Mineral exploration mark

In the process of geological exploration and prospecting, all phenomena and clues that can directly or indirectly indicate the existence or possible existence of minerals are called prospecting indicators. Prospecting indicators are often more widely distributed than ore bodies and are easy to be found. By studying them, we can find ore deposits quickly and effectively. There are many kinds of prospecting indicators, and the common prospecting indicators are:

1. Orebody outcrop

The outcrop of ore body is the part of ore body exposed to the surface. According to the degree of oxidation, it can be divided into primary ore body outcrop and oxidized ore body outcrop. For prospecting, the discovery of ore outcrops does not mean the discovery of deposits with industrial value, and a series of work must be done to determine the practical significance of newly discovered ore outcrops.

(1) outcrop of primary ore body: it refers to the outcrop of ore body exposed on the surface, but not weathered or slightly weathered. The material composition and structure of its ore basically remain in the original state. Generally speaking, ore bodies with stable physical and chemical properties, hard ores and gangue are easy to preserve primary outcrops on the surface. Such as timely veins containing tungsten, Shi Ying veins containing gold and bauxite.

(2) Oxidized orebody outcrop: It refers to the orebody outcrop exposed on the surface, and the mineral composition and structure of the orebody are damaged and changed to varying degrees due to weathering. Most of these outcrops are ore bodies with unstable physical and chemical properties. For example, the ore bodies of various metal sulfides are weathered to form colored oxidized outcrops. From the point of view of geological prospecting, iron hat and weathering crust are more important in the oxidized outcrop of ore bodies. They are not only prospecting indicators of some deposits, but also have industrial value in themselves.

2. Iron hat

Some metal sulfide ore bodies are exposed on the surface, and after weathering, most metal sulfides become soluble sulfates, carbonates and so on. And insoluble limonite, etc. After the change, it covers the upper part of the oxidation zone of the ore body and forms a porous aggregate, which is called the iron cap. Iron cap itself can form a deposit, and it is also an important symbol for finding metal sulfide deposits. The following aspects should be paid attention to when studying iron caps:

(1) Iron cap color: Iron caps with different colors can indicate concealed primary deposits below them, such as brick red indicating pyrite, dark brown and yellowish brown indicating chalcopyrite, ochre orange to chestnut indicating bornite, dark chestnut indicating chalcocite, ochre orange indicating galena, light brown indicating sphalerite, and yellowish brown-chestnut indicating chalcocite.

(2) Structural characteristics of iron cap: Generally speaking, vein-like, massive limonite and limonite-like and slag-like secondary quartzite in silicified fracture zone around rock mass can be used as prospecting indicators for gold deposits; Vein and massive limonite in the silicified fracture zone inside the rock mass can be used as prospecting criteria for copper deposits.

3. Weathered crust

Weathering can lead to the loss of chemically active elements in primary deposits or rocks, while some useful elements with relatively inactive chemical properties are reserved for mineralization, such as iron, aluminum, manganese, nickel, cobalt, kaolin and some rare earth elements. Weathered crust is not only the oxidized outcrop of these minerals, but also their direct prospecting indicator. Weathering crust related to ultrabasic rocks and basic rocks is mainly related to nickel ore and bauxite. Such as weathered residual nickel silicate deposits in Yunnan and other places, and residual bauxite deposits formed by basalt in southern Fujian. Weathering crust related to acid rocks is mainly related to kaolin deposits and rare earth deposits, such as Jiangxi residual kaolin deposits and ion adsorption heavy rare earth deposits. The weathering crust related to carbonate rocks is mainly related to iron ore and manganese ore, such as western-style iron ore, limonite weathering crust above siderite layer in Yunnan-Guizhou area and weathered residual manganese ore deposits in Guangxi, Hunan and Guizhou.

(2) Indirect prospecting criteria

1. Wall rock alteration

In the process of mineralization, the surrounding rocks are also affected by magmatic hydrothermal solution, resulting in alteration. Altered rocks are more widely distributed than ore bodies and are easy to be found, which can indirectly indicate the existence of mines. Different alteration types can indicate different mineralization. In general, strong and widely developed wall rock alteration can indicate the existence of large-scale ore or rich ore. Table 4-3- 1 shows the common types of wall rock alteration.

Table 4-3- 1 Common wall rock alteration types and related minerals

2. The color of surrounding rock near the mine

Due to hydrothermal alteration or supergene action, rocks around ore bodies often show some special colors, such as ochre, green and white. , also used as a prospecting indicator. For example, when looking for porphyry copper deposits in Hebei, "burning skin" is used as a prospecting sign.

