University Of Tasmania
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Geology, hydrothermal alteration, geochemistry, mineral geochemistry, and alteration textures of the lithocap at Bantug, Negros-Philippines

posted on 2023-05-27, 09:30 authored by Jimenez Torres, CA
Negros island is located in a complex tectonic setting in the central Philippines. The lithocap at Bantug is one of several laterally extensive lithocaps in central Negros. Volcanic rocks from the Pleistocene to Holocene Canlaon Volcanic Complex have been altered to advanced argillic and silicic mineral assemblages, and intruded by diorite porphyries and hydrothermal breccias from the Pleistocene Bantug Intrusive Complex (BIC). The lithocap at Bantug extends over 6 km, and has an approximate thickness of 600 m, transitioning downwards into hornfels that has developed in the Pliocene-Pleistocene Caliling Formation. REE and other geochemical anomalies were detected during this study but no significant concentrations of precious and base metals have been found to date. The Bantug lithocap formed in plagioclase-phyric andesites, aphyric andesites, and volcaniclastic andesitic breccias of the Quaternary Canlaon Volcanic Complex. The lithocap is genetically linked to magmatic-hydrothermal activity caused by the BIC. At least four intrusions and two hydrothermal breccias comprise the BIC. The intrusive complex consists of an early diorite porphyry (U-Pb zircon ages: 1.37 ± 0.10 Ma; 1.35 ± 0.30 Ma), and at least three phases of dikes (U-Pb zircon ages: 1.25 ± 0.31 Ma; 1.06 ± 0.29 Ma). The dikes are dioritic in composition, have plagioclase-phyric textures, and some contain miarolitic cavities filled with epidote and quartz. The diorite porphyry is intensely phyllic-altered and the diorite porphyry dikes have undergone moderate propylitic and argillic alteration. Drilling in the lithocap intercepted hydrothermal breccias. The breccia bodies have drill hole intercepts of 5 to 325 m. The hydrothermal breccias display advanced-argillic altered clasts, and quartz-pyrite cemented breccias contain juvenile clasts. Silicic, kaolinite-dominated argillic, montmorillonite-dominated argillic, advanced-argillic, and phyllic hydrothermal alteration assemblages characterise the Bantug lithocap. Surface SWIR analyses over a 2.5 km northeast transect revealed a marked transition from alunite to pyrophyllite and muscovite-bearing samples. Drill hole results transitioned from argillic and advanced argillic alteration associated with hydrothermal breccias in the southwest, to phyllic alteration associated with the diorite porphyry to the northeastern part of the studied area. Index of crystallinity values are higher to the northeast, implying a higher temperature fluid source in this area. Mass balance calculations for the plagioclase-phyric andesites of the Canlaon Volcanic Complex revealed significant losses ofmost elements in silicic-altered samples in the northeast. The same area saw significant gains of SiO\\(_2\\) and TiO\\(_2\\). Alteration index and advanced argillic alteration index values increase markedly to the northeast both in surface and drill hole samples. Magmatism and fragmentation processes produced extensive geochemical anomalism at Bantug. Although base and precious metals results were below ore-grade, trace element concentrations revealed anomalous areas. Rare high copper and gold values from Bantug lithocap are associated with advanced argillic-altered aphyric andesite (i.e., 2,517 ppm Cu) and plagioclase-phyric andesite samples (i.e., 0.56 ppm Au). Systematic analyses of surface and drill hole geochemistry results revealed HFSE, REE, transition metals, and base metals anomalism in the northeastern part of the study area. This coincides with the presence of high temperature minerals (i.e., pyrophyllite, zunyite, and muscovite) and high alteration index values (i.e., 93.6 to 96.3). LA-ICP-MS analyses of alunite, pyrite, magnetite, chlorite and epidote revealed many similarities between the Bantug lithocap, propylitic-altered rocks from Batu Hijau, and the Mankayan lithocap. The variations in alunite compositions from Bantug (i.e., Na, La, Sr, Pb, Sr/Pb, 100 La/Pb, Pb, La, and Pb/Sr) vary systematically with respect to the location of diorite porphyry intrusions intercepted by drill holes. The trace element composition of alunite samples from Bantug compares favourably with that of alunite samples from Mankayan. Trace element variations represent vectors towards potential mineralised centres. The trace element composition of pyrite, chlorite and magnetite samples from Bantug is comparable with that of samples from Batu Hijau, and indicates a potential heat source located at distances from the sample locations greater than 1.5 km, possibly at depth. Similar results were acquired from comparing trace element results for epidote from Bantug with results from Baguio. Patchy-wormy (gusano) textures are known to occur at the base of several lithocaps around the world, and were observed in surface and drill hole samples from Bantug. Patchy textures are characterised by amoeboid-like, sub-rounded to rounded clusters of clay minerals, sulfates, and sulfides, in a fine-grained granular quartz groundmass. Wormy texture is characterised by contorted quartz veins, which commonly occur with patchy texture. Patchy features from Bantug are composed of pyrophyllite, alunite, diaspore, muscovite, APS minerals, zunyite, rutile, pyrite, and quartz. These are interpreted to be the product of dissolution of primary volcanic features due to repetitive exposure to fluxes of acid, halogen-bearing hydrothermal fluids. Sedimentation of the Pliocene-Pleistocene Caliling Formation took place under shallow marine conditions (i.e., 5.33 to at least 1.93 Ma), based on the presence of several limestone horizons along the eastern coast of Negros. The Pleistocene to Holocene Canlaon Volcanic Complex rocks are interpreted to have been deposited above sea level. Magmatism and emplacement of the Pleistocene Bantug Intrusive Complex began around 1.65 Ma. Extensive hydrothermal alteration is genetically linked to emplacement of the BIC. Large areas of silicic and advanced argillic-altered volcanic rocks of the CVC indicate that hydrothermal alteration took place above sea level and that sea water did not dilute hydrothermal acid fluids generated by the BIC. The hydrothermal system at Bantug formed large volumes of silicic-altered volcanic rocks, surrounded by argillic-altered volcanic rocks, which transition to depth to phyllic-altered diorite porphyritic intrusions. Incipient stockwork formed in the phyllic-altered diorite porphyry intercepted by exploration drill holes; however, base and precious metals results in the lithocap and intrusions were below ore grade (i.e., mean values: 147.7 ppm Cu; 9.994 ppm Mo; 0.01499 ppm Au; 0.07056 ppm Ag). The apparent absence of mineralisation at Bantug could indicate that: i) the mineralised centre has not been intersected by drill holes, ii) oreforming physicochemical conditions were inefficient, or iii) the causative intrusion was metaldepleted. Pathfinder element concentrations in vuggy-quartz intercepted by drill holes, hydrothermal breccias, magmatic-hydrothermal breccias, and patchy-wormy textures development at Bantug provide encouragement for continuing exploration in the lithocap for concealed porphyry and epithermal mineral deposits. Parts of the Bantug lithocap remain unexplored and untested by drill holes. Zunyite, high AI values, muscovite, pyrophyllite, and whole rock geochemistry highlight the northeastern area as a priority target for future exploration. Organic-matter rich horizons within the Pliocene-Pleistocene Caliling Formation may also represent traps for hydrothermal fluids and future exploration should consider the potential occurrence of a skarn deposit in the limestone horizons.


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