Compositional variations in ferromagnesian minerals from porphyry copper-generating and barren intrusions of the Western Highlands, Papua New Guinea

: The Mamut deposit of Sabah, East Malaysia, is a porphyry type Cu‐Au deposit genetically related to a quartz monzonite (“adamellite”) porphyry stock associated with upper Miocene Mount Kinabalu plutonism. The genesis of the Mamut deposit is discussed based on petrology of the intrusives in the Mount Kinabalu area combined with ore– and alteration–petrography, fluid inclusion and sulfur isotope studies. Groundmass of the adamellite porphyry at Mamut is rich in K which suggests vapor transport of alkaline elements during the mineralizing magmatic process, while the groundmass of the post‐ore “granodiorite” porphyry at Mamut contains small amounts of normative corundum suggesting depletion in alkaline elements at the root zone of the magma column. Sub‐dendritic tremolitic amphibole rims on hornblende phenocrysts in the Mamut adamellite porphyry suggest interaction between the mineralizing magma and the exsolved fluids. Occurrences of clinopyroxene microphenocrysts and pseudomor‐phic aggregates of shredded biotite and clinopyroxene after hornblende phenocrysts in the barren intrusives imply lower water fugacity and decreasing in water fugacity, respectively. Compositional gap between the core of hornblende phenocrysts and the tremolitic amphibole rims and those in the groundmass of the Mamut adamellite porphyry suggests a decrease in pressure. Higher XMg (=Mg/(Mg+Fe) atomic ratio) in the tremolitic amphibole rims in the Mamut adamellite porphyry compared to those of the barren intrusions suggests high oxygen fugacity. High halogen contents of igneous hydrous minerals such as amphiboles, biotite and apatite in the Mamut adamellite porphyry suggest the existence of highly saline fluids during the intrusion and solidification of the mineralizing magma. Fluid inclusions found in quartz veinlet stockworks are characterized by abundant hypersaline polyphase inclusions associated with subordinate amounts of immiscible gaseous vapor. Both Cu and Au are dispersed in disseminated and quartz stockwork ores. Chalcopyrite and pyrrhotite as well as magnetite are the principal ore minerals in the biotitized disseminated ores. Primary assemblage of intermediate solid solution (iss) and pyrrhotite converted to the present assemblage of chalcopyrite and pyrrhotite during cooling. Subsequent to biotitization, quartz veinlet stockworks formed associated with retrograde chlorite alteration. The Cu‐Fe sul–fides associated with stockwork quartz veinlet are chalcopyrite and pyrite. Overlapping Pb and Zn and subsequent Sb mineralizations were spatially controlled by NNE‐trending fractures accompanying the phyllic and advanced argillic alteration envelope. Sulfur isotopic composition of ore sulfides are homogeneous (about +2%) throughout the mineralization stages. These are identical to those of the magmatic sulfides of Mount Kinabalu adamellitic rocks.

Section: Mineral Chemistry 3 1 Amphibolesmentioning

confidence: 86%

Genesis of the Mamut Porphyry Copper Deposit, Sabah, East Malaysia

Imai¹

2000

“…Magmatic water saturation and highly oxidizing nature have been suggested for the intrusive rocks related to porphyry Cu deposits (Mason, 1978;Chivas, 1981;Imai, 2000aImai, , 2001. Imai et al (1993Imai et al ( , 1996 and Imai (2001) documented the petrologic similarities between the ore-generating intrusion at the Santo Tomas II (Philex) deposit and the pumices erupted from Mount Pinatubo in 1991 in terms of magmatic water saturation and high magmatic f O 2 .…”

Section: Introductionmentioning

confidence: 99%

Metallogenesis of Porphyry Cu Deposits of the Western Luzon Arc, Philippines: K‐Ar ages, SO₃ Contents of Microphenocrystic Apatite and Significance of Intrusive Rocks

