Petrology and mineralisation of the southern Platreef: northern limb of the Bushveld Complex, South Africa
Petrology and mineralisation of the southern Platreef: northern limb of the Bushveld Complex, South Africa
复制标题
Platreef 南部的岩石学和矿化：南非 Bushveld 杂岩体的北缘
DOI：
--
复制
发表时间：
2005
期刊：
影响因子：
--
通讯作者：
Renee de Lange
中科院分区：
文献类型：
--
作者：
J. Kinnaird;D. Hutchinson;L. Schurmann;P. Nex;Renee de Lange
Platreef是Merensky Reef的假定的局部类似物，形成了Bushveld杂岩北方翼镁铁质序列的底板。我们将Platreef定义为“岩性可变单元，以辉石岩为主，辉石岩不规则矿化，含铂族元素、铜和镍，位于德兰士瓦变质沉积岩下盘或基性岩基底与上覆主区辉长岩之间”。我们将边缘远北主带内钙硅酸盐捕虏体周围的矿化定义为“普拉特里夫式”矿化。Platreef（ss）的走向长度为130公里，而Platreef式矿化的走向长度为110公里。Platreef的厚度从南部的400 m到北部的<50 m不等。总体走向为NW或N，地表倾角为40-45°W，向下倾角变浅，普拉特礁南部的总体几何形状似乎受不规则的海底地形控制。南部Platreef的最大厚度发生在农场Macalacaskop和Turfalit的两个子盆地。在岩性上，南部Platreef是异质的，比更北部的部门更多变，虽然主要是辉石岩，但包括纯橄榄岩，橄榄岩和苏长岩周期，斜长岩在中部到上部。在整个包装中可能会出现强烈的蛇纹化区域。断层使Platreef的走向发生偏移：主要是一个南北向的陡倾组，其次是ENE和ESE组，倾角为50-70°S。断层结构是前布什费尔德，也局部控制增厚和变薄的继承。长<1500米的围岩捕虏体很常见。在Macalacaskop，这些通常是石英岩和角岩状带状铁矿石、页岩、泥岩和粉砂岩，而在Turfspruit，也有钙长石或钙硅酸盐捕虏体。硫化物在某些交叉点可能达到>30模态%。这些矿物以磁黄铁矿为主，含少量镍黄铁矿和黄铜矿，少量黄铁矿和微量的多种硫化物。在南部，矿化带的铜品位为0.1-0.25%，镍品位为0.15- 0.36%。块状硫化物通常（但不限于）局限于与下盘变质沉积岩的接触处。岩浆硫化物呈浸染状或网状结构，由几微米至2厘米大小的磁黄铁矿和镍黄铁矿颗粒组成，含黄铜矿和少量黄铁矿。大部分硫化物与粒间斜长石或石英长石合晶岩有关，沿着圆形积云正辉石的边缘。在南部地区的铂族元素存在的碲化物，铋化物，砷化物，锑化物，铋锑化物和复杂的铋碲化物。铂族元素很少包含在硫化物中，但在填隙硫化物周围和蛇纹化带的蚀变组合中以微米级卫星颗粒的形式出现。Pt：Pd比值≥ 1和PGE品位可能与S和贱金属丰度无关。
The Platreef, the putative local analogue of the Merensky Reef, forms the floor to the mafic succession in the northern limb of the Bushveld Complex. We define the Platreef as ‘the lithologically variable unit, dominated by pyroxenite, which is irregularly mineralised with PGE, Cu and Ni, between the Transvaal metasedimentary footwall or Archaean basement and the overlying Main Zone gabbronorite’. We define the mineralisation around calcsilicate xenoliths within the Main Zone in the far north of the limb as a ‘Platreef-style‘ mineralisation. The Platreef (ss) has a strike extent of ∼30 km, whereas Platreef-style mineralisation occurs over a strike length of 110 km. The Platreef varies from 400 m thick in the S to <50 m in the N. The overall strike is NW or N, with dips 40–45°W at surface, shallowing down dip, The overall geometry of the southern Platreef appears to have been controlled by irregular floor topography. The maximum thickness of the southern Platreef occurs in two sub-basins on the farms Macalacaskop and Turfspuit. Lithologically, the southern Platreef is heterogeneous and more variable than sectors further north and, although predominantly pyroxenitic, includes dunites, peridotites and norite cycles with anorthosite in the mid to upper portion. Zones of intense serpentinisation may occur throughout the package. Faults offset the strike of the Platreef: a N–S, steeply dipping set is predominant with secondary ENE and ESE sets dipping 50–70°S. The fault architecture was pre-Bushveld and also locally controlled thickening and thinning of the succession. Country rock xenoliths, <1500 m long, are common. On Macalacaskop, these are typically quartzites and hornfelsed banded ironstones, shales, mudstones and siltstones whereas on Turfspruit dolomitic or calcsilicate xenoliths also occur. Sulphides may reach >30 modal% in some intersections. These are dominated by pyrrhotite, with lesser pentlandite and chalcopyrite, minor pyrite and traces of a wide compositional range of sulphides. In the southern sector, mineralised zones have Cu grades of 0.1–0.25% and Ni 0.15–0.36%. Massive sulphides are localised, commonly, but not exclusively towards the contact with footwall metasedimentary rocks. Magmatic sulphides are disseminated or net-textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and minor pyrite. Much of the sulphide is associated with intergranular plagioclase, or quartz-feldspar symplectites, along the margins of rounded cumulus orthopyroxenes. The PGEs in the southern sector occur as tellurides, bismuthides, arsenides, antimonides, bismuthoantimonides and complex bismuthotellurides. PGM are rarely included in the sulphides but occur as micron-sized satellite grains around interstitial sulphides and within alteration assemblages in serpentinised zones. The Pt:Pd ratio ∼1 and PGE grade may be decoupled from S and base metal abundance.