Skarn Type Research Articles

Mineralogy and petrology of iron skarns in western British Columbia, Canada

Calcic iron skarns present in a discontinuous belt, extending from Northern California through western British Columbia and into the Alaskan peninsula, were formed in an oceanic island-arc environment and later accreted to continental terrane. These iron skarn deposits are of particular interest because of their large size; minor element suite of Au, Co, Cu, and Zn; igneous protoliths; and common association with primitive intrusions and cogenetic volcanic rocks. A comparative study of six calcic iron skarn deposits in western British Columbia documents several important differences from other major skarn types. Compared to other skarn types, plutons associated with calcic iron skarns have a similar wide range of silica contents but are generally more mafic, and they have similar total alkali contents but much higher Na 2 O/K 2 O ratios (avg = 2.6). The iron content of intrusions is inversely proportional to the iron content of calc-silicate minerals in associated skarn, suggesting a magmatic component of iron in skarn. Fluid inclusion analyses suggest that early, relatively iron-poor calc-silicate minerals formed from hot, relatively dilute fluids (T h = 450 degrees -460 degrees C, 3.3 wt % NaCl), whereas later, iron-rich calc-silicate minerals associated with magnetite formed from cooler, more saline fluids (T h = 370 degrees -460 degrees C, 10-50 wt % NaCl, with local KCl). The correlation between fluid inclusion salinity and skarn composition is consistent with experimental evidence suggesting that the molar ratio of Fe/Al in solution should increase with the cube of the chloride concentration.--Modified journal abstract.

Minor elements and ore genesis of the Fankou lead-zinc deposit, China

The Fankou Pb-Zn deposit occurs in the Middle-Upper Devonian and Lower Carboniferous carbonate and argillaceous carbonate formations. In principle, the deposit can be classified as a carbonate-hosted strate-bound deposit. Representative sphalerite, galena, and pyrite separates from Fankou have been analysed. For the purpose of comparison a literature survey on minor elements of other districts have been carried out. The comparison of determined data with the quoted data shows that the Fankou sphalerites are rich in Ga, Ge and Ag, but poor in Se and Te; the Fankou galenas are rich in Ag, Hg, Sb and As, but poor in Se, Te, Tl and Bi; the Fankou pyrites are rich in As, Cd and In, but poor in Se, Te, Co and Ni. Zn/Cd and Se/S×10−4 ratios for sphalerites, Sb/Ag, Sb/Bi and Se/S×10−4 ratios for galenas and Co/Ni ratios for pyrites from Fankou and other districts have been calculated. Ga-Ge-Ag atomic ratios in sphalerites, Sb-Bi-Ag atomic ratios in galenas and Co-Ni relations in pyrites have been plotted. The average value (311) of Zn/Cd ratios for sphalerites from Fankou is similar to values of sphalerites from Gaobanhe, Heqing, Accesa and Broken Hill. The average Sb/Ag ratio (0.74) and the Sb-Bi-Ag atomic ratios in Fankou galenas are similar to those in the syngenetic galena from the British Island. The Ga-Ge-Ag atomic ratios for Fankou sphalerites are similar to those for the syngenetic sphalerites and Gorno sphalerites. The average Co/Ni ratio (1.1) for micro to fine-grained pyrites from Fankou laminated-bedded pyrite ore is similar to that (0.8) for the sedimentary pyrites from other districts. As to the fine to medium-grained pyrites from Fankou massive pyritic ores, their higher Co/Ni ratios (1.6–1.8) may relate to the fact that more Ni is lost than Co, during the reformation or recrystallization. Sphalerite, galena and pyrite from Fankou all are rather poor in Se and have very low values of Se/S×10−4, so they may bear no genetic relation to volcanism. To sum up, the following conclusions can be reached: (1) The Fankou deposit possesses some syngenetic features. (2) Evidently it differs from skarn type, hydrothermal type, and volcanogenic type deposits. (3) Surely it is a reformed sedimentary Pb-Zn deposit.

COMPLEX MODEL OF COPPER METALLOGENY IN MIDDLE-LOWER YANGTZE VALLEY AND ITS SIGNIFICANCE FOR ORE EXPLORATION

The middle-lower Yangtze Valley is a major metallogenic province of copper deposits in China. The major types of copper deposits in this area are: the sedimentary-reformational type, skarn type, porphyry type and volcanic type ore deposits. The complex model of copper metallogeny of the area is a dialectical synthesis in space, time and composition of several types of ore deposits. It is convenient for us in exploring and evaluating one type of ore deposit to consider the possibility of the presence of other types at the same time, and when one kind of ore is discovered, one must consider to search for another. Thus, the new approach to exploration may be developed and the potential of the orefield be expanded.

