The fault that moved during the 2017 Mw 5.4 Pohang earthquake in SE Korea; implications for the EGS project and the generation of the Pohang earthquake

<p>     Korean Government Commission concluded that the stimulation during the Pohang EGS project triggered the 2017 Mw 5.5 Pohang earthquake in SE Korea which propagated far beyond the stimulated zone of ~ 1 km in size in granitic basement where induced earthquakes had occurred (Ellsworth et al., 2019, SRL).  Distributions of aftershocks of the Pohang earthquakes (Kim et al., 2018, Science) and of the induced earthquakes both indicate that the Pohang earthquake fault (i.e., fault that moved during the Pohang earthquake) cuts PX-2 well at a depth of 3,800 m.  We found abundant fault gouge and fine fault breccia in forms of mud balls (round-shaped fragile fragments of fault rocks coated by thin drilling mud) in the PX-2 borehole cuttings at the depths of 3790-3816 m (26 m interval).  The fault rocks in mud balls are very similar to fault gouge and fine fault breccia constituting the fault cores of the Yangsan and Yeongdeok faults, major faults in SE Korea.  The average content of the fault rocks in the coarse cuttings is 48% and this corresponds to a fault zone of several meters in width, even wider than the fault cores (~ a few meters) of the major faults.  Thus the Pohang earthquake fault is a large fault in SE Korea.  Big mud loss occurred at depths of 3815~3850 m during the PX-2 drilling, causing induced seismicity, and the mud loss is likely to have occurred in fractured host rock in the damage zone.  The PX-2 drilling record reports 10~40 % of gouge at 3790~3805 m depths, nearly consistent with our result, but we could not find fault rocks at many other depths where the drilling record reports gouge.</p><p>     It was so unlucky for the EGS project to have a large-scale fault between PX-1 and PX-2 wells.  First, water injection during the EGS stimulation became nearly direct injection into the faut zone, causing the Pohang earthquake with a small amount of injected water (5,841 m^3; injected water minus flow back).  Second, an impermeable gouge zone could have shut down hydraulic connection between the two wells to inhibit water circulation in the EGS project.  On the other hand, the Pohang earthquake can be a prototype earthquake for studying the mechanisms of induced/triggered earthquake because the induced earthquakes occurred within about 1 km in rather homogeneous granitic rocks (simple geology) and fault-rock samples only several hundred meters away from the epicenter of the Pohang earthquake are available for physical property measurements.  Preliminary experiments on the fault rocks at a temperature of 200 degC, pore water pressure of 30 MPa, and effective normal stresses of 10, 20 and 30 MPa revealed friction coefficients of 0.55 to 0.7 with slight velocity weakening.  The frictional properties are distinctly different from those of the surface fault gouge from the Yangsan and Yeongdeok fault zones.  It is important to reproduce the Pohang earthquake by modeling with known injection history and with measured frictional and transport properties.</p>

Paleoproterozoic Iron Oxide Apatite (IOA) and Iron Oxide-Copper-Gold (IOCG) mineralization in the East Arm Basin, Northwest Territories, Canada

The Paleoproterozoic East Arm Basin of Canada hosts polymetallic vein, iron oxide–apatite (IOA), and potential iron oxide–copper–gold (IOCG) mineral occurrences, mainly associated with a belt of ca. 1.87 Ga intermediate-composition sills termed the Compton intrusions. Advances in our knowledge of the East Arm Basin and of IOA and IOCG deposits within the broader context of iron oxide and alkali-calcic alteration systems enables a new regional analysis of this mineralization and facilitates comparison of these mineral occurrences and host rocks to the nearby Great Bear magmatic zone IOCG districts. The Compton intrusions and co-magmatic Pearson Formation volcanic rocks are comparable in age and composition to intrusive plus volcanic rocks of the Great Bear magmatic zone that host IOA–IOCG mineralization. Taking into account fault displacements, emplacement of Compton intrusions and Pearson Formation volcanic rocks are also consistent with the architecture of modern arcs, supporting a direct relationship with the Great Bear subduction zone. Trace element patterns of uraninite contained in IOA occurrences of the East Arm Basin are also similar to the patterns of uraninite from the Great Bear magmatic zone occurrences, consistent with both regions having experienced similar iron oxide and alkali-calcic alteration and mineralization. Our new results indicate that exploration for IOA, IOCG, and affiliated deposits in the East Arm Basin should focus on delineating increased potassic-iron alteration types and fault/breccia zones associated with these systems through field mapping and application of geochemical, radiometric, magnetic, and gravity surveys.

Exceptional Tl-bearing manganese oxides from Zalas, Krakow area, southern Poland

In the Permian rhyodacite quarry at Zalas near Krakow, southern Poland, thallium-bearing Mn oxides occur in a small fault zone cutting Middle Jurassic sandy limestone poorly encrusted by an oxidized polymetallic mineralization. The encrustation comprises sulphides (pyrite, chalcopyrite, chalcocite, covellite, galena, marcasite), native bismuth, hematite, goethite, cuprite, mottramite, iodargyrite, unrecognized Cu sulphates and Bi oxychlorides as supergene minerals, barite and rare tiny grains of gold. It is most likely connected with rejuvenation of Early-Paleozoic faults during the Alpine orogeny on the Oligocene–Miocene boundary. Rare Tlbearing Mn oxides occur in an outside zone of the encrustations, filling small fractures and voids in limestone forming the fault breccia. Tl contents, reaching 20.82wt% as Tl2O, exceed by more than two orders of magnitude those reported in similar minerals before, making the oxides unique on a world scale. The Tl-bearing Mn oxides from Zalas reflect intensive weathering of an older Tl-bearing sulphide mineralization in an arid climate, involving saline fluids delivered to the groundwater system as the nappe structure of the Carpathians was developing during the Sava tectonic phase Oligocene/Miocene boundary.