V. A. Simonov

Genesis of platiniferous massifs in the southeastern Siberian Platform

Data obtained on melt inclusions in Cr-spinel suggest a magmatic genesis of dunite in the Konder and Inagly placer-forming platiniferous massifs in the southeastern Siberian Platform. These data make it possible to evaluate the physicochemical parameters of the magmatic processes that produced these concentrically zoned alkaline-ultrabasic complexes. The comparative analysis of the composition of the Cr-spinel with inclusions highlights remarkable differences between this mineral in the Konder and Inagli massifs, on the one hand, and in ultramafic rocks in ophiolites and the modern oceanic crust, on the other. Minute clinopyroxene crystals included in Cr-spinel from the Konder Massif have a composition and configurations of their REE patterns contrastingly different from those of clinopyroxene in basite-hyperbasite complexes of ophiolite associations but are close to those of clinopyroxene in the Kytlym and Nizhnii Tagil platiniferous massifs in the Urals. The composition of the quenched melt inclusions suggests that the chromite crystallized predominantly from picrite alkaline magmas. The concentrations of most elements in the high-Mg inclusions are close to those in biotite-pyroxene alkaline picrites, a fact testifying to the significant contribution of ultrabasic (picrite) alkaline magmatic systems to the origin of the Konder and Inagli massifs. Ion-probe analyses of the inclusions suggest that the melts were rich in water (up to 0.6 wt %). Data on the distribution of REE and other trace elements in the inclusions provide evidence of the influence of a deep plume. Our simulations with the use of the composition of the melt inclusions suggest that dunite in the Konder and Inagli massifs were produced mainly by water-bearing magmas at temperatures of 1460–1300°C. As the melts evolved to less magnesian ones, olivine continued to crystallize from them until the temperature decreased to 1230°C.

Petrogenesis of the Fe-Ti intrusive complexes in the Sierra Leone region, Central Atlantic

The study of melt inclusions in Cr-spinels from melanocratic troctolites provided the first direct information on the physicochemical parameters of enriched magmatic systems that produced high-Fe and high-Ti intrusive complexes in the Sierra-Leone region (Central Atlantic, 6°N). These complexes are made up of predominating hornblende Fe-Ti oxide gabbronorites and gabbrodiorites with subordinate amount of ultramafics, diorites, quartz diorites, and trondhjemites. The study of melt inclusions and rocks showed that the majority of gabbroids of the Central Atlantic (Sierra Leone area and 15°20′ Fracture Zone) were derived from N-MORB-type melts, whereas differentiated Fe-Ti-oxide rocks were crystallized from other melts, which were preserved as inclusions in the Cr-spinels from the melanocratic troctolites of the Sierra Leone region. The ion-microprobe study of these inclusions yield direct evidence on the elevated water content (up to 1.24–1.77 wt %) in the parental melts of Fe-Ti oxide rocks. Data on trace and rare-earth element distribution together with high (La/Sm)N and (Ce/Yb)N ratios in the inclusions indicate the possible influence of deep plume source on the generation of these magmas. Simulation based on melt inclusion data testifies that high-Fe intrusions of the Sierra Leone area were crystallized from the water-saturated magmas at relatively low temperatures (1020–1240°C). It was shown that the geochemically enriched Fe-Ti melts were presumably formed regardless of N-MORB-type magmatism predominant in Central Atlantic, under the influence of new mantle plume that caused melting of hydrated oceanic lithosphere.

Physicochemical parameters of magmatic and hydrothermal processes at the Yaman-Kasy massive sulfide deposit, the southern Urals

Melt and fluid inclusions in minerals have been studied and physicochemical parameters of magmatic processes and hydrothermal systems estimated at the Yaman-Kasy copper massive sulfide deposit in the southern Urals. It was established that relatively low-temperature (910–945°C) rhyodacitic melts belonging to the tholeiitic series and containing 2.7–5.2 wt % water participated in the formation of the igneous complexes that host the Yaman-Kasy deposit. As follows from ion microprobe results, these silicic magmas had a primitive character. In the distribution of trace elements, including REE, the rhyodacites are closer to basaltic rather than silicic volcanic rocks, and they are distinguished in this respect from the igneous rocks from other massive sulfide deposits of the Urals and the Rudny Altai. Two types of solutions actively took part in the formation of hydrothermal systems: (1) solutions with a moderate salinity (5–10 wt % dissolved salts) and (2) solutions with a low salinity (a value close to that of seawater or even lower). Concentrated fluids with more than 11.5 wt % dissolved salts were much less abundant. Hydrothermal solutions heated to 130–160, 160–270, or occasionally 280–310°C predominated in ore formation. The sequence of mineral-forming processes at the Yaman-Kasy deposit is demonstrated. Mineral assemblages were formed with an inversion of the parameters characterizing ore-forming solutions. An increase in the temperature and salinity of solutions at the early stages was followed by a decrease at the final stages. The evolution of the hydrothermal system at the Yaman-Kasy deposit has much in common with the parameters of black smokers in the present-day Pacific backarc basins.

