Apatite and Zircon Geochemistry in Yao’an Alkali-Rich Porphyry Gold Deposit, Southwest China: Implications for Petrogenesis and Mineralization

The Yao’an gold deposit is located in the middle of the Jinshajiang-Ailaoshan alkali-rich metallogenic belt, and this belt hosts many porphyry-type Cu-Au-Mo deposits formed at 46–33 Ma. Yao’an porphyry gold-mineralization is intimately associated with biotite syenite porphyry, whereas the contemporaneous quartz syenite porphyry is barren. In this study, we compared the major and trace elements of apatite and zircon and isotopic compositions of zircon from the biotite syenite porphyry and quartz syenite porphyry, to explore their geochemical differences that may affect their mineralization potential. The results show that both porphyries were derived from the partial melting of the thickened lower crust, which has been modified by slab-derived fluids, but has different mineral crystallization sequences, magma fluid activities, and magma oxidation states, respectively. REE contents in apatite and zircon can be used to reveal the crystallization sequence of minerals. A rapid decrease of (La/Yb)N ratio in apatite from both porphyries may be caused by the crystallization of allanite. Large variation of Cl contents and negative correlation between F/Cl and (La/Yb)N in apatite from fertile porphyry indicate that it has experienced the exsolution of Cl-bearing hydrothermal fluid. Higher Y/Ho and lower Zr/Hf in zircon from fertile porphyry indicate a stronger fluid activity than barren porphyry. The high S, V, As contents, δEu, low δCe in apatite, as well as high Ce4+/Ce3+ and log(fO2) estimated from zircon geochemistry from fertile porphyry, indicate high a oxidation state of fertile porphyry, similar to other fertile porphyries in this metallogenic belt. High fluid activity and fluid exsolution are conducive to the migration and enrichment of metal elements, which are very important for mineralization. High oxygen fugacity inhibits the precipitation of metal in the form of sulfide, thereby enhancing the mineralization potential of rock. Therefore, the exsolution of Cl-bearing hydrothermal fluid and high oxygen fugacity are the key factors promoting mineralization in Yao’an area.

Petrography of the Mesozoic Alkaline Rocks of the Taezhniy Massif (South Yakutia, Aldan-Stanovoy Shield, Leglier Ore Cluster)

The article is concerned with the petrographie compositions of the Mesozoic alkaline igneous rocks of the Taezhniy massif and its small bodies (dikes). The Taezhniy massif is located in the central part of the Nimnyr block (Aldan-Stanovoy shield). According to our field observations, it was first determined that the Taezhniy massif has a two-phase structure, its rocks of contain syenite-porphyry and leucocratic syenites. The rocks of the massif intrude the Archean granites, and are themselves intruded by late dikes of bostonites and vogesites. Based on petrographic studies, we also identified two phases of intrusion in the Taezhniy massif – syenite-porphyry and leucocratic syenites. The difference of them is as follows: in the syenite-porphyry rocks, plagioclase predominates over K-feldspar, and the content of dark-colored minerals reaches 10%. The rocks of the second phase are characterized by the predominance of K-feldspar over plagioclase, with the content of dark-colored minerals up to 5%. The order of intrusion of the massif formations is determined by the presence of xenoliths of syenite-porphyry rocks in leucocratic syenites. The rocks of the bostonite dike cutting the massif are characterized by the absence of amphibole and an increased content of pyroxene. This is most likely due to the fact that, the rock contains xenoliths of the host rocks of biotite-pyroxene composition. The dikes also conyain xenoliths of the second phase rocks, which indicates a later age of this dike. The bostonite dike located in the immediate vicinity of the massif is almost identical to the dike found in the massif, except an amphibole in the composition of the rocks. The only dike of vogesites that intersects the body of the massif is characterized by the presence of two varieties of amphibole: common hornblende and a sufficient amount of barkevikite, which phenocrysts stand out clearly against the background of the total microcrystalline mass. Also, in the course of crystal-optical studies, it was noted that in the structure of the massif there is an increase in the leucocratic magmatism from the early phase to the late, but with a decrease in this indicator, taking into account the introduction of vogesite dikes at the late stages of magmatism development. Based on this, the conclusion is made about the antidromic development of the rocks of the Taezhniy massif. When comparing the evolution of magmatism of the Taezhniy massif with the Ryabinoviy massif, a possible gold-ore specialization of the studied object is suggested.

Petrology and mineralogy of the Ulaan Del Zr-Nb-REE deposit, Lake Zone, Western Mongolia

The Ulaan Del deposit is located in the Lake Zone, Western Mongolia. In the area, middle-late Devonian alkali dykes of the Khalzan Complex are hosted in the middle-late Cambrian granodiorite-tonalite of the Togthohiinshil Complex. The alkali dykes of the Khalzan complex comprise medium- to fine-grained syenite, microsyenite, syenite-porphyry and trachyte, trachyrhyolite, and trachyandesite. The dykes are replaced to silica, sericite, albite, fluorite and are brecciated. They crosscut by quartz and quartz-carbonate veinlets. The dykes contain zircon (>0.19% Zr) with a total of rare earth elements oxides >0.1%. The host rocks of the Togtokhiinshil complex are mid-K, metaluminous, I- type granite, depleted in HFSE. Based on geochemical and mineralogical data, economic REE mineralization is concentrated in syenite and syenite porphyry of calc-alkaline high K to shoshonite series of A- type granite, emplaced at within a plate setting. Syenite dykes are enriched in REE. Ore minerals are zircon, apatite, sphene, monazite, xenotime, synchysite, parisite, fluorite and REE complex minerals, pyrite, rutile and limonite. Magmatic, metasomatic and hydrothermal processes significantly contributed to the formation of Zr, Nb, REE and Y mineralization at the Ulaan Del deposit.

