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The hydrothermal evolution and genesis of the porgera gold deposit, Papua New Guinea
Abstract
High grade gold mineralization at Porgera is the result of two spatially and temporally distinct mineralization events referred to as stages I and II. Stage I mineralization resulted in the formation of pyrite, Fe-rich sphalerite and galena rich vein sets, spatially associated with intrusions, and hosted in intrusions, the surrounding altered sediment and calcareous black shale. Early sulphide rich veins (A/B veins) contain several grams of Au/tonne but due to their scattered nature do not constitute high grade ore. The fluid responsible for this mineralization contained about 1 molal NaCl (corrected for C02 after Richards and Kerrich, 1993), a minimum of 2 molal C02, had an average Th of about 280°C (Richards et al., 1993) and was relatively reduced. Stage II mineralization which constitutes the bulk of the high grade ore is hosted in banded quartz and quartz-roscoelite pyrite-barite breccia veins (D veins), which are spatially associated with the Roamane Fault. Altered sediment and intrusive rock are the main host to stage II mineralization. Fluid inclusion data from stage II quartz yields a bimodal salinity at 4.2 and 7.8 wt % NaCl equiv,and an average Th of 145oC (Richards et al., 1993). The C02 content of these fluids is extremely variable ranging up to at least 2 molal. High grade ore shoots occur at the intersection of stage I and II veins sets. Munroe(1996) concluded that the Roamane Fault was not active during stage I, but was the main spatial control upon development of stage II mineralization. Stage I mineralization is hosted in and disseminated about A and B veins. In these veins quartz and carbonate are intimately intergrown with stage I sulphide but are rarely observed in equilibrium with sulphide. Textural evidence shows that pyrite, sphalerite, galena and arsenopyrite are aggressively replaced by quartz and carbonate. No evidence was found to support the existence of early A vein quartz and all gangue in these veins is interpreted to postdate pyrite, sphalerite and galena. It is inferred that sulphides formed early,in stage I, and were dissolved during stage II quartz and carbonate deposition. Evidence from this study suggests that all quartz and carbonate in A veins is temporally equivalent to material in the more spatially restricted stage II D veins. Alteration of the sediments was one of the earliest hydrothermal events and predates stage I. It is associated with the bleaching of black shales along thin grey pyrite veinlets termed G stringers, and may be equivalent to Richards and Kerrich's propylitic alteration of the intrusive phases. This alteration is associated with gains in K20, Rb, Ba, and As; and losses in Na20, CaO, C02, Pb, Zn, and Sr. Local geometry, relative paragenetic position and geochemistry suggest that the fluids responsible for this early alteration of the sediments emanated from the currently exposed intrusions. Intrusive rocks are altered next to both AlB (stage I) and D (stage II) veins. Altered selvedges next to both vein types have undergone gains in K20, C02, H20, Pb, As and S and losses in Na20, Sr and Ba. Electron microprobe analyses of stage II D vein pyrite indicate highly anomalous gold and arsenic values of up to 0.9 wt% Au and 7.7 wt% As. The highest values (both Au and As) are found in the high grade M126/Roamane Fault veins as well as A-D intersections. In A vein pyrite, gold is below detection and arsenic ranges from 300 ppm to 2.4 wt %. X-ray images of D vein pyrite for Au, As and Cu show well developed growth banding - with Au and As showing a moderate correlation. These images suggest that Au, As and Cu occur in solid solution within the pyrite structure. Bulk ()34S analysis of Stage I and Stage II pyrite by Richards and Kerrich (1993) gave values of +3 to +5 per mil and -14 to -10 per mil respectively. SHRIMP sulphur isotope analysis (this study) of Stage II pyrite yielded ()34S values ranging from -18 to -14 per mil. Pyrite from the early G stringers returned SHRIMP ()34S values of -4 per mil whereas bulk sulphur analyses returned values ranging from -1.2 to 3.7 per mil. While G stringers are not strongly negative they are more negative than the bulk rock values for stage I sulphides. These data indicate reducing fluids during stage I, oxidizing fluids during Stage II and an intermediate redox state for the early pre-stage I fluids. Since there are no thermodynamic data available for roscoelite and the only data available for other vanadium species are at 25°C, it was necessary to construct a thermodynamic data set to estimate the relative stability of roscoelite and other vanadium species in f02- pH space at temperatures up to 300°C. Results indicate that at neutral to even moderately acidic pH, vanadium is only mobile under oxidizing conditions and that roscoelite is stable under reducing conditions. This suggests that vanadium would be effectively transported by an oxidizing fluid and may be fixed as roscoelite upon reduction. It