the methods of prospecting for placer gold

There are many methods for prospecting for placer gold, and there are five commonly used methods: natural heavy sand method; engineering heavy sand method; old mining investigation; geological and geomorphological analysis; geophysical and aviation new technology methods.

Among them, the first three methods are to find out whether there is placer gold through sampling and investigation, and directly determine whether it is mineralized, which is a direct prospecting method; It is an indirect prospecting method based on certain characteristics of the material to infer whether it may be mineralized. Among them, the geological and geomorphological survey is the basis for the prospecting and analysis of placer gold.

Usually, in determining where to find placer gold deposits and where to arrange sampling projects, the basis is mainly provided by geological and geomorphological analysis.

1. Natural heavy sand method

The natural heavy sand method is based on the characteristics of the high density of placer gold particles, which can be directly selected by panning discs, dig pits in the surface layer or not deep of loose debris deposits, and directly determine whether there is placer gold by panning in the field. a way.

Sampling includes river heavy sand sampling from river system sediments, outcrop sampling from terrace sand and gravel deposits, and heavy sand sampling from residual slope deposits on hillsides. The first two kinds of sampling can be used to understand the gold content of the water system sediments, the approximate distribution range of placer gold, and the grade and thickness of the terrace gold-bearing layer. Sampling from the accretion layer of the residual slope of the hillside is in the Xiaogou mountainous area where gold placer is known to be used to trace the source of placer gold. By digging shallow pits at certain intervals on the hillside and the foot of the slope, sampling and panning are carried out. Distribution range, narrowing the rock gold prospecting target area. Among these three kinds of sampling, the most widely used is the natural heavy sand method of rivers.

The natural heavy sand sampling of rivers is generally carried out from bottom to top along the upper reaches of the river system or along the medium and small branch valleys containing gold. Its advantages are: simple tools (only one shovel and one pan for panning), small sampling workload (dug shallow pits 0.3-0.5m deep, sample weight 20-40kg), simple and easy to operate, and can quickly and directly obtain near-surface gold duster information at . The disadvantage is: because the samples are taken in the shallow and near-surface, it cannot reflect the gold-bearing situation of the deep sand-gravel layer, and placer gold is usually mainly enriched in the lower part of the sand-gravel layer near the bedrock, so the measurement results of the river heavy sand near the surface are in the search for Mine generally only has qualitative significance. The effect of natural heavy sand sampling depends on the selection of sampling points and horizons. In the plane range, the sampling points should be distributed in the place conducive to the enrichment of placer gold.

For example, where the river suddenly widens, where the river turns convex bank, the sandy and gravel deposition area in the shoal of the river bed, near the intersection of the main and tributaries, the rocky ridge and rocky beach in the riverbed, above the rock, at the side beach or the center beach, the water flow In front of the medium and large obstacles, where the slope of the river bed changes from steep to gentle, "Guanmen Mountain", above the river valley or in the accumulation area in front of "Yingmen Mountain", etc. In the vertical section direction, the best position is the bottom of the gravel layer close to the bottom rock. In the glutenite area, it should be taken from the riverbed sediments below the tributary valleys where the glutenite layer is cut. In the mountainous area with multi-level ditch networks, the sampling should be preferentially taken in the branch valleys. When taking terrace sediment outcrops, the samples should be taken as far as possible. Take it at the bottom of the gravel layer or near the bedrock surface.

2. Engineering heavy sand method

The engineering heavy sand method is an effective method to penetrate the loose sedimentary layer and systematically sample by sand drilling or exploratory well engineering to understand the gold-bearing situation of the loose sedimentary and directly determine the grade of the gold-bearing layer. 

Since placer gold and industrial placer gold layers mainly occur at the bottom of loose sedimentary layers, the engineering heavy sand method can find out the deep placer gold enrichment and provide direct prospecting information. The basic point of adopting this method is that the arrangement of sampling engineering points must have sufficient basis and construction feasibility, and secondly, regardless of the main sampling process, the gold-bearing layer must be penetrated and the depth of at least 0.2m below the bedrock surface should be controlled.

The use of sampling engineering for placer gold prospecting must be based on the analysis of geological and geomorphological conditions, and according to the prospecting signs and clues, select favorable parts within the favorable metallogenic zone, and arrange the project according to a certain engineering network. Set and metallogenic laws are determined.

3. Investigation of old gold placer mining traces and civilian mining

In many placer gold areas upstream of rivers or in tributary valleys, there are common old hand-mining traces of placer gold, which are effective signs of placer gold prospecting. According to the old mining traces, further development of peripheral prospecting can often achieve good results. Larger-scale artisanal old mining area tailings piles are often ore bodies of industrial value. In addition, through the investigation of private mining, a lot of valuable information can be obtained about the geological characteristics, regularity and prospecting clues of placer gold mineralization in this area. Therefore, the investigation of private mining of alluvial gold has important prospecting significance.

4. Geological and landform survey

Geological and geomorphological surveys are the basic method for gold placer prospecting, and are mainly used for the analysis of placer gold metallogenic conditions and the prediction of favorable areas for mineralization.

In the prospecting stage, the river valley route survey is mainly carried out. Among them, the natural outcrop method and the river debris observation method can be used in the geological survey, and the known gold-producing ditch rocks in the area can be used for comparison and analogy. At the same time, some natural heavy sand samples can be taken to understand the gold content. Indirectly or directly determine whether there is placer gold supply and the degree of richness or poverty of the supply. During the investigation, attention should be paid to understand the tectonic background of the valley and the geological phenomena related to gold mineralization.

Geomorphic observation mainly divides various geomorphic units of river valley types and determines their distribution, understands their scale, genesis, sediment characteristics and gold-bearing properties, etc., and outlines the geomorphological Quaternary period on a 1:50000 or 1:25000 scale topographic map Geological sketches, draw the boundary lines of main geomorphic units, and provide reference for arranging sampling works and delineating ore bodies later.