Ismail Hasnol Hady

Objectives The serpentinization of the mafic and ultramafic rocks is one of the most significant mechanisms for producing natural hydrogen in the subsurface. The northern part of Borneo is notable for its complex regional geology, particularly regarding Fe-rich rocks in the ophiolite zone of Sabah. This article discusses the mineralogy of ophiolites in Sabah, which could lead to the natural hydrogen feasibility via serpentinization processes. Methods Fieldwork was conducted in selected outcrops to investigate the presence of ophiolites, and standard petrography and x-ray diffraction (XRD) analyses were performed on the mineral composition and composition of the rocks. The investigation area includes the ophiolites found in Ranau, Telupid and Lahad Datu, these ophiolites are composed of crystalline basement and ultramafic- serpentinite rocks that are of Triassic to Early Tertiary age. Results, Observations, Conclusions The rock samples consist mainly of serpentinite with some basalt, dolerite and amphibolite. These rocks are unconformably overlain by younger sedimentary rocks, including the Eocene-Oligocene Crocker Formation and the Paleocene-Eocene Trusmadi Formation in northern Sabah. In central Sabah, the ophiolite is overlain by the Eocene-Oligocene Crocker Formation and the Kulapis Formation, while the ophiolite in southern Sabah is overlain by the Oligocene-Middle Miocene mélange sequence such as the Kuamut, Ayer and Kalumpang Formations. The results of the geochemical and mineralogical investigations show that the serpentinite samples contain a considerable amount of serpentine minerals (Figure 1) such as antigorite, cronstedtite and lizardite as well as some unaltered ultramafic/mafic minerals (e.g. forsterite, fayalite, pyroxene and hornblende) and traces of secondary minerals (such as magnetite, chromite, hematite, siderite and pyrite). The degree of serpentinization in the serpentinite samples ranges from 25% to 79% by volume, with most samples exceeding 45%. The degree of serpentinization gradually increases from North Sabah (average 45%) to Central Sabah (average 59%) and South Sabah (average 74%). Other rock types such as basalt, dolerite and amphibolite show a lower degree of serpentinization, ranging from 2% to 20%. The laboratory analyses indicate that significant serpentinization processes have occurred along the ophiolite zone of Sabah, which could yield significant amounts of hydrogen gas through the geochemical reactions below: 1.     3Fe2SiO4 (fayalite) + 2H2O → 2Fe3O4 (magnetite) + 3SiO2 (quartz) + 2H2 6Fe2SiO4 (fayalite) + 7H2O → 3Fe3Si2O5(OH)4 (serpentine) + Fe3O4 (magnetite) + H2 The overlying sedimentary rock formations could provide reservoir and seal traps for the hydrogen gas accumulation. These results give new insight into potential natural hydrogen resources in Sabah’s ophiolite zone; however, more detailed investigations are needed for further evaluation. Limited subsurface data is a major challenge in studying the entire natural subsurface hydrogen system along the ophiolite zone in Sabah. Novel/Additive Information The study provides fresh insights and new information about the potential of natural hydrogen in the Sabah’s ophiolite zone, Malaysia.