is proposed that Porgera evolved initially during stage I, within the Pb-Zn rich moderately distal reaches of a porphyry hydrothermal system, beneath a regionally extensive tectono-stratigraphic boundary. Base metals and gold were complexed as chloride and sulphide respectively, and were deposited through passive boiling of a C02 rich moderately saline fluid at about 280°C and under pressures approaching lithostatic. Rupture of the Roamane Fault resulted in pressure drop and associated throttling and adiabatic decompression of the fluid, which prematurely ended the stage I hydrothermal system. Fluid that originally deposited base metal sulphide and gold in the early veins, immediately began dissolving both, and simultaneously deposited quartz and carbonate as the fluid flowed into low pressures zones along the Roamane Fault. Here, the same fluid further decompressed and continued to deposit barren quartz and lesser carbonate. Decompression also permitted deeper sourced, fresh, oxidized magmatic fluid to quickly achieve similar levels within the Roamane Fault and mix with the reduced pregnant ore bearing fluid. Mixing of the two fluids at the intersection of stage I and II structures resulted in oxidation of the reduced gold bearing fluid and precipitation of the high grade gold rich quartz-roscoelite layers. That gold was carried in the reduced rather than the oxidized fluid is evidence by the replacement of pyrite by gold within the assemblage quartz-roscoelite-pyrite-barite. Key elements of the model are the carbonate rich host rocks, the C02 rich (high fluid pressure) composition of the early fluid and the presence of a regionally extensive pressure seal. Rupture of the Roamane Fault breached the seal causing rapid decompression of the fluid and loss of both C02 and H2S to the vapour phase which resulted in the dissolution of both sulphide and gold. Most previous studies of boiling hydrothermal systems emphasize the potential of boiling to deposit metals (Drummond, 1981; Drummond and Ohmoto, 1986; Cole and Drummond, 1986; Reed and Spycher, 1985). The current study emphasizes the potential of boiling to dissolve and remobilize both base metals and gold. Whether boiling results in sulphide deposition or dissolution is related to the relative balance between C02 loss (favouring pH increase and deposition) and H2S loss (favouring dissolution). Small amounts of boiling (- 5%) favour sulphide and gold deposition whereas large amounts of boiling/throttling (> 10%) favour dissolution. Reaction path modelling of various boiling scenarios has indicated that both sulphide and gold dissolution may be expected from a boiling fluid whose pH is buffered by the surrounding host rock. At Porgera it is suggested that boiling under similar conditions led to the remobilization of a significant proportion of the widely distributed stage I gold and its subsequent redeposition, through mixing induced oxidation, in a more concentrated form within a much smaller area. In the New Guinea Highlands most of the major Au/Cu deposits show a close spatial relationship to the contact between Tertiary Limestone and underlying Mesozoic clastic sediments. It is suggested that: (1) this contact/decollement surface acted as a major barrier to fluid flow causing a major discontinuity in fluid pressure of evolving hydrothermal systems; and (2) rupture of this contact lead to large scale decompression of large overpressured hydrothermal fluid reservoirs which lead to focussing of large amounts of fluid which simultaneously dissolved pre-existing sulphide and Au. The physical parameters necessary for decompression induced remobilization of gold are commonly achieved in hydrothermal systems. Many of the Carlin type deposits have similar characteristics (to those at Porgera) which include: (1) calcareous shale host rocks beneath regionally extensive tectono-stratigraphic boundary; (2) early widely dispersed disseminated/veinlet hosted mineralization deposited by early C02 rich fluids; and (3) later overprinting spatially restricted gold rich mineralization associated with arsenical pyrite within a relatively oxidized assemblage. In the Carlin type deposits it is possible that remobilization/leaching affected distal sediments as well as early scattered base metal sulphide mineralization (this is also a possibility at Porgera).
... Richards and Kerrich (1993) agreed with this interpretation and argued, on the basis of mineralogical and textural grounds, that vapor phase separation was responsible for stage II ore formation, a model further supported by fluid inclusion gas data (Richards et al., 1997). However, Wall et al. (1995) and Cameron (1998) concluded from studies of vanadium chemistry and mineralogical observations that mixing of an oxidized magmatic fluid and a reduced fluid expelled from carbonaceous sediments was responsible for the formation of high-grade zones. Most recently, Ronacher et al. (2000a, b) documented coexisting vapor-and liquid-rich fluid inclusions in stage II quartz and suggested that boiling was, at least locally, responsible for ore formation. ...
