Evolution of hydrothermal system and ore mineralization at the Southwest prospect, in the vicinity of the Santo Tomas II deposit, Baguio district, Philippines

• Material Type: 博士論文
• Title: Evolution of hydrothermal system and ore mineralization at the Southwest prospect, in the vicinity of the Santo Tomas II deposit, Baguio district, Philippines
• Author/Editor: Avriel Venis, Literal Cirineo
• Author Heading: Avriel Venis, Literal Cirineo
• Publication, Distribution, etc.: 秋田大学
• Publication Date: 2020-03-24
• Publication Date (W3CDTF): 2020-03-24
• Alternative Title: フィリピン、Baguio地域、Santo Tomas Ⅱ鉱床近傍、Southwest鉱徴地における熱水の進化と鉱化作用
• Degree grantor/type: 秋田大学
• Date Granted: 2020-03-24
• Date Granted (W3CDTF): 2020-03-24
• Dissertation Number: 甲第1325号
• Degree Type: 博士（理学）
• Conferring No. (Dissertation): 甲第1325号
• Text Language Code: eng
• Target Audience: 一般
• Note (General): The Baguio district is a premiere gold and copper district in the Philippines known to host numerous porphyry copper, epithermal gold and skarn deposits, including the Santo (Sto.) Tomas II deposit, a 60-year old block-caving mine being operated by Philex Mining Corporation. Recent exploration activities uncovered the occurrence of porphyry-type veinlets that overprinted the complex of porphyry intrusions and breccia facies in the Southwest prospect. A yet to be established genetic connections of this porphyry-type veinlets to the Sto. Tomas II deposit may postulate the occurrence of supposedly blind porphyry copper deposits, thus it is significant to establish a genetic model of the Southwest prospect in the understanding of the porphyry copper deposits in the southern Baguio district. The prospect is located at the southwestern periphery of the Sto. Tomas II orebody, which hosts a copper-gold mineralization related to a complex of porphyry intrusions, breccia facies and overlapping porphyry-type veinlets emplaced within the basement metavolcanics rocks of the Pugo Formation and conglomerates of the Zigzag Formation. The complex is composed of at least four broadly petrographically similar dioritic intrusive rocks that vary in texture and alteration type and intensity. Arranged in chronological order, they are the: (1) coarse-grained biotite quartz diorite (ODP – Old Diorite Porphyry), (2) fine-grained quartz diorite porphyry (EMD - Early Mineralization Diorite), (3) medium-grained porphyritic hornblende quartz diorite (LD – Late Diorite) and (4) hornblende andesite porphyry (DP – Diorite Porphyry). These intrusions were accompanied with at least five breccia facies, namely, (1) PMBx1 - Phreatomagmatic Breccia Facies 1, (2) PMBx2a - Phreatomagmatic Facies 2a, (3) PMBx2b - Phreatomagmatic Breccia Facies 2b, (4) EMDBx - Early Mineralization Diorite Breccia and (5) DPBx - Diorite Porphyry Breccia. The breccia facies overlapped the intrusions and recorded several brecciation episodes that involved explosive brecciation induced by phreatomagmatic activities that reflected the varying pressure-temperature conditions in magmatic-hydrothermal system during the emplacement of the various intrusions. Various quartz-bearing veinlet assemblages overprinted the pre-, syn- and latemineralization porphyry intrusions and breccia facies but are also present as quartz vein fragments in the breccia facies.Hydrothermal alteration assemblages consisting of potassic, chlorite-magnetite, propylitic and sericite-chlorite alteration, and contemporaneous veinlet types were developed on the host rocks. Potassic alteration is characterized by secondary biotitequartz- magnetite- anhydrite-chalcopyrite-bornite. Secondary biotite is the dominant alteration mineral present as fine disseminations in the groundmass and as polygrain aggregates completely altering former ferromagnesian minerals, such as hornblende. It is often accompanied with quartz, anhydrite, magnetite and