Skarn deposits in the Yerington District, Nevada; metasomatic skarn evolution near Ludwig

In the Yerington district of western Nevada, large bodies of skarn formed in a limestone unit of Triassic age adjacent to a Jurassic batholith. This batholith is dominantly granodiorite but contains quartz monzonite stocks and dike swarms that host major porphyry copper deposits. The units enclosing the limestone, an andesitc tuff and a silty limestone, were also extensively metasomatized, as were outlying intrusions of granodiorite.Skarn formation is divided into an early skarnoid stage and a late metasomatic skarn stage. The skarnoid stage formed massive fine-grained granditc garnet in the andesitc tuff, limestone, and silty limestone, with subordinate amounts of pyroxene. The iron content of garnets increases significantly away from the granodiorite, whereas pyroxenes maintain a relatively constant diopsidic composition.Granodiorite apophyses, which occur in the Triassic rocks up to 0.5 km from the main batholith contact, are extensively altered to endoskarn. The main alteration type consists of massive, anhedral granditc garnet with a significant titanium component. The distribution of endoskarn suggests that proximity to limestone was a key factor in endoskarn formation and that one hydrothermal fluid altered both sedimentary and igneous rocks simultaneously.The second stage of alteration most dramatically affected the limestone, forming coarse-grained skarn. Two skarn types are recognized: (1) a magnesium-rich type in dolomitized marble, dominated by pyroxene, and (2) an iron-rich type in calcite marble, dominated by andradite. Age relations between the two are ambiguous, but constraints inferred from T-X (sub CO 2 ) stability relations suggest that magnesium-rich skarn formed first and at high temperatures, followed by iron-rich skarn. During a late stage, intermediate granditc garnet veined both skarn types, and actinolite (+ or - salite) formed locally at the andradite-marble contact.In detail, magnesium-rich skarn nearest the granodiorite is zoned from diopside through serpentine (replacing diopside) + calcite, then from clinohumite + calcite + dolomite to calcite + dolomite marble. Additional phases include monticellite, spinel, magnetite, ludwigitc, and szaibelyite. Farther from the batholith, clinohumite is absent, tremolite separates serpentine from pyroxene, and periclase occurs locally.Pyroxene and serpentine are enriched in iron with distance from the granodiorite, the pyroxene changing from nearly pure diopside to intermediate smite a kilometer away. This compositional variation is consistent with an increase in mu Fe /mu Mg , with time and with distance from the batholith. The presence of magnetite pseudomorphs after hematite and the variation in magnetite composition indicate that the oxidation state decreased with time near the granodiorite.Iron-rich skarn consists of andradite garnet, which on the outer skarn contact directly replaced calcite. Inclusions of wollastonite noted in garnet cores from one sample suggest that initially a wollastonite zone was locally present at the skarn margin. Veins of actinolite and magnetite cut garnet skarn; late actinolite at the garnet-marble contact, locally a site of sulfide deposition, suggests a reversal in the trend toward increasing mu Fe /mu Mg . At the Douglas Hill mine, apatite, quartz, and sulfides replaced andradite.Mineral assemblages and fluid inclusion data indicate that both magnesium- and iron-rich skarn formed at low X (sub CO 2 ) , generally less than 0.1. Temperatures of formation of magnesium-rich skarn are inferred to have been in the range 650 degrees to below 400 degrees C. Temperatures of formation of iron-rich skarn are not constrained by observed assemblages. Fluid inclusion data show that late apatite replaced andradite at temperatures between 120 degrees and 200 degrees C.

