Mineral Inclusions in Sapphire from Basaltic Terranes in Southern Vietnam: Indicator of Formation Model

Abstract

GEMS & GEMOLOGY WINTER 2020 Since the late 1980s, sapphire mines in southern Vietnam have supplied significant quantities of gem materials to the jewelry industry (figure 1). Commercial gem mines in southern Vietnam have been located in four main areas, including Dak Nong, Di Linh, Binh Thuan, and Krong Nang (figure 2) (Smith et al., 1995; Long et al., 2004; Garnier et al., 2005; Izokh et al., 2010; Vu, 2010, 2018). Specifically, the first discovery of abundant gem sapphires was in Dak Nong, followed by Di Linh, Binh Thuan, and Krong Nang, respectively. These sapphires accumulated in Upper-Pleistocene to Quaternary alluvial deposits. In 2017, we carried out field trips in southern Vietnam encompassing Binh Thuan Province, Di Linh (Lam Dong Province), Dak Nong Province, and Krong Nang (Dak Lak Province), where sapphire samples were collected directly from the mines (see figure 2). Gem mining activities in these areas are conducted by a few local miners. Pits are dug with basic tools before washing and hand picking along streams by artisanal miners (figure 3), and mines worked by machinery (figure 4) can also be found. Sapphires in this region usually range from dark blue to bluish green, yellowish green to green, with rare yellow sapphire. The natural intense blue sapphire is the best known and has been specifically recovered from the Dak Nong and Di Linh gem fields (figure 1, left). Other colors such as bluish green and yellowish green to green are common, particularly in the Binh Thuan and Krong Nang gem fields, respectively (figure 1, right). However, these sapphire varieties are generally heat treated for color enhancement. In addition, trapiche-type sapphires (figure 5) are sometimes found in these gem fields.