Rare metal-bearing pegmatites from the Southeastern Desert of Egypt： Geology, geochemical characteristics, and petrogenesis

The pegmatite province of the Southeastern Desert (SED) is part of a pegmatite district that extends from Egypt (extends to 1200 km2). Rare metal pegmatites are divided into (1) unzoned, Sn-mineralized; (2) zoned Li, Nb, Ta and Be-bearing; and (3) pegmatites and pegmatites containing colored, gem-quality tourmaline. The Rb/Sr data reflect a crustal origin for the rare metal pegmatites and indicate that the original SED magma was generated during the peak of regional metamorphism and predates the intrusion of post-tectonic leucogranites. These bodies developed an early border zone consisting of coarse to very coarse muscovite quartz alkali feldspar, followed by an intermediate zone of dominant quartz feldspar muscovite rock. Garnet, tourmaline, beryl, galena, pyrite, amblygonite, apatite and monazite are rare accessories in both zones. Cassiterite tends to concentrate in replacement zones and along fractures in albite quartz muscovite-rich portions. The highest concentrations of cassiterite occur in irregular greisenized zones which consist dominantly of micaceous aggregates of green Li-rich muscovite, quartz, albite and coarse-grained cassiterite. The different metasomatic post-solidification alterations include sodic and potassic metasomatism, greisenization and tourmalinization. Geochemically, the pegmatite-generating granites have a metaluminous composition, showing a differentiation trend from coarse-grained, unfractionated plagioclase-rich granite towards highly fractionated fine- to medium-grained, local albite-rich rock. Economically important ore minerals introduced by volatile-rich, rare metal-bearing fluids, either primarily or during the breakdown of the primary mineral assemblages, are niobium-tantalum oxides, Sn-oxides (cassiterite), Li-silicates (petalite, spodumene, euctyptite, and pollucite), Li-phosphates (amblygonite, montebrasite and lithopilite) and minor REE-minerals (Hf-zircon, monazite, xenotime, thorian, loparite and yttrio-fluorite). The pollucite is typically associated with spodumene, petalite, amblygonite, quartz and feldspar. The primary pollucite has Si/Al (at) ratios of 2.53-2.65 and CRK of 79.5- 82.2. Thorian loparite is essentially a member of the loparite (NaLREETi2O6)-lueshite (NaNbO3)-ThTi2O6-ThNb4O12 quaternary system with low or negligible contents of other end-member compositions. The mineral compositionally evolved from niobian loparite to niobian thorian and thorian loparite gave rise to ceriobetafite and belyankinite with high ThO2 contents. Thorian loparite is metamict or partly metamict and upon heating regains a structure close to that of synthetic loparite NaLaTi2O6.     

The petrology and mineralogy of the pegmatite complex at Bismuth,Torrington, N.S.W.

The pegmatite complex of epi‐Permian age at Bismuth near Torrington, N.S.W., consists of an elongated intrusion of a granitoid quartz‐topaz rock (silexite) together with a series of pegmatites of varying composition. The principal pegmatite consists of orthoclase, biotite, quartz and beryl with concentric zoning passing outwards into fine‐grained biotite‐beryl rock containing a number of ore minerals: arsenides of Co, Fe and Ni, wolframite, bismuth, bismuthinite, molybdenite, joseite, cassiterite, rutile, uraninite and monazite. Small pegmatite veins issuing from this main body contain, in addition to the silicate minerals, high temperature tetrahedrite, chalcopyrite and sphalerite. A second group is characterised by quartz, orthoclase and beryl with occasional patches of tourmaline.

Emplacement at no great depth is indicated by breccia veins and stock‐works filled with pegmatite.

The origin of a silica hydromagma is considered in terms of existing experimental work and in terms of field occurrence. Structural evidence suggests that the quartzose intrusion preceded the injection of the pegmatite fluids, both being derived from the parent Mole biotite granite.     

