Минералого-петрографические особенности и последовательность образования золотосурьмяных руд Удерейского месторождения (Красноярский край)

The article presents results of petrographic and mineralogical studies of gold-antimony ores of the Udereyskoe deposit and their concentration products. The main mineral associations of ores are characterized, mineral forms of occurrence are revealed, and distribution of Au, Pd, Ag is estimated. The paragenetic scheme of mineral formation sequence was clarified and supplemented. The important mineral forms of gold occurrence are goldbearing arsenopyrite and native gold (Au,Ag), as well as rare aurostibite AuSb2 and intermetallides such as antimony gold (Au,Sb), and palladium gold (Au,Pd). The revealed diversity of forms of noble metals and analysis of their relationships with associating sulfides and nonmetallic minerals indicate at least two contrasting early and late stages of ore genesis. Early quartz-pyrite and late pyrite, which are two stages of mineralization, are determened within the early stage. Three consecutive stages of mineralization are established in the late stage: quartz-stibnite, pyrite-stibnite-quartz, and pyrite-arsenopyrite-stibnite.Приводятся данные по петрографическим и минералогическим исследованиям исходных золотосурьмяных руд Удерейского месторождения и продуктов их обогащения. Охарактеризованы главные минеральные ассоциации руд, выявлены минеральные формы нахождения и оценено распределение Au, Pd, Ag. Уточнена и дополнена парагенетическая схема последовательности минералообразования. Важными минеральными формами нахождения золота являются преобладающие золотосодержащий арсенопирит и самородное золото (Au,Ag), а также редкие ауростибит AuSb2 и интерметаллиды – сурьмянистое золото (Au,Sb) и палладистое золото (Au,Pd). Выявленное многообразие форм нахождения благородных металлов и анализ их взаимоотношений с ассоциирующими сульфидами и нерудными минералами указывают на как минимум два контрастных этапа рудогенеза – ранний и поздний. В пределах раннего этапа выделены две стадии минерализации – ранняя кварц-пиритовая и поздняя колчеданная. В позднем этапе выделены три последовательные стадии минерализации: кварц-стибнитовая, пирит-стибнит-кварцевая и пирит-арсенопирит-стибнитовая

The Skaergaard PGE and Gold Deposit: the Result ofin situFractionation, Sulphide Saturation, and Magma Chamber-scale Precious Metal Redistribution by Immiscible Fe-rich Melt

The Skaergaard intrusion, Greenland, is the type locality for Skaergaard-type mineralizations. Mineralization levels are perfectly concordant with igneous layering, up to 5 m thick, internally fractionated, and contain crystallized sulphide droplets and precious metal alloys, sulphides, arsenides and telluride. Immiscible Cu-rich sulphide droplets, formed in a mush zone below the roof, scavenged precious metals. They were subsequently dissolved and transported to the floor in late-formed, immiscible, Fe-rich mush melts. Mineralized stratigraphic intervals of floor gabbro formed in ‘proto-macrolayers‘, owing to local sulphide saturation in melt concentrated between floating plagioclase and sinking clinopyroxene. The floor mineralization is divided into four stratigraphic sections. Formation of the Lower Platinum Group Element Mineralization (LPGEM) involved: (1) crystallization of the bulk liquid liquidus paragenesis and in situ fractionation; (2) sulphide saturation and formation of sulphide droplets in melt in the upper part of ‘proto-macrolayers‘. After further in situ fractionation, the following steps occurred: (3) the onset of silicate–silicate immiscibility and the consequent loss of buoyant and immiscible Si-rich melt; (4) dissolution of unprotected droplets of sulphide melt present in the Fe-rich mush melt; (5) compaction-driven upwards loss of residual mush melt enriched in, for example, Au. The LPGEM preserves upward increasing bulk Pd/Pt (∼6–13) owing to a continued supply of PGE and Au, with high Pd/Pt. The further development of the LPGEM ceased as the supply of precious metals to the floor waned. The Upper PGE Mineralization (UPGEM) subsequently formed from precious metals recycled in the floor. The UPGEM is characterized by increasing Au substitution in PGE phases, and a decrease in total PGE and Pd/Pt owing to upward fractionation in migrating mush melts and exhaustion of Pd and Pt. An upper Au-rich mineralization level (UAuM) was caused by late remobilization of Au and deposition on grain boundaries in fully crystallized gabbro. Cu concentrations (∼150 ppm) are not correlated with PGE and Au. Repeated Cu mineralization levels (CuM), attaining >1000 ppm, occur above the Au levels, caused by local mush layer sulphide saturation. PGE, Au and Cu distributions in the floor mineralization reflect sub-liquidus, but supra-solidus, processes and reactions in mushes at the roof, wall and floor. Constraints provided by a new model for the mineralization provide the basis for re-evaluation of the solidification processes in the Skaergaard intrusion. We have identified the importance of extensive in situ fractionation and intrusion-wide elemental redistributions in immiscible Fe- and Si-rich silicate melts. Our model characterizes the floor cumulates as bulk liquid orthocumulates containing an upwards-increasing proportion crystallized from Fe-rich, immiscible mush melt. The roof-rocks are complementary to the floor, with downwards increasing proportions crystallized from the conjugate Si-rich melt. Petrographic observations and the relative timing of crystallization support the hypothesis that crystallization was restricted to marginal mush zones. Bulk melt remaining in the magma chamber evolved not, as generally assumed, as a result of loss of crystals grown from the bulk melt, but as the consequence of mixing with recycled and evolved melt expelled from the mush by compaction. Redistribution of Fe in immiscible melts may be common to mafic intrusions and puts into question the validity of petrogenetic modelling of bulk liquids in mafic intrusions based only on consideration of floor cumulates

Palladosilicide, Pd2Si, a new mineral from the Kapalagulu Intrusion, Western Tanzania and the Bushveld Complex, South Africa

© 2015 The Mineralogical Society. This document is the author's submitted/pre-refereeing version of the journal article. You are advised to consult the publisher's version if you wish to cite from it