Source tracing of noble metal elements in Lower Cambrian black rock series of Guizhou-Hunan Provinces, China

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1 Vol. 43 No. 6 SCIENCE IN CHINA (Series D) December 2000 Source tracing of noble metal elements in Lower Cambrian black rock series of Guizhou-Hunan Provinces, China LI Shengrong ( ) 1 & GAO Zhenmin ( ) 2 1. Section of Genetic Mineralogy, China University of Geosciences, Beijing , China; 2. Institute of Geochemistry, Chinese Academy of Sciences, Guiyang , China Correspondence should be addressed to Li Shengrong ( lisr@263.net) Received August 2, 1999 Abstract The Lower Cambrian black rock series of South China is abnormally rich in noble metal elements. According to the concentrations, the ratios, the relations, the distribution and partition patterns of noble metal elements, the authors think that the noble metals and other elements are neither directly from extraterrestrial materials, nor from the products of normal marine sedimentation. The abnormal enrichment of noble metal elements is closely related with hydrothermal fluid that flew out on the sea floor through deep cycling and reaction with Proterozoic ultramafic-mafic igneous rocks forming noble metal rich fluid. It is possible to form industrial multiple-element- oredeposits, especially hydrothermal type platinum-group-element-ore-deposits in the region with strong hydrothermal action. Keywords: black shale, noble metal elements, Lower Cambrian, Guizhou-Hunan Provinces, China. The Lower Cambrian black rock series has been found in Asia, Europe and North America. The series in Hunan and Guizhou Provinces of China covers the top of Proterozoic dolomitite with a disconformity in between. A shale dominated bed near the boundary is rich in double peak elements such as Ni, Mo, As, Ba, Au, Ag, Os, Pt and Pd. Abrupt changes marked by the boundary are found in lithofacies, biocoenosis, elements and isotopes, mineralogical assemblage and mineralization characteristics. These changes represent an important catastrophic event. According to element parageneses and Ir anomaly, Fan et al. [1,2] pointed out that platinum group elements, Ni and Co, might be of extraterrestrial origin, and other elements such as Zn, Mo, Ba, U, Cd, Se and Tl might be related with sea floor hot spring. Based on the linear distribution of metal-rich bed in South China, Coveney et al. [3,4] thought that the enrichment of the metals might be the result of syngenetic precipitation of metals from sea floor hot spring related with deep seated fractures. Recently (1995, personal communication) Chen put forth a supergene leaching model for the element enrichment. Based on the REE study of the metal-rich bed and the REE models of marine sediments, the authors once discussed the hydrothermal sediment characteristics of the metal-rich bed [5]. The authors also reported the discovery of hydrothermal chert in the black rock series [6]. In order to further explain the origin of the double peak elements, especially the origin of noble metal elements and the nature of the related boundary event, it is necessary to study directly the source

2 626 SCIENCE IN CHINA (Series D) Vol. 43 tracing geochemistry of noble metal elements. The importance of the source study of noble metal elements is that: (i) if the elements were mainly of extraterrestrial origin, they would be distributed in globe scale with small thickness. They are not of resource value, but indicate a celestial collision event. (ii) If the enrichment of the elements was mainly related with sea floor hydrothermal activity, their distribution would certainly not be homogeneous, and in some regions they would form industrial scale. The origin of elements is, therefore, of significance both to the study of globe change and to the prospecting of new platinum group elements resource. 1 Profile of the black rock series The metal-rich black rock series is best developed in the regions near Zhangjiajie in northwest Hunan Province and Zunyi in north Guizhou Province. The series consists of five lithological members. The lithology and thickness of each member are, in the order from upper to lower: carbonaceous illite shale (30 40 m), carbonaceous-argillaceous chert (upper chert, 0 18 m), metalrich shale bed with calcite dominated and hydroxylapatite dominated nodules (0.2 1 m), argillaceous phosphorite ( m), carbonaceous and argillaceous chert (lower chert, m). 2 Sampling and methods of noble metal elements analyses The samples were mainly collected from Ganziping, Sancha sections near Zhangjiajie and Tianeshan section near Zunyi. Some samples were from the sections near Danzhai and Sandu counties in Guizhou Province. Seven elements, Ru, Rh, Pd, Os, Ir, Pt and Ag, were analyzed at China National Center of Geological Experiments and Analyses. The samples for PGE analyses were first dissolved with Na 2 O 2. After distillation separation, the samples were ready for Os and Ru analyses. Having enriched with thiourea, the samples can be measured for Ir, Rh and Pt, and further extracted with D.D.O. petroleum ether for Pd. The analyses of Os, Ru, and Ir were conducted in Ce-As system with a UV-120 spectrophotometer. The detective levels are ng for Os, ng for Ru and ng for Ir. Rh and Pt were analyzed with JP-2 polarograph and Pd with PE-3030 atomic absorption spectrometer. The detective levels are 0.05 ng/ml, 0.2 ng/ml and ng/ml for Rh, Pt and Pd respectively. The samples for Ag analyses were first dissolved with HCl+HNO 3 and analyzed with a PE-3030 atomic absorption spectrometer. The detective level is 0.25 ng/ml. The accuracy is controlled by double parallel measurements. The analyses of Au were conducted at the Institute of High Energy Physics, Chinese Academy of Sciences with neutron activation method, the working condition as described in ref. [5]. 3 Results of analyses and regional correlations The results of analyses are shown in table 1. Coveney et al. from USA once analysed 2 samples from the metal-rich bed, and they reported of Ir, of Pd and of Au

