Evidence for fluid phase separation in high-grade ore zones at the Porgera gold deposit, Papua New Guinea

Coexisting, liquid-rich and vapor-rich primary fluid inclusions in quartz provide direct evidence for fluid phase separation in high-grade quartz–roscoelite–gold veins and breccias from the Porgera alkalic-type gold deposit. Vapor-rich fluid inclusions are CO₂-rich, and sometimes contain liquid CO₂ at room temperature. The close spatial and paragenetic relationship between these “boiling assemblage” fluid inclusions and gold suggests that gold was precipitated by phase separation, at least locally. Additionally, the occurrence of carbonate and sulfate minerals in high-grade veins (reflecting pH increase and oxidation of the boiled fluid) and the appearance of hydrothermal breccias, are consistent with the process of fluid phase separation. Liquid CO₂-bearing fluid inclusions are rare in near-surface epithermal deposits, and indicate that the Porgera vein system was formed at greater depths and pressures (our estimates suggest pressures between 250 and 340 bars). It is suggested that alkalic-type gold deposits may be distinguished from other epithermal deposit types by the more gaseous nature of the ore-forming fluids, in addition to their association with alkalic magmas. Received: 24 February 2000 / Accepted: 6 April 2000     

巴布亚新几内亚波格拉斑岩型铜-金矿床富Nb碱性火成岩成因新解

巴布亚新几内亚波格拉(Porgera)斑岩铜-金矿床的侵入杂岩岩石类型为碱性玄武岩→夏威夷岩(粗面玄武岩)→橄榄粗安岩套.微量元素总体特征是富集大离子亲石元素(LILE)(Sr、Ba)和高场强元素(HFSE)(Zr、Nb、Th) .在微量元素蛛网图上显示宽的Th、Nb、U峰和Ti、Hf、Y的谷.该矿床夏威夷岩和橄榄粗安岩是一套板内富Nb碱性玄武岩系列,属于大陆OIB碱性侵入岩,而不是洋岛OIB系列,与地幔柱活动相关.波格拉侵入杂岩形成于弧-陆碰撞环境,为新生代主动大陆边缘板内碱性玄武岩的典型代表之一.     

Short life-span of the ore-forming system at the Porgera gold deposit, Papua New Guinea: laser 40Ar/39Ar dates for roscoelite, biotite, and hornblende

The Porgera gold deposit, Papua New Guinea, is associated with sodic-alkalic, hypabyssal intrusions of alkali basaltic to mugearitic composition. The intrusions were emplaced into Cretaceous mudstones and siltstones in the latest Miocene, and both igneous and sedimentary rocks are mineralized. Three types of veins occur: (1) magnetite-sulfide - Au-carbonate veins; (2) base-metal sulfide - Au-carbonate veins; and, economically most important, (3) quartz-roscoelite-Au veins and breccias. Magmatic hornblende and biotite, hydrothermal biotite from alteration selvages associated with the magnetite-sulfide - Au veins, and roscoelite were dated using the laser ⁴⁰Ar/³⁹Ar step-heating technique. Magmatic biotite yielded a date of 5.99ǂ.11 Ma (2C error; MSWD=3.7), and two samples of magmatic hornblende provided dates of 6.35ǂ.23 Ma (MSWD=1.0) and 6.3ǂ.7 Ma (MSWD=2.6). Hydrothermal biotite provided a date of 5.98ǂ.13 Ma (MSWD=2.7), and analyses of two roscoelite samples yielded dates of 5.92ǂ.09 Ma (MSWD=2.0) and 5.92ǂ.17 Ma (MSWD=2.0). The date of the magmatic biotite is interpreted to represent the age of the intrusions more accurately than the hornblende dates because small amounts of excess ⁴⁰Ar were present in the hornblende samples. The date for hydrothermal biotite is interpreted to be the age of the alteration associated with the magnetite-sulfide - Au veins (the paragenetically earliest veins), and thus reflects the age of the onset of the mineralizing activity at Porgera. Based on the two dates for roscoelite, the age of the main ore deposition event is interpreted to be 5.92ǂ.08 Ma. The apparent ages for the intrusive and the mineralizing events are thus identical within error, and suggest that the magmatic and ore-forming system at Porgera was short-lived.     

