Andradite and andradite-grossular solid solutions in very low-grade regionally metamorphosed rocks in Southern New Zealand
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Andradite
Actinolite
Grossular
Pseudomorph
Greenschist
Paragenesis
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Andradite
Protolith
Actinolite
Pyroxene
Metasomatism
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Abstract The Enjerd skarn is located 28 km northwest of Ahar in East Azarbaijan Province, northwestern Iran. Based on mineral composition, paragenesis, and mineral relationships, two stages of hydrothermal alteration and related mineralization have been identified. Stage I was produced by early pulses of Fe-rich, magmatically derived fluids coincident with potassic alteration of the main stock. This event caused the formation of oscillatory zoned garnets, followed by pyroxene (diopside-heden-bergite) + wollastonite + magnetite and rare MoS at high temperatures (∼445°C). Molybdenite was the only sulfide mineral deposited at this stage. Stage II occurred at a lower temperature (∼360°C), and is characterized by tremolite-actinolite, epidote, quartz, calcite, apatite, titanite, and hematite crystallization and by copper deposition. Copper mineralization was due to decreasing fO2 and increasing pH as temperatures fell below 400°C. Fluctuations in the Al+3/Fe+3 ratio of hydrothermal fluids at the Enjerd skarn resulted in complex oscillatory zoning in the garnets from andradite100 to andradite76-grossular24. Individual zones are typically composed either of near-end-member andradite or andradite containing approximately 20 mole % grossular. Introduction of fresh batches of Fe-rich fluid caused crystallization of near-end-member andradite garnet because the system was buffered by the fluid. The Al in the system was derived from Cretaceous marls and limestone sequences, because the Al available from granitoid magmatic solutions was very low and almost constant.
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Molybdenite
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Garnet skarn mineralization was recently studied at the Trohanka locality near Prakovce (Gemeric Unit, Eastern Slovakia). Ca-skarn forms lenticular bodies in green schist environment. It mainly consists of zonal garnets, pyroxenes, amphiboles and magnetite accumulations. Studied garnets are rich in andradite component (up to 89.95 mol. %) with minor grossular component (6.83 - 39.67 mol. %). Strong oscillatory zoning in andradite is caused by substitution of Fe3+ and Al3+. Most pyroxenes are rich in the hedenbergite component. In some cases, euhedral diopside crystals with marginal transition zones (composed of diopside with lower content of Mg2+ and higher content of Fe2+) were found in hedenbergite matrix. Amphiboles are dominantly represented by ferro-actinolite and ferro-hornblende in association with isolated euhedral crystals of ferro-tschermakite and ferro-pargasite. Indistinct chemical zonality of amphibole euhedral crystals is caused by presence of ferro-pargasite in the central parts and ferro-tschermakite in the peripheral parts of crystals.
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Amphibole
Diopside
Actinolite
Almandine
Hornblende
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Andradite
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The Huanggang Sn-Fe deposit is a large-size skarn deposit hosted by Lower Permian marble of Huanggangliang Formation.Its skarn mineral assemblage mainly consists of diopside,andradite-grossular and vesuvianite,and its retrograde minerals include actinolite-tremolite,epidote,chlorite,sericite,grossular etc.Electron microprobe analyses show that the end member of garnet in the early stage is dominated by andradite,while that in the major ore-forming stage is dominated by grossular,and that the end member of pyroxene has changed greatly,composed mainly of diopside and hedenbergite.Components of garnet and pyroxene are Adr28.69～96.44Grs2.00～67.38(Prp+Sps)0.67～5.69 and Di11.8～94.12Hd4.08～81.28Jo1.79～20.02,respectively,their wide compositional variation range suggests that skarns were not formed under the totally enclosed equilibrium condition.The amphiboles mostly belong to magnesium-ferric-calcium amphibole,with only some individuals being ferric-hornblende.The relatively remarkable change of composition is probably attributed to the change of redox conditions resulting in different degrees of AlⅥ Si←→(Na,K)displacement,which belongs to the transforming tendency under solidus.Tetrahedral Si,Al,octahedral Al,Ti and cations in A site of amphibole change greatly,which may be caused by the composition difference of the magma in the contact metasomatic process or the change of physicochemical condition during crystallization.Electron microprobe analysis shows that the early skarns in Huanggang belong to the typical oxidation type,while the late skarns transfer to the reduction type.From the viewpoint of mineral assemblages,the skarn assemblage in this area is similar to the calcareous skarn formation.Composition characteristics