... However, we assumed that precipitation of muscovite occurs under similar conditions as roscoelite because of the chemical similarity of the two micas [KAl 2 (Si 3 AlO 10 )(OH) 2 versus K(V,Al) 2 (Si 3 AlO 10 )(OH) 2 ]; therefore, muscovite was used as a proxy for roscoelite. Cameron (1998) modeled the stability of roscoelite and concluded that at 200°C roscoelite was stable between pH values of 4 and 12 and log fO 2 values between -60 and -35, thus overlapping the muscovite field. ...
... In addition, vaporrich fluid inclusions coexisting with liquid-rich inclusions were observed in stage II quartz intergrown with gold in one sample from near the Roamane fault zone (Ronacher et al., 2000b). In contrast, Wall et al. (1995) and Cameron et al. (1995), based on considerations of mobility of V 3+ versus V 5+ in a hydrothermal fluid, argued that precipitation of roscoelite was a result of reduction (rather than oxidation caused by boiling) and that mixing with a reduced fluid was a likely mechanism of ore formation (see also Cameron, 1998). ...
The ∼20-million-ounce (Moz) Porgera gold deposit, Papua New Guinea, is hosted by 6-m.y.-old alkalic intrusions and Cretaceous sedimentary rocks in which the intrusions were emplaced. Gold-bearing veins occur in three stages: (1) magnetite-sulfide-carbonate ± quartz veins with minor gold (prestage I), (2) base metal-sulfide-carbonate ± quartz ± Au veins (stage I), and (3) quartz-roscoelite-pyrite-gold veins and breccias (stage II). Stage II veins are economically the most significant. Quartz-roscoelite-pyrite-gold veins form high-grade zones associated with the Roamane fault (a late normal fault that crosscuts the intrusive complex) and in the footwall to the fault (the North zone). The North zone mineralization is the main focus of this study. The quartz-roscoelite-pyrite-gold assemblage occurs in three texturally distinct styles: (1) thin (1-5 mm) veinlets in which roscoelite-pyrite-gold are more abundant than quartz and in which roscoelite and gold also occur in the wall rock; (2) veins (5 mm to 10 cm) in which roscoelite-pyrite-gold with minor quartz form a band at the vein edges, followed by coarse-grained quartz and the vein centers commonly filled with anhydrite and carbonate; and (3) breccia veins and breccias in which wall-rock fragments are rimmed by roscoelite-pyrite-gold and minor quartz, followed by vuggy quartz infilling. Fluid inclusions from quartz in these veins and breccias are mostly liquid rich, and average salinities in individual samples range from 7.5 ± 1.0 to 9.6 ± 0.2 wt percent NaCl equiv. In five of 27 samples, an additional cluster of salinities between 4.4 and 6.2 wt percent NaCl equiv was observed. These relatively low salinity inclusions occur toward the vein center and are less abundant than high-salinity inclusions that occur toward the vein margins. Three samples exhibit a continuous salinity trend from 4.5 to 10.2 wt percent NaCl equiv. For samples where CO2 analyses were available average corrected salinities range from 5.1 to 8.0 wt percent NaCl equiv. Average homogenization temperatures (Th) of individual samples range from 127° ± 12° to 167° ± 25°C. The average Th of the low-salinity inclusions (145° ± 9°C) is marginally lower but overlaps with that of all high-salinity inclusions (152° ± 17°C). Gas chromatographic analyses showed that the high-salinity fluid contains up to 2 mol percent CO2, 0.11 mol percent CH4, 0.065 mol percent N2, and traces of C2H4, C2H6, and COS. Concentrations of Cl- (310-609 mM/l), Br- (0.28-0.75 mM/l), Li+ (1.25-8.80 mM/l), Na+ (462-1126 mM/l), K+ (0-81 mM/l), Mg2+ (0-7.0 mM/l), and Ca2+ (0-185 mM/l) were determined by ion chromatography.
... Interestingly, high-grade zones in some epithermal deposits have associated anomalous gangue minerals. Reyerite ([Na,K] 4 Ca 14 (Si,Al)Si 24 O 60 (OH) 5 , 5H 2 O) occurs at Hishikari, Japan and Lebong Donok, Sumatra, (Henley 1991), perhaps reflecting extreme supersaturation conditions; also, the occurrence of roscoelite (vanadiummica) at Porgera, Papua New Guinea (Cameron 1998) may indicate how the excess heat effect amplifies trace metal concentrations derived from local rocks. Corrosive textures are commonly seen in laminated silica-adularia veins, again showing how chemical pathways flip-flop across depositional boundaries. ...
