The Nature of Gold Mineralization in the Archean Sunrise Dam Gold Deposit in Western Australia

호주 Yilgarn Craton의 동부 Laverton Tectonic Zone에 위치하는 Sunrise Dam 금광은 서호주에서 지난 10년간 두번 째로 가장 많은 금을 생산한 광산이다. 광상 주변의 지질은 화산암류와 이들의 상부에 산출하는 저탁 퇴적암류 그리고 이들을 관입하여 산출하는 시생대 및 원생대 석영섬록암 및 암맥류 등으로 구성된다. 이들 암석들은 북북동 방향의 비 대칭 Spartan 배사를 이루고 있으며, 광상은 본 배사의 서쪽 날개부에 발달된 북동-남서 방향의 전단대를 따라 배태된 20여개의 광체로 구성되어 있다. 본 광상은 광물조합 등에 의하여 확실하게 구분되는 5개의 열수광화시기 (D1, D2, D3, D4a, D4b)로 구분된다. 이들 열수광화시기 중 금 광화작용이 가장 우세하게 진행된 시기는 D4a열수광화기이며, D4b열수광 화기에 그 다음으로 우세한 금 광화작용이 야기되었다. 이들 열수광화시기의 금 광화작용은 주로 황철석 및 사면동석의 침전과 밀접하게 관련되어 있다.

The Sunrise Dam gold deposit is located approximately 850 km ENE of Perth, in the eastern part of the Yilgarn Craton, Western Australia. The mine has produced approximately 153 t of Au at an average grade of 4.2 g/t, which stands for the most significant gold discoveries during the last decade in Western Australia. The deposit occurs in the Laverton Tectonic Zone corresponding to the corridor of structural complexity in the Laverton greenstone belt, and characterized by tight folding and thrusting. The mine stratigraphy consists of a complexly deformed and altered volcaniclastic and volcanic rocks. These have been overlain by a turbidite sequence containing generally well-sorted siltstones, sandstones and magnetite-rich shales, which are consistently fining upwards. These sequences have been intruded by quartz diorite, ultramafic dikes, and rhyodacite porphyry (Archean), and lamprophyre dikes (Palaeoproterozoic). These rocks constitute the asymmetric NNE-trending Spartan anticline with north-plunging thrust duplication of the BIF unit. The deposit is located on the western limb of this structure. Transported, fluvial-lacustrine and aeolean sediments lie unconformably over the deposit showing significant variation in relief. Gold mineralization occurs intermittently along a NE-trending corridor of ca. 4.5 km length. The 20 currently defined orebodies are centered on a series of parallel, gently-dipping (~30o) and NESW trending shear zones with a thrust-duplex architecture and high-strain characteristics. The paragenetic sequence of the Sunrise Dam deposit can be divided into five hydrothermal stages (D1, D2, D3, D4a, D4b), which are supported by distinctive features of the mineralogical assemblages. Among them, the D4a stage is the dominant episode of Au deposition, followed by the D4b stage, which is characterized by more diverse ore mineralogy including base metal sulfides, sulfosalts, and telluride minerals. The D4a stage contains higher proportions of microscopic free gold (48%) than D4b stage (12%), and pyrite is the principal host for native gold (electrum) followed by tetrahedrite-group minerals in both stages.