David R. Cooke

Evaluation of the role of Cambrian granites in the genesis of world class VHMS deposits in Tasmania

Abstract An analysis of the distribution, composition and alteration zonation of Cambrian granites which intrude the Mt Read Volcanics of western Tasmania provides evidence that there may have been a direct input of magmatic fluids containing Fe, Cu, Au and P to form the copper-gold volcanic-hosted massive sulphide (VHMS) mineralisation in the Mt Lyell district. Interpretation of regional gravity and magnetic data indicates that a narrow discontinuous body of Cambrian granite (2–4 km wide) extends along the eastern margin of the Mt Read Volcanic belt for over 60 km. The Cambrian granites are altered magnetite series types which show enrichment in barium and potassium, and contrast markedly with the fractionated ilmenite series Devonian granites related to tin mineralisation elsewhere in the Dundas Trough. Copper mineralisation occurs in a linear zone above the apex of the buried Cambrian granite body at the southern end of the belt, from Mt Darwin to the Mt Lyell district over a strike length of 25 km. Gold and zinc mineralisation are concentrated higher in the volcanic stratigraphy more distant from the granite. Overlapping zones of alteration extend from the granite into the surrounding volcanic rocks. An inner zone of K-feldspar alteration is overprinted by chlorite alteration, which passes outwards into sericite alteration. Magnetite ± pyrite ± chalcopyrite ± apatite mineralisation is concentrated in the chlorite alteration zone as veins and low grade disseminations. The Mt Lyell copper-gold stringer and disseminated mineralisation is hosted in felsic volcanic rocks 1 to 2 km west of the interpreted buried granite position. Magnetite-apatite ± pyrite veins in the Prince Lyell deposit at Mt Lyell are very similar to the veins in the halo of the granite, further south, and provide evidence for magmatic fluid input during the formation of the copper-gold VHMS deposits. A model involving deeply penetrating convective seawater, mixing with a magmatic fluid released from the Cambrian granites, best explains the features of VHMS mineralisation in the Mt Lyell district.

Tectonic setting of porphyry Cu – Au mineralisation in the Ordovician – Early Silurian Macquarie Arc, Eastern Lachlan Orogen, New South Wales

Available age data enable the recognition of four groups of porphyries that were emplaced during the ∼50 million year punctuated history of the earliest Ordovician to earliest Silurian intra-oceanic Macquarie Arc in the Eastern subprovince of the Lachlan Orogen. These porphyries were not emplaced during steady-state subduction. Porphyry groups 1 – 3 formed during critical events in the evolution of the arc related to interruptions and resumptions of arc activity. They are pre-accretionary in character, in that they formed in the arc that for most of the Ordovician lay on the Gondwana Plate, above a west-dipping subduction zone, and separated from the Gondwana margin by the Wagga Basin. In contrast, Group 4 porphyries are syn-accretionary and were emplaced into tilted and deformed volcanic and volcaniclastic packages during an extension or relaxation event in the multiphase Early Silurian Benambran Orogeny, attributed to the accretion of the arc and collapse of the former backarc basin. The Lachlan Transverse Zone is a major arc-normal corridor that favoured emplacement of many porphyries commonly, but not exclusively, shoshonitic in chemistry. All porphyry groups are potentially mineralised, with Groups 3 and 4 known to contain world-class Cu – Au deposits.

Shoshonitic magmatism and the formation of the Northparkes porphyry Cu-Au deposits, New South Wales

Four economic porphyry Cu – Au deposits occur at Northparkes, New South Wales: Endeavour 22 (E22), E26, E27 and E48. Mineralisation is centred on thin, pipe-like Late Ordovician to Early Silurian quartz monzonite porphyry complexes. Nine intrusive phases have been recognised, with a common sequence of emplacement recognised in all deposits. Pre-mineralisation intrusions include coarse-grained, equigranular monzodiorite and equigranular to weakly porphyritic biotite – quartz monzonite (U – Pb SHRIMP age 444.2 ± 4.7 Ma). Three variably felsic quartz monzonite porphyry phases comprise the mineralised intrusive complexes: (i) volumetrically minor early and late mineralisation biotite-phyric quartz monzonite porphyry dykes; (ii) abundant synmineralisation K-feldspar-phyric quartz monzonite porphyry intrusions; and (iii) less abundant syn- to late mineralisation augite – biotite K-feldspar-phyric quartz monzonite porphyry intrusions. Post-mineralisation basaltic trachyandesite dykes and augite-phyric monzonite porphyry dykes are also present (U – Pb SHRIMP age 436.7 ± 3.3 Ma), as are younger mafic dykes. The regional volcanic and intrusive rocks define systematic geochemical trends consistent with high-temperature magmatic fractionation of basaltic trachyandesite through trachyte. However, the trace-element compositions and REE patterns of the mineralising intrusions cannot be explained by crystal fractionation alone because there is a return to more mafic compositions in the waning stages of intrusive activity. The intrusive complexes are interpreted to have formed due to the emplacement of mafic alkaline melts into the base of a crystallising, zoned, monzodiorite to monzonite magma chamber. Shallow crustal fault ruptures above the magma chamber probably caused instantaneous depressurisation and simultaneous egress of quartz monzonite porphyry and exsolved aqueous fluid into dilatant zones. Localised fracturing and additional volatile exsolution from the quartz monzonite melt is thought to have led to the formation of the quartz monzonite porphyry complexes and associated Cu – Au-bearing stockwork veins and related orthoclase alteration. Volatile-rich aqueous fluid partitioned LREE preferentially to MREE and HREE resulting in the development of distinctive U-shaped REE patterns of the ore-related intrusions.