Petrogenesis of the island-arc complexes of the Chara zone, East Kazakhstan

The study of clinopyroxenes and melt inclusions provided direct (independent on secondary alteration) information on the petrogenesis of the island arc complexes of the Chara zone, East Kazakhstan. It was shown that magmatism of this zone evolved from primitive island-arc systems with boninites to mature island arc with calc-alkaline melts. In terms of trace and rare-earth element distribution, the melt inclusions in the clinopyroxenes of the Chara zone differ from mid-ocean ridge basalts, being closer to the island-arc calcalkaline series. Based on inclusion composition, the parental melts of the considered complexes crystallized within 1150–1190°C with decreasing iron, magnesium, calcium, and sodium contents. Simulation based on melt inclusion data in clinopyroxenes indicates that the melts contained up to 1 wt % water, which was confirmed by direct ion-microprobe determination of 0.84 wt % H2O in the inclusions. Calculated liquidus temperatures are consistent with homogenization temperatures of the inclusions. Our calculations on the basis of inclusion data testify that the primary melts of the studied basaltic series of the Chara zone were generated from the mantle protolith within temperatures of 1350–1530°C at depths of 50–95 km. Similar parameters are typical of the generation of the tholeiitic and boninitic island-arc magmas in the modern ocean-continent transition zones of the Pacific type. In general, the study of clinopyroxenes and melt inclusions suggests that the considered complexes of the Chara zone were formed with the participation of tholeiitic and calcalkaline volcanogenic systems of basaltic, basaltic andesite, and, possibly, boninitic composition in the paleogeodynamic setting of evolving ancient island arc.

Phase separation of fluid in the Ashadze deep-sea modern submarine hydrothermal field (mid-atlantic ridge, 12°58s’ N): Results of fluid inclusion study and direct observations

1446 The degree of chemistry variation in fluids venting from hot springs and the mechanisms and parameters controlling the evolution of their composition and temperature are the most important question in the study of hydrothermal systems found in the global sys tem of mid ocean ridges. Our knowledge in this sphere was mainly based on the results of short term (during several years) monitoring on hydrothermal fields. The study of fluid inclusions in minerals provided new concepts on the evolution of salinity and temperature of fluids [1].

Hydrothermal Alteration and Sulfide Mineralization in Gabbroids of the Markov Deep (Mid-Atlantic Ridge, 6 ° N)

A new type of sulfide occurrence related to metasomatically altered brecciated gabbroids has been studied at the Sierra Leone site situated in the axial rift valley of the Mid-Atlantic Ridge (Markov Deep, 6° N). Two associations of plutonic, subvolcanic, and volcanic rocks were dredged: (1) mid-ocean ridge basalts (MORB) and their intrusive analogues and (2) rocks of the silicic Fe-Ti-oxide series with dominating gabbronorites and sporadic trondhjemites. Almost all igneous rocks at the Sierra Leone site are enriched in Pb, Cu, U, Ga, Ta, Nb, Cs, and Rb and depleted in Zr, Th, and Hf. The rocks of the Fe-Ti-oxide series are enriched in Zn, Sn, and Mo and depleted in Ni and Cr. The main ore-bearing zone is situated at the foot of the eastern wall of the deep, where it is hosted in cataclastic hornblende gabbro and gabbronorite of the Fe-Ti-oxide series. Ore mineralization in metasomatically altered rocks is composed of quartz-sulfide and prehnite-sulfide veinlets, disseminated sulfide, and veined copper sulfide ore. The ore consists of pyrite, chalcopyrite, sphalerite, pyrrhotite, bornite, chalcocite, and digenite. The δ34S value of sulfides varies from 3.0 to 15.3‰. At the foot of the eastern wall of the Markov Deep, directly downslope from the ore-bearing zone, loose sediments contain grains of native Cu, Pb, Zn, and Sn and intermetallic compounds (isoferroplatinum, tetraferroplatinum, and brass) apparently derived from the ore. Mineral assemblages of ore-bearing metasomatic rocks and fluid inclusions therein were studied. Ore metasomatism developed under a low oxygen potential within a temperature interval from 400 to 160°C, though initial hydrothermal alteration of rocks proceeded at temperatures of 800–450°C. The temperature of stringer-disseminated ore mineralization is estimated at 170–280°C. The hydrothermal fluids are considered to be of magmatic origin; as the hydrothermal system evolved, they became diluted with seawater that was contained in fractured oceanic crust. The ore matter could have been derived from magmatic fluids that were released from water-saturated melts of the Fe-Ti oxide series during their ascent and leached from host gabbroids in the process of metasomatic alteration.

