Structural and compositional characteristics of skarn zinc-lead deposits in the Yeonhwa-Ulchin mining district, Southeastern Taebaegsan region, Korea Part II: the Yeonhwa II mine

第二蓮山鑛山은 그 스카른硫化鉛ㆍ亞鉛鑛床의 體系的인 分布狀態로 特徵지어졌으며 鑛床은 中傾斜로 기울어진 板狀乃至 렌즈狀鑛體群으로 代表되며 東北東으로 走向하는 豊村石灰岩 및 猫峯粘板岩의 衝上斷層面에 따라 貫入한 石英몬조니斑岩의 岩床 및 이로부터 分枝한 岩脈의 接觸帶에 따라 發達되었다. 이들 鑛床은 母岩 및 火成岩과의 關係에 따라 (1) 貫入岩床의 下盤鑛體群(月谷下盤鑛體) (2) 同 上盤鑛體群(月谷上盤鑛體) (3) 岩脈接觸帶와 이로부터 石灰岩層間에 따라 延長된 鑛體群(仙谷鑛體)으로 三大分된다. 鑛床은 石灰硅酸鹽(스카른鑛物)과 硫化鑛物로 構成돼 있는데 硫化鑛石으로는 閃亞鉛石을 主로 하고 方鉛石 및 黃銅石이 包含되며 硫化脈石으로는 磁硫鐵石을 主로 한다. 農村石灰岩과 石英몬조니岩과의 月谷下盤接觸帶에 發達된 外成 및 內成스카른鑛物은 -120坑에서 다음과 같은 對稱帶狀分布를 보인다. 즉 外成스카른의 中心에 柘榴石-石英帶, 이 帶의 兩側바같으로 輝石-鑛石帶, 그리고 더욱 外側으로 猫峯粘板岩쪽으로는 含綠簾石ㆍ綠泥石 혼펠스帶와 火成岩쪽으로는 綠簾石을 主로 하는 內成스카른帶가 配列한다. 이는 스카른形成에 있어서의 두가지 效果 즉 (1) 原岩의 差異(堆積岩과 火成岩) 및 (2) 스카른分帶는 이들 原岩의 交代變質過程에 있어서 內測帶로부터 外側帶로 向한 漸進的 移動이 있었음을 보여주고 있다. 電子線分析에 依하면 輝石은 灰鐵輝石質이고 中心帶로 부터 外側帶를 向하여 鐵分이 增加하는데 反하여 柘榴石의 鐵分은 增加함으로서 輝石과 柘榴石에 吸收된 鐵分의 量이 서로 反比例함을 보여준다. 準輝石類에 硅灰石이 안나타나고 代身 파이록시망가이트, 장미휘석, 버스타마이트가 우세한 點은 스카른溶液의 높은 含망간性을 말하며 柘榴石이 흔히 螢石에 依해 交代되었음은 할로겐의 活動이 매우 强했음을 가르친다. 硫化亞鉛은 스카른帶中 특히 輝石帶에 親近하게 隨伴되며 이는 灰鐵輝石이 硫化亞鉛의 沈澱을 促進하는 還元環境을 造成하는데 起因된 것으로 解析된다. 地質構造的으로 開放된 環境에 있었던 月谷上盤 및 仙谷鑛化帶에 있어서는 金屬品位의 變動이 甚한데 反해 두 개의 貫入岩床사이에 閉鎖되었던 月谷下盤接觸帶內에서는 金屬品位의 分布가 比較的 一定하다.

The Yeonhwa Ⅱ zinc-lead mine is characterized by a dozen of moderately dipping tabular orebodies of skarn and zinc-lead sulfides, developed in accordance with the ENE-trending bedding thrusts and bedding planes of the Pungchon Limestone and underlying Myobong Formation, mostly along the contacts of a ENE-trending sill and a NW-trending dike of quartz mononite porphyry. The orebodies occur in three groups: (1) the footwall Wolgok orebodies with respect to the sill, (2) the hangingwall Wolgok orebodies, and (3) the Seongok orebodies extended from dike contacts into carbonate beds. Mineral compositions of these orebodies are dominated by calc-silicates (skarn) associated with ore minerals of sphalerite, galena, and chalcopyrite, as well as sulfide gangue of pyrrhotite. A pair of exo- and endo-skerns in the Wolgok footwall contact aureole between the Pungchon Limestone and quartz monzonite porphyry on the -120 level represents a well-developed symmetrical pattern of mineral zoning: a garnet/quartz zone in the center of exoskarn, two zones of pyroxene with ore minerals on both sides of the garnet/quartz zone, further outwards-an epidote/chlorite-bearing hornfelsic zone in the Myobong slate beyond a zone of unaffected limestone, and an epidote-dominated zone of endo skarn on the opposite side toward fresh quartz monzonite porphyry. These features indicate a combination of two effects on the skarn formation: (1) differences in composition of the host rocks (sedimentary and ignous), and (2) progressive outward migration of inner zones on outer zones on the course of metasomatic replacement of the pre-existing minerals. Microprobe analyses of garnet, pyroxene, pyroxenoids, epidote, and chlorite for nine major elements on a total of 23 mineral grains revealed that: the pyroxenes are hedenbergitic, in most zones, with a gradual decrease of Fe- and Mn-contents toward the central zone, whereas the garnets are andraditic in outer zones, but are grossularitic in the central zone. This indicates a reverse relationship of Fe-contents between pyroxene and garnet across the exoskarn zones. Pyroxenoids are lacking in wollastonite but are dominated by pyroxmangite, rhodonite and bustamite, indicating a Mn-rich nature in bulk chemistry. Pseudomorphic fluorite after garnet occurs abundantly reflecting a fluorine-enhanced evidence of the skarn-forming fluids. Epidote contains 0.19-0.25mole fraction of pistacite, and chlorite is Mn-rich but is Mg-poor. Sulfide mineralization took place with the most Fe-rich pyroxene rather than with garnet as indicated by the fact that the highest value of hedenbergite mole fraction occurs in the ore-bearing pyroxene zone. The Yeonhwa Ⅱ ores are characterized by high zinc and low lead in metal grade, with minor quantity of copper content in almost constant grade. The hangingwall Wolgok and Seongok orebodies, that formed in a more open environment with respect to their local configurations of geologic setting, are more variable in metal grades and ratios, than are the footwall Wolgok orebodies formed in a more closed condition in a narrow interval of sedimentary beds.