川西高原砂金矿床形成规律

砂金富集规律可归纳为五条:(1)砂金比重大,体积小,一般下沉于砂砾层底部基岩之上,形成富矿带。(2)河流流速减缓处是砂金富集之场所,如河流内湾处;两河会流处;河谷由宽突变窄处;河谷由窄突变宽处。(3)河床起伏不平是砂金富集之主要条件。最理想的河床是软硬相间之岩层,如砂页岩互层,且走向横穿河谷,倾斜较陡,这样,便形成具有无数隔梁与隔槽的起伏不平的河床,砂金受阻,易于停积。(4)位于谷旁之古老河床沉积阶地是无水患的易采砂金矿床,不容忽视。(5)红黄杂色铁砂及石英碎屑常为砂金富集标志。

THE REGULARITIES OF FORMATION OF THE PLACER GOLD DEPOSITS IN WEST SICHUAN PLATEAU

The West Sichuan Plateau belongs to the eastern marginal part of the Qinghai-Xizang Plateau. From east to west there are 3 large rivers, Dadu, Yalong and Jinsha rivers running through from north to south. The topography of the plateau is sufferec to the deep erosion by these rivers and their tributaries.The stratigraphy in this region chiefly represents the flysch type deposits of sandstone and shale formations of the Triassic age. It was strongly folded and abundant granites have emplaced of the Indosinian-Yanshanian Cycle outcroped along the flanks of on the anti-clinoria. The gold bearing quartz veins which are related genetically with the grauites are widely distributed every where. They are usually in the short lenticular forms scattered here and there controlled by the abundant irregular fissures in the rocks. After the weasthering and erosion of the veins the materials as the residual and slope deposits were formed and then became the fluvial placer gold deposits with economic values through the transportation by the brooks and rivers.Five regularities of formation of the placer gold deposits may be summarized as the following which may be useful for the exploration and mining of the placer gold in any places.1. The characteristic of the gold bearing gravel bed. The gold is large in specific gravity and small in size. It usually sinks down to the bottom of the gravel bed above the bedrock forming the rich ore zones or pay streaks which are the target beds for the exploration of the sand gold with a thickness of 0.5-5m. Some muddy layers of lenticular form are usually included in the middle parts of the gravel bed acting as a local pseudo-bedrock and some local ore deposits formed upon it.2. The regions where the rivers are reduced in flow velocity are the ones of rich accumulation of the placer gold. It includes 4 kinds of regions: 1) the inner bank of the sinuous river, 2) the confluence of two rivers, 3) the section of the river where it is suddenly narrowed. 4) the section of the river where it is suddenly widened in course.3. The unevenness of the river bed is the chief factor for rich accumulation of the placer gold. River bed is the natural chute. If it is smooth the gold be bearing gravel is very easily washed away along its course and cannot be deposited. But if it is uneven like the artificial chute with many alternate troughs and tidges the gold bearing gravel would be deposited in the troughs. The most favorable river beds are those which are composed of the alternate soft and bard rocks like the alternating beds of sandstone and shale and its strike across the river course and dipping steeply (dipping down the river is much better than that of river-se). In this way there would be numberless ridges and troughs formed which are favorable for the deposition of the placer gold.4. Old gravel beds are the favorable and easily working placer gold deposits. In the regions of plateau the rivers are strong for the down erosion due to the periodically uplifting the old grave] beds are usually deep cutted to form the high and low residual gravel terraces by the later river erosion. The higher the gravel terraces the older they are but the same terrace world be low in the upper course and high in the lower part due to different volume of water and different intensity of erosion. In general we may see 3-4 order terraces along the river bank: 5-30m, 20-60m, 40-100m and 100-200m. The bedrock of the gravel terrace is generally exposed over the river bank and make the water soaked in the gravel bed flow out along the surface of the bedrock to form a dry gravel bed which is favorable for the mining along the berock and free from the influence of the underground water.5. variegated iron sands and quartz detritus are usually the indicators of rich accumulation of placer gold. Gold grains are usually accomponied with the pyrites in the quartz veins, the pyrite is easily weathered into the Jimonite and hematite leaving the quartz veins with much depresions like the honeycombs. It stained the white quartz in a yellow brown a