geochemistry of the REE and trace elements of the Emeishan Permian basalts, there has been only limited research on the platinum group elements (PGE). Preliminary studies have been carried out on the PGE geochemistry of the basalts in the Emeishan area (Qingying Power Station profile) and Xinjie area (Zhang and Li, 1998), and in the Shuicheng and Weining areas in Guizhou (Li et al., 2003). There are no PGE analysis data of the basalts in the Jinping area, which is also genetically part of the Emeishan Permian basalts. The Permian basalts in the Jinping area are distributed in the southwest side of the Ailaoshan structure zone, and concentrated in the area between the west of the Dalaotang area and the east of the Sanjia River and in the belt extending along the east margin of the NW-striking Menla River and the Tengtiao River fault in the Jinping area (Fig. 1). The basalts extend southward to the northwest of Vietnam, combining with the Song-Da rift. In general, the basalts strike in the NW direction which is in line with the regional structure. The basalts in the Jinping area share many common geochemical characteristics with that in the Lijiang-Binchuan area and many authors consider that the Jinping area was moved to its current position from these areas (Xiao et al., 2003; Ren and Jin, 1996; Wu, 1993; Zhong et al., 1989).

Figure  1.  Sketch map showing the distribution of Permian basalts in the Jinping area and sampling locations. 1. Fault; 2. Cu-Ni deposit; 3. Sampling line and the location. P₂β. Late Permian basalt; ν. mafic and ultramafic intrusions.

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GEOLOGY

The Permian basalts in the Jinping area are distributed to the southwest of the town of Jinping and east of the NW-trending Tengtiaohe structure zone. The basalts have a maximum thickness of up to 4 530 m in the southeast, becoming thinner westwards to the east of the Tengtiaohe structure zone, with a thickness of 1 221 m. The basalts are dominated by lavas consisting of porphyritic basalt, amygdaloidal basalt, massive basalt and trachybasalt, while thevolcaniclastic rocks are composed of volcanic breccia with basaltic clasts and basalt tuff. The basalts can be divided into two cycles consisting of rock sequences from volcanic breccia, porphyritic or amygdaloidal basalts, massive basalts and trachybasalts to basalt tuff.

SAMPLING AND ANALYTICAL TECHNI QUES

All the samples tested in this paper were collected from the basalt section of a road cutting between Jinping and the town of Nafa to the southeast. As the basalts are well-weathered, the samples are mostly taken from the middle to bottom parts of the section, covering a thickness of about 4 000 m. After careful microscopic determination, 12 samples were selected for PGE testing. The PGE were analyzed by the National Geological Experiment and Analysis Center of the Chinese Academy of Geological Sciences.

The PGE were determined by NiS-fire assay pre-concentration and the ICP-MS method. The sample was mixed with sodium carbonate, sodium borate, borax, glass powder, nickel powder, sulfur and flour, and placed into a melting pot. A suitable amount of Os-dilution solution was added and the mixture was melted at a temperature of 1 150 ℃. The melt was put into an iron model and cooled to produce an NiS button. The NiS button was dissolved in HCl acid and the PGE sulfide remained in the undissolved material (slag). The slag was then dissolved in aqua regia and Te was added to co-precipitate it. It was then filtered, the precipitate was determined for Os in the media of HCl-H₂O₂ by using the distilling isotope dilution method. After treating with the HNO₃ acid, the remaining solution was tested for the contents of Pt, Pd, Rh, Ir, Ru and Au by the ICP-MS method. The precision and accuracy are better than 5% for Rh, Pd and Ir, and 10% for others. The detection limit is 0.2 ng/g for Pt, Pd and Au; 0.001 ng/g for Ir, Rh and Os; 0.1 ng/g for Ru.

PGE GEOCHEMISTRY

The Abundance of the PGE

The data in Table 1 show that the PGE contents and the total PGE content of the Permian basalt in the Jinping area are relatively low when compared to the other Emeishan basalt provinces. Pd contents are higher than Pt contents, which is the typical characteristic shared with the middle basalt districts of the Emeishan basalt province; however, Pt contents are lower than Pd contents in basalts from the east districts in Shuicheng, Weining and the Qingying Power Station areas of the Emeishan basalt province. Also, the PGE values of Permian basalts in the Jinping area are generally lower than those of the primitive mantleand upper primitive mantle and chondrite meteorite, which shows their relatively depleted characteristics.

The PGE assay data and their characteristic ratios of the Permian basalts in the Jinping area, and those of the remaining Emeishan basalt provinces are listed in Table 1.