Physicochemical parameters of crystallization of dunite from the Guli ultrabasic massif (Maimecha Kotui province)

On the basis of analysis of molten inclusions in chrome-spinelide, physicochemical parameters of dunite crystallization were defined. Experimental, analytical studies directly indicate that dunite was formed from high-temperature melts close in petrochemical composition and high-temperature characteristics to meimechite magmas. Successive evolution of magmatic systems compositions in a course of intra-chamber crystallization of dunite was established: from picriteâ “meimechite (with olivine formation at 1590–1415°C and chrome-spinelide crystallization at 1405–1365°C) to picrate-basalt and basalt.

Conditions of crystallization of dunite from the Nizhnii Tagil platinum-bearing ultrabasic massif (Urals)

Platinumbearing dunite–clinopyroxenite–gabbro complexes of the Urals have attracted the attentionof researchers for a long time and are currently beingstudied in detail. In addition, despite the increasingvolume of information, the conditions of petrogenesisof these associations are still debatable. Of specialimportance are the problems of the formation of dunitecontrolling chromite–platinum mineralization [1–4].The problems of genesis of these ultrabasic rocks wereconsidered in numerous papers; most of them containdata on the significant role of magmatic processes incrystallization of dunite [3, 5–8]. At the same time,most published papers providing evidence for themagmatic nature of dunites from zoned platinumbearing complexes of the Urals practically do not contain direct proofs of the participation of hightemperature silicate melts.Investigations of basic–ultrabasic complexes ofSiberia, the Urals, the Far East, and the Central Atlantic demonstrate that analysis of chrome spinellids andmelt inclusions in them is the most promising way toobtain direct information on the genesis of ultrabasicrocks [9–13]. This approach was applied for study ofthe condition of crystallization of dunite from theNizhnii Tagil ultrabasic massif (Platinumbearing Beltof the Urals).Information on the geological structure of theNizhnii Tagil massif, petrography, mineralogy, and orepotential has been discussed in many publications [1–4, 6, 7, 14]. It forms an ellipselike body in plan with thecentral part composed of a large dunite core surrounded by a clinopyroxenite rim of complex morphology. Petrostructural investigations allowed theauthors of [7] to distinguish protogranular, porphyroclastic, and mosaic–granoblastic types of microtextures. The first type is considered as primary corresponding to the class of protomagmatic adcumulatetextures. The two other types are formed later in theprocess of hightemperature plastic deformation andsyntectonic recrystallization.Investigation of dunite samples from the NizhniiTagil massif mainly corresponding to the protogranular (primary) type allowed us to discover and studymelt inclusions in accessory chrome spinellids. Theconsidered chromites are almost intransparent and donot provide direct observations during heating;because of this feature, a special methodology [10, 12]was applied. Chrome spinellid grains were heated upto 1320–1330°C in graphite microcontainers of a special microthermocamera in the course of hightemperature experiments. After being kept for 10 min,they were quenched in water in order to obtain glass ininclusions. Glasses of quenched inclusions andchrome spinellids were studied on a CamebaxMicroXray microanalyzer and on a LEO 1430 VP scanningelectron microscope at the Institute of Geology andMineralogy, Siberian Branch, Russian Academy ofSciences (Novosibirsk).According to the values of the chromium and magnesium mole fractions, chrome spinellids (with meltinclusions) from dunite of the Nizhnii Tagil massif arepractically consistent with chromites from dunite ofthe platinumbearing Konder and Inagli massifs(Siberian Platform). The studied chrome spinellidsdiffer from minerals of ultrabasic rocks from modernand ancient (ophiolites) oceanic structures. As awhole, chrome spinellids from platinumbearing massifs are characterized by a high chromium mole fraction (Cr# = 78–89%). Chromites from oceanic complexes provide a single trend of Cr# decrease (from65 to 45%) at a significant decrease in Mg# (from 78to 28%).The studied primary melt inclusions (10–40 µm)are regularly located in grains of accessory chromite.Inclusions have round morphology, being in equilibrium with host chromite, and are often slightly faceted. Before experiments they contained a set of vari