3 No. 6 SOURCE TRACING OF Au-Ag-PGE IN BLACK SHALES, CHINA 627 for the bed in Zhangjiajie and of Ir, of Pt and of Au for the bed in Zunyi. These data are much similar to those in table 1. Table 1 Concentrations of noble metal elements in the Lower Cambrian black rock series(w B /10 6 ) Lithology No. Os Ru Rh Ir Pt Pd Au Ag Metal-rich bed G G S14B S14B T T T T05B Illite shale G < G <0.001 < < G09 < <0.001 < G < T < T < DZ <0.001 <0.001 < SD < Chert G03 <0.001 <0.001 < G04 < <0.001 < < Phosphorite G < < G, S, T, DZ and SD mean the samples are from Ganziping, Sancha, Tianeshan, Danzhai and Sandu respectively. It is noticed from table 1 that the metal-rich bed (or sulfide-rich black shale) is the preferential stratum for the enrichment of noble metal elements. The concentration coefficients of noble metal elements relative to earth crust [7] are for Os, for Ru, for Rh, 1 43 for Ir, for Pt, for Pd, for Au and for Ag. A correlation between Zunyi and Zhangjiajie shows that the concentrations of noble metal elements in the metal-rich beds in Zunyi are much higher than those in Zhangjiajie (table 2). Calculation of previous data makes the same conclusion. It is interesting to note that the noble metal elements in the metal-rich bed of Zunyi, Danzhai and Sandu of Guizhou Province and Ganziping and Sancha of Hunan Province show small grade of variations with high concentrations. This is a favorable condition for the utilization of this potential new type of resource of platinum group elements. 4 Correlation with extraterrestrial materials 4.1 Concentrations and ratios of elements In the 6 platinum group elements, the concentrations of Pt, Pd and Os are much higher than those or Ru, Rh and Ir. Most of the data of Ir obtained here are much lower than those ( ± ) [8] of the K/T boundary clay in Denmark which is known to be related with collision between the earth and extraterrestrial materials. The ratios of elements and element associations are also quite different between extraterrestrial materials and the metal-rich bed. The ratio

