Overprinting porphyry‐type veinlets on the intrusive rocks and phreatomagmatic breccias in the Southwest prospect, southwestern Sto. Tomas II (Philex), Baguio District, Philippines

IF 1.1 4区 地球科学 Q3 GEOLOGY Resource Geology Pub Date : 2020-10-13 DOI:10.1111/rge.12242
Avriel Venis Literal Cirineo, A. Imai, R. Takahashi, Redempta P. Baluda, N. Oliveros, V. B. Maglambayan, Roy Ronald C. Luis, Maria Lourdes M. Faustino, Jacky P. Almadin
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引用次数: 4

Abstract

The Southwest prospect is located at the southwestern periphery of the Sto. Tomas II porphyry copper–gold deposit in the Baguio District, northwestern Luzon, Philippines. The Southwest prospect hosts a copper‐gold mineralization related to a complex of porphyry intrusions, breccia facies, and overlapping porphyry‐type veinlets emplaced within the basement Pugo metavolcanics rocks and conglomerates of the Zigzag Formation. The occurrences of porphyry‐type veinlets and potassic alteration hosted in the complex are thought to be indications of the presence of blind porphyry deposits within the Sto. Tomas II vicinity. The complex is composed of at least four broadly mineralogically similar dioritic intrusive rocks that vary in texture and alteration type and intensity. These intrusions were accompanied with at least five breccia facies that were formed by the explosive brecciation, induced by the magmatic–hydrothermal processes and phreatomagmatic activities during the emplacement of the various intrusions. Hydrothermal alteration assemblages consisting of potassic, chlorite–magnetite, propylitic and sericite–chlorite alteration, and contemporaneous veinlet types were developed on the host rocks. Elevated copper and gold grades correspond to (a) chalcopyrite–bornite assemblage in the potassic alteration in the syn‐mineralization early‐mineralization diorite (EMD) and contemporaneous veinlets and (b) chalcopyrite‐rich 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 Late Diorite (LD). High XMg of calcic amphiboles (>0.60) in the intrusive rocks indicate that the magmas have been oxidizing since the early stages of crystallization, while a gap in the composition of Al between the rim and the cores of the calcic amphiboles in the EMD and LD indicate decompression at some point during the crystallization of these intrusive rocks. Fluid inclusion microthermometry suggests the trapping of immiscible fluids that formed the potassic alteration, associated ore mineralization, and sheeted quartz veinlets. The corresponding formation conditions of the shallower and deeper quartz veinlets were estimated at pressures of 50 and 30 MPa and temperatures of 554 and 436°C at depths of 1.9 and 1.1 km. Temperature data from the chlorite 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. Evidence from the vein offsetting matrix suggests multiple intrusions within the EMD, despite the K‐Ar ages of the potassic alteration in EMD and hornblende in the LD 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 alteration. The constrained K‐Ar ages also indicate earlier formed intrusive rocks in the Southwest prospect, possibly coeval to the earliest “dark diorite” intrusion in the Sto. Tomas II deposit. In addition, the range of δ34S of sulfide minerals from +1.8‰ to +5.1‰ in the Southwest prospect closely overlaps with the rest of the porphyry copper and epithermal deposits in the Sto. Tomas II deposit and its vicinity. This indicates that the sulfides may have formed from a homogeneous source of the porphyry copper deposits and epithermal deposits in the Sto. Tomas II orebody and its vicinity. The evidence presented in this work proves that the porphyry copper‐type veinlets and the adjacent potassic alteration in the Southwest prospect are formed earlier and at a shallower level in contrast with the other porphyry deposits in the Baguio District.
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西南远景区侵入岩与渗透岩浆角砾岩上的套印斑岩型脉脉。托马斯二世(Philex),碧瑶区,菲律宾
西南远景区位于Sto的西南边缘。菲律宾吕宋岛西北部碧瑶地区的托马斯二斑岩型铜金矿床。西南远景区有一个与斑岩侵入体、角砾岩相和叠置斑岩型脉脉有关的铜金矿化,这些斑岩侵入体位于基岩Pugo变质火山岩和锯齿组砾岩中。斑岩型脉脉的出现和钾蚀变被认为是Sto内存在盲斑岩矿床的迹象。托马斯二世附近。该杂岩由至少4块矿物学上大致相似的闪长质侵入岩组成,它们的结构、蚀变类型和强度各不相同。这些侵入体还伴有至少5个角砾岩相,这些角砾岩相是由各种侵入体侵位过程中岩浆热液作用和呼吸岩浆活动诱发的爆炸角砾岩形成的。热液蚀变组合主要为钾质蚀变、绿泥石—磁铁矿蚀变、丙质蚀变和绢云母—绿泥石蚀变组合以及同生脉蚀变类型。铜和金品位的升高对应于(a)同矿化早期闪长岩(EMD)钾化蚀变中的黄铜矿-斑铜矿组合和同时期的细脉;(b)与同时期的绿泥石-磁铁矿-绿泥石-放线石±绢云母细脉相关的黄铜矿富矿化。在成矿晚期闪长岩(LD)中也存在不稳定的显著金浓度。侵入岩中钙质角闪石的高XMg值(>0.60)表明岩浆从结晶早期开始就处于氧化状态,而EMD和LD中钙质角闪石边缘与岩心之间存在Al组成的间隙,表明这些侵入岩在结晶过程中出现了某种程度的降压。流体包裹体显微测温表明,形成钾蚀变、伴生矿化和片状石英脉的不混溶流体被圈闭。在1.9 km和1.1 km的压力为50 MPa和30 MPa,温度为554℃和436℃的条件下,估计了较浅和较深石英脉的形成条件。绿泥石的温度数据表明,与绿泥石-磁铁矿蚀变相关的富含黄铜矿的矿化形成于比钾蚀变低得多的温度(约290℃)。尽管EMD中钾质改变的K - Ar年龄与LD中角闪石的K - Ar年龄大致相同,均为3.5±0.3 Ma,但来自静脉偏移基质的证据表明EMD中存在多个侵入。钾化蚀变的K - Ar年龄可能由于热液的叠印蚀变而被热重置。限定的K - Ar年龄也表明西南远景区形成较早的侵入岩,可能与Sto最早的“暗闪长岩”侵入岩同时期。托马斯二世存款。西南远景区硫化物矿物δ34S范围为+1.8‰~ +5.1‰,与其余斑岩型铜矿和浅成热液矿床有密切的重叠。托马斯二世矿床及其附近。这表明该矿床的硫化物可能是在该区的斑岩铜矿床和浅成热液矿床中形成的。托马斯二世矿体及其附近。研究结果表明,与碧瑶地区其他斑岩矿床相比,西南远景区斑岩型铜脉及其钾蚀变形成时间较早,层位较浅。
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来源期刊
Resource Geology
Resource Geology 地学-地质学
CiteScore
2.30
自引率
14.30%
发文量
18
审稿时长
12 months
期刊介绍: Resource Geology is an international journal focusing on economic geology, geochemistry and environmental geology. Its purpose is to contribute to the promotion of earth sciences related to metallic and non-metallic mineral deposits mainly in Asia, Oceania and the Circum-Pacific region, although other parts of the world are also considered. Launched in 1998 by the Society for Resource Geology, the journal is published quarterly in English, making it more accessible to the international geological community. The journal publishes high quality papers of interest to those engaged in research and exploration of mineral deposits.
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