... Thompson & Arehart (1990) speculated that the banding in replacement style ores in the Leadville district, Colorado resulted from autocatalytic reactions. Cameron (1998) has documented dissolution of auriferous sulfide from early high-grade vein material at Porgera, Papua New Guinea, again indicating the flip-flop from supersaturated to undersaturated conditions during vein formation. Sulfide replacement and pyrite formation rates may also be dependent on oscillations in a redox state manifested by the concentration of thiosulfate (Howd & Barnes 1975, Schoonen & Barnes 1991; recent data (Dadze & Sorokin 1993) suggest that may be more common in hydrothermal systems than generally assumed. ...
... For example, discontinuities in rock properties and consequent focused heat transfer effects during deformation may localize bonanza mineralization. The Porgera gold deposit (Papua New Guinea), for example, is located immediately below the regional-scale 1200 m-thick Dari Limestone (Cameron 1998). In the Carlin district, detailed mapping and logging have enabled costly deep drilling to locate discrete ore bodies closely associated with the interfaces of distinct lithologies (Vikre et al 1997). ...
Epizonal base and precious metal deposits makeup a range of familiar deposit styles including porphyry copper-gold, epithermal veins and stockworks, carbonate-replacement deposits, and polymetallic volcanic rock-hosted (VHMS) deposits. They occur along convergent plate margins and are invariably associated directly with active faults and volcanism. They are complex in form, variable in their characteristics at all scales, and highly localized in the earth’s crust.
More than a century of detailed research has provided an extensive base of observational data characterizing these deposits, from their regional setting to the fluid and isotope chemistry of mineral deposition. This has led to a broad understanding of the large-scale hydrothermal systems within which they form. Low salinity vapor, released by magma crystallization and dispersed into vigorously convecting groundwater systems, is recognized as a principal source of metals and the gases that control redox conditions within systems. The temperature and pressure of the ambient fluid anywhere within these systems is close to its vapor-liquid phase boundary, and mineral deposition is a consequence of short timescale perturbations generated by localized release of crustal stress.
However, a review of occurrence data raises questions about ore formation that are not addressed by traditional genetic models. For example, what are the origins of banding in epithermal veins, and what controls the frequency of oscillatory lamination? What controls where the phenomenon of mineralization occurs, and why are some porphyry deposits, for example, so much larger than others?
The distinctive, self-organized characteristics of epizonal deposits are shown to be the result of repetitive coupling of fracture dilation consequent on brittle failure, phase separation (“boiling”), and heat transfer between fluid and host rock. Process coupling substantially increases solute concentrations and triggers fast, far-from-equilibrium depositional processes. Since these coupled processes lead to localized transient changes in fluid characteristics, paragenetic, isotope, and fluid inclusion data relate to conditions at the site of deposition and only indirectly to the characteristics of the larger-scale hydrothermal system and its longer-term behavior. The metal concentrations (i.e. grade) of deposits and their internal variation is directly related to the geometry of the fracture array at the deposit scale, whereas finer-scale oscillatory fabrics in ores may be a result of molecular scale processes.
Giant deposits are relatively rare and develop where efficient metal deposition is spatially focused by repetitive brittle failure in active fault arrays. Some brief case histories are provided for epithermal, replacement, and porphyry mineralization. These highlight how rock competency contrasts and feedback between processes, rather than any single component of a hydrothermal system, govern the size of individual deposits. In turn, the recognition of the probabilistic nature of mineralization provides a firmer foundation through which exploration investment and risk management decisions can be made.
... Anhydrous silicates (e.g. diopside and K-feldspar) may form at this stage and the processes typically release fluids previously bound in hydrous or carbonate alteration assemblages during mode I. Mode II is noticeably episodic with many (100s to 1000s) of overprinting events recorded as multiple stages of brecciation, crack-seal microstructures, laminated veins, compositional zoning in minerals, fluid inclusion compositions and multiple stages of gold deposition and, sometimes, gold dissolution (Cameron 1998) and remobilization (Fougerouse et al. 2016). For examples of these modes of operation in hydrothermal systems, see Oreskes & Einaudi (1990), Wilkinson & Johnson (1996), Barnicoat et al. (1997), Henley & Berger (2000), Baker et al. (2013), Wilson et al. (2009Wilson et al. ( , 2013, Schaubs & Zhao (2002) and Zhu et al. (2011). ...
