The Sericitic to Advanced Argillic Transition: Stable Isotope and Mineralogical Characteristics from the Hugo Dummett Porphyry Cu-Au Deposit, Oyu Tolgoi District, Mongolia
Khashgerel, B; Rye, RO; Kavalieris, I; Hayashi, K
Late Devonian porphyry Cu-Au deposits within the Oyu Tolgoi mineral district, Mongolia, occur in a north-notheast-trending zone 22 km long. they are related to quartz monzodiorite instrusions, and hosted by augite basalt lavas. The porphyry systems have been preserved beneath overturned and allochthonous stratigraphic sequences and geologic relationships suggest that fold-thrust belt deformation and tectonic burial occurred soon after their formation. Eight known separate porphyry centers currently contain a measured and indicated resource of 1,390 Mt at 1.33 wt percent Cu and 0.47 g/t Au, and an inferred resource of 2,200 Mt at 0.83 wt percent Cu and 0.37 g/t Au (at 0.6 wt % cu equiv cutoff). Advanced agrillic alteration is present for 6 km along a north-northeast trend and is characterized by minerals that include andalusite, corundum, diaspore, residual quartz, alunite plus aluminum-phosphate-sulfate minerals, zunyite, topaz, pyrophyllite, kaolinite, anhydrite, gypsum, and dickite but is dominated by pyrophyllite. This alteration is exposed at surface in the Central deposit and in the subsurface between 50 to 1,500 m depth at Hugo Dummett South and NOrth deposits. The advanced argillic zone at the Hugo Dummett deposits closely envelopes high-grade porphyry Cu-Au mineralization, and overpoints a deep zone of sericitic alteration. Mineralogical investigations show that pyrophyllite replaces fine-grained muscovite as coarse crystals (up to 25 mu m long), or replaces coarse muscovite crystals (50 mu m long) along clevage. Muscovite (delta(18)O = 6.7-10.4 parts per thousand, delta D = -116 to -92 parts per thousand, n = 13) and pyrophyllite (delta 18O = 5.9-12.2 parts per thousand, delta d = -122 to -87 parts per thousand, n = 28) have similar measured isotope compositions. The calculated parent fluid compositions for all phyllosolicate minerals (muscovite, chlorite, pyrophyllite: delta(18)O(H2O) = 1-7.3 parts per thousand, delta D(H2O) = -102 to -67 parts per thousand) suggest they were predominantly magamtic with a minor component of meteoric water. The composition of younger dickite (delta(18)O = -3.2 to +5.7 parts per thousand, delta D = -165 to -129 parts per thousand, n = 13) shows evidence of magmatic fluid mixing with meteoric water, and provides an estimate of delta D = -160 per mil for Late Devonian meteoric water. Sulfides (delta(34)S = -16 to -1.4 parts per thousand) are isotopically light, whereas sulfates (delta(34)S = 4.2-17.9 parts per thousand) are heavy, and the distribution suggest that initial fluids were SO(2)-dominant and ratios of reduced to oxidised sulfur species of fluids were buffered at near 1:1. Alunite is related to condensed magmatic vapor (delta(18)O((SO4)) = 7.1-20.1 parts per thousand (n = 14), delta D = -92 to -37 parts per thousand (n = 13), delta(34)S = 8-17.9 parts per thousand (n = 18)) with a component of meteoric water, and contrasts in O and H isotope values from pyropyllite, which is similar to muscovite. Pyrophyllite at shallo levels replaces earlier advanced argillie minerals, including alunite, whereas at deep levels it replace muscovite in quartz monzodiorite, or chlorite-muscovite-illite (after biotite) in basaltic wall rocks. Field relationships, mineralogy, and stable isotopes suggest that pyrophyllite is derived from late magmatic-hydrothermal fluids during cooling.