• Economic and Environmental Geology
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• v.55 no.3
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• pp.295-307
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• 2022

We studied the Cretaceous Jeonggaksan Formation to determine depositional processes of pyroclastic density currents entering into the lacustrine environments. This formation is composed largely of sandstone-mudstone couplets and (tuffaceous) normally graded sandstones deposited in lacustrine environments, interbedded with two pyroclastic beds: welded massive lapilli tuff and normally graded lapilli tuff. The welded massive lapilli tuff (10 m thick) is composed of poorly sorted, structureless lapilli supported by a welded ash matrix. The normally graded lapilli tuff (4 m thick) is characterized by moderately to well sorted natures and multiple normally graded divisions in the lower part of the bed with internal boundaries. The contrasting depositional features between these lapilli tuff are suggestive of different physical characteristics and depositional processes of pyroclastic density currents in the lake. Overall poorly sorted and massive natures of the thick, welded massive lapilli tuff are interpreted to have been formed by rapid settling of pyroclastic sediments from highly concentrated and sustained pyroclastic density currents. In this case, the pyroclastic density currents were able to displace lake water from shoreline and the pyrolclastic density currents preserved their own heat except for frontal parts of the currents. As a result, welded textures can be formed despite entrance of pyroclastic density currents into the lake. The internal boundaries of the normally graded lapilli tuff reflect unsteady natures of the pyroclastic density currents at the time of the deposition and the pyroclastic density currents can not provide sufficient pressure to displace lake water. As a consequence, the pyroclastic density currents transformed into water-saturated turbidity currents, forming relatively well sorted, normally graded lapilli tuff.

• Journal of the Mineralogical Society of Korea
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• v.28 no.3
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• pp.279-292
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• 2015

New occurrences of large-scaled Zn-Pb deposits are recently found in the Danjang-myeon, Milyang. They are skarn-type deposits which replaced the intercalated limestone beds in the Jeonggaksan Formation. This study aims at characterizing occurrences, mineralogy, and chemistry of Zn-Pb ores and skarn minerals. Skarn orebodies are mainly found in 3 areas, named Gukjeon-ri, Gorye-ri, and Gucheon-ri orebodies, where sphalerite found as main ore mineral in 200-300 m in height and amount of galena increases as altitude does. Ores are dark grey to dark green in color and closely related with clinopyroxene zone. They occur with hedenbergite, grossular, actinolite, epidote, and small amounts of axinite, calcite, and quartz. Main ore mineral is sphalerite which includes tiny spotted grains of galena and chalcopyrite and becomes rich in grade in association with clinopyroxene and epidote. FeS contents in sphalerite show relatively wide range between 1.53 and 23.07 mole%, whose contents intend to increase towards biotite granite known as ore-related igneous rocks. CdS contents are in the range of 0.22-0.93 mole%, showing decrease tendency from southwest (Gukjeon-ri) to northeast (Gucheon-ri). Zn-Pb deposits developed in Danjang-myeon reveal decrease in temperature with increase of altitude, leading to gradual changes in compositions of ore and skarn minerals.