Episodic fluctuations in fluid pressure and temperature are characteristic of the behaviour of orogenic gold systems and are commonly attributed to processes external to the system, such as seismic events and associated adiabatic fault valve or suction pump/piston behaviour; any temperature changes are attributed to the adiabatic nature of the process. Such processes are commonly associated with fluctuations in fluid pressure with little, if any, changes in temperature. We describe aseismic, non-adiabatic processes internal to the hydrothermal system that lead to episodic modes of temperature and fluid pressure behaviour and the deposition of gold. These gold deposition processes are essentially controlled by localized changes in temperature; such temperature-dependent gold deposition processes are normally thought of as inconsequential. We propose that internal episodic behaviour is fundamental in hydrothermal mineralizing systems associated with orogenic gold deposits. Importantly, the time period for these hydrothermal events appears to be small relative to metamorphic systems, with 1-2 myr as an upper limit. This has important ramifications for rates of heat production and for the resultant kinetics of mineral reactions during alteration and mineralization. We explore these systems as non-linear, nonequilibrium dynamic, open flow systems.
Porgera is a ~974-metric ton (t) Au, low-sulfidation, alkalic, epithermal gold deposit located in Papua New Guinea. The deposit is spatially associated with 6 Ma stocks of the mafic alkalic Porgera Intrusive Complex, which were emplaced within Cretaceous carbonaceous mudstones in a transpressional orogenic setting linked to continent-island arc collision. As with many other major magmatic-hydrothermal ore deposits in New Guinea, deep-seated, arc-normal transfer structures have been suggested as controls on intrusion emplacement through the creation of a localized extensional environment favorable for magma ascent. Gold mineralization occurred in two distinct phases, both within ≤0.2 m.y. of emplacement of the Porgera Intrusive Complex. Stage 1 mineralization of intrusion-related carbonate-base metal association consists of extensional vein swarms dominated by coarse intergrown pyrite ± galena and sphalerite, generally hosted within or proximal to the intrusive bodies of the Porgera Intrusive Complex. These veins represent the lowest grade and economically least significant mineralization phase. Overprinted high-grade epithermal Stage 2 mineralization consists of roscoelite, pyrite, and quartz veins and breccia veins ± subordinate amounts of barite, marcasite, sphalerite, tetrahedrite, galena, hematite, and tellurides. Gold mineralization is commonly associated with the roscoelite-rich coatings on vein walls or breccia clasts. Stage 2 mineralization is controlled by a deposit-scale extensional fault-fracture mesh and displays a variety of textural styles including: (1) <5-mm veinlets dominated by roscoelite, pyrite, and gold; (2) thicker veins up to 10 cm wide with roscoelite, pyrite, and gold on the margins with central bands of alternating crustiform quartz and thin layers of roscoelite-pyrite-gold; (3) hydrothermal breccias with roscoelite, pyrite, and gold coating breccia margins and internal clasts, with crustiform quartz forming the matrix. The giant endowment of the Porgera gold system is attributed to its favorable tectonic location and local extensional setting, its vertical extent, the oxidized nature of the mineralizing fluids, and highly efficient gold precipitation.
Laser ablation-inductively coupled plasma-mass spectrometry was used to traverse hydrothermal vein sphalerite from different ore-forming stages of the Porgera Au-Ag mine, Papua New Guinea. Elements were measured in situ over the growth of crystals to investigate the greatly varying concentrations of cations in sphalerite and their positions in the lattice. Traverse profiles for 16 elements were obtained and aligned to transmitted light images where possible. Each sample contained an array of elements, with many displaying orders of magnitude concentration differences.
Results show the simultaneous incorporation of Cu and Sn in sphalerite, as well as Cu and Ag, In and Sn, As and Sb, Fe and Mn, and Cu and Ga. The relation [4Zn2+ ↔ 2Cu+ + Sn2+ + Sn4+] is proposed to explain the 1:1 Cu–Sn correlation. Further relations can be seen, including a Ga “ceiling” or Cu “floor”, where Ga incorporation becomes dependent on Cu concentrations. Furthermore, silver was also observed to correlate with Au, Mn, Ni, Pb, and Bi.
