Geometry, kinematics and regional significance of faulting and related lamprophyric intrusion in the mineralised zone at the Pu Sam Cap complex, Northwest Vietnam

Abstract

Mineralisation; Lamprohyric dykes; Syenitic intrusives; Strike-slip fault Tóm tắt: The alkali volcanics and intrusive rocks, dated at around 35-33Ma, are cut by mineralised northeast and east trending faults showing predominant evidence for strike slip. Mineralisation includes haematite-Au-Cu and is accom­panied by iron-rich alteration of the volcanic rocks. Detailed assessment of the geometty of the fault system at Pu Sam Cap suggests that the faults formed as a Riedel shear system during left-lateral slip within the Song Hong-Song Chay shear zone and the numerous contemporaneous northwest trending faults to the south: the northeast trending faults are interpreted as dextral "book-end" faults between major northwest trending faults enclosing the Pu Sam Cap massif. As mineralisation is hosted within these faults and is also associated with lamprohyric dykes it confirm s a thermal event younger than the alkaline volcanics and syenitic intrusives at Pu Sam Cap, suggesting a hidden, young porphyry system. The age of faulting, and thus the maximum age for this young intrusive event, is attributed to the 23-21 Ma period of late-stage left- lateral strike-slip motion across northwest Vietnam.