Geology, geochemistry and genesis of the Namosi porphyry Cu-Au deposits, Fiji
Creators: name: Orovan, EA
Publisher: name: University Of Tasmania
Year: 2023

description: The porphyry Cu-Au(-Mo) deposits of the Namosi district are located 30 km west-northwest of Suva, southeastern Vitu Levu, Republic of Fiji. The district comprises three principal porphyry deposits (Wainaulo, Waisoi West, and Waisoi East) and several other peripheral porphyry, polymetallic vein, and skarn prospects. The combined Wainaulo, Waisoi West, and Waisoi East resources are in excess of 7.86 million tonnes of copper and 7.97 million ounces of gold. The Namosi porphyry district formed after a major perturbation in the Australian-Pacific plate tectonic regime during the Late Miocene. The collision of the Melanesian Border Plateau with the Vitiaz Arc led to a localized reversal of arc polarity, wherein the South Fiji Basin began to subduct northeastward along the Matthew-Hunter Trench. This short-lived subduction system had a low-angle component beneath the Fiji Platform that may have triggered the generation of the Namosi district porphyry deposits. A north-dipping and locally flat-lying orphaned slab from the fossil subduction zone currently abuts the westward subducting Pacific plate ~ 350 km beneath the Fiji platform. The Namosi district consists of a gently dipping, andesitic, volcanic and volcaniclastic succession ‚Äöv¢‚Ä¢ 1 km thick (known as the Namosi Andesite Formation of the Medrausucu Group), which was deposited on the submerged flank of a shoaling seamount in the Late Miocene. The Namosi Andesite Formation unconformably overlies steeply dipping, low-K tholeiitic submarine basaltic andesite volcanic and volcaniclastic rocks of the Late Oligocene to Middle Miocene Wainimala Group. The Namosi porphyry deposits are temporally and genetically associated with a sequence of medium-K calc-alkalic porphyritic rocks that intruded both the Wainimala Group and Namosi Andesite Formation. Diorite plutons and a dike complex were emplaced at Wainaulo, quartz-diorite stocks were intruded at Waisoi West, and dacite stocks and dikes were emplaced at Waisoi East. The high 143Nd/144Ndi (~ 0.51304) and low 87Sr/86Sri (~ 0.7036) values of the Namosi district porphyries suggest a primitive mantle source with no crustal input. Neodymium isotopic compositions infer more primitive sources throughout the evolution of the Namosi district, interpreted to reflect the onset of rifting in the vicinity of the Fiji Platform in the Early Pliocene. Two contrasting styles of porphyry deposits have been recognized within the Namosi district. The first style is a variation on the plutonic-hosted porphyry deposit model (Wainaulo and Wainaulo West), consisting of dike complexes emplaced into pre-mineralization porphyritic diorite plutons. The ore bodies are localized around dikes and are vertically attenuated (&gt; 800 m). The second type are classic volcanic-hosted porphyry deposits (Waisoi West and Waisoi East), consisting of multiphasic porphyritic stocks that intruded the volcanic host sequence. The ore bodies are shallow and annular, occurring in and around the cupola of the porphyritic stocks. Alteration assemblages at Wainaulo are mineralogically complex, but have well-constrained zonation patterns. A central zone of biotite ‚Äö- albite ‚Äö- magnetite ‚Äö- actinolite ¬¨¬± muscovite ¬¨¬± rutile and chalcopyrite ¬¨¬± molybdenite bearing veins is spatially and temporally associated with two early-mineralization diorite phases. Separating the central biotite-bearing assemblage from the peripheral chlorite ‚Äö- albite ‚Äö- epidote ‚Äö- calcite ¬¨¬± montmorillonite ¬¨¬± magnetite assemblage is a zone of actinolite ‚Äö- albite ‚Äö- chlorite ‚Äö- magnetite ¬¨¬± epidote ¬¨¬± rutile ¬¨¬± titanite alteration. Overprinting the central biotite-bearing assemblage are domains of chlorite ‚Äö- albite ‚Äö- quartz ‚Äö- actinolite ‚Äö- epidote alteration that are spatially and temporally