3. Indicator minerals

In the process of deposit formation, some typical minerals are often associated, which can also be used as a sign to find some minerals. For example, in skarn deposits formed by contact metasomatism, the most common typical minerals are garnet, pyroxene, epidote and actinolite. In diamond deposits, chromium-containing magnesium aluminum garnet and chromium-containing spinel are the main indicator minerals.

4. Geophysical and geochemical anomalies

The difference of physical properties between ore bodies and surrounding rocks will produce various geophysical anomalies (geophysical anomalies for short), such as magnetic anomalies, IP anomalies, gravity anomalies and so on. During the formation or weathering of ore deposits, the migration of ore-forming elements and associated elements changed the normal distribution of elements in surrounding rocks, soil, river sediments, water, atmosphere and organisms (plants) near ore bodies, and increased their contents. This phenomenon of increasing elements is called geochemical anomaly (geochemical anomaly for short). When there are hidden underground ore bodies in an area, the physical properties of ore bodies and surrounding rocks can be obviously different, and some elements are highly enriched, which becomes one of the important means of indirect prospecting.

5. Old mine remains

China has developed mining and smelting industry since ancient times, and old cave dwellings, waste rock piles, smelting slag and other old mine relics abound. They are not only reliable indicators of mineral distribution, but also good prospecting indicators. Due to the backward mining technology in ancient times, it is impossible to continue mining or lack the ability to identify mineral assemblages. If we re-evaluate with modern technical and economic conditions, we will sometimes find deposits of great industrial value.

6. Signs of special place names

Special place names mean that some place names were named by ancient miners according to the properties, names, colors, uses and shapes of local minerals. This is of indirect significance for us to choose the mining area (section). Such as Xitieshan (lead-zinc mine) in Qinghai, Dexing Copper Factory (copper mine) in Jiangxi and Xikuangshan (antimony mine) in Hunan.

7. Indicating plants

The distribution of plant species or the ecological changes of plant shape, size, color and growth rate can be used as a sign of prospecting. For example, in the copper area in the middle and lower reaches of the Yangtze River in China, Elsholtzia Haizhou, an indicator plant of copper, is commonly known as copper grass.

The various prospecting indicators listed are often not isolated. In prospecting, it is necessary to make a comprehensive study of all kinds of signs found in order to achieve better results.

(3) Metallogenic prediction

1. Concept and significance of metallogenic prognosis

Under the guidance of metallogenic geological theory, metallogenic prognosis is to summarize the metallogenic regularity (or metallogenic model) and prospecting methods, predict the potential mineral resources in the predicted area, delineate the metallogenic prospect section, and put forward suggestions for further prospecting.

Metallogenic prediction is not only the theoretical basis of mineral exploration, but also an important technical means. The significance of metallogenic prognosis is that it is an important way to realize scientific prospecting. Its scientific manifestations are as follows: (1) Metallogenic prediction must be based on in-depth understanding of known deposits; It must be based on in-depth study and summary of regional metallogenic laws; We must make full use of geological, geophysical, geochemical and remote sensing data to make them in the best combination state. The prospecting effect and economic benefit are the key to measure the success or failure of metallogenic prognosis.

2. Metallogenic prediction method

There are many methods for mineral prediction, which can be roughly divided into the following four categories:

(1) Empirical mode prediction: Empirical mode prediction method is based on analogy theory and is generally realized by modal analogy. This method is based on the deposit description (existence) model and is implemented through the practical experience of geologists.

(2) Theoretical model prediction: The theoretical model prediction method is based on the genetic (conceptual) model of the deposit and applies modern geological theory to predict. At present, because geological theory is still in the development stage, there are still some difficulties in forecasting practice, and this method is still being explored. Once there is a big breakthrough in geological theory, there will be a big breakthrough in theoretical prediction.

(3) Statistical analysis and prediction: The statistical analysis and prediction method is based on the basic statistical model established by mathematical statistics, and then extrapolated and predicted. Usually, in the exploration area, a statistical model is established with known ore deposits as standard statistical samples. Extrapolating and forecasting in unknown areas.

(4) Comprehensive method prediction: Comprehensive method prediction, also known as comprehensive information metallogenic prediction method, is a method to establish comprehensive information prospecting model and carry out metallogenic prediction according to the metallogenic information obtained from the theory of forecasting prospecting model and prospecting technology.