Geology of the Kapit NE and coastal ore zones, Lihir gold deposit, Papua New Guinea

The Lihir gold deposit is the world's largest alkalic low-sulfidation epithermal gold deposit in terms of contained gold (50 Moz). The deposit formed over the past ~500 kyr and records a progression from porphyry to epithermal-style hydrothermal activity. Lihir is located on the eastern shore of Lihir Island, in the TLTF island arc of New Ireland Province, Papua New Guinea. Its formation has been coincident with alkalic magmatism along arc-oblique extensional structures and sinistral rotation in the back-arc of the New Britain subduction zone. The deposit is centered on Luise Volcano, a well-preserved Plio-Pleistocene alkalic volcano with an elliptical, north-northeast-trending sector collapse amphitheater in its core. The major ore zones that make up the deposit (Lienetz, Minifie, Kapit, Coastal and Kapit NE) are situated in the footwall of the sector collapse detachment surface. The Kapit NE and Coastal ore zones are located immediately northeast of the Lienetz open pit and have yet to be mined. The host rocks in these ore zones are dominated by a thick package of northeast-dipping, volcaniclastic debris flows that were deposited on the submarine flank of the locally emergent Luise volcanic edifice. On the same stratigraphic level at Minifie, the volcano-sedimentary sequence is dominated by south-dipping, intercalated volcanic debris flow deposits and minor pyroclastic facies that indicate subaerial deposition close to a vent. Both sequences were intruded by basaltic to andesitic sills and dikes that reflect emplacement into an unconsolidated and water-saturated sedimentary substrate in a submarine setting. A voluminous, north-trending intrusive complex composed predominantly of porphyritic monzonite and latite stocks and dikes crosscuts the volcano-sedimentary sequence in Coastal and Kapit NE. Its emplacement spans the range of magmatic-hydrothermal activity at Lihir. New whole-rock geochemical results confirm that these intrusions have an alkalic and shoshonitic affinity and were emplaced in a supra-subduction zone tectonic setting. Biotite-feldspar-phyric, nepheline-bearing monzonites in Coastal contain abundant magmatic-hydrothermal features (e.g., miarolitic cavities) that provide evidence for volatile phase exsolution; however, there is no evidence that this intrusive phase is linked to gold mineralization. The intrusive complex hosts well-developed stockwork veins and breccia veins, whereas the predominantly clastic volcano-sedimentary sequence typically hosts mineralization of a more diffuse character, including discontinuous veins and alteration patches. Evidence for porphyry-style hydrothermal activity is preserved at depth in the Kapit NE and Coastal ore zones. Porphyry stage features include pervasive alteration domains composed of phlogopite ‚Äö- K-feldspar ¬¨¬± magnetite (potassic) and chlorite ‚Äö- calcite (propylitic) assemblages, and widespread, low grade (<1 ppm Au and <0.3 wt% Cu) anhydrite-rich breccias and veins. In Kapit NE, there is an apparent zonation of pervasive alteration from proximal potassic to distal propylitic assemblages surrounding crystal-poor feldspar-phyric trachyte dikes. No single intrusive center has been identified that can account for the distribution of alteration across the Lihir deposit. New geochronological age constraints suggest that porphyry-style hydrothermal activity occurred in Kapit NE between ~481 ka and 345 ka. The transition from porphyry to epithermal conditions occurred as a result of volcanic sector collapse, which removed at least 1 km of volcano-sedimentary rocks from the top of Luise Volcano. This catastrophic mass wasting event depressurized rocks that had previously been at depth, resulting in extensional fracturing and increasing the hydraulic conductivity of the rock mass. The epithermal stage in Kapit NE and Coastal was characterized by quiescent periods when banded vein infill and small-scale hydraulic breccias were formed, and brief, chaotic periods of tectonism and phreatic brecciation. The epithermal stage generated significant gold mineralization at shallow levels, which is hosted in veins and breccias containing gold-bearing pyrite and marcasite with variably abundant gangue minerals (quartz, chalcedony, adularia, carbonate, anhydrite and/or illite) and up to 10% open space. These veins and breccias are typically accompanied by intense, pervasive adularia ‚Äö- pyrite or quartz ‚Äö- illite ‚Äö- pyrite alteration. Brittle fault zones have locally concentrated or diluted porphyry and epithermal-stage mineralization. Gold is typically refractory, occurring within the crystal lattice of pyrite. Minor native gold and precious metal tellurides occur locally. A period of maar-diatreme volcanism disrupted the Luise amphitheater during the latter stages of epithermal mineralization (between 151 and 55 ka). Phreatomagmatic and phreatic activity was triggered by the intrusion of feldspar-phyric andesite dikes into the ore-forming hydrothermal system and produced seven large volcanic-hydrothermal breccias focused along north- to northeast-trending structures. The diatreme breccia complex truncated several of the epithermal ore zones and was crosscut locally by Au-bearing calcite ‚Äö- quartz ‚Äö- pyrite epithermal veins in Kapit and Kapit NE. Modern geothermal activity has produced a layer of steam-heated smectite ‚Äö- kaolinite (argillic) and kaolinite ‚Äö- alunite (advanced argillic) alteration near surface. Downward percolating steam-heated fluids leached gold from large areas of mineralized rocks, which was reprecipitated within structures in the steam-heated alteration zone, producing thin gold-rich pyrite rims around earlier-formed sulfides. Significant compositional variability exists in pyrite at Lihir, as recorded by LA-ICP-MS raster maps and lines. A principal component analysis of transformed pyrite data and subsequent K-means clustering of the principal components has identified six pyrite clusters that can be linked to different stages of the hydrothermal paragenesis. The porphyry stage is associated with Cluster 1 (high Cu, Co, Pb, Bi and Se; low Sb and Tl) and Cluster 3 (high Ni, Cr, V and Co; low As), which have relatively low concentrations of gold (xvÉvë = 2.7 ppm Au; maximum 1,445 ppm Au) and total trace elements (xvÉvë = 5,429 ppm). Texturally, these pyrites form aggregates of coarser-grained, euhedral, cubic and zoned crystals with corrosion vugs and rounded margins. The epithermal stage is associated with Cluster 4 (high Mo, Te, Pb, Ag and Au; low Mn and Zn), Cluster 2 (high Ag and Au; low Co and V), Cluster 6 (high Zn, As, Mn, Tl and Sb; low Au, Cu and Pb) and Cluster 5 (high Tl, Sb, Mn, As, Zn and Au; low Ag and Mo). These pyrites commonly form rims around earlier-formed pyrite, and also occur as finegrained, anhedral to euhedral (equant, acicular, dendritic), porous and inclusion-rich aggregates with colloform, botryoidal or granular habit. Despite their textural variability, epithermal-stage pyrites are ubiquitously associated with high total trace element contents (xvÉvë = 22,258 ppm). Clusters 2, 4, and 5 contain high concentrations of gold (xvÉvë = 51.3 ppm Au; maximum 131,619 ppm Au), whereas Cluster 6 contains very low concentrations of gold (below detection limit). Epithermal stage veins and breccias display a characteristic progression from Cluster 4 to Cluster 2 to Cluster 5 pyrite, demonstrating that there was a repeated compositional evolution of the mineralizing fluids throughout the epithermal life cycle.