4 628 SCIENCE IN CHINA (Series D) Vol. 43 Os/Ir is 1.1 in solar system [9], 0.84 in iron meteorite, 2.25 in chondrite (calculated from ref. [10]) and 1.07 in carbonaceous chondrite (calculated from ref. [11]). The ratio Os/Ir in extraterrestrial materials, therefore, is less than In the metal-rich beds of Hunan and Guizhou Provinces, the ratio reaches and averages 112 (with one datum to be 3.3, table 2) which greatly excess those of the extraterrestrial materials mentioned above. Taking carbonaceous chondrite, which is known to be the best representative of the solar system in composition, as the representative of extraterrestrial materials. Its ratios Au/Ir, Ag/Au, Pd/Pt and (Pt+Pd)/(Os+Ru+Rh+Ir), which are respectively 0.31, 1.44, 0.57 and 0.86 (calculated from ref. [12]), except Pd/Pt, are not of the same order as those of the metal-rich beds (table 2). Table 2 Characteristic parameters of noble metal elements in the metal-rich bed Location No Ganziping G Ganziping G Sancha S14B Sancha S14B Tianeshan T Tianeshan T Tianeshan T Tianeshan T05B Average ΣPGE( 10-6 ); 2. (Pt+Pd)/(Os+Ru+Rh+Ir); 3. Pd/Pt; 4. Ag/Au; 5. Os/Ir; 6. Au/Ir. 4.2 Relations of elements Previous reports show that the element couples such as Ir-Os, Pt-Ru and Ru-Ir are positively correlated in iron meteorite. In Ni-rich iron meteorite Au with Pd and Os with Ru are positively correlated, and Au with Pt, Os, Ir, Ru is negatively correlated. In addition, Pt with Au in chondrite is positively correlated [12]. In the black rock series of Hunan and Guizhou, however, even though some very abnormal data have been eliminated, the relations of the said element couples, except Pt-Ru (r=0.609, 14 samples), are still very poor, showing clear difference with iron meteorite, Nirich iron meteorite and chondrite in terms of the noble metal elemental relationship. 4.3 Distribution patterns of platinum group elements Supposing that the noble metal elements in the thin metal-rich bed of several centimeters thickness were supplied directly by noble-metal-element-rich extraterrestrial materials, the 6 elements will show no clear differentiation and the distribution pattern will be consistent with that of extraterrestrial material. In view of the broad distribution of the metal-rich bed, it might be inferred that the extraterrestrial material was great both in volume and mass and its composition bore the characteristics of solar system. Compiling distribution patterns of platinum group elements normalized with C1 chondrite (fig. 1), it is noticed that the metal-rich bed is rich in Pt, Pd, Os and Rh and poor in Ir and Ru. The pattern curves are strongly W shaped, showing no consistence with those of extraterrestrial materials.

5 No. 6 SOURCE TRACING OF Au-Ag-PGE IN BLACK SHALES, CHINA Correlation with related geological bodies Examining from source point of view, except extraterrestrial materials, the geological bodies related with the noble metal elements in the black rock series should be the terrigenous marine sediments and mafic and ultramafic rocks under the seafloor. The medium carring the noble metal to move should mainly be the marine water or hydrothermal fluid cycling under sea floor. In as early as Precambrian, the Fig. 1. C1 chondrite normalized distribution patterns of PGE in metal-rich bed. region had already developed a series of NE deep seated fractures. Along the fractures, multiple periods of mafic and ultramafic magmatism happened during Proterozoic and early Palaeozoic Era, forming various mafic and ultramafic rocks such as splite, pyrolite, pyroxenite, diabase, kimberlite and minette. As we know, mafic and ultramafic rocks are still the best for platinum group elements prospecting. Seeing that these types of rocks are related with the black rock series both temporally and spatially, they should consequently be examined as a possible major supplier of noble metal elements in the black rock series. In addition, the black rocks are special submarine sediments, their similarities and differences in geochemistry with other submarine sediments of known origins are important clues for tracing their material source. Considering the mentioned factors, the following discussion will focus on the relations of the noble metal elements in the black rock series with mafic-ultramafic rocks and submarine sediments. 5.1 Relations with mafic-ultramafic rocks Li and Gao [13] revealed that three typical partition patterns exist for platinum group elements, namely Ru-Pt type, Ru-Os type and Pt-Pd type. These partition patterns represent respectively the products of different stages of the earth evolution and the products of different concentration processes of the earth. The core and its representative iron meteorite, and the mantle and its representative chondrite show Ru-Pt type of partition pattern, reflecting the concentration state of platinum group elements at the initial stage of the earth evolution. The partition pattern of the crust is of Pt- Pd type, reflecting the concentration state at the present stage. The partition curve of the mantle occurs in between those of the core and the crust, reflecting in a certain degree the transition from the core to the crust. Magnesium ultramafic rocks have special Ru-Os partition pattern, representing the relic of partial melting of the mantle materials. This pattern is distinctly different from the Pt-Pd type for ferruginous mafic-ultramafic rocks. The similarity of ferruginous mafic-ultramafic rocks and the crust in platinum group elements partition pattern is due to that both the rocks and the crust are the result of further evolution of mantle materials. This study displays that platinum group elements partition pattern is of great help in geochemical tracing.

