Abstract
Magmas containing the components needed to form porphyry copper deposits are relatively common within arcs, yet mineralising events are uncommon within the long-lived magmatic systems that host them. Understanding what causes the transition from barren to productive intrusions is critical to the development of conceptual deposit models. We have constrained the tempo of pre- and syn-mineralisation magmatic events in relationship to the thermal evolution of the plutonic body that underlies one of the world's youngest exposed plutonic–porphyry systems, the Inamumu Zoned Pluton, Koloula Porphyry Prospect, Solomon Islands. High precision ID-TIMS U–Pb dates of texturally and chemically characterised zircons indicate pluton emplacement over 50% crystals) state, past the point of rheological lock-up. We estimate that thermal rejuvenation of the deeper high-crystallinity magma and generation of a mobile melt fraction may have occurred ≤10 kyr before its transport and emplacement within the porphyry environment. The underlying pluton likely cooled and returned to high-crystallinity states prior to subsequent remobilisation-emplacement events. Titanium-in-zircon geothermometry and whole-rock geochemistry suggest pre-mineralisation intrusions were remobilised by mixing of a silicic magma with a high-temperature, less-evolved melt. In contrast, syn-mineralisation melts were most likely remobilised by the percolation of hot volatiles exsolved from contemporaneous less-evolved intrusions cooling beneath the crystalline silicic magma. The evidence for the rapid thermal rejuvenation and long term storage of highly crystalline silicic magmas is consistent with previous studies that indicate two components of exsolved volatiles contribute to ore forming fluids. We conclude that the liberation of crystal-rich porphyry copper deposit forming magmas, and the addition of the chemical components required for ore formation, are intrinsically linked to the volatiles released during the recharge of less-evolved melt into a highly crystalline silicic magma.
Highlights
Porphyry copper deposits (PCDs) are fundamental to meeting present and future global demand for metals (Sillitoe, 2010)
Traditional models of porphyry copper formation, in which large, single melt batches exsolve volatiles upon cooling and/or decompression (e.g. Candela and Holland, 1986; Shinohara and Hedenquist, 1997), are inconsistent with our data set that demonstrates that PCDs in the Inamummu Zoned Pluton (IZP) followed the transient thermal perturbation of a large, high-crystallinity pluton generated during the first ca. 150 kyr of magmatic activity
We suggest that the upwards percolation of hot, S ± metal bearing volatiles released from underlying, less-evolved magma is intrinsically linked to the thermal rejuvenation of the high-crystallinity silicic magma, allowing the magmatic transport of ore forming components to the PCD environment within crystalrich porphyritic melts
Summary
Porphyry copper deposits (PCDs) are fundamental to meeting present and future global demand for metals (Sillitoe, 2010). Constraining the thermal-crystallinity evolution of source magmas with time is essential to our interpretation of PCD formation, yet the difference in emplacement depth between PCDs and their parental plutonic systems makes connected observations within modern and ancient systems problematic. To negate this issue, this study utilises high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) U–Pb geochronology of geochemically (Ti-in-zircon thermometer; Ferry and Watson, 2007) and petrographically characterised zircon populations to constrain residence times and thermal fluctuations in magmas prior to their eruption or emplacement In this study we examine: the conditions of magma storage preceding PCDs; the duration and triggers of mineralising magmatic–hydrothermal events; and the role of less evolved magmas in generating ore deposits
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