Geology and Hydrothermal Alteration of the Milyang Pyrophyllite Deposit, Southeast Korea

Abstract: The Milyang pyrophyllite deposit, which is embedded in the Late Cretaceous Yuchon Group of the Kyongsang Supergroup, is one of the largest hydrothermal clay deposits in the Kyongsang basin, southeast Korea. Host rocks of the deposit are porphyritic andesite lava and minor andesitic lapilli tuff. In the Milyang district, a hydrothermally altered zone is about 2 × 3 km in extent; we can recognize the concentric arrangement of advanced argillic, propylitic, and sericitic alteration zones from the central to peripheral parts of the zone. The Milyang pyrophyllite deposit forms a part of the advanced argillic alteration zone. The Milyang pyrophyllite deposit is subdivided into the following four zones based on mineral assemblages: the pyrophyllite zones 1, 2, 3, and the silicified zone. The pyrophyllite zone 1, which occupies the central part of the deposit, comprises mainly pyrophyllite, kaolinite, and diaspore without quartz. Diaspore nodules often concentrate in beds 40–50 cm thick. Andalusite, dumortierite, and tourmaline locally occur as network veins, crack‐filler, or small spherulitic spots. The Al₂O₃ content of the ore ranges from 27 to 36 wt%. The pyrophyllite zone 2, which constitutes a major part of the deposit, comprises mainly pyrophyllite, kaolinite, and quartz. The Al₂O₃ content of the ore ranges from 15 to 24 wt%. The pyro‐phyllite zone 3 is the hematite‐rich marginal facies of the deposit. The silicified zone, which occurs as beds and septa, is mostly composed of quartz with minor pyrophyllite and kaolinite; the SiO₂ contents range from 79 to 90 wt%. Comparing chemical compositions of the high‐Al ores with those of unaltered host andesite, the Fe, Ca, alkalis, HFSE, and HREE contents are significantly depleted, whereas S, B, As, Sr, and LREE are enriched. The hydrothermal alteration of the Milyang pyrophyllite deposit can be classified into the following four stages: 1) extensive sericitic and propylitic alteration, 2) medium‐temperature (200–250°C) advanced argillic alteration, 3) high‐temperature (250–350°C or more) advanced argillic alteration, and 4) retrograde low‐temperature alteration. The heat and some volatile components such as B and S would be derived from the Pulguksa Granite intruded underneath the deposit.     

Rb-Sr Dating of Pyrite and Quartz Fluid Inclusions and Origin of Ore-forming Materials of the Jinshan Gold Deposit,Northeast Jiangxi Province,South China

The Jinshan gold deposit is located in the Northeast Jiangxi province,South China,which related to the ductile shear zone.It contains two ore types,i.e.the alteration-type ore and the goldbearing quartz vein ore.Rb-Sr age dating is applied to both gold-bearing pyrite in the alteration-type ore and fluid inclusion in the gold-bearing quartz vein to make clear the time of the gold mineralization of the Jinshan deposit.Analytical results of this study yielded that the age of the alteration-type ore bodies is about 838±110Ma,with an initial 87Sr/86Sr value of 0.7045±0.0020.However,the age of the gold-bearing quartz vein-type ore is about 379±49Ma,and the initial 87Sr/86Sr is 0.7138±0.0011.Based on the age data from this work and many previous studies,the authors consider that the Jinshan gold deposit is a product of multi-staged mineralization,which may include the Jinninian,Caledonian,Hercynian,and Yanshanian Periods.Among them,the Jinninian Period and the Hercynian Period might be the two most important ore-forming periods for Jinshan deposit.The Jinninian Period is the main stage for the formation of alteration-type ore bodies,while the Hercynian Period is the major time for ore bodies of gold-bearing quartz vein type.The initial values of the 87Sr/86Sr from this study,as well as the previous isotope and trace element studies,indicate that the ore-forming materials mainly derived from the metamorphic wall rocks,and the ore-forming fluids mainly originated from the deep metamorphic water.     

