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Chromite compositional variability and associated PGE enrichments in chromitites from the Gomati and Nea Roda ophiolite, Chalkidiki, Northern Greece
Mineralium Deposita ( IF 4.4 ) Pub Date : 2022-04-06 , DOI: 10.1007/s00126-022-01109-z
Micol Bussolesi 1 , Alessandro Cavallo 1 , Giovanni Grieco 2 , Niccolò Storni 2 , Federica Zaccarini 3 , Evangelos Tzamos 4
Affiliation  

The Gomati and Nea Roda ophiolites are located into the Serbo-Macedonian massif of the Chalkidiki peninsula (Northern Greece). The present work focuses on the variability of platinum-group elements (PGEs), geochemistry, spinel mineral chemistry, and platinum-group minerals (PGMs) — base metal minerals (BMMs) assemblage in chromitites of three Gomati localities (St. George, Tripes, and Limonadika) and Nea Roda. The studied chromitites show variable textures and are heavily altered. Primary silicates are almost completely replaced by chlorite, and chromite rims are altered into ferrian chromite. The variability of spinel mineral chemistry in terms of Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)], and the PGE contents, argues for a genesis in a supra-subduction setting (SSZ), at different stratigraphic positions in the ophiolite section. Chromitites from Tripes have the lowest Cr# (0.5–0.6) and the highest PGE contents (3516 ppb), similar to some chromitites formed in small magma chambers in the cumulate sections above the Moho. The high PGE contents of Tripes chromitites are due to an IPGEs-enriched melt derived from critical melting of mantle peridotites. Limonadika and St. George show the highest Cr# (0.77–0.96 and 0.74–0.87, respectively) and variable PGE contents (175 ppb and 383 ppb on average respectively), compatible with a genesis from boninitic magmas in the mantle section. Nea Roda chromitites have intermediate to high Cr# (0.66–0.75) and low PGE contents (135 ppb on average) and show similarities to other intermediate chromitites formed from evolving magma sources at subduction initiation. BMMs detected in both ophiolites are primary (pentlandite) and secondary (mainly millerite and heazlewoodite) sulfides. All the detected PGMs are primary, crystallized from the melt, and entrapped into chromite, and they are mainly laurites. In the studied chromitites, the absence of alloys indicates that the circulating fluids during chloritization were at high fS2 and fO2, and did not remobilize the PGEs. The same fluids are probably responsible for the low-T crystallization of an uncommon suite of arsenides and antimonides at St. George.



中文翻译:

来自希腊北部哈尔基迪基的 Gomati 和 Nea Roda 蛇绿岩的铬铁矿组成变异性和相关的 PGE 富集

Gomati 和 Nea Roda 蛇绿岩位于哈尔基迪基半岛(希腊北部)的塞尔维亚-马其顿地块中。目前的工作重点是铂族元素 (PGE)、地球化学、尖晶石矿物化学和铂族矿物 (PGM) 的变异性 — 三个戈马蒂地区 (St. George, Tripes) 铬铁矿中的贱金属矿物 (BMM) 组合和 Limonadika) 和 Nea Roda。所研究的铬铁矿显示出不同的纹理并且发生了很大的变化。原生硅酸盐几乎完全被绿泥石取代,而铬铁矿边缘则变成了铬铁矿。尖晶石矿物化学在 Cr# [Cr/(Cr + Al)] 和 Mg# [Mg/(Mg + Fe 2+ ) 方面的变化)] 和 PGE 含量,论证了在蛇绿岩剖面不同地层位置的超俯冲环境 (SSZ) 中的成因。来自Tripes的铬铁矿具有最低的Cr#(0.5-0.6)和最高的PGE含量(3516 ppb),类似于在莫霍面上方堆积层的小岩浆房中形成的一些铬铁矿。Tripes 铬铁矿的高 PGE 含量是由于地幔橄榄岩的临界熔融产生的富含 IPGE 的熔体。Limonadika 和 St. George 显示出最高的 Cr#(分别为 0.77-0.96 和 0.74-0.87)和可变的 PGE 含量(平均分别为 175 ppb 和 383 ppb),这与地幔剖面中软质岩浆的成因相符。Nea Roda 铬铁矿具有中到高 Cr# (0.66–0. 75)和低 PGE 含量(平均 135 ppb),并显示出与俯冲开始时演化岩浆来源形成的其他中间铬铁矿的相似性。在两种蛇绿岩中检测到的 BMM 是原生硫化物(镍黄铁矿)和次生硫化物(主要是钙铝石和石榴石)。检测到的所有铂族金属都是原生的,从熔体中结晶出来,夹带在铬铁矿中,主要是月桂石。在所研究的铬铁矿中,没有合金表明绿泥石化过程中的循环流体处于高 fS2和 fO 2,​​并且没有重新动员 PGE。相同的流体可能是圣乔治罕见的一组砷化物和锑化物的低 T 结晶的原因。

更新日期:2022-04-06
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