与碱性岩有关的浅成低温热液型金矿特征与控矿因素——以巴布亚新几内亚波尔盖拉金矿为例

与碱性岩有关的浅成低温热液型金矿在全球特别是环太平成矿带中具有重要意义,波尔盖拉金矿位于新几内亚造山带内较为典型的该类型金矿床。矿床的矿化与碱性的波尔盖拉侵入杂岩体有关,矿化主要发育在岩体与白垩纪Chim组的接触部位,矿化作用分为2期,矿化形态主要为脉状和角砾岩矿化,成矿物质和成矿流体具有明显的岩浆来源的特征。与成矿有关的波尔盖拉杂岩体侵入时代为5.99Ma,与成矿时间一致,并来源于被交代富集型的上地幔,该类碱性岩浆对于金的富集具有重要的意义。通过总结该类矿床的控矿因素和找矿标志为中国企业在该地区寻找同类型金矿提供参考。     

巴布亚新几内亚利希尔岛拉多拉姆(Ladolam)金矿矿床地质特征及找矿标志

巴布亚新几内亚利希尔岛拉多拉姆金矿处于太平洋板块与印澳板块的板块边界部位,为西南太平洋地区最大的碱性侵入岩有关的低硫型浅成低温热液型金矿。矿床赋存在Luise火山内,成矿与更新世碱性二长闪长岩侵入体有关,该岩体形成于俯冲带环境下被流体交代的上地幔的部分熔融。矿床的矿化可分为三个阶段,在岩体侵入后形成斑岩铜矿化,并在之后逐渐过渡为浅成低温热液型金矿化。该类矿床在成矿过程和矿体的分布明显受到区域性构造、蚀变带和成矿有关角砾岩等成矿结构面的控制,在找矿标志上具有特定的物化探和地质特征,具有较好的找矿远景。     

Comparison between magmatic activity and gold mineralization at Conical Seamount and Lihir Island, Papua New Guinea

Summary Grab samples from the submarine Conical Seamount, located about 10km south of the giant Ladolam gold deposit, Lihir Island, reveal the highest gold concentrations yet reported from the modern seafloor. Lavas from Conical Seamount are characterized by high K₂O contents, high K₂O/Na₂O ratios, and high Ce/Yb ratios, which are typical of high-K igneous rocks from oceanic (island) arc-settings. The primitive character of the rocks from Conical Seamount implies a magmatic evolution related to a single eruptive phase, which contrasts with the more evolved rocks forming Lihir Island. Geochemical as well as mineral chemical data suggest that the melts from both Conical Seamount and Lihir Island originate from the same magma source. In common with the samples from Lihir Island, elevated oxygen fugacities of 0.7–2.5log units above the FMQ buffer are recorded from the Conical Seamount lavas.There are distinct differences between the mineralization styles at Conical Seamount and at the Ladolam gold deposit, Lihir Island. While early-stage pyritic stockwork mineralization at Conical Seamount is hosted by clay-silica altered basaltic rocks with local alunite±kaolinite alteration, main-stage Au-mineralization occurs in sericite-alkali feldspar altered polymetallic sulfide-bearing siliceous veins. By contrast, early-stage pyritic stockwork mineralization at Ladolam is restricted to biotite–magnetite ± silica-altered monzodiorite, while the main-stage bulk-tonnage mineralization occurs as auriferous pyrite-bearing hydrothermal breccias which, in places, are cut by quartz–chalcedony–illite–adularia–pyrite±marcasite veins containing isolated bonanza gold grades.     

巴布亚新几内亚奥克泰迪铜金矿床成矿特征和控制因素

巴布亚新几内亚西部Fubilan山奥克泰迪矿床是一个世界级铜金矿床,在大地构造上位于新几内亚造山带的巴布亚褶皱带。该矿床的铜金矿化赋存于Fubilan二长斑岩及其周边的磁铁矿夕卡岩和硫化物夕卡岩中。矿石类型以原生硫化物矿石为主,金属矿物包括磁铁矿、黄铁矿、磁黄铁矿、白铁矿、黄铜矿、斑铜矿等。蚀变类型包括夕卡岩化、钾化、泥化和青盘岩化。矿床氧化次生富集带发育,表生矿石矿物为蓝辉铜矿、辉铜矿、自然铜、铜蓝和银金矿。成矿作用主要受区域构造、侵入杂岩体、Darai组灰岩地层、断裂等因素的控制。根据矿床的主岩、矿石特征、蚀变特征和控矿因素,认为该矿床成因类型属于较为典型的夕卡岩—斑岩型矿床。     

