THE STRUCTURE OF ANHYDRITE VEINS AND CEMENTED BRECCIAS AT THE GIANT LIHIR EPITHERMAL GOLD DEPOSIT, PAPUA NEW GUINEA

The Lihir gold deposit in Papua New Guinea is the world’s largest alkalic gold deposit, with a 60 Moz resource. Lihir evolved from an early magmatic-hydrothermal porphyry system to a shallow-level gold-rich epithermal system. This evolution was facilitated via northeast-directed sector collapse of the volcanic edifice [1, 2]. The ore deposit is situated in the immediate footwall of the sector detachment surface.

The epithermal and porphyry alteration domains of Lihir are defined by three sub-horizontal alteration zones that are parallel to the detachment surface. With increasing depth, the alteration zones include: (1) a barren near-surface steam-heated clay blanket; (2) a high-grade, refractory sulfide zone with a broadly tabular morphology; and (3) a zone of intense anhydrite veining and brecciation with erratic high-grades that extends to depths beyond the limits of resource drilling. Controls on gold distribution within the anhydrite-rich zone are poorly defined.

This paper documents the structural evolution of anhydrite ± carbonate vein arrays exposed in the deepest levels of the Lienetz open pit. The principal array comprises massive and banded shear veins (up to 3 m thick) with shallow (< 30°) northward dips. Linking these massive veins are sets of conjugate extension to hybrid vein arrays and hydraulic breccias that have local high-grades and which record a history of north-to-northeast-block-down extension. Near the top of the anhydrite zone, veins record volume loss and dissolution. This is evident by collapsed wallrock fragments, vugs and stylolitic bands. Above these dissolved veins are open-space cavities that contain high-grade epithermal gold mineralization.

The anhydrite ± carbonate veins are interpreted to have formed under a high fluid pressure regime associated with an early deep-seated (> 1 km) magmatic-hydrothermal porphyry system. Widespread vein dissolution, extension and collapse are interpreted to have either contributed to, or to be the expression of, the sector collapse event(s) responsible for the telescoping and formation of the giant Lihir gold deposit.

[1] Carman, G.D., 1994, Genesis of the Ladolam gold deposit, Unpublished PhD thesis, Monash Uni, 381

[2] Sillitoe, R.H., 1994, Erosion and Collapse of Volcanoes - Causes of Telescoping in Ore-Deposits, Geology, 22, 945-948