Imai

2002

112

K-Ar ages of the following porphyry Cu deposits in the western Luzon arc are determined: Lobo-Boneng (10.5±0.4 Ma), Santo Niño (9.5±0.3 Ma), Black Mountain (2.1±0.1 Ma), Dizon (2.5±0.2 Ma) and Taysan (7.3±0.2 Ma). Microphenocrystic apatite in the late Cenozoic intermediate to silicic intrusions associated with porphyry Cu deposits in the western Luzon arc contains sulfur as SO 3 detectable by electron probe microanalyzer. Sulfur is supposed to have been accommodated dominantly as oxidized species in oxidizing hydrous magmas that generated porphyry Cu deposits. Likewise, such high SO 3 contents in microphenocrystic apatite are common characteristics of the intermediate to silicic magmatism of the western Luzon arc, from tonalitic rocks of the Luzon Central Cordillera of about 15 Ma to an active magmatism at Mount Pinatubo. Thus, the western Luzon arc has been generating porphyry Cu mineralization associated with oxidizing hydrous intermediate to silicic magmatism related to eastward subduction, since Miocene to the present day. Intermediate to silicic rocks since 15 Ma to present-day western Luzon arc generally show high whole-rock Sr/Y ratio ranging from 20 to 184. However, porphyry Cu deposit is not necessarily related to the rocks that show higher Sr/Y ratios compared to the other barren rocks in the western Luzon arc. The characteristics of the intermediate to silicic magma associated with porphyry Cu deposit are not attributed to the composition of the source material of the magma, but to the properties defined by the high activity of oxidized species of sulfur in the fluid phase that is encountered during the generation of intermediate to silicic magmas.

“…The "genetically related intrusion" is presumably responsible for the origin of ore fluids and metals (Holland, 1972;Burnham, 1979;Holland, 1984, 1986;Urabe, 1984Urabe, , 1985. Mason (1978) and Chivas (1981) have distinguished the ore-generating intrusions from the barren ones in terms of hornblende/calcic amphibole mineral chemistry. Similar hornblende-calcic amphibole characteristics were documented at the Mamut deposit and related Mount Kinabalu intrusions, Sabah, Malaysia (Imai, 2000a).…”

Section: Introductionmentioning

confidence: 99%

Generation and Evolution of Ore Fluids for Porphyry Cu‐Au Mineralization of the Santo Tomas II (Philex) Deposit, Philippines

Imai¹

2001

Abstract:The Santo Tomas II (Philex) deposit is a porphyry Cu-Au deposit, located in the southern part of the Baguio mineral district, Benguet Province, northern Luzon, Philippines. The Santo Tomas II deposit is associated with an intrusive complex consisting of four rock types that are distinguished based on petrography. They are 1) post-ore clinopyroxene-bearing hornblende andesite porphyry, 2) ore-generating hornblende andesite porphyry, 3) hornblende quartz diorite porphyry and 4) porphyritic hornblende quartz diorite. K-Ar age of hydrothermal biotitization was estimated to be 1.5±0.4 Ma.A number of intrusive bodies having broadly similar petrography and K-Ar age occur in the vicinity of the Santo Tomas II deposit, such as at Clifton, Ligay (Binang), Bumolo (Waterhole) and Philex Main Camp areas. The intrusions at the Santo Tomas II deposit and in the vicinity are characterized by high X Mg (Mg/[Mg+Fe] atomic ratio, about 0.7 or higher) of mafic silicate phenocrysts such as hornblende, and high sulfur contents (> 0.2 wt% as SO 3 ) in accessory microphenocrystic apatite, suggesting a highly oxidizing condition. Sulfur is accommodated dominantly as oxidized species since the crystallization of phenocrysts. Sub-dendritic rim of tremolitic amphibole on hornblende phenocryst in the ore-generating andesite porphyry at the Santo Tomas II deposit suggests interaction of magma and aqueous fluid(s) exsolved due to decompression during intrusion.Dissemination of magnetite is associated with hydrothermal biotitization and is followed by sheeted and stockwork quartz veinlets having silician magnetite and rare titanohematite instead of Cu-Fe sulfides. The silician magnetite-rich quartz veinlet was formed at f O2 near the hematite-magnetite buffer at nearly magmatic temperature, where sulfur dominantly existed as oxidized species such as SO 2 . Chalcopyrite and bornite, which commonly exhibit micrographic texture often accompanying Pd telluride and native gold/Au-rich electrum, are associated with subsequent anhydrite (-quartz) veinlets and stringers. Both intermediate solid solution (iss) and bornite solid solution (bnss) are thought to have coprecipitated primarily at above 500°C based on fluid inclusion microthermometry and sulfur isotope thermometry applied for anhydrite and associated chalcopyrite and bornite. The initial iss is considered to have converted to chalcopyrite partly replacing bnss during cooling. The hypersaline polyphase fluid inclusions abundantly found in the sheeted and stockwork quartz as well as anhydrite veinlets with scarce gaseous inclusions suggest that they have been trapped in the two aqueous fluid immiscible region.The western Luzon arc associated with porphyry Cu mineralization is characterized by oxidized hydrous magmatism and shallow emplacement, and by the source of sulfur enriched in 34 S.