The origin of tin deposits in the Mount Wells region,Pine Creek Geosyncline,Northern Territory

Tabular steeply dipping cassiterite‐bearing lodes in the Mount Wells region are hosted by lower greenschist fades metasediment of the Pine Creek Geosyncline within the contact aureole of late orogenic granitoids. The latter are predominantly I‐type, but S‐type phases are developed near the sediment‐granitoid contact.

Quartz, cassiterite, pyrite, arsenopyrite, chalcopyrite and pyrrhotite are the main minerals. Two types of lodes are present: (i) Sn‐quartz lodes containing 5–10 vol% sulphide minerals; and (ii) Sn‐sulphide lodes containing ～ 70 vol% sulphide minerals. At the surface, the former appear as normal quartz veins and the latter as hematite‐quartz breccia resulting from the collapse of original sulphide‐rich lodes as a consequence of volume reduction due to oxidation and leaching.

Two stages of quartz veining are recognized in both types of lodes. Cassiterite is present in stage I while stage II is composed of barren quartz with minor pyrite. Late stage III carbonate veinlets are present in Sn‐sulphide lodes. The lode‐wallrock contact is sharp with weak alteration effects confined to the fringe of the lodes. The alteration minerals include sericite, quartz, tourmaline, chlorite, pyrite and minor K‐feldspar.

Four types of fluid inclusions are present in vein quartz and cassiterite: Type A (CO₂ ± H₂O ± CH₄); Type B (H₂O+～ 20% vapour); Type C (H₂O+ < 15% vapour) and Type D (H₂O+ < 15% vapour + NaCl). Early ‘primary’ inclusions represented by Types A and B are present in stage I only and have a well‐defined temperature mode at ～300°C and a salinity range of 1–20 wt% eq NaCl. Types C and D inclusions are ‘secondary’ in stage I and primary in stage II and have a temperature mode at 120–160°C and salinities from about 1 to more than 26 wt% eq NaCl. Variable H₂O‐CO₂ ratios of Type A inclusions and homogenization in CO₂ or H₂O phase at near identical temperature indicate entrapment at the H₂O‐CO₂ solvus and define a pressure of ～ 100 MPa. The melting sequence of frozen inclusions suggests that the ore fluids were mainly H₂O‐CO₂‐CH₄‐Na‐Ca‐Cl brines. This is also confirmed by Raman Laser Spectrometry.

Oxygen and sulphur isotope data are consistent with a magmatic origin of the ore fluids. The δD values are up to 20%0 higher than those expected for magmatic fluids and probably resulted from interaction of the latter with the carbonaceous strata. This interpretation is supported by δ¹³C data on the fluid inclusion CO₂.

Fluid inclusions, stable isotope and mineralogical data are used to approximate the physico‐chemical parameters of the ore fluids which are as follows: T 300°C, m Cl～2, fO₂ ～ 10^‐35, mSS ～ 0.01, Sn ～ 1 ppm, Cu ～ 1 ppm and pH ～ 5.5.

It is suggested that fluids of granitic parentage interacted with the enclosing sediment and picked up CO₂, CH₄ and possibly Ca. The granitic phases became reduced due to this interaction and developed S‐type characteristics. Tin was probably partitioned into the CH₄‐bearing reduced fluids. At some stage the fluid overpressure exceeded the lithostatic lode enforcing failure of the carapace and the intruded rocks by hydraulic fracturing causing CH₄ and CO₂ loss resulting in the precipitation of the ore minerals.     

Genesis of tin and tantalum mineralization in pegmatites from the Bynoe area,Pine Creek Geosyncline,Northern Territory

