[S-CG48] Petrology, Mineralogy & Resource Geology

[SCG48-06] Discovery of barite and native gold from mafic microgranular enclaves in the Morobe Granodiorite, Papua New Guinea

*Takashi Hoshide¹, Tatsuki Ishikawa¹, Angela Lynett Bokuik² (1.Faculty of International Resource Sciences, Akita University, 2.Mineral Resources Authority, Papua New Guinea)

Keywords:magma mixing, porphyry-type ore deposits, shoshonite, fluid inclusions, subduction zone

The Morobe Granodiorite in the New Guinea Mobile Belt is a composite batholith that was intruded into the Owen Stanley Metamorphic rocks during the Middle Miocene. Large-scale porphyry copper-gold deposits such as Hidden Valley and Wafi-Golpu are in the vicinity. The granodiorite shows calc-alkaline and metaluminous affinities typical of I-type granites. The high whole-rock Mg-number of the granodiorite suggests that the magma originated from partial melting of the mafic lower crust. The granodiorite contains ellipsoidal microgranular mafic enclaves (MMEs). The MME consists of euhedral hornblende, biotite, plagioclase, and anhedral K-feldspar, with minor amounts of quartz, opaque minerals (magnetite and sulfide minerals), apatite, and titanite. The K-feldspar displays a poikilitic texture with abundant inclusions of euhedral hornblende, biotite, and plagioclase. Plagioclase in MME (=An_61-70) is more calcic than that in the surrounding granodiorite (=An_37-48). The whole-rock compositions of MME are shoshonitic, and rich in K and Ba. Barite occurs as inclusions of hornblende and biotite in MME. Barite, which occurs as daughter crystals of fluid inclusions in hornblende, is Sr-rich (SrO=˜3.9 wt%) and commonly accompanied by Cl-rich chlorite. Barite also occurs with iron oxides (or iron hydroxides) and pyrite by replacement of a primary mineral crystallized from magma. In addition, native gold (81-83% Au and 17-19% Ag) occurs within cracks and voids in hornblende and plagioclase. We found 18 native gold grains (1-2 micrometer in size) in a 3 cm² area of MME. The hornblende geothermobarometer (Ridolfi, 2021) was applied to hornblende in the MME and surrounding granodiorite. The temperatures and pressures of the hornblende rims are concentrated in the range of 780-820 ℃ and 110-150 MPa (= 4-6 km) for both the MME and its host granodiorite. This is the emplacement depth of the granodiorite body. The hornblende cores, on the other hand, show higher temperature and pressure ranges of 840-890 ℃ and 200-400 MPa (= 8-16 km) in some of the hornblende cores of the MME, although the temperature and pressure ranges of cores partly overlap those of rims in both the MME and its host rocks. Considering the result of geothermobarometry and the occurrence and textures of MME, the MMEs should be crystallized products of magma brought from a depth deeper than the emplacement depth of the granodiorite body. The presence of fluid inclusions containing Barite daughter crystals suggests that the MME-forming magma was rich in H₂O, S, Ba, and Sr. The Ba-bearing fluid separated from the MME-forming magma oxidized pyrite crystallized from the magma and replaced it with Barite and Fe-oxides. The native gold in MME may have been released from the decomposed pyrite by the Barite-forming reaction. The MMEs in the Morobe Granodiorite may be the products by a precursor activity of the magma from which the gold in the surrounding porphyry-type ore deposits originated.