摘要
玄武岩浆携带深源包体、蓝宝石和其他重砂“巨晶”矿物高速从火山口喷出,离开火山口后由于压力突然降低、流动速度急剧减小,以致于在火山日附近沉淀许多深源包体和重矿物;而离开火山口愈远,重砂矿物愈轻、颗粒愈小。依此规律,确认海南蓬莱岭为一火山口。玄武岩区同一层的人工重砂矿物特别是比重较大的磁铁矿“巨晶”大量富集于火山口附近,而蓝宝石和锆石却常离火山口1～3。5km,宝石分布区常处于几个火山口之间。对主要宝石矿区牛姆岭的残坡积层,沿勘探网(100m×100m)取样,不同点重砂矿物与不同层位玄武岩中人工重砂及岩石对比证明,牛姆岭砂矿以残积为主,坡积为辅。
The basaltic magma carrying deep-seated source inclusions, sapphireand other placer 'megacryst' minerals extruded from crater at a highspeed. As a result of erupt drop of pressure and strong decrease in flowvelocity depart from the crater a lot of deep-seated spurce inclusions andheavy minerals were precipitated. With an increase in distances departfrom the crater placer minerals have become lighter and smaller.Accordingto above-mentioned regularities, it has proved that the Penglai ridge is acrater. Artificial placer minerals in the same layer, especially magnetite'megacrysts', were largely concentrated near the crater, whereassapphire and zircon were concentrated at a distance 1 to 3.5 km from thecrater. In general, the gem area is located among several craters.Through collecting samples in clay-gravel layer within the main gemlocality____Niumu ridge along the prospecting net (100m×100m) and bycomparison between the placer minerals in different placer samples andthe artificial heavy minerals and rocks of different basalts, it is shownthat the Niumu ridge placer deposit is mainly of the eluvium type andsecondarily of the slope wash type.
