Mechanisms of generation and exudation of Tibetan medicine Shilajit (Zhaxun)

Shilajit, also named as Mumie, Zhaxun, is called in Tibetan medicine, meaning the‘juice of rock’ or ‘the essence of the rock’ [1, 2]. The water extract of Shilajit is mainly used for heat related syndrome in Tibetan medicine [3]. It occupies an important position in Tibetan prescribed preparations with a rank of sixth in the most frequently used medicine [4]. The commonly used well-known prescriptions containing Shilajit include Jiu Wei Shilajit Pills, Twenty-Five Wei Yu Ganzi Pills, Zhituo Jiebai Pills and Eighteen Wei Hezi Diuretic Pills. Besides, Shilajit is also widely used by many other ethnic groups in China as well as other traditional medical systems all over the world, for example, Indian Ayurvedic medicine [5].

In China, Shilajit is mainly distributed in Aba Tibetan and Qiang Autonomous Prefecture, Ganzi Tibetan Autonomous Prefecture, Liangshan Yi Autonomous Prefecture in Sichuan Province, Bomi County in Tibet and Qinghai Province [6]. Shilajit is also widely distributed in other parts of the world [1, 7], such as the southern foothills of the Himalayas [8] (from southern Tibet in the east to Kashmir in the west), the Pamir Plateau, the Altai Mountains, the Ural Mountains [9], and the Hindu Kush [10]. It has been reported in Bhutan, Egypt, Mongolia, Nepal, India, Norway, Pakistan [11], Russia, Afghanistan, Australia [7], Tajikistan [12] and some Commonwealth of Independent States. The chemical composition of Shilajit from different regions are similar, mainly including organic matter, humic acid, fulvic acid, volatile and fat-soluble components such as taxol, verbenol, α-pinene, cypress Brain [6]. Shilajit mainly can be found on steep cliff at an altitude of 2000to 4000 m [13] and is usually mixed with animal fences, leading it difficult to study it’s source, which remains unclear and controversial.

The existing hypotheses about the source of Shilajit can be divided into two types: hypothesis of rock source and that of biological source. Scholars of both Tibetan Medicine [14] and Ayurvedic medicine [15] supported hypothesis of rock source and believed that Shilajit was a melt of metal elements such as gold, silver, copper, iron. Indian researchers [13] suggested that Shilajit was originated from marine invertebrates. Russian scholar Scholz-Böttcher reported that ‘Mumie’ was derived from the fossils of higher plants [16]. The hypothesis of biological source believed that Shilajit was derived from the dry fecal coagulum of Trogoupterus xanthotis, Ochotana erythrotis, and the fecal and urine conjugate of the squirrel [17, 18], as well as the secretions of the plant Euphorbia royleana Boiss., Trifolium repens L. and some bryophytes [10]. However, none of the current theories can either clarify clearly the source of Shilajit or can be accepted by the traditional Tibetan medicine practitioners.

The present study came up with a new hypothesis of organic matter source based on the previous research findings and the evolution rule of organic matter. Previous study showed rich organic humic acids in Shilajit presented an outflow-like characteristic in the exudation points [19]. Meanwhile, according to evolution rules [20], organic matter will pass through various stages from humic acid to kerogen, oil, natural gas and residual carbon under high temperature and pressure. Therefore, this study suggested that Shilajit was derived from the organic matter that was exuded from rock layers as a result of geological activity.

Nevertheless, to the best of our knowledge, traditional regular methods of pharmacognosy research have been unable to study the source of Shilajit. So, this study took advantages of geological research methods. Hence, a series of investigations including geological environment of the exudation points, physical and chemical characteristics of the mother rocks, storage condition and exudation process of organic matter were conducted in this paper to study the exact origin of Tibetan medicine Shilajit.

The investigation of the exudation points showed that Shilajit was mainly distributed in Duke River, Dajinchuan River, Gesheza River, Xiaojinchuan River, Dawei River, Fubian River, Jiaomuzu River, Suomo River and Heishui River in Aba Prefecture, Sichuan province, and Dingqu River and Aqu River Basin in Ganzi Prefecture. The geographical location of the Shilajit exudation points and the measurement project information of the background rocks were presented in Table 1. The location of the survey point and long-term observation point were shown in Fig. 1.

According to the dynamic characteristics of Shilajit exudation area, the paleogeographic environment of sedimentary tectonic structure was Triassic tectonic paleogeographic pattern, which belonged to the residual ocean basin (OB)-spreading ridge (Sr) environment in ocean basin. The sedimentary environment of Triassic was characterized by gradual evolution of shelf slopes and shelf ridges, semi-deep sea slope valleys and slope fans (skirts) in terrigenous clastic shallow sea. At the end of the Triassic, due to collisional orogeny on north side, the ocean basin was closed. The paleogeographic features in the area were transformed into intracontinental environment, which was transformed into Late Triassic foreland basin and developed a thick turbidite system. The sedimentary environment of deposits was alluvial fan-river facies, and the latter was dominated by reticulated rivers. The Yanshanian medium-acid magmatic intrusive activity was strong, and the late Yanshanian intrusive rock was mainly distributed in the stress concentration area or regional fault activity zone [33].

The stratigraphic sequence of Shilajit exudation area was incomplete. While the Triassic strata were mainly distributed in large areas in sedimentary basins, and the Triassic rock combination was dominated by thick semi-deep sea turbidites and contourite (sand slate). In the early and late periods, some distant Yuanbin mudstone, siltstone and sandstone were distributed. After entering the Cenozoic, when marine environment was over, a small fault basin accumulation was formed, representing by river glutenite, siltstone and mudstone combination, and river–lake-phase coal-bearing clastic rock combination.

According to the geological and historical background, combined with the analysis of the research results, the formation mechanism of Shilajit was somewhat complicated. There were several possibilities, which need further confirmation by geochemical research.

In this study, it was found that Shilajit mainly distributed on sunny steep slopes and cliffs with a slope of 60° or above at altitude of 2000–4000 m. The control structure surface of the exudation points included rock layer, joint, fracture surface, fault control, and it developed into micro-geomorphology such as concave cavity, wind erosion hole and steep cliff.

The lithology character of the Shilajit exudation area were mainly various metamorphic rocks of sedimentary rocks and a small amount of granite in the Yanshanian period that were rich in organic carbon. Some of rocks developed into intergranular pores, dissolution pores, cracks and joints for storing and transporting organic matter. The organic matter in Shilajit was found to flow out naturally from rocks along pore, structural plane and even accumulate on the surface of rock as a result of storage environment (temperature, stress, structural surface) change caused by rock tectonic action. Further geochemical research is required to confirm the source of organic matter in rocks.