Alteration and mineralization in the Ruth porphyry copper deposit near Ely, Nevada

The Ruth-Ruth Extension porphyry copper deposit in the Robinson mining district, east central Nevada, represents the eastern extremity of a once-continuous, large sulfide system that has been fragmented by normal faulting in early mid-Tertiary time. Mineralization and alteration are confined to a miogeoclinal sedimentary sequence ranging in age from Devonian to Permian and to Early Cretaceous quartz monzonite porphyry apophyses of a larger pluton that underlies the district. Prior to intrusion the sedimentary rocks were folded into a northwest-trending, locally overturned anticline. Bedding plane thrusting greatly reduced the thickness of the fiat-dipping upper limb of the anticline, and at least six tectonic units separated by fiat faults can be distinguished. Ore-grade hypogene copper mineralization only occurs within the steeply dipping lower limb of the anticline.Alteration and mineralization are spatially and temporally related to the emplacement of an early quartz monzonite porphyry pluton. In the mine area this intrusion is cylindrical and plunges 25 degrees west with smaller offshoots penetrating the overlying sedimentary rocks. At the lower west end of the cylinder this early quartz monzonite porphyry was intruded by, and graded into, a weakly altered and weakly mineralized late quartz monzonite porphyry. A granodiorite porphyry sill exposed in the southwest wall of the Ruth pit postdates mineralization. Intrusion of the early quartz monzonite porphyry resulted in the formation of biotite-andalusite hornfels in the Mississippian Chainman Shale and massive garnet skarn in Pennsylvania Ely Limestone. In contrast to the sulfide-rich garnet-pyroxene skarns elsewhere in the district, the predominant skarn type at Ruth is a massive garnet skarn low in sulfides. Following crystallization of this early porphyry phase a major fracturing event shattered the porphyry and surrounding hornfels but left the massive garnet skarn unaffected. At this time an extensive zone of biotite-orthoclase alteration formed in early porphyry and in the directly surrounding hornfels, which subsequently was largely obliterated by pervasive quartz-sericite alteration. Limestone within the zone of quartz-sericite alteration was replaced by an assemblage of quartz, chalcedony, and pyrite, locally termed silica-pyrite rock which at higher elevations gives way to pyritic marble. Chainman hornfels adjacent to Ely skarn contains epidote, actinolite, diopside, garnet, calcite, pyrite, chalcopyrite, and magnetite along fractures. At the Ely-Chainman contact there is a magnetite-rich zone which also contains pyrite, pyrrhotite, and chalcopyrite.Early quartz monzonite porphyry outside the quartz-sericite zone of alteration has been argillized. An inner zone with predominantly kaolinite and an outer zone--the clay-calcite zone with montmorillonite, kaolinite, and calcite--have been distinguished. Limestone within the kaolinite zone was converted to a pyritic marble, whereas in the clay-calcite zone it was only recrystallized. The marble halo extends for 460 m south of the Ruth deposit and contains minor amounts of wollastonite and tremolite in addition to calcite and chert. Within the marble halo minor structurally controlled lead-zinc-silver mineralization occurs. Early quartz monzonite porphyry apophyses and Permian Rib Hill Sandstone above the top of the porphyry cylinder contain quartz, kaolinite, alunite, zunyite, pyrite, and minor sericite, minerals characteristic of advanced argillic alteration.Early quartz monzonite porphyry in the core of the hydrothermal system shows biotiteorthoclase alteration and contains chalcopyrite mineralization. An increase in quartz vein density at the upper margin of the cylinder resulted in formation of a zone of pervasive silicification at the interface of the quartz-sericite and biotite-orthoclase alteration zones. This high silica zone contains high-grade chalcopyrite mineralization. Ore-grade hypogene copper mineralization (>0.4%) is largely restricted to the early quartz monzonite porphyry in the Ruth and Ruth Extension deposits and forms a 25 degrees west-dipping tabular ore zone. Following the crystallization of the late quartz monzonite porphyry, the early quartz monzonite porphyry within the biotite-orthoclase zone was intensely argillized. The unfractured late quartz monzonite porphyry is only weakly affected by this event.A pronounced vertical alteration zoning in the hydrothermal system can be demonstrated by the distribution of alteration assemblages in Ely Limestone. In areas above the hypogene ore zone, silica-pyrite rock, pyritic marble, and marble are found as one moves upward and outward from the center of the system. Laterally away from the ore zone, garnet skarn and marble alternate, but silica-pyrite rock is absent and pyritic marble is only rarely found. The asymmetric distribution of mineralization and alteration assemblages with respect to the porphyry cylinder and the inclined attitude of the ore zone and alteration zones are not the result of postmineral low-angle faulting and rotation. The alteration and metal distribution patterns indicate that hydrothermal fluids entered the fractured porphyry at its lower west end and migrated toward the top of the cylinder to the east. Middle Tertiary normal faulting exposed the high-grade hypogene mineralization to oxidation and leaching, producing a mature chalcocite blanket.

