Origin, evolution and significance of anhydrite-bearing vein arrays and breccias, Lienetz orebody, Lihir gold deposit, Papua New Guinea

The Lihir gold deposit (also known as Ladolam), has a 56 Moz resource and is the world’s largest alkalic gold deposit in terms of contained gold. It is located on Lihir Island, part of the Tabar-Lihir-Tanga-Feni island chain, New Ireland Province, Papua New Guinea. The Tabar-Lihir-Tanga-Feni island chain formed in a complex tectonic environment over the past four million years. Docking of the colossal Ontong Java Plateau at the Melanesian trench during the Miocene caused near-complete cessation of magmatism and a reversal of subduction polarity, forming the markedly northward-convex New Britain Trench. Pliocene to Recent subduction along the New Britain Trench was coeval with sinistral transposition of New Ireland relative to New Britain, and the formation of the Tabar-Lihir-Tanga-Feni island chain. Lavas of Lihir and other islands in the chain are shoshonitic, alkali- and volatile-rich, silica-undersaturated, and highly oxidized with elevated large-ion lithophile element contents. Their hybrid geochemical characteristics are consistent with partial melting of an already metasomatized, oxidized and hydrous mantle wedge originally formed by the Miocene Melanesian subduction zone. Lihir Island is composed of five volcanic centers, presently inactive. The Luise volcano consists of a 4 × 3.5 km wide amphitheater, elongated and breached to the northeast. This is inferred to be a remnant of the original ~ 1.1 km high volcanic cone that underwent sector collapse(s). The Lihir gold deposit is situated in the foot wall of the sector collapse detachment surface and consists of several adjacent and partly overlapping orebodies (Lienetz, Minifie, Kapit, Kapit NE, etc.). The sector collapse event(s) marked an important stage in the deposit’s evolution by superimposing late-stage, gold-rich, alkalic low-sulfidation epithermal mineralization upon early-stage, porphyry-style alteration. A broad, three-fold vertical alteration zonation at Lihir is interpreted to represent this evolution. With increasing depth, the alteration zones consist of: (1) a ~ 0.2 Ma to 0.0 Ma, surficial, generally barren, steam-heated clay alteration zone that is a product of modern high-temperature geothermal activity; (2) a ~ 0.6 to 0.2 Ma, high-grade (> 3 g/t Au), refractory sulfide and adularia alteration zone that represents the ancient epithermal environment; and (3) a ~ 0.9 to 0.3 Ma, comparatively low-grade (< 1 g/t Au) zone rich in anhydrite ± carbonate, coupled with biotite alteration, that represents the ancient porphyry-style environment. Recent volcanism has occurred during the modern geothermal-stage, with the emplacement of several diatreme breccia bodies. Early porphyry-style hydrothermal activity in the Lienetz orebody resulted in a magmatic-hydrothermal breccia complex and associated hydrothermal veins and breccia veins, most of which contain abundant anhydrite. An eight-stage vein paragenesis, linked with a five-stage breccia paragenesis, records the transition from porphyry-style to epithermal conditions. A spectacular anhydrite ± carbonate vein array is exposed in the deeper levels of the Lienetz open pit. They reveal a dynamic structural evolution, where veins were reactivated, but with grossly similar geometries and kinematic histories. Overall, discrete sets of veins record a history of early compression and protracted, or multistage, northwest-directed extension, with predominant east-northeast and northeast strikes for both veins and faults. Early northwest and/or southeast-directed compression and west-northwest-directed extension is evident from low-angle thrust faults and tensile vein arrays with both sub-vertical and sub-horizontal dips. Early vein formation occurred in the porphyry-style environment, under low differential stress, an oscillating sub-horizontal to sub-vertical σ1, and temporarily elevated fluid-pressures that resulted from mineral sealing. Protracted, or multistage, northwest-directed extension with a mostly sub-vertical σ1 predominated for the rest of the porphyry-stage and into the epithermal-stage vein paragenesis at Lienetz. This is best documented by the principal vein array at Lienetz, which consists of large hybrid and 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 north- to northwest-directed, top-block down sense of shear. Significant modification of the early formed veins and breccias occurred during the transition from porphyry-style to epithermal conditions, leading to recrystallization, dissolution seams, stylolites, volume loss and solution collapse breccias. Modification was most likely facilitated by anhydrite dissolution and recrystallization, probably due to changing temperatures ± pressures ± salinities. The modified veins localized shearing, and their sub-horizontal to low-angle northward-dipping geometry may have had some control on the geometry of, and lubrication for, the sector collapse event(s). However, the modified veins appear not to be kinematically linked to the northeast-directed collapse event(s) due to their top-block down to the northwest or north-northwest sense of shear. High-grade, epithermal-style, gold mineralization followed vein modification and sector collapse(s). Mineralization was partly facilitated by preconditioning provided by porphyry-stage events, as auriferous hydrothermal fluids utilized permeable and porous open spaces and cavities that were created by the dissolution of early formed anhydrite. Mineralization was also localized at depth by northeast-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 northeast-striking tensile to hybrid veins and breccia veins with high-angle dips and rhombic dilational jogs that localized high-grade gold. The northeast to east-northeast-striking structural grain, evident at both the regional 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 are considered to have contributed to the northeast-elongation of the volcanic amphitheater, and were fundamental for the structural control of vein formation and gold localization. The δ\(^{34}\)S values of anhydrite and pyrite from Lihir are consistent with deposition from oxidized magmatic-hydrothermal fluids (δ\(^{34}\)S\(_{sulfate}\) from 7.2 to 13.6 ‰, and δ\(^{34}\)S\(_{sulfide}\) from −13.0 to 3.6 ‰). \(^{87}\)Sr/\(^{86}\)Sr values indicate a primitive (mantle) source. The δ\(^{34}\)S\(_{sulfate}\) values increased (+1.3 ‰) from porphyry-style to epithermal conditions with time. The δ\(^{34}\)S\(_{sulfide}\) values of pyrite grains are heterogeneous or bimodal, and vary significantly at the microscopic scale, and in time and space. Late-stage epithermal mineralization superimposed on early porphyry-style alteration created complications with regards to ore processing, specifically with regards to the difficulties in mineral processing of the refractory gold-rich pyritic ore. Early generations of coarse-grained pyrites are interpreted to have formed under porphyry-style conditions. Later generations of pyrites display oscillatory zones and are interpreted to have formed under epithermal conditions. The porphyry-stage pyrite grains are relatively trace element poor, except for Co, Ni and Se, whereas the epithermal pyrite grains are enriched in As, Mo, Ag, Sb, Au and Tl. Most of the pyrite grains in the anhydrite-rich zone at Lienetz are composite grains, and display some textural and geochemical evidence of modification. The composite pyrite grains have porphyry-stage trace element-depleted cores, and epithermal-stage delicate banded rims enriched in gold, arsenic and other trace elements. Because gold is concentrated only along their rims, these pyrite grains can be subjected to a shorter period of oxidation and leaching in order to liberate most of their gold. In contrast, for areas dominated by high-grade epithermal-stage mineralization, pyrite grains are arsenic- and gold-rich throughout, and thus require longer oxidation and processing time. Understanding gold deportment in telescoped deposits is therefore essential for optimizing mineral processing and can impact significantly on the economics of mining these complex, hybrid ore deposits. The unique characteristics of the Lihir gold deposit, in particular the preserved relationships of hybrid ore and volcanic architecture, provides insights into transitional processes between porphyry and epithermal end-members. Reactivated structures and anhydrite dissolution were significant factors in gold mineralization at Lihir. As such, they should be regarded in the exploration and understanding of other magmatic-hydrothermal ore deposits.