Meta-stable solid solutions between pairs such as Cu, Ag; Fe, Mn; As, Sb; and In, Sn are also suggested. Each of these pairs are neighbors on the periodic table of elements, which suggests that simple solid solution can occur, and positive correlations for all four solid solutions were found in one sample alone.
While the concept of charge-specific solid solutions in sphalerite has been discussed in the literature with reference to monovalent cations, the results presented herein also indicate solid solutions of higher oxidation states, containing many cations. Furthermore, while cations in charge-specific solid solutions have been proposed to compete for lattice sites in sphalerite, simultaneous in situ coupled concentrations at Porgera suggest otherwise.
Cationic substitution equations displaying decimal ratios of each element in solid solution can then provide a novel method to distinguish between solid solution concentrations in different samples. For example, displaying 1:1 ratios of Cu–Ag and Sb–As: [2Zn2+ ↔ (Cu+0.5, Ag+0.5) + (As3+0.5, Sb3+0.5)], or for a 100:1 Fe–Mn ratio: [Zn2+ ↔ (Fe2+0.99, Mn2+0.01)].
This Chapter provides 42 Tables summarizing the characteristic features of global gold-copper deposits associated with potassic igneous rocks for which substantial information is available.
Examples of direct associations between potassic igneous rocks and copper-gold deposits described in this Chapter are, in order of increasing age: the Quaternary Ladolam gold deposit, Lihir Island, Papua New Guinea; the Tertiary Emperor gold deposit, Viti Levu, Fiji; the Pliocene Grasberg copper-gold deposit, Irian Jaya, Indonesia; the Pliocene Misima gold deposit, Misima Island, Papua New Guinea; the Miocene Agua Rica copper-gold deposit, Catamarca Province, Argentina; the Miocene Bajo de la Alumbrera copper-gold deposit, Catamarca Province, Argentina; the Miocene Dinkidi copper-gold deposit, Didipio district, Philippines; the Miocene El Indio gold deposit, Chile; the Miocene Porgera gold deposit, Papua New Guinea; the Miocene Skouries copper-gold deposit, Chalkidki Peninsula, Greece; the Miocene Sungun copper deposit, Iran; the Miocene Yulong copper deposit, Tibet, China; the Oligocene Cripple Creek gold deposit, Colorado, USA; the Oligocene El Abra copper-molybdenum deposit, Chile; the Eocene Bingham copper-gold deposit, Utah, USA; the Cretaceous Peschanka copper-gold deposit, Siberia, Russa; the Jurassic Mount Milligan copper-gold deposit, British Columbia, Canada; the Devonian Oyu Tolgoi porphyry copper-gold deposit, Mongolia; and the Ordovician Cadia and Northparkes copper-gold deposits, New South Wales, Australia. This Chapter also includes new colour photos of ore samples from gold-copper deposits hosted by potassic igneous rocks.
Processes responsible for high-grade gold enrichment are of significant economic interest and have long been elusive. Structural and geophysical models postulate that earthquake-induced fault-valve processes are frequently responsible for gold mineralization in structurally controlled hydrothermal systems, although a comprehensive geochemical record of these processes has been lacking. The Porgera gold deposit (Papua New Guinea) provides an opportunity to examine the complex processes surrounding high-grade Au mineralization in structurally controlled hydrothermal systems. New high-resolution trace element and sulfur isotope data obtained in situ by laser ablation-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe-stable isotope analysis of pyrite across a well-constrained paragenetic sequence reveal a temporal evolution in fluid chemistry. The data uncover a stratigraphy of repeated high-Au negative delta S-34 and low-Au positive delta S-34 zones within individual pyrite crystals present in the highest grade gold event (stage II). Results provide the first clear geochemical documentation of rapid, complete switching in ore-forming processes, preserved in individual pyrite crystals throughout an evolving hydrothermal system. It is postulated that geochemical variations are attributable to rapid Au deposition during pressure release due to fault failure followed by switching to more reduced rock-buffered (metal poor) fluids from adjacent sediments as the rupture seals. The cyclicity of this process is reflected in the pyrite zonation seen at the Porgera deposit. This study demonstrates the usefulness of applying contemporary analytical techniques to pyrite in hydrothermal systems to gain new insights into ore genesis and fluid pathways, revealing that deposition of exceptionally high grade ore at the Porgera deposit was the result of multiple events in a fluctuating ore-forming environment.
The tables in this chapter summarize the characteristic features of some gold-copper deposits associated with potassic igneous rocks for which substantial information is available.
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