Application of cathodoluminescence and trace element analysis of quartz for understanding ore forming process of the Asachinskoe epithermal gold deposit
|Location||International Geological Congress,oslo 2008|
|Author||Takahashi, Ryohei۱; Muller, Axel۲; Van den Kerkhof, Alfons۳; Kronz, Andreas۳; Okrugin, Victor۴; Matsueda, Hiroharu۱|
|Holding Date||29 September 2008|
The Asachinskoe epithermal gold deposit in South Kamchatka, Russia, is a low-sulfidation type deposit which consists of Au-Ag bearing quartz veins. Cathodoluminescence (CL) analysis using optical microscope (OM) and scanning electron microscope (SEM) and trace element analysis of quartz using electron probe micro-analyzer (EPMA) were performed to elucidate the relationships between CL structures, trace element concentrations of different quartz generations, and ore forming process of the Asachinskoe deposit.
The deposit consists of approximately forty N-oriented quartz-adularia-illite veins, which are mainly hosted by the dacite-andesite stock intrusions of Miocene-Pliocene age. Four mineralization stages are macroscopically recognized in bonanza zone, where K-Ar age determination shows Stage I (4.7±0.2 Ma) and Stage III (4.5-4.4±0.1 to 3.1±0.1 Ma). Stage I is a low grade ore mineralization, and Stage II consists of abundant illite and manganese minerals. Principal Au-Ag mineralization is observed in Stages III-IV, where fluid inclusion study indicates the fluid boiling (ca. 160-190 °C and 170-180 °C, respectively). Hydrothermal brecciation is associated with Au-Ag mineralization in Stage IV.
The CL analysis revealed five sequences (Seq. 1-5) of quartz crystallization corresponding to Seq. 1 (Stage I), Seq. 2 (Stage II), Seq. 3 (Stage III) and Seq. 4-5 (Stage IV). Seq. 3 shows colloform and microcline texture of quartz with moderate to dull red-brown CL, coexisting with electrum, naumannite-aguilarite and polybasite-pearceite. Seq. 4 fills open cavities of a few millimeter scale, where colloform quartz has bright yellow CL suggesting the vein re-opening at initial timing of hydrothermal brecciation and rapid precipitation of amorphous silica. Seq. 5 shows almost non-luminescent, dark brown CL in quartz that forms the matrix of the hydrothermal breccia. EPMA analysis revealed that the most distinctive trace elements are Al (av. 1463 ppm) and K (av. 350 ppm) in quartz through all sequences, while abundant Fe (av. 80 ppm) and Mn (av. 194 ppm) were detected in Seq. 2. The EPMA results permit to distinguish between the sub-sequences Seq. 5a and 5b by their trace element signature. Seq. 5a shows a peculiar characteristics of higher K (av. 684 ppm) than Al (av. 489 ppm), which would be a key to understand the metal precipitation mechanism associated with hydrothermal breccia. Trace element concentrations of Seq. 5b are below the limit of detection for K, Al, Fe, Ti and Mn. Diagrams (Al/K, Al/Fe vs. Al content) for trace elements in quartz clearly distinguished Seq. 1-3 and Seq. 4-5. A positive correlation between Al/Fe ratio and Al in Seq. 4-5 is interpreted as a result of increasing oxidized iron in the quartz-forming solution. A relatively high fo2 condition (at most, the hematite-magnetite buffer) is suggested based on occurrences of selenide minerals in Seq. 3. Seq. 4-5 might have obtained much higher fo2 than Seq. 3.