巴布亚新几内亚奥克泰迪铜金矿床成矿特征和控制因素

巴布亚新几内亚西部Fubilan山奥克泰迪矿床是一个世界级铜金矿床,在大地构造上位于新几内亚造山带的巴布亚褶皱带。该矿床的铜金矿化赋存于Fubilan二长斑岩及其周边的磁铁矿夕卡岩和硫化物夕卡岩中。矿石类型以原生硫化物矿石为主,金属矿物包括磁铁矿、黄铁矿、磁黄铁矿、白铁矿、黄铜矿、斑铜矿等。蚀变类型包括夕卡岩化、钾化、泥化和青盘岩化。矿床氧化次生富集带发育,表生矿石矿物为蓝辉铜矿、辉铜矿、自然铜、铜蓝和银金矿。成矿作用主要受区域构造、侵入杂岩体、Darai组灰岩地层、断裂等因素的控制。根据矿床的主岩、矿石特征、蚀变特征和控矿因素,认为该矿床成因类型属于较为典型的夕卡岩一斑岩型矿床。     

Geochemical exploration at yandera porphyry copper prospect, Papua New Guinea

A case history is presented describing geochemical exploration of the porphyry Cu-Mo system at Yandera in the Bismark Ranges of Papua New Guinea. Three phases of geochemistry are discussed: (1) stream sediment, (2) ridge and spur, and (3) detailed rock and soil from contour trails. Results of each phase are presented and their relationship to drill-indicated mineralisation discussed. The effectiveness of the three techniques and various elements as geochemical guides to primary mineralisation is compared.Copper in stream sediment samples at a density of one sample per 1–2 km² effectively delineated the porphyry Cu system. The extent of ridge and spur sampling was limited, therefore its effectiveness is uncertain. Detailed sampling along contour trails indicated that Au and Mo are the most effective geochemical tools. Copper geochemistry is of limited use as its distribution is largely a function of recent processes and dispersion from supergene mineralisation. No clear relationship exists between Ag geochemistry and mineralisation. Lead and Zn are distributed peripheral to the porphyry Cu system.The effectiveness of Au and, to a lesser extent, Mo as geochemical guides to ore in the steep mountainous terrain of the prospect area where high rainfall, deep weathering and rapid erosion prevail is due to the relatively high stability of these metals in the soils and the oxide zone.     

胶东牟平邓格庄金矿床流体包裹体研究

邓格庄金矿是胶东牟平-乳山金成矿带内第二大石英脉型金矿,金主要产于黄铁矿和多金属硫化物石英脉/细脉中。流体包裹体研究表明,邓格庄金矿不同蚀变带岩石和各成矿阶段金矿石中的流体包裹体主要有三种类型：H2O-CO2包裹体、CO2-H2O±CH4包裹体和H2O溶液包裹体。早期乳白色石英中主要赋存原生的H2O-CO2包裹体和次生的CO2-H2O±CH4包裹体;成矿期黄铁矿石英脉和多金属硫化物石英脉中的CO2-H2O±CH4包裹体主要为原生,随机分布,气液比变化较大,有时出现不同相比例的包裹体共存现象,而H2O溶液包裹体明显沿愈合裂隙分布;在成矿晚期的石英和方解石中主要发育原生H2O溶液包裹体。显微测温结果显示,成矿前（第1阶段）H2O-CO2包裹体的完全均一温度（Th,TOT,至液相）为254℃至365℃,成矿期（第Ⅱ和Ⅲ阶段）CO2-H2O±CH4包裹体的完全均一温度（Th,TOT,至液相）为195～317℃,成矿后（第Ⅳ阶段）H2O溶液包裹体的均一温度（Th,TOT;至液相）为156—219℃。成矿的初始流体富CO2,主成矿期有CH4流体加入,成矿晚期则演化为低温的水溶液流体。水／岩反应及流体不混溶可能是邓格庄金矿金沉淀的主要原因。     

The transition from porphyry- to epithermal-style gold mineralization at Ladolam, Lihir Island, Papua New Guinea: a reconnaissance study