6 630 SCIENCE IN CHINA (Series D) Vol. 43 There are more than ten kinds of Proterozoic and early Palaeozoic mafic-ultramafic rocks not far from the region with black rock series in Guizhou Province [14], but the variation of their m/f values are not great ( ). These rocks are all ferrigenous and their platinum group elements partition pattern should be of Pt-Pd type. Supposing these rocks were the major supplier of the noble metal elements, the ultimate emplacement of the elements would have passed a stage of hydrothermal transference. In the metastable environment between shelf sea and marginal sea, the magma of mantle origin ascending along the deep seated fractures was generally rich in noble metal elements. When the magma effused on sea floor or intruded into submarine rocks, some of the noble metal elements would mix with sea water and precipitate together with terrigenous materials in the sea water. At the early stage of this process, magmatic water from the intrusions mixed with underground water, forming metal-rich fluid, or the underground cycling hot water extracted noble metal elements from Proterozoic mafic-ultramafic rocks, forming metal-rich fluid, the fluid then moved up along fractures into the sea, contributing to the enrichment of noble metal elements in the sediments on sea floor. The effusions, in another aspect, directly released metalrich fluid, contributing to the enrichment of noble metal elements in the sediments on sea floor. According to materials of many hydrothermal platinum group element deposits of China [15,16], the enrichment sequence of the elements is Pt/Pd>Os>Rh>Ru>Ir. Studies by Keays et al. [17], Rowell and Edgar [18] also reveal that in broad geological environments including aqueous solution and carbonate-rich solution, Ir is not active. Combining all these materials, it is deduced that the partition of platinum group elements in the ore-forming metal-rich fluid mentioned above should be more enriched with Pt and Pd compared with that in the mafic-ultramafic rocks not far from the region with black rock series. If the said magma erupted into the sea and volcanic dust precipitated forming noble metal-rich bed, the 6 platinum group elements would not repartition and would keep the partition pattern of the original mafic-ultramafic magma. According to Chen Nansheng (oral communication), thin bedded volcanic tuff really occurs in the Lower Cambrian strata of Guizhou, then it is possible for magma of deep origin contributed to the noble metal enrichment in this way. No matter what was the way of the enrichment, the partition pattern would keep Pt-Pd type. The values of (Pt+Pd)/(Os+Ru+Rh+Ir) and Pd/Ir, of course, would be greater for hydrothermal transference than for volcanic precipitation. Table 1 shows clear Pt-Pd type of platinum group elements partition pattern of the metal-rich bed, which is consistent with that for hydrothermal transference. Even so, we still cannot determine the nature of hydrothermal origin for noble metal elements enrichment and the relation of the elements with the mafic-ultramafic rocks, because platinum group elements precipitated by normal sea water may show the same type. It is necessary, therefore, to examine the relation of platinum group elements with marine sediments. 5.2 Relations with marine sediments The great difference in concentrations of platinum group elements in marine sediments of normal origin and hydrothermal origin supplies a clue to understand the origin of the noble metal

