华南氧化锰矿晚新生代大规模成矿

^(40)Ar/^(39)Ar Constraints on Pervasive Supergene Manganese Mineralization of the Late Cenozoic Age Throughout South China

氧化锰矿是我国仅次于沉积型碳酸锰矿的重要锰矿床类型,主要分布在广西、湖南、广东和福建等南方省区,但由于定年手段的限制,迄今对我国氧化锰矿形成时代的了解还极其有限。作者近几年利用激光阶段加热40Ar/39Ar同位素分析技术对华南地区典型氧化锰矿进行了系统和精确的年龄测定,获得了一大批可靠的成矿年龄,年龄值介于23.64±0.29 Ma和0.48±0.17 Ma(2σ),表明华南氧化锰矿的大规模成矿至少在渐新世末-中新世初就已开始并一直持续到更新世末期。基于多数氧化锰矿的上部矿石已被剥蚀或开采殆尽这一事实,可将华南氧化锰矿的成矿时间上溯到渐新世早期甚至始新世末期。研究发现,氧化锰矿的保存程度和年龄分布明显受新构造运动的影响;如果剔除后期剥蚀程度和采样系统性的影响,可以认为华南氧化锰矿的形成在区域上具有同时性。本次研究还说明,次生氧化锰矿的成矿是一个极其漫长的过程,形成一个具有工业意义的氧化锰矿可能需要上千万年的时间;这明显有别于多数内生热液矿床的成矿。本项研究为我国氧化锰矿以及其他类型的风化矿床(如红土型金矿和红土型镍矿)的年代学研究提供了新的思路和途径。

Supergerne Mn-oxide deposits are widely distributed in Guangxi, Guangdong, Hunan, and Fujian Provinces of southern China and represent the second most important Mn ore types, following the sedimentary Mn-carbonate deposits in China. However, knowledge on the timing of supergene Mn enrichment is very limited due to lack of radiometric dating methods that are applicable to this type of deposit. In the last few years, the authors have performed a comprehensive geochronological study of supergene Mn-oxide deposits by using the ^40 Ar/^39 Ar method. Incremental heating analysis of around 110 mineral separates of K-bearing Mn-oxides yielded reliable plateau ages that range from 23.64 ± 0.29 Ma and 0.48 ± 0.17 Ma （2σ）. The age data thus indicate that pervasive supergene Mn enrichment in South China commenced at least in the latest Oligocene and persisted into the present time. Given the fact that the uppermost parts of most Mn-oxide deposits have been removed or destroyed by later denudation and/or mining, the supergene mineralization event may date back to early Oligocene or even the Eocene. The age distribution of supergene Mn-oxide deposits in South China shows a spatial trend, with the youngest ages recorded in the west, whereas the oldest ages were preserved in the east. This reflects how tectonic stability has affected the preservation of weathering profiles and Mn-oxides deposits therein. The results from this study also demonstrate that formation of an industrial supergene Mn oxide deposit may have spanned tens of million years, in great with most hydrothermal deposits that may be formed in only 10s to 100s thousand years. The approaches cedures employed in this study can be used in the study of geochronology of other weathering related ore such as lateritic gold and nickel deposits.