Table  1.  The PGE and Au data of Permian basalts in the Jinping area, Yunnan  10^-9

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As the individual element of the PGEs display different geochemical behaviors in the magma evolution, ratios such as the Pd to Ir ratio, the (Pd+Pt) to (Os+Ir+Ru) ratio and the (Pd+Pt) to (Ru+Rh+Ir) ratio are of great significance in considering the genesis of the magma. The ratio of (Pd+Pt) to (Os+Ir+Ru) of basalts in the Jinping area ranges from 0.66 to 19.9, averaging 5.78, which is higher than that of the upper primitive mantle (1.57) and that of chondrite (0.90). The ratio of (Pd+Pt) to (Ru+Rh+Ir) ranges from 0.73 to 13.95, averaging 4.33, which is higher than those of the chondrite (1.09), upper primitive mantle (1.39) and primitive mantle (1.37), but lower than the average (Pd+Pt) / (Os+Ir+Ru) value of the basalts from the Qingying Power Station Section, Shuicheng, Weining and Xinjie areas.

The Pd/Ir values of the Jinping basalts range between 1.52 and 144.67, averaging 42.72, which is higher than those of the basalts in the Shuicheng and Weining areas in the east and the Huili and Xinjie areas in the middle of the Emeishan basalt province, but lower than those of the basalts in the Qingying Power Station area. The Pd/Ir ratios of basalts in the Jinping area are higher than those of the PM (1.22), CM (1.01) and UPM (1.52) (Table 1), and also higher than that of the komatiite (3.98) with a high melting degree. They are close to the ratios of N-MORB (40) (Naldrett and Barnes, 1986) and continental tholeiite (120) (Zhang and Li, 1998) derived from the mantle with a lower melting degree (the Pd/Ir of basalts in the middle of the basalt section in the Jinping area is 144.56).

The variation in Pd/Ir values can reflect the melting degree of the mantle. The smaller the Pd/Ir values, the higher the melting degree of the mantle (Barnes et al., 1985). Both the Jinping basalts and the Emeishan basalts have similar high Pd/Ir values which indicate that they are both derived from the basaltic magma in the upper mantle with a lower partial melting degree. Wang et al. (1993) held that the primitive magma of the Emeishan basalts derived from the upper mantle with 10%-17% partial melting, which is in accordance with the conclusions from the abundance and differentiation of the PGE of the basalts in Jinping and the rest of the Emeishan basalt province.

The Pd/Ir values can also reflect the oxygen fugacity when the primitive magma was formed. The negative Ir anomaly was formed when Ir was kept in the depths of the mantle, with a lower oxygen fugacity (Wang et al., 2003; Chu et al., 2001; Orberger et al., 1998; Pattou et al., 1996). The Pd/Ir values of the basalts in the Jinping area and the rest of the Emeishan basalt province are relatively high, suggesting a lower oxygen fugacity when they are formed.

Primitive Mantle Normalization Distribution Pattern of the PGE

The PGE are the relatively inert elements. Their primitive mantle normalization patterns can be an indicator of the genesis source of magma in the same way as REE because of their common characteristics and differentiation and their evolutionary relation to the parent magma.

There are three basic distribution patterns for the PGE, i.e. ① the Ru-Pt type which is the typical pattern for chondrite meteorite and iron meteorite from the mantle and earth core, featuring relatively equal contents of refractory elements (Os, Ru, Ir) and elements with a low melting degree such as the Pt and Pd, there generally are two peaks as the Ru and Pt peaks with almost equal height; ② the Ru-Os type, in which the refractory elements as the Os, Ru and Ir are dominant in the magnesian ultramafic rocks, and have a significantly low content of Pt and Pd, the distribution pattern has a weak or no Pt peak, and ③ the Pt-Pd type, which generally has far higher Pt and Pd contents than the contents of Os, Ir, Ru and Rh in the ferric ultramafic rocks, the distribution pattern has a significant Pt and Pd peak, suggesting that the rocks are derived from basaltic magma, and the PGE are dominated by Pt and Pd with a low melting point (Institute of Geochemistry of Chinese Academy of Science, 1981). The PGE of the basalts in the Jinping area share similar distribution patterns with the basalts in the east of the Emeishan basalt province. They both have the Pt-Pd enriched type-distribution pattern and they also share similar distribution patterns with the Xinjie layered mafic-ultramafic intrusion (both the top and bottom walls of the intrusions are basalts) (Zhang and Li, 1998) and the ferric ultramafic intrusion in Bushveld and Sudbury (Taylor and McLeannan, 1985). However, they have significantly different distribution patterns to those of the magnesian ultramafic intrusions, which have an Ru-Os enriched PGE distribution pattern, and the chondrite meteorite or the upper primitive mantle with an Ru-Pt enriched PGE distribution pattern.

