Gold commonly occurs in pyrite (FeS2) as "invisible" or refractory gold, as is the case at the giant Lihir (i.e., Ladolam) hybrid alkali-type gold deposit in Papua New Guinea. The Lihir gold deposit is also unique as it the exemplar of a telescoped ore deposit, whereby volcanic sector collapse led to superimposition of shallow-level gold-rich epithermal mineralization upon preexisting, but genetically related, porphyry-style alteration. While this superimposition led to a giant 56 Moz gold resource, it also created complications with regards to ore processing, specifically with regards to the difficulties in mineral processing of the refractory gold-rich pyritic ore.We have analyzed trace element zonation and composition of pyrite grains, using LA-ICP-MS imaging coupled with NaOCl etching, from a subset of spatially and paragenetically constrained pyrite-bearing samples from the Lienetz orebody. Pyrite grains belong to either porphyry-or epithermal-stages, or are composite pyrite grains with a multi-stage history. Trace element zonation and metal contents are unique to pyrite from each paragenetic event, providing insights into the nature of the mineralizing fluids. Early generations of coarse-grained pyrites that formed under higher temperature porphyry-style conditions have low trace element contents compared to epithermal-stage pyrites, except for Co, Ni and Se. Later generations of oscillatory zoned pyrites that formed under lower temperature epithermal conditions are comparatively enriched in trace elements such as As, Mo, Ag, Sb, Au and Tl. The composite pyrites are relatively coarse-grained and display textural and geochemical evidence of modification (i.e., dissolution and re-precipitation). They are interpreted to be porphyry-stage pyrite grains that have been overgrown by rims of delicate banded epithermal-style pyrite enriched in gold, arsenic and other trace elements.The composite pyrite grains are volumetrically dominant in the deep-seated anhydrite zone at Lienetz. Because gold is concentrated only along the rims of these pyrite grains, they can be subjected to a shorter period of oxidation and leaching to liberate most of their gold. This is in contrast for areas dominated by high-grade epithermal-stage mineralization where 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 optimising mineral processing and can impact significantly on the economics of mining these complex, hybrid ore deposits.

The Lihir gold deposit (also known as Ladolam), Papua New Guinea, has a 56 Moz resource and is the world's largest alkalic gold deposit in terms of contained gold. The gold deposit is in an amphitheater, inferred to be a remnant of the original ~ 1.1 km high volcanic cone that underwent northeast-directed sector collapse(s) and prolonged tropical weathering. The ore deposit is situated in the foot wall of the sector collapse detachment surface and consists of several orebodies.Late-stage, gold-rich, alkalic low-sulfidation epithermal mineralization was superimposed upon early-stage, porphyry-style alteration. A three-fold 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, steam-heated clay alteration zone that is a product of modern hightemperature 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 magmatichydrothermal 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 sub-vertical and sub-horizontal dips. This occurred in the porphyry-style environment, under low differential stress, an oscillating sub-horizontal to sub-vertical σ 1 , and temporarily elevated fluidpressures 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 northwest-directed extension with a mostly sub-vertical σ 1 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.3 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 trans...

Identifying the location of intrusions is a key component in exploration for porphyry Cu ± Mo ± Au deposits. In typical porphyry terrains, in the absence of outcrop, intrusions can be difficult to discriminate from the compositionally similar volcanic and volcanoclastic sedimentary rocks in which they are emplaced. The ability to produce lithological maps at an early exploration stage can significantly reduce costs by assisting in planning and prioritization of detailed mapping and sampling. Additionally, a data-driven strategy provides opportunity for the discovery of intrusions not identified during conventional mapping and interpretation. We used Random Forests (RF), a supervised machine learning algorithm, to classify rock types throughout the Kliyul porphyry prospect in British Columbia, Canada. Rock types determined at geochemical sampling sites were used as training data. Airborne magnetic and radiometric data, geochemistry, and topographic data were used in classification. Results were validated using First Quantum Minerals’ geologic map, which includes additional detail from targeted location and transect mapping. The petrophysical and compositional similarity of rock types resulted in a noisy classification. Intrusions, particularly the more discrete, were inconsistently predicted, likely due to their limited extent relative to data sampling intervals. Closer examination of class membership probabilities (CMPs) identified locations where the probability of an intrusion being present was elevated significantly above the background. Indeed, a large proportion of mapped intrusions correspond to areas of elevated probability and importantly, areas were highlighted as potential intrusions that were not identified in geologic mapping. The RF classification produced a reasonable lithological map, if lacking in resolution, but more significantly, great benefit comes from the insights drawn from the RF CMPs. Mapping the spatial distribution of elevated intrusion CMP, a soft classifier approach, produced a map product that can target intrusions and prioritize detailed mapping for mineral exploration.

The Lihir gold deposit, Papua New Guinea, is the world’s largest alkalic low-sulfidation epithermal gold deposit in terms of contained gold (50 Moz). The deposit formed over the past million years and records a progression from porphyry- to epithermal-style hydrothermal activity. The early porphyry stage was characterized by biotite-anhydrite-pyrite ± K-feldspar ± magnetite alteration and weak gold ± copper mineralization and produced abundant anhydrite ± carbonate veins and anhydrite ± biotite-cemented breccias. These features collectively characterize the deep-seated anhydrite zone at Lihir. Several hundred thousand years ago, one or more catastrophic mass-wasting events unroofed the porphyry system after porphyry-stage hydrothermal activity ceased. Mass wasting may have been facilitated in part by dissolution of porphyry-stage anhydrite veins. Epithermal mineralization occurred after sector collapse, resulting in phreatic and hydraulic brecciation and veining, widespread adularia-pyrite ± carbonate alteration, and formation of mineralized zones at Lienetz, Minifie, Kapit, Kapit NE, Coastal, and Borefields. A NE- to ENE-striking fault array localized several of these orebodies. The pyrite-rich veins and pyrite-cemented breccias that formed during epithermal-stage hydrothermal activity define the sulfide zone at Lihir. This zone mostly contains refractory gold in pyrite, with minor free gold and precious metal tellurides hosted in late-stage quartz veins. A period of diatreme volcanism disrupted the Luise amphitheater during the latter stages of epithermal mineralization. The diatreme breccia complex truncated several of the epithermal ore zones and was crosscut locally by late-stage epithermal veins. Recent geothermal activity produced a steam-heated clay alteration blanket that has overprinted the refractory sulfide-rich epithermal assemblage near the present-day land surface. Gold was remobilized downward from the steam-heated zone into the sulfide zone during argillic and advanced argillic alteration, producing thin gold-rich rims around pyrite grains. This process produced a high-grade tabular enrichment zone immediately beneath the base of the clay blanket.