From crucible to graben in 2.3 Ma: A high-resolution geochronological study of porphyry life cycles, Boyongan-Bayugo copper-gold deposits, Philippines

The Boyongan and Bayugo porphyry copper-gold deposits are part of a belt of gold-rich copper deposits in the Surigao district of northeast Mindanao, Philippines. The detailed age relationships described in this study provide insight into the geologically short life cycles that characterize porphyry formation in dynamic arc environments. Since their late Pliocene emplacement (2.3-2.1 Ma; SHRIMP [sensitive highresolution ion microprobe] U-Pb zircon dating) at depths of 1.2-2.0 km, these deposits were exhumed, deeply weathered, and buried. Weathering of these deposits led to the development of the worlds deepest known porphyry oxidation profile (600 m thick) at Boyongan, and a modest (30-70 m) oxidation profile at adjacent Bayugo. This earlymiddle Pleistocene supergene event followed a period of rapid uplift and exhumation in northeast Mindanao (2.5 km/Ma; [U-Th]/He apatite age-elevation spectrum). Subsequent rapid subsidence (≥0.34 km/Ma; radiocarbon age-elevation spectrum) and burial of these deposits are attributed to a mid-Pleistocene shift from transpressional tectonics to the present-day transtensional setting in northeast Mindanao. During this period, debris flows, volcanic material, and fluviolacustrine sediments accumulating in the actively extending Mainit graben covered the weathered deposits, preserving the supergene profiles beneath 50-500 m of cover. This detailed geochronological study documents the geologically short (<2.3 Ma) time scales over which these major intrusion-centered mineral deposits evolved from emplacement, exhumation, deep oxidation, and burial, highlighting the dynamism of tectonic processes in environments such as the Philippine Mobile Belt.

Geochemistry and age of magmatic rocks in the unexposed Narromine, Cowal and Fairholme Igneous Complexes in the Ordovician Macquarie Arc, New South Wales

Much of the northern and southern sections of the Junee – Narromine Volcanic Belt of the Ordovician Macquarie Arc in central-western New South Wales are buried beneath the sediment cover of the Great Artesian Basin. Exploration drilling of these aeromagnetically defined blocks has provided important new material to assess the temporal and magmatic affinities of the rocks in the Narromine Igneous Complex (northern end) and the Cowal and Fairholme Igneous Complexes (southern end). Basement rocks are representative of Phase 1 magmatism in the Macquarie Arc, and consist of Lower Ordovician basalts and andesites and common volcaniclastic rocks, all with high-K calc-alkaline affinities. These are very similar compositionally to the Lower Ordovician Nelungaloo Volcanics that outcrop in the central part of the Junee – Narromine Volcanic Belt west of Parkes. Intruding the basement volcanic – volcaniclastic package are three distinct igneous suites. The oldest of these, dated at ca 466 – 460 Ma (Middle Ordovician) and representative of Phase 2 magmatism in the Macquarie Arc, consists of stocks of monzogabbro, monzodiorite and monzonite with high-K calc-alkaline affinities, and is well represented in the Narromine and Cowal Igneous Complexes. Phase 2 suite rocks were, in turn, intruded in the Bolindian (445 ± 5 Ma) by kilometre-size stocks, narrow sheets and dykes made up of hornblende gabbro, diorite and granodiorite, and including quartz + plagioclase + hornblende-phyric dacitic porphyries in the Narromine Igneous Complex. These medium-K calc-alkaline rocks, assigned to the Phase 3 Copper Hill Suite that also occurs in the Molong and Rockley – Gulgong Volcanic Belts further east, crystallised from andesitic or more evolved magmas, probably derived via partial melting of low-K rocks in the arc basement, or amphibolites in the subducting slab. The youngest rocks in these igneous complexes are believed to be the high-level shoshonitic intrusives in the Cowal and Fairholme Igneous Complexes, which are correlated on petrographic and compositional grounds with the Macquarie Arc Phase 4 (Bolindian and Llandovery) magmatic products best represented by the Northparkes Igneous Complex in the central part of the Junee – Narromine Volcanic Belt.