The tin‐ and tantalum‐bearing pegmatites of the Bynoe area are located in the western Pine Creek Geosyncline. They are emplaced within psammopelitic rocks in the contact aureole of the Two Sisters Granite. The latter is a Palaeoproterozoic, fractionated, granite with S‐type characteristics and comprises a syn‐ to late‐orogenic, variably foliated, medium‐grained biotite granite and a post‐orogenic, coarse‐grained biotite‐muscovite granite. The pegmatites comprise a border zone of fine grained muscovite + quartz followed inward by a wall zone of coarse grained muscovite + quartz which is in turn followed by an intermediate zone of quartz + feldspar + muscovite. A core zone of massive quartz is present in some occurrences. Feldspars in the intermediate zone are almost completely altered to kaolinite. This zone contains the bulk of cassiterite, tantalite and columbite mineralization. Fluid inclusions in pegmatitic quartz indicate that early Type A (CO₂ + H₂O ± CH₄) inclusions were trapped at the H₂O‐CO₂ solvus at P～100 MPa, T～300°C (range 240–328°C) and salinity ～6 wt% eq NaCl. Pressure‐salinity corrected temperatures on Type B (H₂O + ～20% vapour), C (H₂O + < 15% vapour) and D (H₂O + halite + vapour) inclusions also fall within the range of Type A inclusions. Oxygen and hydrogen isotope data show that kaolin was either formed in isotopic equilibrium with meteoric waters or subsequent to its formation, from hydrothermal fluid, underwent isotopic exchange with meteoric waters. Fluid inclusion waters from core zone quartz show enrichment in deuterium suggesting metamorphic influence. Isotope values on muscovite are consistent with a magmatic origin. It is suggested that the pegmatites were derived from the post‐orogenic phase of the Two Sisters Granite. Precipitation of cassiterite took place at about 300°C from an aqueous fluid largely as a result of increase in pH due to feldspar alteration.     

Geological,geochemical, stable isotope,and fluid inclusion characteristics of epithermal gold mineralization,Velvet District,Nevada

Gold mineralization in the Velvet District occurs in an eastward dipping sequence of late Tertiary rhyolitic ash-flow tuffs, flows, and tuffaceous sediments in northwestern Nevada. Minor gold and silver concentrations are associated with irregular zones of brecciation, argillic alteration, and quartz veining along north-northeast trending normal faults. Reaction of mineralizing fluids with wallrock produced an argillic alteration assemblage of illite, mixed-layer clays, smectite, and kaolinite. Illite alteration and highest gold concentrations appear to be associated with zones of high water/rock ratios. Kaolinite, smectite, alunite, and opal are postulated to have formed during a steam-dominated episode of alteration.Fluid inclusion studies indicate that the quartz veins were deposited in the temperature range 230 to 280°C from fluids which had salinities equivalent to 0.2–0.8 weight percent NaCl. δ ¹⁸O of quartz veins varies from ?2.5 to $\text{+6.7 ‰}$ and indicates that the ore fluid must have been Tertiary meteroric water. Stable isotope data appear to define a zone of concentrated fluid flow and potential subsurface mineralization in the southeastern part of the district. Fluid inclusion and isotope studies can be used in combination with more standard geochemical, geophysical, and geological information to provide site-specific targets for epithermal metal concentrations.     

Geological,geochemical, and geochronological characteristics of Weilasituo Sn-polymetal deposit,Inner Mongolia,China