As the PGE and Au share some common chemical properties and a strong siderophile property, Au is placed in the same group for discussion. The PGE distribution patterns are drawn by the primitive mantle normalized PGE and Au contents in the decreasing order of the melting point (Naldrett et al., 1979) (Os, Ir, Ru, Rh, Pt, Pd and Au). The PGE are divided into two groups according to the correlation between the elements, i.e. the Ir group (IPGE) : Os, Ir, Ru and Pd group (PPGE) : Rh, Pt, Pd and Au (Naldrett and Barnes, 1986), and the curve inclination of the primitive mantle normalized PGE distribution pattern is determined by the Pd/Ir values (Chu et al., 1999).

The PGE distribution pattern can be divided into two types. One is the nearly-horizontal flat type which shows little PGE differentiation. In general, the primitive mantle, mantle xenolith, and alpine-type peridotite have this kind of PGE distribution pattern. The other is the steeply left-dipping curve which shows strong PGE differentiation. Middle ocean ridge basalt, continental tholeiite, low-Ti lavas and ferric ultramafic intrusions exhibit this kind of PGE distribution pattern (Chu et al., 1999; Naldrett and Barnes, 1986).

The primitive mantle normalized PGE distribution patterns of the basalts in the Jinping area are shown in Fig. 2 and can be compared to similar patterns in the Emeishan basalt province shown in Fig. 3. They both belong to the steeply left-dipping curves of the PGE distribution patterns, and they both have a steep positive inclination. The patterns show a significant Ru peak and a weak Pd peak, which belong to the transitional patterns between the Ru-Os and Pt-Pd types. The IPGE of the basalts in the Jinping area are depleted, which is similar to the basalts in the Shuicheng, Weining (Li et al., 2003) and Qingying areas (Zhang and Li, 1998), and close to the PGE differentiation trend (Barnes et al., 1985) of common continental tholeiite and middle ocean ridge basalts.

Figure  2.  Primitive mantle-normalized patterns of the PGE and Au of Permian basalt in the Jinping area.

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Figure  3.  Primitive mantle-normalized patterns of the PGE and Au of Permian basalt in the Emeishan basalt province (primitive data after Li et al. (2003) and Song et al. (1998)).

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The PGE of the basalts in the Jinping area exhibit a different feature from that of the basalts with the strong Ir depletion in the Qingying area, but show a strong Ru enrichment which is similar to that of the basalts in the Shuicheng and Weining areas.The Jinping basalts also have strong Rh depletion, which shows that they are different from the basalts derived from low-level melting, which generally show a significant negative Ir anomaly.

Pd/Ir values higher than that of meteorite reflect not only the melting degree of the mantle, but also the PGE differentiation. As for Os, Ir to Pd, they exhibit properties extending from highly compatible to highly incompatible. Compared to Ir, Pd is easily enriched in the magma, and Ir remains in the residual mantle peridotite. So the derived magma has higher Pd/Ir values than meteorite, and the Pd/Ir value of the magma increases significantly as the partial melting level decreases. The continental basalts and ocean basalts generally have high Pd/Ir values of up to 100 and more, and the PGE distribution patterns generally have a steep positive inclination (Barnes et al., 1985). The Pd/Ir values of the basalts in the Jinping area range from 1.52 to 144.67. The basalt in the middle of the profile has a higher Pd/Ir value and the basalt on the top of the profile has a lower Pd/Ir, which shows that the source of the basalts in the mantle experienced high-level melting in the early and late stages and low-level melting in the middle stage of the mantle evolution. The PGE shows a higher differentiation in the middle stage during the eruption of the basalts.

CONCLUSIONS

In general, compared to the east Emeishan basalt province, the PGE of the basalts in the Jinping area exhibits little differentiation. The weak Pt-Pd type of the primitive mantle normalized PGE distribution patterns with a steeply left-dipping feature and steep positive inclination show the transitional feature of the Ru-Os type and Pt-Pd enrichment type. The low-level PGE differentiation of the basalts in the Jinping area may account for the low PGE contents in the ores of the magmatic Cu-Ni sulfide deposit.

The PGE analysis data are limited in this study, and the samples are taken from the Jinping basalt profile only; so they may not represent the basalts as a whole in the Jinping area. As such, the conclusions from this study are only preliminary and a further study is required.