Petrogenesis of volcanic and intrusive rocks of the Zhuanqiao stage, Luzong Basin, Yangtze metallogenic belt, east China: implications for ore deposition

The Mesozoic Luzong volcanic basin is located in the Lower Yangtze River fault-depression, along the northern margin of the Yangtze Block. Volcanic and plutonic rocks are widely distributed in the basin and are spatially related to Cu, Fe, Au, Pb, and Zn ore deposits. The magmatic rocks are dominated by an alkali-rich shoshonitic suite of volcanics and intrusives, including both a trachybasalt-basaltic trachyandesite-trachyte series and a diorite-monzonite-syenite-alkali feldspar granite series. Volcanic sequences within the Luzong Basin are subdivided into four stages, namely the Longmenyuan, Zhuanqiao, Shuangmiao, and Fushan stages with magmatism occurring between 136 and 124 Ma. Most mineral deposits were formed during the Zhuanqiao stage from about 134 to 131 Ma, during a longer (140–130 Ma) period of transition from compression to extension within the Luzong Basin. Major element concentrations in the Zhuanqiao volcanic rocks range from 53.97 to 62.59 wt.% SiO2, 0.57–0.94 wt.% TiO2, 3.52–6.07 wt.% Na2O, and 3.60–8.66 wt.% K2O. Combined K2O + Na2O totals lie between 7.87 and 12.49 wt.%, and K2O + Na2O/Al2O3 ranges between 0.47 and 0.71. In comparison, the Zhuanqiao intrusions have SiO2 concentrations between 52.19 and 67.70 wt.%, TiO2 between 0.26 and 1.13 wt.%, Na2O between 1.33 and 6.55 wt.%, and K2O between 2.15 and 8.25 wt.%, with K2O + Na2O values ranging from 5.92 to 12.39 wt.% and K2O + Na2O/Al2O3 ratios between 0.37 and 0.75. The Zhuanqiao volcanic rocks have an average ∑REE (rare earth element) content of 223.17 ppm, whereas the average ∑REE of the Zhuanqiao intrusives is 250.06 ppm. Weak negative Eu anomalies (Eu/Eu* between 0.69 and 1.00) characterize the volcanics, whereas a larger range with more significant negative Eu anomalies (Eu/Eu* values of 0.35–0.94) is present in the plutonics. The Zhuanqiao volcanic rocks and plutons have (Nb/Th)PM ratios of 0.05–0.14 and 0.02–0.14 and (La/Sm)PM of 3.33–4.95 and 3.36–7.29, respectively. Initial 87Sr/86Sr and 143Nd/144Nd ratios and ϵSr(t) and ϵNd(t) of the Zhuanqiao volcanic rocks are 0.7059–0.7067, 0.51194–0.51218, 22.0–33.3, and −10.2 to −5.5, respectively; those of the intrusives are 0.7060–0.7082 and 0.51203–0.51214 and 23.3–54.7 and −8.6 to −6.4, respectively. The geochemistry presented here suggests that both volcanic and intrusive Zhuanqiao stage magmatic rocks were cogenetic and were probably derived from an enriched mantle (EM) source, most probably metasomatized mantle relating to the EM type-I (EM-I). The Zhuanqiao stage magmatism apparently was controlled by differentiation and assimilation of crustal material in a high-level magma chamber. Formation of the Zhuanqiao stage magmatic rocks coincided with the formation of much of the Cu and Fe mineralization within the Luzong Basin. This coeval magmatism and mineralization during the Zhuanqiao stage can be subdivided into three differing mineral deposit affinities: magmatism associated with magmato-hydrothermal vein Cu (e.g. the Jingbian, Shimenan, and Chuanshandong deposits), porphyry-hosted Fe (e.g. the Luohe and Nihe deposits), and skarn Fe (e.g. the Longqiao deposit). Combining geological characteristics with variations in SiO2, K2O + Na2O, and MgO enabled the division of the Zhuanqiao stage magmatism in the Luzong Basin into geochemically distinct units associated with these differing styles of mineralization.