The recently discovered Weilasituo Sn-polymetal deposit is located in the southern part of the Great Xing'an Range of Inner Mongolia, NE China, which is belonged to the eastern part of the Central Asian Orogenic Belt (CAOB). Sn-polymetal mineralization is closely related to the emplacement of the Early Cretaceous fine- to medium-grained quartz porphyry. Three types of mineralization have been recognized at Weilasituo with the disseminated and stockwork Sn-polymetal mineralization mainly hosted by the quartz porphyry, the vein-type Sn-polymetal mineralization hosted by NE-trending and WE-trending fractures and faults in the upper and outer part of the porphyry, and breccia mineralization occurred within a steep cryptoexplosive breccia pipe. The ore-related alteration typically consists of Na-Ca-Sr alteration and greisen.In order to understand the petrogenetic link between the Sn-polymetal mineralization and the host quartz porphyry, this paper presents new whole-rock geochemistry for the quartz porphyry, EPMA analysis of ore and gangue minerals, and in situ U-Pb dating of cassiterite. The Laser Abrasion Multiple Collector Inductively Coupled Plasma Mass Spectrometer (LA-MCICP-MS) cassiterite U-Pb dating yields two well-defined isochron ages of 138 ± 6 Ma for disseminated ore and 135 ± 6 Ma for the vein-type ore, which could be regarded as the ore-forming age. The cassiterite U-Pb ages (ca. 138–135 Ma) determined in this study, together with previous data, reveals the close temporal and genetic relationship between the mineralization event and the emplacement of the quartz porphyry.The quartz porphyry is characterized by high SiO₂, Na₂O, and A/CNK values (1.09–1.21). REE tetrad effect combined with extremely high Rb/Sr, K/Ba ratios and low K/Rb, Zr/Hf, La/Nb, La/Ta and Eu/Eu* ratios indicate that the quartz porphyry is a highly fractionated peraluminous I-type granite that is hydrothermally altered. Low εNd(t) values of 4.27–0.28 and the two-stage depleted mantle Nd model ages (T_2DM = ca. 1279–908 Ma) for the quartz porphyry, are similar to granites in Precambrian microcontinents of the eastern part of the Central Asian Orogen (CAOB). This suggests that the quartz porphyry was derived from the remelting of juvenile crust and Precambrian rocks in an extensional setting. Therefore, the highly fractional crystallization and magmatic-hydrothermal interactions of the quartz porphyry have contributed to the formation of the Weilasituo Sn-polymetal deposit.     

Geology and Geochemistry of the Bianbianshan Au‐Ag‐Cu‐Pb‐Zn Deposit,Southern Da Hinggan Mountains,Northeastern China

The Bianbianshan deposit, the unique gold-polymetal (Au-Ag-Cu-Pb-Zn) veined deposit of the polymetal metallogenic belt of the southern segment of Da Hinggan Mountains mineral province, is located at the southern part of the Hercynian fold belt of the south segment of Da Hinggan Mountains mineral province, NE China. Ores at the Bianbianshan deposit occur within Cretaceous andesite and rhyolite in the form of gold-bearing quartz veins and veinlet groups containing native gold, electrum, pyrite, chalcopyrite, galena and sphalerite. The deposit is hosted by structurally controlled faults associated with intense hydrothermal alteration. The typical alteration assemblage is sericite + chlorite + calcite + quartz, with an inner pyrite - sericite - quartz zone and an outer seicite - chlorite - calcite - epidote zone between orebodies and wall rocks. δ34 S values of 17 sulfides from ores changing from –1.67 to +0.49‰ with average of –0.49‰, are similar to δ34 S values of magmatic or igneous sulfide sulfur. 206Pb/204Pb, 207Pb/204Pb and 208Pb/ 204Pb data of sulfide from ores range within 17.66–17.75, 15.50–15.60, and 37.64–38.00, respectively. These sulfur and lead isotope compositions imply that ore-forming materials might mainly originate from deep sources. H and O isotope study of quartz from ore-bearing veins indicate a mixed source of deep-seated magmatic water and shallower meteoric water. The ore formations resulted from a combination of hydrothermal fluid mixing and a structural setting favoring gold-polymetal deposition. Fluid mixing was possibly the key factor resulting in Au-Ag-Cu-Pb-Zn deposition in the deposit. The metallogenesis of the Bianbianshan deposit may have a relationship with the Cretaceous volcanic-subvolcanic magmatic activity, and formed during the late stage of the crust thinning of North China.     

Mineral Paragenesis of the Lepanto Copper and Gold and the Victoria Gold Deposits, Mankayan Mineral District, Philippines

