Trace element geochemistry of the 1991 Mt. Pinatubo silicic melts, Philippines: Implications for ore-forming potential of adakitic magmatism

Abstract

The dacite pumice erupted from Mt. Pinatubo on June 15, 1991 (whole-rock, rhyolitic groundmass glasses and homogenized melt inclusions) has been analyzed using inductively coupled plasma-mass spectrometry (ICP-MS), nanosecond and femtosecond laser ablation ICP-MS and secondary ion mass spectrometry (SIMS) to evaluate its ore-forming potential. Data suggest that adakite magmas are metal-rich and concentrate ore metals during magmatic differentiation. Sulfides segregate in limited amounts under the hydrous, oxidizing conditions typical of adakitic magmas resulting in incompatible behavior for Au (6-22 ppb), Cu (26-77 ppm), and Pb, Mo, As, and Sb in melts of dacitic to rhyolitic compositions. Metal transfer from this adakite magma to the coexisting aqueous phase was favored by the peraluminous composition of the rhyolitic melt and high aqueous chloride concentrations. Mass balance calculations suggest that the pre-eruptive aqueous phase could have extracted a minimum of 100 t Au and 5 · 10 5 t Cu from the Mt. Pinatubo magma. Our data suggest that intrusives having adakitic signatures are genetically associated with Au-Cu and Cu-Mo mineralization, auriferous porphyry copper deposits, and epithermal gold veins. High H 2 O, Cl, Sr/Y, Pb/Ce, Mo/Ce, As/Ce and Sb/Ce in Mt. Pinatubo melts reflect the contribution of deep fluids derived from subducted sediments and altered MORBs in the dacite genesis. The slab-derived fluids carrying mobile elements are likely responsible for the enrichment of adakite magmas in gold, associated metals and H 2 O, and may explain the exceptional ore-forming potential of adakite magmatism.

Is gold solubility subject to pressure variations in ascending arc magmas?

“…Besides the fact that we avoided starting samples containing anhydrite crystals, such low S contents are likely due to volatilization and loss of sulfur during the preparation by double melting of the starting rock glass powder at 1400°C in air. The bulk gold contents of our samples are quite low (12 and 38 ppb; Table 1), close to the detection limits (which are generally ≤ 10 ppb) of the analytical equipment (ICP-MS, Toulouse, France); these Au concentrations are in the same range as those measured by Borisova et al (2006) in the rhyolitic matrix glass of the Pinatubo 1991 white dacite (22 ± 8 ppb). Note that the typical adakite Bal2 is the most Au-enriched.…”

Controls on gold solubility in arc magmas: An experimental study at 1000°C and 4kbar

“…The bulk gold contents of our samples are quite low (12-38 ppb; Table 1), close to the detection limits (which are generally 10 ppb) of the analytical equipment used (ICP-MS, Toulouse, France); these Au concentrations are in the same range as those measured by Borisova et al (2006) in the rhyolitic matrix glass of the Pinatubo 1991 white dacite (22 ± 8 ppb). Note that the typical adakite Bal2 is the most Au-enriched.…”

“…In comparison, amphibolitic rocks (i.e., archean greentones and modern oceanic metabasalts) generally contain <2 ppb Au ( [Greenough and Fryer, 1990] and [Togashi and Terashima, 1997]). Borisova et al (2006) estimated that a bulk slab composed of 90 wt% fresh MORB + 5 wt% altered MORB + 5 wt% sediment has Au concentrations up to 5 ppb Au. Nevertheless, it is worth stressing that an average subducting oceanic crust is likely to be as well composed of at least several wt% of polymetallic massive sulfides deposits.…”

“…Herzig and Hannington (1995) report that gold concentrations are locally high in mid-ocean ridge deposits (up to 6.7 ppm Au, with an overall average of >1.2 ppm Au), and may reach concentrations of more than 50 ppm Au in massive sulfides from particular areas, like immature back-arc rifts. Thus, assuming an average amount of 5 wt% massive sulfides (with an average gold concentration of 1.2 ppm) in the subducting slab, 5-20% partial melting of the bulk slab may lead to a sulfur-rich slab melt gold content ranging between 300 and 1200 ppb, provided that residual sulfide is minimal (i.e., most of sulfides dissolve into the partial melt; Mungall and Naldrett, 2008) and Au behaves as a highly incompatible element (Borisova et al, 2006). Therefore, and although the calculations need to be refined especially by taking into account the mobilities of Au and S in slab-dehydration fluids which are presently virtually unknown, the subducted slab clearly represents a potentially favourable protolith for the genesis of magmas specialized with respect to gold.…”

Processes controlling the 2010 Eyjafjallajökull explosive eruption

“…In our system, the ablated material was carried in a flow of helium gas, which was mixed with argon in the transporting tube connected to a quadrupole ICP-QMS (Agilent 7500ce). We used a femtosecond Ti:Sapphire laser (Amplitude Pulsar 10, Amplitude Technologies) operating at 800 nm and providing 55 fs pulses at a maximum output of 12 mJ/pulse; details of this method are described by Borisova et al [2006Borisova et al [ , 2008Borisova et al [ , 2010aBorisova et al [ , 2012. The laser was fired at a repetition rate of 5 Hz and the Gaussian beam was focused (using a  15 objective) onto a Table S3 for the complete set of analytical parameters).…”