短语词组短语gold placer -mining 采金场placer gold mining 砂金矿开采placer r mining 砂矿淘采beach placer mining industry 海滨砂矿开采业beach placer mining 海滨砂矿开采method of littoral placer mining 滨海砂矿开采法hydraulic mining of placer 砂矿露天水力开采 返回 placer mining