copper iron sulfides. Chloritemagnetite alteration overprinted the potassic alteration. It is defined by the assemblage of sericite-chlorite-carbonate- magnetite±K-feldspar and abundant chalcopyrite-magnetitechlorite- actinolite±sericite veinlets. Propylitic alteration is exhibited by LD. It is characterized by chlorite-epidote-calcite±illite. The shallower portions dominated by the PMBx2a and PMBx2b were altered by chlorite-illite-pyrite±chalcopryite with associated quartz-gypsum cement. Elevated copper and gold grades are present in the (1) chalcopyrite-bornite assemblage in the potassic alteration in the syn-mineralization EMD and contemporaneous veinlets including the sheeted quartz veinlets and (2) chalcopyriterich mineralization associated with the chalcopyrite-magnetite -chlorite- actinolite±sericite veinlets contemporaneous with the chlorite-magnetite alteration. Erratic remarkable concentrations of gold were also present in the late-mineralization LD.High XMg of calcic amphiboles (>0.60) in the intrusive rocks indicates that the magmas have been in oxidizing conditions since the early stages of crystallization, while a gap in the of Al content between the rims and the cores of the calcic amphiboles in the EMD and LD indicates decompression during the crystallization of these intrusive rocks from the magmas. Meanwhile, XMg of secondary biotites and chlorites are also high (>0.70) with low to undetectable Cl and F contents.Fluid inclusion microthermometry suggests the trapping of fluids that formed the potassic alteration, associated ore mineralization and sheeted quartz veinlets within the subcritical immiscible region at pressures between 50 and 30 MPa and temperatures between 554 and 436 °C, at depths between 1.9 km and 1.1 km. Temperature data from the biotite and chlorite geothermometers indicate that the chalcopyrite-rich mineralization associated with the chlorite-magnetite alteration was formed at a much lower temperature (ca. 290 °C) than the potassic alteration dominated by secondary biotite (ca. 470°C).Evidences from the vein off-setting matrix suggest multiple intrusions within the EMD, despite the K-Ar ages of the potassic alteration in EMD and hornblende in the LD to be of about the same age at 3.5±0.3 Ma. The K-Ar age of the potassic alteration was likely to be thermally reset, as a result of the overprinting hydrothermal activities. Alternatively, these may also indicate that the two events occurred within short time interval. The constrained K-Ar ages also indicate an earlier, if not co-eval intrusive rocks in the Southwest prospect with that of the earliest “dark diorite” intrusion in the Sto. Tomas II. In addition, the range of δ34S of sulfide minerals from +1.8 ‰ to +5.1 ‰ in the Southwest prospect closely overlaps with those of the Sto. Tomas II, Clifton and Bumolo porphyry copper deposits and epithermal vein prospects within the vicinity, indicating a homogeneous source of sulfur.
• DOI: 10.20569/00005244; https://doi.org/10.20569/00005244
• Persistent ID (NDL): info:ndljp/pid/11583606; https://dl.ndl.go.jp/pid/11583606
• Collection: 障害者向け資料
• Collection (Materials For Handicapped People:1): テキストデータ
• Collection (particular): 国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文; https://dl.ndl.go.jp/collections/A00014
• Acquisition Basis: 博士論文（自動収集）
• Date Accepted (W3CDTF): 2020-12-08T06:37:43+09:00
• Date Created (W3CDTF): 2020-11-30
• Format (IMT): PDF; application/pdf
• Access Restrictions: 国立国会図書館内限定公開
• Service for the Digitized Contents Transmission Service: 図書館・個人送信対象外
• Availability of remote photoduplication service: 可
• Periodical Title (URI): http://dx.doi.org/10.20569/00005244; http://hdl.handle.net/10295/00005244
• Data Provider (Database): 国立国会図書館 : 国立国会図書館デジタルコレクション; https://dl.ndl.go.jp

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2023-04-26 再収集

2023-04-26 再収集