碎斑熔岩在黑矿型矿床成矿作用中的地位

白银矿田火山碎屑岩及赋矿岩石的深入研究表明，该地区存在一种长英质侵出相火山岩——碎斑熔岩。其主要特征是：①产于矿田酸性火山岩穹的火山活动中心或古火山喷口位置，代表中酸性火山作用晚期岩浆活动的产物，其典型者如折腰山石英角斑质碎斑熔岩体，规模为2×0．5km2，有次火山相石英钠长斑岩和各种火山碎屑岩如晶屑凝灰岩、集块岩及熔岩等分布在其四周；②碎斑熔岩中碎斑为长石和石英，具破而不碎、碎而不离或离而不远的特征，③基质具霏细结构，但多数已发生绢云母化、绿泥石化和黄铁矿化等；④岩石化学上同该地区喷出相石英角斑岩相当，SiO2均大于70ωB％，具有较高的Al2O3含量和Na2O／K2O值，分别为11．16～14．90和1．78～7．43，具I型与S型的混合型特征，⑤稀土元素配分型式具平坦型和富集型两种，分别与白银矿田东西两段酸性火山岩成因类型相对应，微量元素地球化学研究表明碎斑熔岩与区内石英角斑岩属同源岩浆的衍生物．海相火山岩中碎斑熔岩的确立，不仅增加了海相火山作用形式，即便出作用，而且因其占据火山活动中心或岩浆通道位置，在成岩或火山作用期后便成为海底热液对流循环成矿作用中心即热液喷流口．它既标志着黑矿型块状硫化物矿床成矿的相对时期，是在火山作用晚期侵出相碎斑熔

MARINE CLASTOPORPHYRITIC LAVA: SIGNIFICANCE TO METALLOGENESIS OF KUROKO TYPE DEPOSIT

Following the discovery of hot spring metallogenism on ocean floor, revolutionary advances have been made in researches on Kuroko type massive sulfide deposit. This has brought about progress in researches on the host volcanic rocks. In the 70s, there were arguments between quartz porphyry and acid tuff or tuff containing crystal pieces and between rhyolite and alaskite (K. H. Wolf, 1978). In the 80s, Japanese petrologists used the terms such as white rhyolite or leucorhyolite dome to discribe the subvolcanic rocks which are formed during late volcanism or before mineralization. Carefull investigation on volcaniclastic rocks of Baiyin orefield suggests that there exists such extrusive body termed clastoporphyritic lava, which is of great significance to the positioning and controlling of metallogenesis.The main characteristics of clastoporphyritic lava of Baiyin orefield are as in the followings: 1) They occur at the volcanic centres or ancient craters such as Zheyaoshan and Ganggou, representing the late acid magmatism. 2) Quartz and albite are the main clastoporphyritic crystals of the lava which are broken but not into pieces and/or not departed and/or not far from each other. 3) The groundmass is felsophyric, of which most parts have been altered by convective circulating hydrothermal mineralization, consists mainly of albite,sericite and chlorite. 4) clastoporphyritic lava and quartz keratophyres are chemically identical and consistent of mixings of the I -and the S-type clastoporphyritic lava with SiO:>70 WB %, higher Al2O3 (11.16%-14. 90 WB% ) and Na2O/K2O ratio (1. 78-7. 43) (Wang Den,1993). 5) There are two types of REE pattern, flat and LREE enriched, corresponding respectively to the petrological genetic types of acid volcanic rocks from the eastern and western parts of Baiyin orefield.The confirmation of clastoporphyritic lava in marine volcanic rocks first gives a new type of marine volcanism, that is, an extrusive phase with underground explosion. Second, as occurring at volcanic centre and/or conduit, the lava become the centre of convetive circulating hydrothermal mineralization. This suggests that metallogenesis of massive sulfide deposit started at the late stage of volcanism, after the extrusion of the lava. Besides, the clastoporphyritic lava can be an important ore--containing rock because of its porosity (Tao Kueiyuan,et al. 1985). Comprehensive researches on metallogeny and petrology of Baiyin orefield indicate that the mainbody of Zheyaoshan Cu-Zn type deposit positioned in clastoporphyritic lava under the water--rock boundary. Part of the deposit and hydrothermal sedimentary rocks such as ferruginous and ferrimanganese silicic rocks occurred in volcani elastic robs. They are all the products of convetive circulating hydrothermal mineralization and the extention and continuation of endogenous process under epigenetic condations.