Refractory chromitites recovered from the Eretria mine, East Othris massif (Greece): Implications for metallogeny and deformation of chromitites within the lithospheric mantle portion of a forearc-type ophiolite

The chrome ores of the abandoned Eretria mine of the East Othris ophiolite occur within a pervasively serpentinized and sheared harzburgite body. They consist of massive chromitites with mylonitic fabric in imbricate shaped pods. Modal analyses of these ores average at about 90-95% chromian spinel (Cr-spinel) and 5-10% secondary silicates. Chromian spinel compositions vary in Cr# [Cr/(Cr + Al) x 100] and Mg# [Mg/ (Mg + Fe2+) x 100] from 44 to 62 and from 59 to 81, respectively. Trace element (Ti, Ni, V, Mn, Zn, Sc, Co and Ga) contents in Cr-spinel do not show significant variations from grain cores to grain boundaries. However, Cr-spinel compositions show depletions in Ti, Zn and Sc when compared to the composition of accessory Cr-spinel from typical mid-ocean ridge basalts (MORE). Mineral inclusions hosted in Cr-spinel comprise a range of (hydrous and anhydrous) silicate and base metal (BM) minerals occasionally intergrown with phosphate minerals and rare intermetallic compounds. A number of these inclusions have Cr-spinel rims with higher Cr# (63-68) than those of the enclosing Cr-spinel grains.The absence of dunite sheaths around chromitites is interpreted as an artifact of dunite structural obliteration during prolonged ductile shearing within harzburgite. The microtextural characteristics of a number of inclusions in Cr-spinel imply that they were initially fully molten. Furthermore, primary hydrosilicate (amphibole, phlogopite) inclusions in Cr-spinel indicate that chromitites crystallized from a water-bearing melt. Chromian spinel rims around silicate inclusions probably represent early crystals generated from a primitive magma produced by melting of a depleted mantle source.Geochemical calculations demonstrate that the parental melts of chromitites had intermediate affinity between MORE and arc-related magmas. Our preferred hypothesis for the genesis of the Eretria chromitites is that they were formed from a melt originated within the hydrated mantle wedge beneath a nascent forearc basin during subduction initiation.

东奥瑟里斯蛇绿岩中废弃的埃雷特里亚矿的铬矿石产于普遍蛇纹石化和剪切的方辉橄榄岩体中。它们由具有糜棱状构造的块状铬铁矿组成，呈叠瓦状矿囊。这些矿石的模态分析平均约为90 - 95%的铬尖晶石（Cr - 尖晶石）和5 - 10%的次生硅酸盐。铬尖晶石成分的Cr# [Cr/(Cr + Al)×100]和Mg# [Mg/(Mg + Fe²⁺)×100]分别在44到62和59到81之间变化。Cr - 尖晶石中微量元素（Ti、Ni、V、Mn、Zn、Sc、Co和Ga）含量从晶粒核心到晶界没有显著变化。然而，与典型大洋中脊玄武岩（MORB）中的副矿物Cr - 尖晶石成分相比，Cr - 尖晶石成分在Ti、Zn和Sc方面表现出亏损。铬尖晶石中的矿物包裹体包含一系列（含水和无水）硅酸盐和贱金属（BM）矿物，偶尔与磷酸盐矿物和罕见的金属间化合物共生。许多这些包裹体具有比周围铬尖晶石颗粒更高Cr#（63 - 68）的铬尖晶石边缘。铬铁矿周围没有纯橄榄岩鞘被解释为在方辉橄榄岩内长期韧性剪切过程中纯橄榄岩结构被磨灭的结果。铬尖晶石中许多包裹体的微观结构特征表明它们最初是完全熔融的。此外，铬尖晶石中的原生氢硅酸盐（角闪石、金云母）包裹体表明铬铁矿是从含水熔体中结晶出来的。硅酸盐包裹体周围的铬尖晶石边缘可能代表由亏损地幔源熔融产生的原始岩浆所生成的早期晶体。地球化学计算表明，铬铁矿的母岩浆在大洋中脊玄武岩和弧相关岩浆之间具有中间亲和性。我们对埃雷特里亚铬铁矿成因的首选假设是，它们是由俯冲起始期间在新生弧前盆地下方的水化地幔楔内产生的熔体形成的。