1. Application conditions
When applying water system sediment geochemistry for mineral prospecting, it is necessary to consider the type of mineral deposit, census scale and natural landscape conditions. Aqueous sediment geochemical prospecting methods are widely used to find non-ferrous metals, rare metals, precious metals, ferrous metals and some non-metallic deposits, especially for Cu, Pb, Zn, Ni, Co, Mn, V, Cr, Sb, Hg , W, Sn, Mo, Be, Au, Ag, P, B, Nb, Ta and other minerals have remarkable effects.
Figure 4-31 Seasonal change chart of dispersion flow content in Songjiang Copper Mine
In terms of searching for the above minerals, geochemical prospecting of river sediments is mainly used in regional geological surveys and mineral resources census stage. It can be carried out simultaneously with the geological survey at the same scale during the 1:200,000 scale geological survey and the 1:50,000 scale mineral survey; it can also be carried out separately and in advance at the corresponding scale. Especially in the process of 1:50,000 scale mineral survey, reconnaissance and measured profiles are usually carried out at the same time as water system sediment geochemical prospecting; while the water system sediment anomaly inspection work is officially carried out after the 1:50,000 scale geological survey Carry out during work.
The natural landscape conditions for the application of water system sediment geochemical prospecting are mountainous areas with strong terrain cutting and developed water systems. The prospecting effect in such areas is relatively significant. Low mountainous and hilly areas (such as coastal areas) also have certain effects; before full application in alpine areas and desert edge areas, method testing must be carried out systematically.
In the geochemical prospecting of river sediments, studying the distribution of chemical elements in the area can not only achieve the purpose of prospecting, but also provide a geochemical basis for agriculture, animal husbandry, endemic disease prevention and environmental protection material. Therefore, water system sediment geochemical prospecting is actually regional geochemical exploration with prospecting as the main purpose.
2. Field work methods
The basic working links and working methods of aquatic sediment geochemical prospecting mainly include the following aspects.
(1) Sampling work layout
There are three ways to arrange the sampling work in the geochemical measurement of river sediments: ① Arrange according to the drainage basin, and arrange controlled sampling points in each major water system . This method is suitable for small-scale, large-area scanning work. ② Arrange sampling points at first-level intervals along the longitudinal direction of the water system in the survey area, focusing on first-level tributaries. This method is suitable for medium-scale census work. ③ Arrange sampling lines longitudinally along the water system and horizontally in conjunction with the river valley. It is suitable for abnormality inspection, tracing mineralization sources and delineating detailed investigation of prospect areas.
In addition, there is a highly efficient centroid sampling method that can be used based on systematic quantification of water systems. Sampling is a continuous bisection of the drainage basin area to achieve the purpose of narrowing the target area. This method requires timely determination of the indicator element content in samples in the field in order to successfully implement the sampling plan. This sampling method is an innovation of the traditional sampling method (grid method or approximately equally spaced method). For example: In a certain area of ??South Carolina, USA, only 5 to 11 samples need to be collected within an area of ??109km2 to delineate the mineralization source area related to gold mines.
Traditional geochemical measurements of river sediments include line spacing (the spacing between sampling water systems), point spacing (the spacing between samples distributed longitudinally along the water system) and sampling density (the number of sample points per square kilometer). basic parameters of the job. It is also subject to the measurement scale. The relationship between the sampling density and the corresponding scale of geochemical measurements of river sediments at home and abroad is listed in Table 4 10.
Table 4-10 Sampling density and corresponding scale table
The above data is a general experience summary. The sampling points can be appropriately diluted in the mainstream and longer tributaries; The tributaries (first-level water systems) or dry valleys should be controlled by sample points; sampling should be done above the entrance where the tributaries merge into the mainstream. In order not to miss the ore bodies near the watershed and to facilitate tracing anomalies, sampling should be as close as possible to the source of the water system.
At present, foreign water system sediment geochemical measurements tend to reduce density, reduce the number of analyzed samples, and increase element determination items to meet the needs of rapid scanning and multi-purpose. In 1977, my country's Regional Geochemical Exploration Work Conference (Huangshan Conference) recommended a measurement density of 1:200,000 of 1 to 2 units/km2. The lower limit of the density is used in areas with a warm and humid climate, year-round water flow, and developed river systems; the upper limit of the density is used in areas with a dry climate, intermittent water flow, and wadi development. In this way, the dispersed flow of large and medium-sized mineral deposits can be found; if the sensitivity and accuracy of the analytical technology are high, small and medium-sized mineral deposits will not be missed.
At present, another trend in sampling layout is to distribute the sample points as evenly as possible within each kilometer grid on the premise that they are not far away from the water system, in order to facilitate data processing.
During specific sampling, samples should be taken near the designed sample points based on the distribution of sediments in the river valley, and then the actual material map of the river sediment measurement should be filled in according to the actual sampling point locations. The allowable error in the last column of Table 4 10 refers to the error between the point on the actual material map and the actual sampling position (fixed point error).
(2) Sample collection and processing
Water system sediment geochemical measurement samples are mainly composed of silt or fine sand sediments.
