Ballooning emplacement and alteration of the Chah-Musa subvolcanic intrusion (NE Iran) inferred from magnetic susceptibility and fabric

The porphyritic diorite Chah-Musa subvolcanic intrusion is located in the Toroud-Chah Shirin magmatic arc in the northern Central Iranian structural zone. The elliptical Chah-Musa body hosts a copper deposit and intrudes an Eocene sequence of volcanic breccia, agglomerate and red tuffaceous sediment. High magnetic susceptibility values are attributed to the presence of magnetite as a magnetic carrier. Changes in bulk magnetic susceptibility correlate with zonation of alteration in the intrusion. Although the degree of anisotropy of magnetic susceptibility decreases due to hydrothermal alteration, the field observations confirm that this parameter can be used as a strain marker. Strongly oblate magnetic ellipsoids are found in the eastern half of the intrusion where isolated outcrops of flat-lying tuffaceous host cover dioritic rocks (roof zone). Stations with prolate ellipsoids mostly belong to the centre of the intrusion where the magnetic lineations plunge steeply. They are interpreted as indicating the main feeder zone. The concentric fabric pattern at the periphery of intrusion, the oblate magnetic ellipsoids at the roof, the highest anisotropy degree along the small diameter of the intrusion, and an intense deformation of the host rocks, especially at the western margin, all are evidence that the intrusion was ballooning during the late stages of its emplacement. Ascent and emplacement of the Chah-Musa body is ascribed to the tensional space provided by a dextral shear zone created by the regional left-lateral movement on the bounding Anjilow and Toroud strike-slip faults.

PETROGENESA BATUAN INTRUSIF GUNUNG BERJO - BUTAK, DAERAH GODEAN BERDASARKAN DATA PETROGRAFINYA

Daerah Godean merupakan bagian dari vulkanisme Miosen, yang tersisa pada daerah Yogyakarta bagian tengah. Vukanisme pada daerah Godean, diperkirakan sebagai produk dari busur gunungapi kepulauan dengan diferensiasi yang terbentuk mulai dari andesit basaltik, andesit dasitik, dasit, riodasit, dan kembali menjadi andesit basaltik. Penelitian dilakukan pada batuan beku pada gunung Berjo, gunung Butak, batuan intrusi subvulkanik dengan orientasi sebaran selatan- timurlaut. Analisis data menggunakan petrografi sebanyak 17 sayatan tipis untuk melihat mineralogi dan tekstur khusus yang terdapat dalam batuan. Morfologi daerah penelitian berupa bukit terisolir dengan kelerengan 51 – 56 %, merupakan produk denudasional yang dikelilingi endapan kuarter dari fluvio-vulkanik gunung Merapi. Litologi pada gunung Berjo dan gunung Buthak terdiri dari andesit piroksen, dasit, andesit, basalt piroksen, yang merupakan batuan intrusi dangkal (subvolcanic intrusion) dengan tekstur utama berupa intersitial, mortar, porfiritik yang diikuti tekstur khusus berupa oscilating zoning, sieve, dan miarolitic cavities terisi epidot-feldspar-kuarsa sebagai indikasi proses magmatic-hydrothermal pada suhu ± 200 - 400ºC diikuti alterasi hidrotermal berupa phyllic dan propylitic alteration yang terbentuk pada pH 4-5 dengan temperatur 200-3000C. Petrogenesa batuan gunung Berjo dan Butak diinterpretasi terbentuk sebagai batuan intrusi dangkal pada kedalaman 500 m dari permukaan purba, afinitas magma kapur alkali busur gunung api kepulauan dengan konten air tinggi, yang diikuti proses fraksinasi kristalisasi dengan pergerakan magma intrusi yang relatif melambat dan menerobos batuan sedimen karbonat. Petrogenesa daerah Godean masih perlu diperkuat dengan riset mineralogi lebih detil dan data lain yang mendukung.

The geology, petrology, and geochronology of the Archean Côté Gold large-tonnage, low-grade intrusion-related Au(–Cu) deposit, Swayze greenstone belt, Ontario, Canada

The Archean Côté Gold Au(–Cu) deposit is the first large gold deposit discovered in the Swayze greenstone belt of the Abitibi Subprovince. The deposit is a low-grade, large-tonnage type with a combined indicated and inferred resource of 8.65 M oz Au (245.2 t Au). The deposit is hosted by the Chester intrusive complex (CIC), a multiphase, subvolcanic intrusion composed of low-Al tonalite, diorite, and quartz diorite, plus magmatic and hydrothermal breccia bodies. The age of the tonalite and dioritic phases is constrained at 2741–2739 ± 1 Ma using high-precision isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon geochronology. Although these phases are co-temporal and co-spatial, they appear to be petrogenetically unrelated. The CIC was emplaced into mafic metavolcanic rocks of the Arbutus Formation whose geochemistry reflects a back-arc environment. The tonalite of the CIC is coeval and co-genetic with the felsic to intermediate metavolcanic rocks of the Yeo Formation. Emplacement of the CIC into a shallow crustal level is inferred based on the incorporation of screens and inclusions of the Yeo Formation and is supported by the presence of textures in tonalite and dioritic rocks (e.g., granophyres, miarolitic cavities, and pegmatites), as well as Al-in-hornblende geobarometry results of ≤1.3 ± 0.6 kbars (1 kbar = 100 MPa). The CIC is petrologically similar to other subvolcanic, low-Al tonalite–trondhjemite–diorite intrusions that underlie volcanogenic massive sulphide (VMS)-type deposits and which themselves may contain syn-intrusion mineralization. Several geochemically unrelated dykes and deformation events crosscut and postdate the CIC.

Geologic interpretation of aeromagnetic and chemical data from the Oaks Belt, Wabigoon subprovince, Minnesota: implications for Au-rich VMS deposit exploration

The Oaks Belt (OB) is a Neoarchean volcanic complex located in northwestern Minnesota, USA. It is part of the Wabigoon granite–greenstone terrane that hosts the world-class Rainy River gold deposit in nearby Ontario, Canada. Rocks in the OB form a north-dipping homocline in the fault-bounded pressure shadow of a sigma-shaped volcano-plutonic wedge that spans east–west for 220 km across the Minnesota, USA – Ontario, Canada border. Exploration drilling in the area delineated pyrrhotite–pyrite massive sulfide deposits, iron formation, chert, and semi-massive sphalerite mineralized zones. High-resolution aeromagnetic data indicate a large (∼60 km2) composite subvolcanic intrusion underlies these iron-rich strata in the OB. The position of this inferred intrusion elucidates the low base metal content of known massive sulfide deposits, as they were too far away (6–10 km) from a heat source to have been favorable sites for base metal deposition. The relative abundance of Au and Zn in the OB, alongside correlation coefficients between metals in massive sulfide deposits, iron formation, and chert, indicates the rocks were affected by a low-temperature hydrothermal system under relatively shallow water conditions (<1000 m). Negative correlation between Na2O and CaO in basalt, and their mutual moderate positive correlation with immobile corundum (Al2O3), implies alteration in the upper part of the volcanic pile did not result in substantial element mobility in most samples. Geochemical data from mafic and felsic volcanic rocks plot mainly in the calc-alkaline field. Thus, the OB is most prospective for hosting Au-rich VMS deposits and future exploration should focus on paleo-thermal corridors and favorable stratigraphic horizons near the newly inferred composite subvolcanic intrusion.