Abstract

Occurrence of opal is being reported here from the Mesoarchean Bangur chromite mines area in the Boula–Nuasahi ultramafic complex (BNUC) of Odisha, India. The opal shows colour bands in mm to cm scales. From the X-ray diffraction pattern, it is identified as a variety of opal-CT consisting dominantly of α-tridymite and α-cristobalite with very minor quartz. This is the first report of opal from BNUC area. High-resolution field emission scanning electron microscopy (FE-SEM) reveals that this opaline silica is nano-crystalline and consists of silica spherules (10–20 nm) with occasional ill-defined cubic and tetragonal crystallites. Selected area electron diffraction (SAED) pattern obtained through transmission electron microscope (TEM) reveals that it is polycrystalline in nature. Multi-point analysis by electron micro-probe (EMP) indicates its composition to be ~95 wt.% SiO₂. From its mode of occurrence in the field and the type of mineral inclusions in the opal, its genesis can be coined with the second phase of magmatic event (the Bangur gabbro intrusion) and the related hydrothermal alterations. We interpret that the silica has been derived from the host mafic and ultramafic rocks at a high-temperature regime (1000–500°C) during the Bangur gabbro intrusion. During this magmatic event when the host rocks were hydrothermally altered, SiO₂ was released and precipitated as opal-CT.

Graphical abstract

Research highlights

• This is the first report of occurrence of opaline silica in form of opal-CT from Boula–Nuasahi ultramafic complex, Odisha, India.

• This opal-CT is semicrystalline, and primarily consists of nano-crystallites (10-20 nm) of α-tridymite and α-cristobalite.

• The silica material could have been released from the mafic–ultramafic host rocks of the area in relatively high-temperature regime (1000–500°C) due to hydrothermal activity caused by the second phase of magmatic intrusion of ‘Bangur gabbro’.

• The opal-CT is interpreted to have formed due to rapid cooling from a siliceous sol/gel.

中文翻译：

---

印度辛格哈姆邦克拉通地区中古统系班古尔铬铁矿矿床中乳白硅石的赋存与成因

摘要

据报道，印度奥里萨邦Boula-Nuasahi超镁铁矿综合体（BNUC）的Mesoarchean Bangur铬铁矿产区存在蛋白石。蛋白石显示色带，以毫米至厘米为单位。根据X射线衍射图谱，可以确定为多种蛋白石CT，主要由α-鳞石英和α组成。-方石英与非常小的石英。这是BNUC地区蛋白石的首次报道。高分辨率场发射扫描电子显微镜（FE-SEM）显示，这种不透明二氧化硅是纳米晶体，由二氧化硅球（10–20 nm）组成，偶有不确定的立方和四方晶体。通过透射电子显微镜（TEM）获得的选择区域电子衍射（SAED）图谱表明它本质上是多晶的。通过电子微探针（EMP）进行的多点分析表明其组成为〜95 wt。％SiO ₂。从其在野外的发生方式和猫眼石中矿物包裹体的类型来看，其成因可以与岩浆事件的第二阶段（Bangur gabbro侵入）和相关的热液蚀变相吻合。我们解释说，二氧化硅是在Bangur gabbro侵入过程中的高温状态（1000–500°C）下从基质铁镁质和超镁铁质岩石中提取的。在岩浆事件中，当主岩发生水热蚀变时，SiO ₂释放并沉淀为蛋白石-CT。

图形概要

研究重点

• 这是印度奥里萨邦Boula–Nuasahi超镁铁复合物以蛋白石CT形式出现乳白硅石的第一个报道。

• 该蛋白石CT是半结晶的，主要由α-鳞石英和α-方石英的纳米晶体（10-20 nm）组成。

• 由于“ Bangur gabbro”岩浆侵入的第二阶段引起的热液活动，硅质物质可能在相对较高的温度范围（1000–500°C）下从该地区的铁镁质-超红壤质宿主岩中释放出来。

• 蛋白石-CT被解释为是由于硅质溶胶/凝胶的快速冷却而形成的。