Since the distribution of material components in river valley sediments is very uneven, special attention should be paid to observing changes in sediment composition when sampling, and attention should be paid to collecting silt and clay materials with strong adsorption capacity for metal elements. When the sediment is predominantly sandy, fine sand or silt material or a mud-sand mixture should be collected. Except for mineral deposits where the main purpose of measurement is to find mineral deposits with stable physical and chemical properties, sampling is generally done in the surface layer. If there is obvious possibility of contamination, take samples from the surface 10 to 15 cm. The sample locations are generally at the bottom of the river bed (wadi), near river bed depressions, floodplain depressions, and places along the river banks where river water often overflows. In rivers with fast flowing water, go to the place where the flow speed slows down. Samples are collected behind obstacles (turning stones, river bed bulges, gentle slopes) and inside the river channel transition where more fine-grained matter accumulates.
In order to ensure the representativeness of the sample, when the collection points are sparse, several samples can be collected near one sample point to form one sample. in certain favorable locations. In order to check for abnormalities after they are discovered and reduce the cost of traveling to and from intensive sampling, 1 to 2 more "preparatory samples" can be appropriately collected between the two designed sample points in order to supplement the measurement of this "preparatory sample" when abnormalities are found in the main sample. Prepare sample". Check in time and understand the abnormal content changes.
The original mass of the sample is generally 50-100g; for samples of precious metal elements, it should be increased to 100-200g. When sampling in the upstream wadi, special attention should be paid to distinguishing alluvial deposits from residual slope landslides (such as loess landslides on river bed slopes). Because their representativeness is different, samples should be correctly collected from alluvial deposits.
The requirements for sampling river sediments are relatively strict. Detailed records must be recorded when sampling, including the geology and landform characteristics near the sampling point, the geological location, terrain location and geographical location of the sample, the material composition of the sample, the material composition of the sediment, sampling depth, vegetation conditions, original sample quality, sample number, Sampler and sampling time. Cataloging should gradually move towards standardization and standardization, and record cards in a unified format should be produced to facilitate the use of computers for storage and retrieval.
Water system sediment samples are generally wet, so they should be dried, air-dried or sun-dried immediately after returning to the base. If the sample needs to be screened to separate different particle sizes, the sample should be kneaded during the drying process to prevent the clay material from clumping, or beaten with a wooden hammer after drying to disintegrate the clay particles without destroying the natural particles.
After the sample is dried, it is ground completely through a 150-mesh sieve and divided into half. Then use an agate mortar to grind it finely until the powder has no gritty feel but only a greasy feel when kneaded on your fingers. Send for spectral analysis or chemical analysis.
The sieve used for sieving should be nylon sieve, stainless steel sieve or aluminum sieve. Do not use copper sieves or tin sieves to avoid contamination. For precious metals with special ductility (such as Au, Pt, etc.), it is not suitable to pass through a fine sieve, and tests should be conducted to determine the processing technology solution.
Experience has proven that disc-type steel disc mills inevitably bring serious pollution, resulting in a significant increase in Fe, Mn, Ti, Ni, Cr, Mo and other elements, making them unsuitable for geochemical exploration. Therefore, in order to ensure the authenticity of the trace element content in the sample, attention must be paid to improving abrasive tools and using non-metallic hard abrasive tools to improve processing and analysis accuracy, avoid contamination, and eliminate errors.
3. Inspection, interpretation and evaluation of abnormalities
After discovering abnormalities in water system sediments, the existence of true abnormalities should first be verified before subsequent work can be carried out. Checking for abnormalities focuses on two aspects: ① Check whether there are false abnormalities caused by errors or contamination during sample processing and analysis and measurement. During the inspection, the original and duplicate samples of the abnormality can be processed and analyzed repeatedly according to strict technical requirements, and the results can finally be compared. ② Check whether there are false abnormalities caused by sampling errors or contamination. To this end, it is necessary to visit the site, conduct repeated sampling, and send the samples for processing and testing. If during field sampling, preliminary samples are collected between the basic sample points, then this batch of preliminary samples adjacent to the abnormality will first be sent for analysis to confirm the existence of the abnormality.
After repeated sampling, repeated processing and repeated testing (abnormal samples), when it is proved that the abnormality does exist, encrypted samples will be placed above the water system where the abnormal samples are located to further identify the abnormal strength and indicator element combinations. , combined with regional geological conditions to make a preliminary qualitative explanation of the anomaly, indicating the possible source of the abnormal material in the river sediments, whether it is some special lithological rock masses, strata, or mineralization and deposits of economic significance. If it belongs to the latter, then the mineralization prospect area will be determined according to the watershed ridge of the water system where the abnormal sample is located, and will be used as a detailed investigation area. To this end, it is necessary to identify the characteristics of the dispersed flow of the ore deposit, including identifying the ore-forming elements and their associated element combinations, content and spatial variability of the content, the shape and scale (length) of the dispersed flow, etc.
The anomaly detailed investigation uses a combination of double-line encrypted upstream samples and cross-valley profile sampling. It can be combined with soil geochemical measurements or heavy sand measurements in residual slope accumulations to find mineralization sources. Determine the position, length, and change characteristics of indicator element content of the dispersed flow in the ore deposit to distinguish the flow zone and head of the dispersed flow. The drainage basin of the water system where the dispersed flow head is located is delineated as the area where mineralization (or deposits) may exist, and corresponding methods are used for quantitative evaluation.