Metallogenesis in the Japanese island arc system

Ore deposits in the Mesozoic and Cenozoic orogenic belts of the Japanese island arc system consist essentially of stratabound, vein and skarn types. The stratabound base-metal deposits, which are the main source of copper, are divided into the lead-free, chalcopyrite-pyrite, Besshi-type, which occur mainly in glaucophane schists (metamorphosed basaltic pyro-clastic rocks) of the Sanbagawa belt, and the lead-bearing, polymetallic, massive sulphide, Kuroko-type, associated with rhyolite extrusion. Both types are regarded as having been formed syngenetically in a submarine environment. Vein-type deposits are widely distributed and economically the most important for metals, containing more than 50 per cent of the available tin, tungsten, molybdenum, gold, silver and manganese. The ore deposits are genetically related to either 'geosynclinal basalt' or an andesite-dacite-rhyolite and plutonic association of island arc magmatism. Special attention is given to deposits of plutonic affinity. Most Japanese plutons are granitoids. Ore deposits associated with Cretaceous-Palaeogene granitoids in southwest Japan constitute two metallogenic provinces, characterised by W-Sn-Cu and Mo-Pb-Zn veins and skarns. Northeast Japan shows a similar, but reversed, zoning, with no tin. Miocene granitoids in southwest Japan are accompanied by tin and some copper mineralization. The Japanese granitoids consist of approximately equal proportions of magnetite-series and ilmenite-series rocks. The magnetite-series is thought to be derived from a parental magma generated from mafic igneous rocks of the upper mantle—lower crust and related to major subduction. The ilmenite-series, with which tin deposits may be associated, is ascribed to magma originating in the lower crust from sedimentary, metamorphic and granitoid rocks. These magma types appear to be the most important factors in producing the various metallogenic provinces in Japan.

Distribution patterns of lead-zinc mineralization in Gorno-Altay

The map and chart of correlations of the region's polymetallic ores with geologic, tectonic, and other variables, on the geochronological scale, indicate the end of the Hercynian geologic-tectonic cycle as the time of the regional appearance of the lead-zinc mineralization and also, locally of some deposits of copper ores. However, many of the Zmeyinogorsk (Middle Carboniferous), formed during the main stage of the Hercynian folding, carry ores only in the inherited Hercynian synclinoria or in zones of deep ruptures. Their polymetallic and other ores are of the skarn type, as a rule. -- AGI Staff.

Colloform magnetite in a contact metasomatic iron deposit, Vancouver Island, British Columbia

Colloform magnetite has recently been found in the magnetite orebodies on the Empire Development property at the northern end of Vancouver Island, British Columbia. Magnetite with colloform habit is very unusual and none, as far as the writers know, has been described from limestone. The colloform magnetite occurs as pockets of nodular masses within pipe-like orebodies of predominately coarsely crystalline magnetite in limestone close to a diorite stock. The magnetite is associated with typical skarn minerals.The colloform magnetite possesses all the textural features ascribed to deposition through colloidal processes, and the deposits appear to have been formed by gel metasomatism of limestone. The writers conclude that the iron for the colloform magnetite was carried in HCl solution, replaced the limestone, and was precipitated as a colloid during an intermediate state of aggregation that existed between the state of ionic solution and the precipitate.

A NEW MORPHOLOGIC TYPE OF SKARN FORMATION IN THE TUR'YINSK COPPER DEPOSITS

A new type of skarn was discovered as a result of mining operations in the Tur'yinsk District of the Urals, the classical area of cupriferous skarns in the U. S. S. R. This previously unknown type consists of protruding ledges of a pyroxene (salitie) skarn rising steeply from the nearly flat zone of contact between aluminosilicate rocks and limestones, and protruding into the limestones over appreciable distances. The skarn pyroxene is largely replaced by chalcopyrite. The origins of the new skarn type remain to be determined. The ledges may lie over the pinched out dikes or in fissure systems that cut across the contact. The skarn may be the product of contactbimetasomatic environments or of the contact-infiltrational process. V. P. Sokoloff

