Research Status, Existing Problems and Countermeasures in China

In 1960s and 1970s, China set off a climax of searching for "five elements" deposits and platinum group elements in black rock series, and made progress and breakthrough in the south. It is preliminarily determined that there are large and super-large deposits in the black rock series of the following 9 horizons; ① Lower Kunyang Group in Mesoproterozoic; ② Neoproterozoic Waitoushan Formation; ③ Dawu Formation and Nantuo Formation in Early Sinian; ④ Doushantuo Formation in Late Sinian; ⑤ Early Cambrian Niutitang Formation; ⑥ the ambition of leaving a name in the early and middle period; ⑦ Late Devonian; 8 Early Permian; Pet-name ruby late tertiary Miocene series, etc. The deposits are concentrated in the early Cambrian and late Devonian strata, forming five super-large and numerous large and medium-sized tin, antimony and barite deposits. It is preliminarily found that the layers with high PGE content appear in the Doushantuo Formation in the late Sinian and Niutitang Formation in the early Cambrian.

Doushantuo Formation is widely distributed in Anhui, Zhejiang, Jiangxi, Hunan, Hubei, Sichuan and northern Chongqing. The black rock series rich in organic matter is more than 20 meters thick, belonging to tidal flat facies and swamp facies, accompanied by stone coal, phosphate rock and rhodochrosite. At present, the types of deposits found in Doushantuo Formation are: sedimentary silver-vanadium deposit (Ginkgo source in Xingshan, Hubei); Fine disseminated gold-antimony deposit (Fangshan, Chongyang County, southern Hubei Province); Contact metasomatic Ag-Pb-Zn deposit (Sanbao, Qimen County, Anhui Province); Sedimentary reformed molybdenum deposit (Zhangjialing, Anhua, southern Hunan). According to previous investigations, the mineralization of platinum group elements has only been found in Chengkou, Chongqing and Wanyuan, Sichuan.

Early Cambrian black rock series is the product of large-scale anoxic events during the geological and historical transition period in southern China, which has the characteristics of wide distribution and stable horizon. It is usually composed of carbonaceous shale, carbonate rock, sandstone, phosphorite and siliceous rock, and in some areas, stone coal and basic-ultrabasic volcanic tuff are also found. The organic carbon content of black rock series is 5.6% ~ 18.7%, and generally it also contains metal sulfides dominated by pyrite. Sheng et al. (1986) classified the multi-element deposits in this horizon into the following three types according to the lithofacies and ore-forming element combination characteristics of black rock series: ① Ni-Mo multi-element deposits; ② Vanadium deposit; ③ Sedimentary transformation of Cu-Mo-V-U multi-element deposit. The results of PGE verification show that the mineralization with industrial significance mainly occurs in Ni-Mo multi-element deposits (Table 3-4).

Nickel and molybdenum multi-element deposits are distributed in Zunyi, Guizhou, Deze, Dayong, Cili, Zhuji, Fuyang and Tonglu, Zhejiang. They occur at the bottom of carbonaceous siliceous and black rock series, and occur in layers and lentils, with small thickness (0.5 ~ 1.8m in Dayong and 0. 1 ~ 1m in Zunyi). The sulfide assemblage of multi-element deposits of nickel and molybdenum is very complex, including pyrite, white iron ore, molybdenite, nickel disulfide, sphalerite, goethite, nickel arsenite, nickel pyrite, purple pyrite, chalcopyrite, tetrahedrite, sphalerite, silver gold ore, ilmenite and so on. No independent platinum group minerals have been found, and it is speculated that there are ultrafine particles or carbonaceous adsorption. Coal seam grade: nickel 1% ~ 5% (average 3.0 1%), molybdenum 1% ~ 20% (average 3.48%), copper 0.09%, selenium 0.28%, gold 0.054 ~ 0.38g/ton.

In terms of genesis, Ni-Mo deposit used to be considered as a sedimentary deposit with organic matter, but the study of stable isotopes and fluid inclusions in recent years supports the view of submarine jet hot water deposition. Re-Os isotopic ages are 560Ma and 54 1Ma, respectively (Mao Jingwen et al., 2002); The formation temperature of pyrite measured by inclusions is115 ~185℃; The diagenetic temperature calculated according to organic heat is 154 ~ 2 10℃. The homogenization temperature of calcite and calcite is 130 ~ 200℃. Both mineralization and sedimentation are controlled by finite basins.

The study of PGE in China black rock series highlights the phase equilibrium of multicomponent system under low temperature geochemical conditions. In the experimental simulation, it is concluded that at the temperature of 25 ~ 200℃, Pt can be transported by the complex anion of Cl, and its amount can reach × 10-9, while Pd can be one order of magnitude higher. In NaCl/H2SO4 solution at 300℃, Pt can migrate and precipitate as sulfide. In addition, in black rock series, Pt and Ir are often mixed with natural gold and appear in ultrafine state. The above experiments and tests explain the objective phenomenon that no independent platinum group minerals have been found in black rock series.

Table 3-4 PGE Verification Results of Black Rock Series Deposits in South China (wB/ 10-6)

Although the knowledge that PGE in black rock series can be enriched in integrated ore has been confirmed theoretically and the prospect of economic development has been affirmed, that is, it will be developed and utilized sooner or later, there are still many problems at present:

1. Analysis test technology failed. Foreign technology is kept secret, and no corresponding research has been carried out in China, which has seriously affected exploration and evaluation.

