Sediment‐Hosted Gold Deposits in Southeast Asia

Abstract

AbstractSediment‐hosted gold (SHG) deposits are becoming more widely recognized in the Southeast Asian region. They are characterized by very fine‐grained gold, typically occurring in stratabound jasperoid replacement and structurally controlled orebodies in calcareous or carbonaceous sedimentary rocks, which are commonly distal to porphyry copper–gold deposits. Gold mineralization is associated with anomalous As‐Sb‐Hg‐Tl and is normally concentrated in the arsenic‐rich rims of pyrite and arsenopyrite grains (commonly refractory). The examples discussed in this paper are located in Southeast Asian countries, and all occur in island or continental arc terrains. SHG deposits have many features comparable to the Carlin deposits in Nevada; however, they differ in tectonic setting, their shallow level of emplacement, fluid composition, and probable metal source. As such, the Southeast Asian examples described in this compilation may be classified as “Carlin‐like” but not “Carlin‐type.” Furthermore, they appear to represent a subclass of low‐temperature epithermal deposits, with most occurrences showing late‐stage colloform quartz vein events. Indeed, they are better described as sediment‐hosted epithermal gold deposits.Several examples are multimillion ounce deposits, such as Sepon, Laos, with its endowment of more than 4 million ounces of gold. Mesel, Sulawesi, and Bau (Jugan and Tai Parit), Sarawak, also possessed multimillion ounce resources prior to mining. Two centuries of Chinese alluvial and eluvial mining at Buduk in northwest Kalimantan has produced a significant unrecorded amount of gold. Structurally controlled feeder zones frequently contain higher‐grade gold mineralization (e.g. Mesel), and constitute valid exploration targets. More recently discovered deposits, such as those in Myanmar (Gegalaw, Taungzaw, and Katsena) and the Philippines (Hijo and Las Cogon), remain to be thoroughly explored. Most of the deposits (Sepon, Mesel, Abong, Sihayo, Jugan, Tai Parit, Hijo, and Lascogon) can be directly linked spatially and genetically to gold–copper porphyry intrusive centers. It is postulated that localities with no obvious connection to known porphyry deposits (Buduk, Ban Bak, Kyaukpahto, Gegalaw, Taungzaw, and Katsena) may represent district‐scale exploration targets for porphyry copper–gold deposits. In terms of discovery techniques, of the 13 SHG deposits discussed here, several were sites of historical mining (Mesel, Bau, and Kyaukpahto). The majority, however, are virgin deposits discovered in the past 25 years by intuitive area selection followed by systematic regional geochemical exploration programs and routine geological inspections. At the project scale, electrical resistivity techniques, such as controlled‐source audio frequency magnetotellurics (CSAMT) and induced polarization (IP), have proven to be useful for mapping blind jasperoid bodies and associated feeder zones and silicified structures. Traditional field mapping and stream float sampling in tropical regions are also effective exploration tools. Deposits that have significant jasperoid development may have conspicuous positive topographic relief, depending on erosion level.