Några aspekter av skarnmalmsproblemen i Bergslagen

Abstract Some aspects of the skarn ore problems in Central Sweden. According to the interpretation of the non-manganiferous skarn-bearing iron ores of Central Sweden that was given by Magnusson and the present writer 15 years ago and has later been repeated (Geijer and Magnusson 1944, 1952 a, 1952 b), this group is not genetically homogeneous. It contains >primary skarn ores>, of pyrometasomatic origin, and more numerous >reaction skarn ores> that were originally formed by sedimentation or by replacement at low or moderate temperatures, and later changed through interior reactions caused by regional heating. This metamorphism, like the development of primary skarn ores, took place during the epoch when the containing leptite formation was folded and invaded by the Earlier group of Svionian granites. Further, the >magnesia metasomatism> that accompanied these events has wrought great chemical changes in many of the earlier formed deposits. In a textbook (Magnusson 1953), however, these views have been presented in a somewhat modified form, the possibility being indicated that the deposits interpreted as such with primary skarn may all be reaction skarn deposits altered by the magnesia metasomatism. Here is therefore presented the chain of evidence on which interpretation of certain skarns as primary is based. It starts from those deposits that carry borates, chiefly ludwigite. It is shown, with reference to later works by Watanabe (1943), Tilley (1951) and Holser (1950), on deposits in other countries, that the paragenesis of the Swedish deposits in question is the same characteristic one that is found in various occurrences of such borates the world over. Further there are gradations to other skarn types, also with a pronounced magnesian nature. Some special examples are discussed in detail. It is maintained that new scientific developments have corroborated the earlier interpretation. As to the original nature of the non-manganiferous reaction skarn ores, the writer shares Magnusson's view that they probably were, in most cases, originally quartzbanded sedimentary ores deposited in close connexion with beds of limestone or dolomite. The fact that such skarn ores occur chiefly in areas of extremely sodic leptites, in which ordinary quartz-banded ores are rare, should thus mean that in this setting sedimentary deposition of iron and silica took place only in close connexion with such of carbonate rocks. There are also indications that the phosphorus percentage was lower than it is normally in the quartz-banded ores proper. Among proofs of an originally sedimentary origin of a skarn ore deposit there have been cited the occurrence of magnetite pseudomorphic after hematite, and stratification (Magnusson 1953). The former criterion is here shown to fail, as the texture of the only known case of such pseudomorphs in the region is incompatible with the interpretation that the hematite plates once formed part of a bedded deposit, but is analogous to occurrences in pyrometasomatic deposits. As to stratification, true such of sedimentary origin is certainly common among the reaction skarn ores. Caution is necessary, however, when extending this interpretation, as selective replacement may produce deceptively similar results, special reference being made to the banding of ludwigite-bearing ore in the Banat and to the banded >tactite> described by Holser (l. c.) from the Philipsburg district in Montana.

ドロマイト接觸帶スカルンに伴うマグネシウム硼酸塩鉱物の産状と共生について

In the course of the study of ores of skarn type found in some dolomite granite contact aureoles, the writer discovered several new occurrences of magnesium-borate minerals such as kotoite, suanite, ludwigite, paigeite, warwickite, szaibelyite, etc. in Japan and Korea.The magnesium-borate minerals usually occur at close to contact of granitic rocks with dolomites as boron metasomatic products.The paragenesis of the borate minerals of many localities are briefly summarized. Genesis of the boron bearing minerals in the contact aureoles are discussed.

PAIGEITE (FERROLUDWIGITE) FROM THE KAMAISHI IRON MINE, IWATE PREFECTURE, JAPAN

Paigeite from Kamaishi, Iwate Prefecture, with a composition near to the Fe end-member of ludwigite group, has been described. The mineral, a product of boron metasomatism related with a granodioritic intrusion, is found as fibrous aggregates associated with magnetite, hedenbergite, andradite, phlogopite, pyrrhotite and chalcopyrite in the iron ores of the skarn type. Its X-ray diffraction pattern is very similar to those of minerals of the ludwigite group. Oremicroscopic observations have been made. Chemical analysis gave the following percentages and formula: SiO2 1.16, TiO2 0.11, Fe2O3 29.84, FeO 48.72, MnO 0.62, MgO 1.51, CaO 1.60, Na2O 0.29, B2O3 13.24, CO2 1.14, H2O(+) 0.70, H2O(-) 0.52, S 0.09, Total 99.54; (Fe″, Mg, Mn″) 2Fe′′′BO5.