The exploration technology of platinum group element deposits in black rock series is similar to Carlin-type gold deposits. Thirty years ago, geologists and authors in China knew nothing about Carlin-type gold deposits. Later, through on-the-spot investigation abroad and a lot of investment in domestic prospecting work, they finally made a major breakthrough. Soon, two large Carlin-type gold mineralization concentrated areas were established in the border areas of Yunnan, Guizhou and Guangxi provinces in southwest China and Shaanxi, Gansu and Sichuan provinces in Qinling Mountains, and a large number of gold deposits were discovered and obtained. It can be said that Carlin-type gold deposits are blooming everywhere in China. One of the important reasons why Carlin-type gold deposits have made such a major breakthrough is the breakthrough in analysis and testing technology, especially the rapid analysis technology of "trace gold", which makes similar technologies "stupid", and uses the "trace gold analysis box" to help geologists complete the field prospecting "reconnaissance" work.

At present, PGE is also facing a similar situation, that is, the technology suitable for the analysis and testing of PGE content in black rock series has not passed the laboratory test, let alone the rapid analysis in the field. In practical work, we have carried out comparative tests on samples of ore-bearing black rock series collected from Sichuan Sulphur Factory and Sandawan respectively. From the reproducibility of the standard sample and the comparison with the recommended value of the standard sample, we can see the quality of each test, but there is a certain gap with the results of the previous two tests (Table 3-5). This shows that the method of enriching platinum group elements by matte gold assay (Sun Yali et al., 1997) is not suitable for PGE in black rock series, but there is no other method to replace it at present.

Table 3-5 Comparison of twice analysis results of the same sample in Sandawan and Sulfur Plant

Through more than 30 years' research, Russian scientists have also clearly pointed out that the conventional method and the current universal fire assay method are not suitable for the analysis of PGE in black rock series, because PGE is likely to "lose" with the volatilization of element C during the heating process of the sample! Although Russian scientists have mastered the corresponding analytical techniques, they are strictly confidential and there is no disclosure of relevant detection techniques in publications or on the Internet. Although domestic researchers have repeatedly suggested that the relevant departments set up special analysis and testing research topics, they have not been approved, so that the field prospecting work has stagnated. Therefore, we suggest that China Geological Survey and other relevant departments start the analysis and testing of PGE in black rock series. This is not only a project problem, but also an important practical problem related to whether platinum group element deposits can be found in the black rock series widely distributed around the Yangtze platform in China and in the eastern Tianshan Mountains and western Tianshan Mountains in Xinjiang.

2. Lack of long-term unremitting research and geological prospecting.

Sun Daguang, the former Minister of Geology and Mineral Resources, emphasized at the 1977 National Geological Work Conference that platinum group metals are the mineral resources urgently needed by the country, and geological prospecting should be strengthened. Subsequently, in the late 1970s, a small upsurge of searching for PGE deposits was set off nationwide, which promoted the successful exploration of two PGE deposits in Yangliuping, Sichuan and Jinbaoshan, Yunnan. However, in the 1990s, the geological prospecting of PGE deposits basically stopped. From 265438 to the beginning of the 20th century, with the implementation of the geological survey project, the prospecting work of platinum group element deposits was carried out again, and the geological prospecting evaluation of Chongqing Chengkou black rock series platinum group element deposits was established, but the analysis and test ended without results.

Looking at the geological prospecting experience of platinum group element deposits at home and abroad, no deposit has found out its resources in a short time. Even in Stillwater, USA, although it took only one or two years to find ore under the guidance of metallogenic model, it took more than 10 years to find out its resource status, and it took more than 20 years from finding out the resource status to developing and utilizing it. Even the mined deposits will bring twists and turns to the development of mines because of comprehensive evaluation and other reasons. For example, the Ile Lake palladium mine in Canada started mining from 1993 and suffered losses year after year until 1998 was almost closed. Later, on the basis of re-summarizing the data of secondary development and metallogenic regularity, the resources were greatly expanded through re-evaluation, and the mining concept similar to "low-grade large-scale ore" of porphyry copper mine was selected to bring it back to life (Shi Junfa et al., 2005). Therefore, for platinum mines, it is unlikely that there will be a breakthrough in prospecting for short, flat and fast projects.

3. The research on mining, mineral processing and metallurgy is seriously lagging behind.

The mineralization of platinum group elements in black rock series has the characteristics of "low grade and large reserves", so mining, dressing and metallurgy have become the key to develop this kind of mineral resources. But at present, we have not reported the research results in this field, especially the adsorption of PGE by C material needs to be solved urgently. It is found in this study that PGE in carbonaceous rock series in Sandawan area may be adsorbed by carbon, which is similar to the case of "carbon adsorption of gold". How to find out the "adsorption" mechanism between PGE and "C" and desorb PGE from "C" is not only the genetic problem of the deposit, but also the technical problem of the deposit development. It is suggested that the combined determination of PGE and C should be carried out first, that is, the contents of PGE and C (including different types of "C") in the sample should be determined at the same time to find out the correlation between them, and then the adsorption and reverse adsorption between C and PGE should be studied to provide scientific basis for the recovery of PGE in black rock series.