7 No. 6 SOURCE TRACING OF Au-Ag-PGE IN BLACK SHALES, CHINA 631 elements in the black rock series. The sedimentary structures display the enrichment of the metal elements in the metal-rich beds to be mainly the result of syngenetic marine sedimentation. Statistics disclosed that normal marine sediments carry modern normal marine sediments carry of Pt [19], abyssal sediments carry of Pd [20] and of Ir [19]. The ocean edge sediments related with submarine volcanic and hydrothermal eruptions in the East Pacific Ocean, however, carry as high as of Pd [20]. The concentrations of Pt, Pd and Ir in the metal-rich bed of the black rock series, even the concentrations of Pt and Pd (4 180) in the black illite shale of the series, greatly exceeds those in normal marine sediments. We, therefore, consider the enrichment of the noble metal elements in the black rock series has nothing to do with normal marine sedimentation, but closely related with sea floor hydrothermal eruption. 6 Conclusions The metal-rich bed at the bottom of the lower Cambrian black rock series of South China is the product of pre-cambrian-cambrian boundary event of abrupt change in paleoenvironment (anoxic). The author s study of chert (silicalite), rare earth elements and other trace elements in the black rock series reveals that the noble metal and other elements in the metal-rich bed of the black rock series were not mainly supplied by extraterrestrial materials, and that they were not the product of normal marine sedimentation, but closely related with sea floor hydrothermal eruption. In the areas with strong hydrothermal fluid eruption, therefore, industrial multiple elements deposits, especially new type of platinum group elements deposits other than the mafic-ultramafic type of deposits might form. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos , ). Help in the study from Professors Chen Nansheng, Fan Delian, Zhang Aiyun and Academicians Tu Guangchi and Ouyang Ziyuan are sincerely acknowledged. References 1. Fan, D., Polyelements in the Lower Cambrian Black Shale Series in Southern China, in The Significance of Trace Elements in Solving Petrogenetic Problems and Controversies, Greece: Theophrastus Publications S A, 1983, Fan, D., Yang, R., Huang, Z., The Lower Cambrian Black Shale Series and the Iridium Anomaly in South China, in Academia Sinica: Developments in Geoscience, Contribution to 27th International Geological Congress, Moscow, Beijing: Science Press, 1984, Coveney, Jr. R.M., Chen, N., Ni-Mo-PGE-Au-rich ores in Chinese black shales and speculations on possible analogues in the United States, Mineral. Deposita, 1991, 26: Coveney, Jr. R. M., Mourowchick, J. B., Grauch, R. I. et al., Gold and platinum in shales with evidence against extraterrestrial sources of metals, Chemical Geology, : Li, S., Gao, Z., REE Characteristics of black rock series of Niutitang Formation in Hunan-Guizhou Area with discussion on REE model of marine hydrothermal sedimentation, Mineralogica Sinica (in Chinese with English abstract), 1995, 15(2): Li, S., Gao, Z., Silicolites of hydrothermal origin in the Lower Cambrian black series of South China, Chinese Journal of Geochemistry, 1996, 15(2): Li, T., Some statistic characteristics of element abundance in the earth crust, Geology and Exploration (in Chinese), 1992,

8 632 SCIENCE IN CHINA (Series D) Vol (10): Ganapathy, R., A major meteorite impact on the Earth 65 million years ago: Evidence from the Cretaceous-Tertiary boundary clay, Science, 1980, 209: Orth, C.J., Quintana, L. R., Gilmore, J. S. et al., Pt-Group metal anomalies in the Lower Mississippian of Southern Oklahoma, Geology, : Yang, M., Ren, Y., Deng, Y., Platinum Group Elements and Platinum Ore Geology (in Chinese), Beijing: Science Press, 1973, Wasson, J. T., Meteorites: Their Record of Early Solar System History, New York: W H Freeman and Company, 1985, Liu, Y., Cao, L., Li, Z. et al., Element Geochemistry (in Chinese), Beijing: Science Press, 1984, Li, S., Gao, Z., Some typical partition and distribution patterns of platinum group elements, Mineralogical magazine, 1994, 58(A): Guizhou Bureau of Geology and Mineral Resources, Regional Geology of Guizhou Province (in Chinese), Beijing: Geological Publishing House, 1987, Yang, X., Li, X., Yang Z. et al., China Platiniferous Mafic-ultramafic Rocks and Platinum (Group) Ore Deposits (in Chinese), Xi an: Xi an University of Communications Press, 1993, Institute of Geochemistry, Academia Sinica, China Platiniferous Geological Bodies, Platinum Group Element Geochemistry and Platinum Group Ore Deposits (in Chinese), Beijing: Science Press, 1981, Keays, R. R., Nickel, E. H., Groves, D. I. et al., Iridium and palladium as discriminants of volcanic-exhalative, hydrothermal, and magmatic nickel sulfide mineralization, Economic Geology, 1982, 77(6): Rowell, W. F., Edgar, A. D., Platinum-group element mineralization in a hydrothermal Cu-Ni sulfide occurrence, Rathbun Lake, Northeastern Ontario, Economic Geology, 1986, 81(5): Hodge, V., Stallard, M., Koida, M. et al., Determination of platinum and iridium in marine waters, sediments and organisms, Analytical Chemistry, 1986, 58: Cracket, J. H., Neutron activation analysis for noble metals in geochemistry, in Activation Analysis in Geochemistry and Cosmochemistry (eds. Brunfelt, A. O., Steinnes, E.), Oslo: Universitets Forlaget, 1973,

Worked Example of Batch Melting: Rb and Sr

Worked Example of Batch Melting: Rb and Sr Basalt with the mode: Table 9.2. Conversion from mode to weight percent Mineral Mode Density Wt prop Wt% ol 15 3.6 54 0.18 cpx 33 3.4 112.2 0.37 plag 51 2.7 137.7