The exceptionally large gold resource at Ladolam (>1,300 metric tons of gold), Lihir Island, resulted from the transition of an early-stage, low-grade porphyry gold system to a low-sulfidation epithermal gold event. This transition was probably triggered by rapid decompression during the partial slope failure of Luise stratovolcano and accompanied by the ingress of seawater. The original porphyry stage is indicated by remnant hydrothermal breccia clasts of strongly biotite-magnetite altered monzodiorite with disseminated pyrite - chalcopyrite and poorly developed pyrite - quartz stockwork veins. The breccias are overprinted by biotite-magnetite alteration and their matrix is strongly mineralized with disseminated auriferous pyrite. The breccias are cut by late-stage epithermal quartz-chalcedony-illite-adularia-pyrite veins and associated illite-adularia alteration that locally contain bonanza gold grades of up to 120 g/t. Isotope data suggest a magmatic source of sulfur in the gold-bearing fluids at Ladolam. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00126-001-0230-y     

Petroleum Resources of Papua New Guinea

Papua New Guinea’s petroleum sector has substantially grown in the last 2 years. There is an increase in exploration expenditure, which also sees a corresponding increase in the number of exploration licences. Two new petroleum projects have been commissioned and these have added to production of the existing projects. The oilfields are small by world standards. The sector is the second major contributor to the export income and taxation revenue of the country, after minerals.     

Undersaturated lavas from Ambittle Island,Papua New Guinea

Ambittle is part of a gently arcuate chain of Quaternary volcanic islands east of the Bismarck Archipelago. Lavas include basanities, tephrites, ankaramitic lavas and trachyte. Phenocrysts of olivine, clinopyroxene and plagioclase are found in bauonite along with hauyne. Olivine and less commonly hauyne are absent from tephrite while amphibole and biotite are common. Nodules in the lavas are composed of phenocryst phases and are thought to represent crystal clots or cumulates. Low Ti, high Al, low Ni and Cr and high Sr, Rb mark these lavas as being very different from basanites of intra-continental regions. Suggested parallels are shoshonitic lavas from the Aeolian Islands and feldspathoidal lavas from Indonesia. Although the structure in the region of Ambittle is not well known it seems likely that the magmas were linked to tectonic processes along a subducting plate margin. The presence of basalts of transitional affinity in other islands of the chain suggests a petrochemical link with the undersaturated types. Derivation of undersaturated lavas form partial melts formed in the mantle only seems possible if ol + cpx are fractionated at moderate pressures in order to enrich residual liquids in Al. If the undersaturated lavas are related to a parental transitional tholeiite magma then fractionation processes alone are insufficient to explain their composition and processes of volatile differentiation may have occurred.     

Ce-anomalous lavas from Rabaul caldera,Papua New Guinea

REE abundances for a suite of Rabaul caldera lavas ranging from basalt to dacite are reported. Three of the lavas contain negative Ce anomalies. These ‘anomalous’ lavas are older and have a geochemistry different from the younger lavas which form the main Rabaul caldera. The consistent geochemical differences between the two groups of lavas suggest that the Ce anomaly is inherited from the source material from which the magmas were derived and is not due to differences in crystallization or fractionation paths. Ocean floor lavas containing zeolite assemblages have been shown to possess a Ce anomaly and it is concluded that those lavas having a Ce anomaly were derived from lithosphere which had been depleted in Ce during weathering and hydrothermal processes.     

Hydrothermal Ni: Doriri Creek,Papua New Guinea

The Doriri Creek (DC) Ni–Pd–Pt prospect was discovered in 1966 in the Papuan Ultramafic Belt (PUB) in PNG. The DC was interpreted as a hydrothermal Ni accumulation. The DC is located in the southern proximity of Mt Suckling (~ 180 km SE of Port Moresby), where local intrusive rocks are intermediate to acid dykes and small stocks, within the tec tonized contact zone of the Australian and Woodlark Plates. The active volcanoes of Mount Victory and Waiowa indicate recent thermal activity in the area.The Doriri Creek prospect is the result of episodic hydrothermal fluid flow running through the Doriri prospect, that resulted in Ni concentration of up to 1.55 wt.%, formed by alteration of an ultramafic unit of peridotites/pyroxenites within a Mg-rich gabbronorite envelope. Ni was concentrated in chlorite and serpentine group minerals in addition to Fe oxides, with a minor amount in pentlandite in locally sulfidic samples. Ore mineralogy is also associated with a high phosphorous content as apatite, that concentrates LREE (light rare earth elements). Palladium concentrations are up to 0.37 ppm. Platinum is present in concentrations up to 0.06 ppm within the ore.The alteration halo associated with Doriri Creek mineralization is ~ 100 m in width. Primary mineralogy comprises pyroxene, olivine and plagioclase, which have been altered extensively to amphibole and chlorite–serpentine group minerals. This halo is characterized by enrichments of U, K and W over background values.Local magnetite concentration is up to ~ 35% of whole rock, which is very pronounced in the sulfide rich area of the system. The top part of the DC system is overprinted by tropical weathering at metric scale, which displays LREE enrichment and positive Ce anomalies.The Papuan Ultramafic Belt is described as a highly prospective ground for hydrothermal Ni systems based on its availability of Ni, active thermal flow engines, and the geologic regional context dominated by mafic rock suites and the presence of carbonate/siliciclastic units.     