associated with the main-stage quartz-diorites and abundant quartz ‚Äö- magnetite ‚Äö- bornite and quartz ‚Äö- sulfide veins. The last significant ore-bearing event at Wainaulo consists of epidote ‚Äö- sulfide ‚Äö- anhydrite ‚Äö- calcite ‚Äö- hematite veins with conspicuous K-feldspar ‚Äö- epidote ‚Äö- phengite vein halos that overprint the main-stage diorite and quartz-diorite complex. Structurally focused illite ‚Äö- muscovite ‚Äö- anhydrite ‚Äö- calcite ‚Äö- pyrite and kaolinite ‚Äö- montmorillonite ‚Äö- ankerite ¬¨¬± hematite, as well as ubiquitous chlorite ‚Äö- illite-smectite assemblages overprint the earlier alteration domains throughout the deposit. The alteration assemblages at both Waisoi deposits consist of an inner biotite ‚Äö- albite ‚Äö- magnetite ¬¨¬± actinolite ¬¨¬± K-feldspar core that grades out to a peripheral chlorite ‚Äö- albite ‚Äö- magnetite ¬¨¬± calcite ¬¨¬± epidote domain. The bulk of the copper mineralization is hosted in a sulfide-bearing quartz stockwork that overprinted the earlier formed alteration assemblages. Paragonite ‚Äö- quartz ‚Äö- pyrite and chlorite ‚Äö- illite-smectite alteration is ubiquitous, but is most intensely developed overprinting the intrusions associated with mineralization at each deposit. High-resolution CA-TIMS U‚Äö-Pb dating on zircons and Re‚Äö-Os dating on molybdenite from each of the principal deposits has revealed that mineralization in the Namosi district occurred rapidly over a span of ‚Äöv¢¬ß 520,000 years from the Late Miocene to Early Pliocene. Re‚Äö-Os ages overlap with U‚Äö-Pb ages of interpreted causative intrusions at Wainaulo (5.690 ¬¨¬± 0.023 Ma) and Waisoi West (5.473 ¬¨¬± 0.022 Ma), whereas at Waisoi East molybdenum mineralization post-dates the interpreted source intrusion by ‚Äöv¢‚Ä¢ 2000 years (5.248 ¬¨¬± 0.022 Ma). These strong correlations demonstrate the genetic link between mineralization and intrusive activity at each deposit. The sulfur isotopic compositions at Wainaulo indicate sulfides precipitated from a redox neutral to SO42--dominant exsolved magmatic fluid that had probably mixed with seawater. There is a zonation of sulfur isotopic compositions at Wainaulo (i.e., ˜í¬•34Ssulfide values range from &gt; 1 ‚ÄövÑ‚àû within and directly above the deposit core to &lt; -3 ‚ÄövÑ‚àû at the deposit periphery; ˜í¬•34Ssulfate values vary from &gt; 11.5 ‚ÄövÑ‚àû in the core of the deposit to &lt; 11.5 ‚ÄövÑ‚àû distally), whereas at the progressively more reduced deposits at Waisoi West (˜í¬•34Ssulfide from -2.9 to +0.6 ‚ÄövÑ‚àû; mean = -1.4 ‚ÄövÑ‚àû) and Waisoi East (˜í¬•34Spyrite from +1.8 to +3.4 ‚ÄövÑ‚àû; mean = +2.4 ‚ÄövÑ‚àû), no sulfur isotopic zonation was discernible. The calculated ˜í¬•18O and ˜í¬•D composition of water in isotopic equilibrium with gangue minerals from Wainaulo, Waisoi West and Waisoi East reflect the involvement of three different fluid sources during the evolution of the Namosi district porphyry deposits: exsolved magmatic fluid, Late Miocene seawater and a local meteoric component. Results from a Sr isotopic study on Namosi district epidote separates confirm the minor involvement of seawater during hydrothermal alteration. The Namosi district records a rapid evolution of contrasting porphyry deposit styles (low tonnage-high grade to large tonnage-low grade) within a time period of ‚Äöv¢¬ß 520,000 years. These findings have significant implications to porphyry exploration strategies that need to be considered when working at a district scale. Geology, hydrothermal alteration, geochemistry, structural trends, mechanisms for ore concentration, deposit footprints and deposit size can vary significantly over a short period of time and within a confined space. The rapid evolution of the Namosi district reflects the dynamic tectonic setting during ore-formation. The recognition of tectonic environments that reflect arc fragmentation and rotation can be valuable targets for identifying new metallogenic belts worldwide.