Physicochemical parameters of the melts participating in the formation of chromite ore hosted in the Klyuchevsky ultramafic massif, the Central Urals, Russia

The results of melt inclusion study are reported for chromites of the Klyuchevsky ultramafic massif, which is the most representative of all Ural ultramafic massifs localized beyond the Main Ural Fault Zone. The massif is composed of a dunite-harzburgite complex (tectonized mantle peridotite) and a dunite-wehrlite-clinopyroxenite-gabbro complex (layered portion of the ophiolitic section). The studied Kozlovsky chromite deposit is located in the southeastern part of the Klyuchevsky massif and hosted in serpentinized dunite as a series of lenticular bodies and layers up to 7–8 m thick largely composed of disseminated and locally developed massive ore. Melt inclusions have been detected in chromites of both ore types. The heated and then quenched into glass melt inclusions and host minerals were analyzed on a Camebax-Micro microprobe. The glasses of melt inclusions contain up to 1.06 wt % Na2O + K2O and correspond to melts of normal alkalinity. In SiO2 content (49–56 wt %), they fit basalt and basaltic andesite. The melt inclusions are compared with those from chromites of the Nurali massif in the southern Urals and the Karashat massif in southern Tuva. The physicochemical parameters of magmatic systems related to the formation of disseminated and massive chromite ores of the Klyuchevsky massif are different. The former are characterized by a wider temperature interval (1185–1120°C) in comparison with massive chromite ore (1160–1140°C).

Anomalously high concentrations of metals in fluid of the Semenov modern hydrothermal system (Mid-Atlantic Ridge, 13°31′ N): LA-ICP-MS study of fluid inclusions in minerals

How and how much do hydrothermal fluids trans� port metals from the area of their involvement in the oreforming process to the zone of their precipitation? What should the composition of fluids, which may transport metals in the proportions necessary for the formation of mineral deposits, be? These problems have been studied by more than one generation of researchers in the field of ore deposits. An important role in the solution of this problem was played by the experimental and thermodynamic studies of mineral solubility in aqueous fluids at high temperatures. In spite of significant achievements in these areas, we cannot assume that the obtained results reflect the chemical composition of the natural oreforming fluid adequately, since modeling of metal behavior in it is limited by our knowledge of the thermodynamic con� stants of minerals and components of fluid. Significant achievements in improving our knowledge on the chemistry of mineralforming fluids were gained dur� ing the study of modern hydrothermal systems and fluid inclusions in minerals of hydrothermal deposits (1). A huge volume of data was obtained after the dis� covery of modern hydrothermal sources precipitating sulfide ores on the floor of the World Ocean (21° N) on the East Pacific Rise in 1978. During the past 35 years, more than 300 hydrothermal sites have been discov� ered and studied by submersibles. The major data on the chemistry of fluid and concentrations of metals in it were mostly based on the results of the analyses of its samples collected on the hydrothermal fields by sub� mersibles. However, study of fluid inclusions in miner� als from modern sulfide edifices has shown that the conditions of deposition, the evolution of salinity, and the temperature of fluids discharged on the sea floor were much more variable than followed from direct measurements (2-4). Development of new highpre� cision apparatus and analytical methods has provided completely different possibilities for the study of a wide spectrum of chemical elements in fluids trapped by fluid inclusions during mineral crystallization. Study of fluid inclusions in minerals from a number of ore deposits and analysis of the concentrations of met� als in fluids precipitating ores have been performed (5-7). Using these methods, it is very interesting to analyze the fluid captured by minerals during the for� mation of modern sulfide ores. For this purpose, we carried out investigations of fluid inclusions in miner� als from the Semenov modern ore cluster.