of skarns in the Huanggang Sn-Fe deposit are similar to those of Cu-Fe deposits of calcareous-magnesian skarn formation,while the altered mineral assemblage is close to W-Sn deposits of the calcareous skarn formation,indicating a new skarn formation.Massive intrusion of granitic magma brought a lot of metallogenic materials and heat energy needed for mineralization.In addition,the well developed faults in the Huanggang ore district also provide a channel for fluid migration.A lot of laminar skarn ores are developed in this area,and as this uniform banded structure cannot be observed in peripheral marbles,these laminar rocks seemed to be a self-organization phenomenon in the process of metasomatism and were not formed by sedimentary-exhalation.Mn/Fe ratios of pyroxenes in the Huanggang ore district range from 0.15 to 0.44,suggesting that the possibility of finding polymetallic mineralization in this area.The johannsenite in pyroxene changes remarkably in composition,and its content ranges from 1.79% to 20.02%.There is no Mn in the marble of Huanggangliang Formation of the ore-bearing strata in the Huanggang Sn-Fe ore district,and the possibility of the derivation of Mn from the strata is very small.In contrast,the pyroxenes are Mn-enriched in the stratiform ore body away from the contact zones or ore bodies containing no Mn.MnO content(0.02%～0.05%)in Huanggang granites is significantly lower than the average content of MnO(0.07%)in A-type granite,and there exist no Mn-enriched accessory minerals such as ilmenite,indicating that there might have been the participation of some Mn composition of granitic magma in the skarn due to contact metasomatism.Therefore,the Mn-enriched pyroxenes probably resulted from the evolution of Mesozoic magmatic fluid,and Mn-enriched pyroxene skarns were formed by infiltration metasomatism of magmatic fluid along fracture zones between layers.These phenomena are similar to things of the skarn Pb-Zn deposits in central Fujian Province.Mn-rich pyroxene might serve as the indications for Sn,Cu,Zn and many other metallic ores in this area,and the outer contact zone of skarn and its periphral marble seem to be favorable positions for polymetallic mineralization.
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Andradite
Amphibole
Pyroxene
Actinolite
Tremolite
Diopside
Paragenesis
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Almandine
Bornite
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Andradite
Actinolite
Amphibole
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Prehnite, an ubiquitous alterat ion product in caldc systems, was found in an unreported endoskarn occurrence, at Surduc (Banat Moun tains). This endoskarn is developed at the contact betwee~ monzodiorites belonging to a second phase of the Surdue 1 intrusive stage (Coniadan Maastrichtian in age) and gabbros belonging to an earlier intrusive phase of the same stage. The primary skarn paragenesis consists of andradite (andr. 91.04 to andr. 98.36), caldc plagioclase (Ah. 44.79 Or. 4.02 An. 51.19) and diopside. Prehnite is clearly pseudomorph onplagioclase, occur ring as a late hydrometasomatic product. The average of three microprobe analyses gave : Si02 = 40.87 %, Ah03 = 27.03 %, FeO = 0.39 %, Fe203 = 1.20 % (assumed after wet-chemical tests), CaO = 25.50 %, MgO = 0.32 %, MnO = 0.14 %, H20 = 4.31 % (thermogravimetric}. The mineral is white
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Pseudomorph
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The Jiaoli Ag-Pb-Zn-W skarn deposit is located in southern Jiangxi Province, China. The orebodies occur in the exocontact zone between a Yanshanian (171. 6-173. 3 Ma) granodiorite and Upper Cambrian metasandstone and crystalline limestone. Skarn mineralization zoning in the deposit is very pronounced. From the intrusive contract zone to country wall rocks, two ore-bearing skarn zones may be distinguished: proximal scheelite-bearing calcic skarn zone and Ag-Pb-Zn-bearing manganoan skarn zone. The W-bearing calcic skarn is composed of grossular, andradite, diopside , wollastonite, scheelite, and f luorite; while the Ag-Pb-Zn-bearing manganoan skarn consists mainly of manganoan grossular, spessartine, manganoan actinolite, and manganoan vesuvianite, associated with sphalerite, galena, pyrrhotite, argentite, silver, and minor scheelite. Study of the fluid inclusions suggests that ore-bearing fluids flowed from the deep contract zone of the intrusion in the west to the shallow depth in the east. With decreasing temperatures and salinities, the peak values of homogenization temperatures in the W-bearing calcic skarn are 420-340℃. Their salinities range from 12. 7% - 8 % (NaCleq ). In the Ag-Pb-Zn-bearing manganoan skarn, the peak values of homogenization temperatures are 360°-320℃ with salinities being 11. 7%-4. 5% (NaCleq). While for late ore-bearing retrograde hydrothermal metasomatic products, fluolite and quartz, their homogenization temperatures range from 380 to 180℃.
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Scheelite
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