Occurrence, geochemistry and provenance of REE-bearing minerals in marine placers on the West Coast of the South Island, New Zealand

The occurrence, concentration and geochemistry of REE-bearing minerals in beach sands along the West Coast of the South Island of New Zealand is reported as a proxy for the composition of adjacent uplifted Holocene marine terraces. The strandlines of 13 beaches along this coast contain REE-rich allanite (average: 15% REO), monazite (82% REO) and xenotime (83% REO), with allanite being most common and xenotime having a restricted range. Other REE-rich phases are apatite (average: 2259 ppm), zircon (1756ppm), epidote (761 ppm), titanite (699 ppm) and scheelite. Geographical trends to the distribution of these minerals are not entirely clear, but there is an influx of titanite and allanite north of the Grey River and an increase in apatite, allanite, monazite and epidote north of Little Wanganui River. In-situ trace elements and/or ⁸⁷Sr/⁸⁶Sr isotopes indicate allanite, monazite and xenotime to be likely mainly derived from exhumed Paleozoic and Cretaceous granites, with minor amounts from the Alpine Schist metasedimentary rocks. The main source of the analysed apatite is likely also the granitoids, with contributions from the Greenland Group and Alpine Schist. When the REE abundances in the beach sands are compared to the size of the uplifted adjacent Holocene terraces, Waimangaroa terrace may contain the highest concentration of REE, followed by the Little Wanganui and Barrytown terraces.

本文报道了新西兰南岛西海岸滨海砂中含稀土矿物（REE-bearing minerals）的产出特征、含量及地球化学性质，以此作为邻近抬升全新世海阶组成的代用指标。该海岸沿线13处海滩的滨线中赋存富稀土褐帘石（allanite，平均稀土氧化物REO含量15%）、独居石（monazite，82% REO）及磷钇矿（xenotime，83% REO）；其中褐帘石最为常见，磷钇矿的分布范围则较为局限。其余富稀土物相包括磷灰石（平均2259 ppm）、锆石（1756 ppm）、绿帘石（761 ppm）、榍石（699 ppm）及白钨矿。这些矿物的空间分布并未呈现完全清晰的地理规律，但格雷河以北区域出现了榍石与褐帘石的富集，而小旺格努伊河以北区域的磷灰石、褐帘石、独居石及绿帘石含量有所升高。原位微量元素与/或⁸⁷Sr/⁸⁶Sr同位素分析结果表明，褐帘石、独居石及磷钇矿主要源自抬升暴露的古生代与白垩纪花岗岩，仅少量来自阿尔卑斯片岩变沉积岩。本次分析的磷灰石主要物源同样为花岗岩类岩石，另有部分来自格林兰群（Greenland Group）与阿尔卑斯片岩。将滨海砂中的稀土元素丰度与邻近抬升的全新世海阶规模进行对比后可知，怀曼加罗海阶的稀土含量可能最高，其次为小旺格努伊海阶与巴里敦海阶。