1981 Volume 31 Issue 169 Pages 351-365
The Mamut porphyry copper deposit of upper Miocene age is located on the northern end of the island of Borneo. Copper mineralization is associated with an adamellite porphyry intrusion, and is localized in the intrusion as well as in the wall rocks which are composed of serpentinite and clastic sedimentary rocks. This paper deals with the serpentinite which has been altered hydrothermally by the mineralization.
The next four types of alteration for serpentinite are seen in the Mamut ore deposit: (1) weakly altered type, (2) chlorite type, (3) talc type and (4) tremolite type. The alteration process is considered to have proceeded in the following succession: (1) serpentinization of olivine and pyroxene, (2) formation of talc and chlorite from serpentine minerals, and (3) formation of tremolite from talc and chlorite. The alteration and mineralization process caused by the steps (1) and (2), which are considered to have been related to the intrusion of adamellite porphyry, is characterized by pervasive silicification and weak dissemination of chalcopyrite. Many fractures have been developed along the margin of the adamellite porphyry apophysis as the result of its cooling and shrinking. The process of the step (3) may be correlated with the introduction of mineralizing fluid, which has a high salinity and a high copper concentration, into the zone of well fractured adamellite porphyry and contiguous wall rocks. This alteration and mineralization process related to the step (3) is characterized by passive silicification and has completed the building up of the Mamut copper ore deposit.
