Abstract

Late Devonian porphyry Cu-Au deposits within the Oyu Tolgoi mineral district, Mongolia, occur in a north-northeast–trending zone 22 km long. They are related to quartz monzodiorite intrusions, 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 argillic 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 overprints a deep zone of sericitic alteration.

Mineralogical investigations show that pyrophyllite replaces fine-grained muscovite as coarse crystals (up to 25μm long), or replaces coarse muscovite crystals (50 μm long) along cleavage. Muscovite (δ18O = 6.7–10.4‰, δD = −116 to −92‰, n =13) and pyrophyllite (δ18O = 5.9–12.2‰, δD = −122 to −87‰, n = 28) have similar measured isotopic compositions. The calculated parent fluid compositions for all phyllosilicate minerals (muscovite, chlorite, pyrophyllite: δ18OH2O = 1–7.3‰, δDH2O = −102 to −67‰) suggest they were predominantly magmatic with a minor component of meteoric water. The composition of younger dickite (δ18O = −3.2 to +5.7‰, δD = −165 to −129‰, n = 13) shows evidence of magmatic fluid mixing with meteoric water, and provides an estimate of δD = −160 per mil for Late Devonian meteoric water. Sulfides (δ34S = −16 to −1.4‰) are isotopically light, whereas sulfates (δ34S = 4.2–17.9‰) are heavy, and the distribution suggest that initial fluids were SO2-dominant and ratios of reduced to oxidized sulfur species of fluids were buffered at near 1:1.

Alunite is related to condensed magmatic vapor (δ18O(SO4) = 7.1–20.1‰ (n = 14), δD = −92 to −37‰ (n = 13), δ34S = 8–17.9‰ (n = 18)) with a component of meteoric water, and contrasts in O and H isotope values from pyrophyllite, which is similar to muscovite. Pyrophyllite at shallow levels replaces earlier advanced argillic minerals, including alunite, whereas at deep levels it replaces 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.

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