Geological evolution and metallogenesis of the Ordovician Macquarie Arc, Lachlan Orogen, New South Wales

The Ordovician andesitic association in central-western New South Wales has been known for over half a century (Packham 1969; Webby 1976) and formed the basis of plate tectonic interpretations in the 1970s and 1980s that invoked west-dipping subduction and the presence of an Ordovician island arc in the Lachlan Fold Belt (now Lachlan Orogen) (Oversby 1971; Solomon & Griffiths 1972; Scheibner 1973; Powell 1983). Subsequent mapping by Geological Surveys, exploration companies and university departments has shown that these rocks exist outside the Wellington – Orange region and occur in three major structural belts. From west to east, these are the Junee – Narromine Volcanic Belt, passing through Parkes, the Molong Volcanic Belt, through Orange, and the Rockley – Gulgong Volcanic Belt, through Oberon and Sofala; the Kiandra Volcanic Belt in the Snowy Mountains of New South Wales and just crossing the border into Victoria may be a southern extension of the Junee – Narromine Belt. In the late 1980s and early 1990s, the concept that these calc-alkaline rocks formed from Ordovician subduction was challenged by Wyborn (1992), who suggested that most of these Ordovician rocks were shoshonitic in character, and formed instead from the melting of Cambrian enriched mantle during Ordovician delamination or stretching. However, Geological Survey mapping programs of the 1990s suggested that mediumand high-K calc alkaline rocks were present as well as shoshonitic rocks (see also Pemberton & Offler 1985) and supported the concept that the three belts were fragments of a single Ordovician intraoceanic arc, the Macquarie Arc (Glen et al. 1998). We now consider that Ordovician and Early Silurian volcanic, volcaniclastic and intrusive rocks in these four structural belts form relics of the now-fragmented Macquarie Arc that was accreted to Gondwana in the Early Silurian to form an important element of the Eastern Province of the Late Cambrian to Carboniferous Lachlan Orogen. These arc rocks host large numbers of small-scale mineral deposits. World-class Cu – Au mineral deposits occur at NorthParkes, Cadia and Cowal, with other smaller but significant Cu – Au and Au deposits occurring at Peak Hill, Gidginbung, Copper Hill, Junction Reefs and Marsden. The combined resource of the Cadia district porphyries (729 Mt at 0.79 g/t Au and 0.30% Cu) makes Cadia the world’s seventh largest known porphyry gold system and the largest known gold resource in an alkalic porphyry system (574 t Au), and Cadia Ridgeway and Cadia Far East are two of the world’s highest-grade porphyry gold resources. The papers in this thematic issue report the outcomes of a project entitled ‘Origin and metallogenesis of Ordovician volcanic belts of central western New South Wales: a

The nature of magmatism at Palinpinon geothermal field, Negros Island, Philippines: implications for geothermal activity and regional tectonics

10 billion resource’, carried out jointly by staff and graduate students from CODES Special Research Centre of the University of Tasmania, and staff from the Geological Survey of New South Wales, between 1999 and 2001. This project had the enthusiastic support from exploration groups that included North Ltd (subsequently Rio Tinto Exploration), Newcrest Exploration, Alkane Ltd, Goldfields Ltd, Hargraves Resources and Homestake Ltd, and was supported by the ARC as a SPIRT (Strategic Partnership with Industry for Research and Training) project. Our work builds on and extends mapping carried out by the Geological Survey of New South Wales (GSNSW) and the Bureau of Mineral Resources [BMR, subsequently the Australian Geological Survey Organisation (AGSO) and now Geoscience Australia (GA)]. The Kiandra Volcanic Belt was mapped by BMR (Owen & Wyborn 1979) as part of the Tantangara and Brindabella 100 000 map sheets, aimed at updating the geology of the Canberra 1:250 000 map sheet. Progressive discoveries of Au – Cu deposits led to government agencies mapping Ordovician volcanic-related rocks regionally in New South Wales. After the initial discovery of the Northparkes deposits in 1976, the central part of the Junee – Narromine Volcanic Belt was mapped by Clarke and Sherwin (1990) and Krynen et al. (1990). Outcrops of Ordovician volcanic and volcaniclastic rocks on the Cootamundra sheet were mapped by GSNSW in the mid1990s (Warren et al. 1995, 1996). These two maps were the first in New South Wales to show interpretation of aeromagnetic features under cover. Most of the three northern belts, that is, the Junee – Narromine Volcanic Belt, Molong Volcanic Belt and Rockley – Gulgong Volcanic Belt, were remapped at 1:100 000 scale in the middle to late 1990s as part of the Bathurst, Dubbo and Australian Journal of Earth Sciences (2007) 54, (137 – 141)

