Secondary Geochemical Characteristics And Corresponding Geochemical Survey Methods For Swamp-Forest Area In Northeast China

For more than 20 years, studies on methods of geochemical reconnaissance surveys in the swamp-forest areas on the Northeast China have been carried out for several rounds and with different methods and technologies suggested at different phases. However, interference due to organic matters, which is widely exists in swamp-forest areas, has not been completely removed for the suggested methods. Two problems were thus resulted for the geochemical reconnaissance surveys: (1) the regional geochemical variations were not so significant and thus difficult to be traced; (2) the anomalies were weakened and the regularities of geochemical distributions related to the anomalies could not be clearly picked up. Consequently, effects of ore-finding with the geochemical data so obtained were not as good as expected. One of the reasons for this is that the mechanics of element dispersion and migration under secondary conditions in swamp-forest landscape has not been well understood, and therefore the methods have not been suggested under firm theoretical basis. As we know, secondary geochemical actions are very strong in swamp-forest areas. Organic matters play an important role in the actions, and to a great extent they exert an influence, directly or indirectly, on occurrence modes and migration capability of elements. A key point for developing effective geochemical methods in swamp-forest areas is to further understand secondary geochemical mechanics, especially the mechanics of actions due to organic matters, in swamp-forest areas. Aimed at the problems mentioned above, the characteristics of geochemical distributions of elements in various secondary media in swamp-forest areas have been studies in this paper. Also studied in the paper have been the distribution and composition of organic matters in various secondary media and their roles in the process of element dispersion and enrichment, as well as the mechanics of interference from the organic matters. Based on the understanding of features and degrees of the interference from the organic matters, the ways for removing the interference have been suggested. The results of the study show that distributions of elements in a coarsely grained portion of various secondary media ( > 40 mesh for soils, > 60 mesh for stream sediments ) are closely associated with and their own chemical properties and corresponding geological settings, little influence from organic matters could be observed. Whereas in a fine-grained portion, no matter for soils or stream sediments ( < 40 mesh for soils, < 60 mesh for stream sediments ), the contents of organic matters obviously increases and their influences on secondary geochemical distribution pattern of elements become much stronger. Many elements could be occluded directly by the organic matters on one hand, resulting in secondary enrichment of the elements. On the other hand, the main compositions of the organic matters are humic acid, fulvic acid and humin. And the content of organic matters in fine-grained stream sediments is 29 times higher than that in coarse-grained stream sediments, the content of organic matters in layer A of soils is 8 times higher than that in layers B and C of soils. Great changes in physical-chemical condition of the secondary media are thus resulted with such high contents of organic matters, such as decrement of pH value, increment of conductivity, enhancement of activity and migrating capability of elements. The geochemical distribution pattern of elements in secondary environment is finally changed. It shows from our study that almost all of elements in a fine-grained portion in peat are obviously enriched, though the enrichment degree varies from element to element. The elements in grain-size of –80 mesh are mainly occurred in modes of amorphous femic oxides and organic matters, while little in water-soluble mode. The increment for proportion of organic matters is mainly due to the increment of organic carbon. The occlusion by organic matters in anomalous zones is higher that in background areas; it varies for the other two modes ( femic oxides and water-soluble modes ) depending on elements. Although the contrast of anomaly is lowered because of the significant enrichment of elements by organic matters in anomalous zones, enriching centers are still there, therefore it is feasible to track regional geochemical anomalies by taking peat as sampling medium. Based on the results above, the method for geochemical surveys on a scale of 1:200,000 in swamp-forest areas has been suggested. Summarily, the grain size for sampling is –10 ~ + 60 mesh for getting rid of interference from organic matters. The sampling density is one sampling spot every 4 km2, and making the spots mainly spread on 2nd ~ 3rd stream systems and valley mouths of long streams. It is thus possible to sample sandy and granular materials and avoid segments with peat accumulated. While for geochemical surveys more detail than regional surveys, stream sediments and peat could be sampled with grain size selected according to individual circumstance and sampling density adopted based on corresponding regulations. In summary, the geochemical characteristics, especially the mechanics of influence of organic matters on secondary geochemical distribution of elements, for swamp-forest areas have been systematically studies in this paper. And the method for geochemical surveys on a scale of 200,000 has been suggested in swamp-forest areas based on the results of the study. At the same time, the trials for tracking regional geochemical anomalies by taking peat as sampling medium have been carried out with rather good results obtained. The results of the study are not only helpful for practical geochemical surveys, but also prepare a firm ground for further theoretical study in such a kind of landscape.