The structure and significance of anhydrite-bearing vein arrays, lienetz orebody, lihir gold deposit, Papua New Guinea

The Lihir Au deposit (also known as Ladolam), Papua New Guinea, has a 56-Moz resource and is the world’s largest alkalic Au deposit in terms of contained Au. The Au deposit is in an amphitheater, inferred to be a remnant of the original ~1.1-km-high volcanic cone that underwent NE-directed sector collapse(s) and prolonged tropical weathering. The ore deposit is situated in the footwall of the sector collapse detachment surface and consists of several orebodies.

Late-stage, Au-rich, alkalic low-sulfidation epithermal mineralization was superimposed upon early-stage, porphyry-style alteration. A threefold vertical alteration architecture at Lihir is interpreted to broadly represent this evolution. With increasing depth, the alteration zones consist of (1) a surficial, generally barren, steamheated clay alteration zone that is a product of modern high-temperature geothermal activity, (2) a high-grade (>3 g/t Au), refractory sulfide and adularia alteration zone that represents the ancient epithermal environment, and (3) a comparatively low-grade (<1 g/t Au) zone rich in anhydrite ± carbonate, coupled with biotite alteration, that represents the ancient porphyry-style environment.

Early porphyry-style hydrothermal activity in the Lienetz orebody resulted in a magmatic-hydrothermal breccia complex and associated hydrothermal veins, most of which contain anhydrite. A spectacular anhydrite ± carbonate vein array is exposed in the deeper levels of the Lienetz open pit and reveals a dynamic structural evolution where veins were reactivated but with grossly similar geometries and kinematic histories. Early localized compression is evident from low-angle thrust faults and tensile vein arrays with both subvertical and subhorizontal dips. This occurred in the porphyry-style environment under low differential stress and an oscillating subhorizontal to subvertical σ1 and temporarily elevated fluid pressures that resulted from mineral sealing. The variable stress conditions and nucleation of vein arrays with low-angle dips are interpreted to relate principally to instabilities in the volcanic-magmatic environment, triggered in part by volume changes within the underlying magma chamber and exsolution of hydrothermal volatiles. Protracted or multistage NWdirected extension with a mostly subvertical σ1 predominated for the rest of the porphyry- to epithermal-stage evolution, possibly reflecting the greater control of far field tectonic stresses and instabilities in the shallow volcanic environment. This is best documented by the principal vein array at Lienetz, which consists of large hybrid to shear veins with low-angle dips (~30°) to the north. Linking these large, low-angle veins are sets of tensile to hybrid veins and breccia veins with high-angle dips (~65°) to the northwest. Kinematic indicators record dominantly extensional displacement, with N- to NW-directed, top-block down sense of shear. Significant modification of the early formed veins and breccias occurred during the transition from porphyrystyle to epithermal conditions, leading to recrystallization, dissolution seams, stylolites, volume loss, and solution collapse breccias. The modified veins localized some shear and may have had some control of lubrication for the sector collapse event(s); however, they are not kinematically linked.

High-grade, epithermal-style, Au mineralization followed vein modification and sector collapse(s). Mineralization was partly facilitated by preconditioning provided by porphyry-stage events, as auriferous hydrothermal fluids flowed through cavities created by the dissolution of early formed anhydrite. Mineralization was also localized at depth by NE-striking faults. Continued extension with top-block down to the northwest preferentially reactivated the principal vein array with low-angle dips to the north. Porphyry-stage veins were modified during epithermal mineralization due to reactivation under extensional conditions. Reactivation produced NE-striking tensile to hybrid veins and breccia veins with high-angle dips and rhombic dilational jogs that localized high-grade Au.

The NE- to ENE-striking structural grain, evident at both the island scale and the deposit scale, was inherited from the basement. These structures were weaknesses that were reactivated throughout the evolution of Lienetz. Similarly oriented deep-seated faults contributed to the northeast elongation of the volcanic amphitheater and were fundamental for the structural control of vein formation and Au localization.

The unique characteristics of the Lihir Au deposit, in particular the preserved relationships of hybrid ore and volcanic architecture, provide insights into transitional processes between porphyry and epithermal end members. Reactivated structures and anhydrite dissolution were significant factors in Au mineralization in the Lienetz orebody. As such, they should be regarded in the exploration and understanding of other magmatichydrothermal ore deposits.