Petrogenetic–metallogenetic setting and temporal–spatial framework of the Yueshan district, Anhui Province, east-central China

The Palinpinon geothermal field, Negros Island, Philippines is a high-temperature, liquid-dominated geothermal system in an active island-arc volcanic setting. This paper presents a regional context for the Palinpinon geology, discusses the petrogenetic evolution of magmatism in the district and assesses the genetic relationships between intrusion and geothermal circulation. The oldest rock formation, the Lower Puhagan Volcanic Formation (Middle Miocene), is part of a volcanic sequence that is traceable throughout the Visayas region and is related to subduction of the Sulu Sea oceanic basin in a southeasterly direction beneath the Sulu arc. Late Miocene to Early Pliocene times mark a period of regional subsidence and marine sedimentation. A thick sequence of calcareous sediments (Okoy Formation) was deposited during this period. Magmatism in Early Pliocene to Recent times coincided with commencement of subduction at the Negros-Sulu Arc. This produced basaltic andesites and andesites belonging to the Southern Negros and Cuernos Volcanic Formations. During this time the Puhagan dikes and the Nasuji Pluton intruded Middle Miocene, Late Miocene and Early-Late Pliocene formations. Based on radiogenic (40Ar/39Ar) dating of hornblende, the Puhagan dikes are 4.1-4.2 Ma and the Nasuji Pluton 0.3-0.7 Ma. This age difference confirms these intrusions are not genetically related. The Early Pliocene age of the Puhagan dikes also confirms they are not the heat source for the current geothermal system and that a much younger intrusion is situated beyond drill depths. Igneous rock formations in southern Negros are the products of regional island-arc magmatism with medium K, calcalkaline, basaltic to dacitic compositions. Their adakitic affinity implies that the melting of subducted oceanic basalt has influenced magmatism in this region. Considering the regional tectonic history the most likely scenarios for the generation of slab melts are: (1) during the Middle Miocene, by the melting of relatively young ( less than 20 Ma) oceanic crust; (2) during Early Pliocene times, by the initiation of subduction along the Negros-Sulu Trench; and (3) during Late Pliocene times, by the melting of young (less than 10-20 Ma) oceanic crust. The adakitic composition of the magmas at Palinpinon has promoted the formation of a porphyry copper-style magmatic-hydrothermal system that is comparable to mineralised porphyry deposits elsewhere in the Philippines.

Five questions for fun and profit: A mineral system perspective on metallogenic epochs, provinces and magmatic hydrothermal Cu and Au deposits

The Yueshan district is located in the Anqing–Guichi ore deposit area of the Middle–Lower Yangtze Metallogenic Belt. Two groups of intrusive rocks and three main types of ore mineralization occur in this district: diorite plutons (e.g. Yueshan, Zongpu, Wuheng, and Yangshan) and granite plutons (Hongzhen and Dalongshan), Cu–Au–(Fe) skarn deposits (e.g. Anqing, Tiepuling), Cu–Mo–Au–(Pb–Zn) hydrothermal vein-type deposits (Tongniujing), and hydrothermal uranium mineralization (Dalongshan). Detailed geological and geochemical work suggests that the Cu–Au–(Fe) skarn deposits and the Cu–Mo–Au–(Pb–Zn) hydrothermal vein-type deposits have a close spatial and genetic relationship with the dioritic plutons, whereas the hydrothermal uranium mineralization is associated with A-type granite plutons. Based on the highly precise dating of metal deposits and related plutons in the Yueshan district, such as the molybdenite Re–Os, Os–Os dating, 39Ar–40Ar dating of potassium-bearing minerals and quartz, several Rb–Sr isochrons, SHRIMP zircon U–Pb dating + single-grain zircon U–Pb dating, and the SHRIMP zircon U–Pb dating of Hongzhen granite pluton, we suggest that the extensive magmatism and mineralization in the Yueshan district took place in two episodes: (1) the first episode involved the mineralization of both skarn and vein-type hydrothermal deposits, c.a. 136–139 Ma, related to diorite plutons emplaced at 138.7 ± 0.5 Ma; (2) the second episode attended the hydrothermal uranium mineralization at 106.4 ± 2.9 Ma, related to granite intrusive activity at 126.8 ± 1.0 Ma. These two times of Yueshan petrogenetic–metallogenetic development appear to be consistent with a tectonic environment transition from compression to extension.