High-temperature pumice flows from the Rabaul caldera Papua,New Guinea

The Rabaul caldera is at the northeastern tip of the island of New Britain, Papua New Guinea. Unwelded pumice flows and air fall pumice of andesite, dacite and rhyolite drape the caldera. They contain sparse phenocrysts of plagioclase, pyroxene and rarely amphibole, together with microphenocrysts of titanomagnetite and ilmenite; apatite and pyrrhotite are also present. The equilibration temperature of the iron-titanium oxides range from 1035° to 835° C. Estimates of sulphur fugacity are obtained from the composition of the pyrrhotites which contain about 1% Cu and 0.3% Mn. Calculations show that the fugacity of SO₂ may be several tens of bars at 1000° C. An estimate of the activity coefficient of Fe₃O₄ in titanomagnetite was obtained, and within the limits of error, can be taken as unity in the temperature range 835–1035° C and the composition range 22. 6–42.5% ulvospinel. Calculations suggest that the phenocrysts of orthopyroxene and titanomagnetite in the rhyolitic pumice equilibrated at pressures (P _total) of between 2.2 and 2.6 kilobars. Estimates of pH₂o are unreliable because of the presumed later hydration of the pumice.     

Isotopic and trace-element profiles across the New Britain island arc,Papua New Guinea

New Pb-, Sr-, and Nd-isotopic data have been obtained for the rocks of volcanoes overlying a wide range of depths (100–580 km) to the Wadati-Benioff Zone (WBZ) in the New Britain island arc, Papua New Guinea. Well-defined trends consistent with two-component mixing are observed in combined Pb-isotope/trace-element plots. One of the components is believed to represent a slab contribution whose isotopic signature, unlike those noted for several other arcs, appears to be dominated by subducted, altered, oceanic crust rather than by sediment. This conclusion is consistent with the results of a recent Be–B study of New Britain rocks. The influence of the slab component is considered to decrease as depth to the WBZ increases. Higher abundances of high-field-strength elements correlate with increasing depths to the WBZ, and may be indicative of smaller degrees of partial melting of the mantle wedge as WBZ depths increase. Abundances of other incompatible elements appear to reflect a complex interplay between the slab-derived flux and melting process.     

The Mineral Resources of Papua New Guinea

Abstract. Papua New Guinea is geologically highly prospective for minerals. There are three world class, open pit mines located at Ok Tedi, Porgera and Lihir, and two medium-scale underground operations at Tolukuma and Kainantu. Two other mining projects - Ramu and Simberi are fully licensed and will be mined in the next 6–12 months. The mining sector is the major contributor to the export income and taxation revenue of the country.     

Rayleigh wave dispersion in Papua New Guinea

Phase and group velocity dispersions of fundamental mode Rayleigh waves for the path Port Moresby-Rabaul have been computed. The structural interpretation of dispersion curves for the Solomon Sea reveals the channel shear velocity as about 4.35 km/s and the channel starts around 95 km below the earth's surface. In other words, the elastic velocities-depth structure beneath the Solomon Sea is very similar to that beneath the northern part of the Western Mediterranean Basin. Further, group velocity dispersion of fundamental mode Rayleigh waves at the PMG and the RAB seismic stations have been computed. The structural interpretation of a dispersion curve for the orogenic belt of Papua New Guinea reveals a crust of about 62.5 km thick and uppermost mantle shear velocity of about 4.3 km/s.