Mineralogy and petrography of the Keban Pb-Zn-(Cu) skarn deposit, Elaziğ, eastern Turkey

<p>The Keban Pb-Zn-(Cu) skarn deposit in the Elazığ region, Turkey, was formed at the contact zone of the Permo-Triassic metamorphics and the Late Cretaceous plutonic rocks in the Eastern Taurus orogenic belt. The mineralization is hosted by skarn and meta-clastic/carbonate rocks of the Keban Metamorphics intruded by alkali syenite porphyry, which is associated with the Pb-Zn-(Cu) mineralization. The rock units in the region are partly hydrothermally altered graphite calc-schist containing crystallized limestone interlayers and lenses, meta-pellitic rocks (phylitte/calc-phyllite), dolomitic limestone, calc-silicate hornfels, marble and plutonic rocks. Calc-silicate hornfels is an initial metamorphic product occurred in contact zone of the intrusive unit. Results of mineralogical studies indicate that garnet and pyroxene-rich skarn formed in early (prograde) stage of skarnization whereas epidote, chlorite, tremolite, phlogopite, muscovite, calcite, quartz and fluorite are typical minerals of the retrograde stage. Using the Raman spectroscopy investigations, garnets in alteration zone are subdivided into two groups. Garnets in andradite composition are zoned and occur close to the intrusion reflecting high-temperature conditions and those of grossular composition represent low-temperature conditions. The sill/dykes and stock-like Keban plutonic rocks hosting foid syenite porphyry and nepheline syenite are of holocrystalline hipidiomorph porphyritic texture including large nepheline and plagioclase phenocrysts. Metallic minerals comprise sphalerite, galena, chalcopyrite, magnetite, bornite, pyrite, fahlore and hematite, which mainly occur as dissemination, vein and massive forms and crosscut by late-stage quartz, fluorite and calcite veinlets. Sphalerite is medium-coarse grained, semi-euhedral and contain chalcopyrite inclusions. Blebs of chalcopyrite are widely recognized in sphalerite (chalcopyrite disease). Galena replaces sphalerite and in some cases, it hosts several sulfo-salt minerals. Magnetite partly or completely transforms to limonite and chalcopyrite inclusions in sphalerite occur among the magnetite grains.</p><p><strong>Key words: </strong>Keban, Pb-Zn-(Cu) skarn deposits, Mineralogy, Petrography, Ore Microscopy, Raman Spectroscopy</p>

An integrated study of the electrical parameters of the samples of magnetite ores in relation to their genesis and mineral composition (on the example of the Goroblagodatskoe skarn-magnetite deposit)

The temperature dependences of electrical resistance (at DC and AC voltage at 1 kHz frequency) of the samples of the magnetite ore and magnetite from the contact of the syenite-porphyry with the volcanic-sedimentary rocks from the Goroblagodatskoye iron-ore deposit in the temperature range 20—800 °C have been studied. The frequency dependences of the active electrical resistance and dielectric losses in the range 0,01—100 kHz have been obtained at 20 °C. For the magnetite ore and magnetite from the syenite-porphyry contact with volcanogenic sedimentary rocks in the studied temperature and frequency ranges, the relationship between electrical resistance (lgR) and dielectric losses (lgtgδ) has been revealed. The character of the relations is different, that allows to separate uniquely the magnetite ore and magnetite. The parameters of high-temperature conductivity (activation energy Eoand electrical resistance coefficient lgRo)nave been obtained. The parameters of the studied samples of pyroxene-orthoclase-magnetite, garnet-magnetite, epidote-chlorite-magnetite ores form a straight line, as if forming its different parts. The correlation between the parameters of E0and lgRo, samples of these ores has the following form: lgRo, — 2,2—6,6 Eo. The parameters of magnetite samples from the contact of syenite porphyry with volcanogenic-sedimentary rocks also form a straight line, as if forming its different parts, with the correlation form as following: lgRo— 2,1—6,6 E0. It has been found that with the increasing distance to the syenite intrusion, the electrical parameters of magnetite ore change: EQ increases, lgRo, decreases. For magnetite from the contact of the syenite-porphyry and volcanic-sedimentary rocks there is another picture — the further you are from the syenite-porphyries, the smaller E0is, and the bigger the lgRo, is. The T0temperatures, at which the electrical resistance at the constant voltage becomes equal to the active resistance at the alternating voltage for the samples of garnet-magnetite ore with the different magnetite content, P,%, have been revealed. The correlation between the T0 parameter and the magnetite content in the ore, P(Fe304, %) = 323,4 — 47,4 ln( T0), R2= = 0,93, has been established.