Abstract: Mineral paragenesis of the alteration, ore and gangue minerals of the Lepanto epithermal copper‐gold deposit and the Victoria gold deposit, Mankayan Mineral District, Northern Luzon, Philippines, is discussed. The principal ore minerals of the Lepanto copper‐gold deposit are enargite and luzonite, with significant presence of tennantite‐tetrahedrite, chalcopyrite, sphalerite, galena, native gold/electrum and gold‐silver tellurides. Pervasive alteration zonations are commonly observed from silicification outward to advanced argillic then to propylitic zone. The ore mineralogy of the Lepanto copper‐gold deposit suggests high f_S2 in the early stages of mineralization corresponding to the deposition of the enargite‐luzonite‐pyrite assemblage. Subsequent decrease in the f_S2 formed the chalcopyrite‐tennantite‐pyrite assemblage. An increase in the f_S2 of the fluids with the formation of the covellite‐digenite‐telluride assemblage caused the deposition of native gold/electrum and gold‐silver tellurides. The principal ore minerals of the Victoria gold deposit are sphalerite, galena, chalcopyrite, tetrahedrite and native gold/electrum. The alteration halos are relatively narrow and in an outward sequence from the ore, silica alteration grades to illitic‐argillic alteration, which in turn grades to propylitic alteration. The Victoria gold mineralization has undergone early stages of silica supersaturation leading to quartz deposition. Vigorous boiling increased the pH of the fluids that led to the deposition of sulfides and carbonates. The consequent decrease in H₂S precipitated the gold. Gypsum and anhydrite mainly occur as overprints that cut the carbonate‐silica stages. The crosscutting and overprinting relationships of the Victoria quartz‐gold‐base metal veins on the Lepanto copper‐gold veins manifest the late introduction of near neutral pH hydrothermal fluids.     

湘东桐木山云英岩型锡矿床锡石及共生石英中流体包裹体研究

桐木山云英岩型锡矿床是湘东锡田锡多金属矿田中一个典型矿床,在详尽的野外考察、矿石结构观察以及流体包裹体岩相学研究的基础上,采用流体包裹体组合的研究方法,利用冷热台、激光拉曼等测试手段,对矿床中锡石中流体包裹体进行直接测定,同时开展与锡石共生的石英及切割矿体的后期石英脉石英中流体包裹体对比研究。结果显示,锡石中流体包裹体的组分、均一温度、盐度与共生的石英存在明显差异,表明锡石与石英形成的P-T-X条件不同。切割矿体的石英脉为成岩成矿后流体作用的产物。与锡石形成相关的流体为中高温、中低盐度的Na Cl-H_2O流体体系,与石英形成相关的流体为复杂的含碳流体。流体体系的冷却作用及流体与围岩的反应可能是导致锡石沉淀的主要因素,与石英形成相关的流体在演化过程中则经历了明显的流体不混溶作用。     

Geology and Hydrothermal Alteration of the Duobuza Gold‐Rich Porphyry Copper District in the Bangongco Metallogenetic Belt,Northwestern Tibet

The Duobuza gold‐rich porphyry copper district is located in the Bangongco metallogenetic belt in the Bangongco‐Nujiang suture zone south of the Qiangtang terrane. Two main gold‐rich porphyry copper deposits (Duobuza and Bolong) and an occurrence (135 Line) were discovered in the district. The porphyry‐type mineralization is associated with three Early Cretaceous ore‐bearing granodiorite porphyries at Duobuza, 135 Line and Bolong, and is hosted by volcanic and sedimentary rocks of the Middle Jurassic Yanshiping Formation and intermediate‐acidic volcanic rocks of the Early Cretaceous Meiriqie Group. Simultaneous emplacement and isometric distribution of three ore‐forming porphyries is explained as multi‐centered mineralization generated from the same magma chamber. Intense hydrothermal alteration occurs in the porphyries and at the contact zone with wall rocks. Four main hypogene alteration zones are distinguished at Duobuza. Early‐stage alteration is dominated by potassic alteration with extensive secondary biotite, K‐feldspar and magnetite. The alteration zone includes dense magnetite and quartz‐magnetite veinlets, in which Cu‐Fe‐bearing sulfides are present. Propylitic alteration occurs in the host basic volcanic rocks. Extensive chloritization‐silicification with quartz‐chalcopyrite or quartz‐molybdenite veinlets superimposes on the potassic alteration. Final‐stage argillic alteration overlaps on all the earlier alteration. This alteration stage is characterized by destruction of feldspar to form illite, dickite and kaolinite, with accompanying veinlets of quartz + chalcopyrite + pyrite and quartz + pyrite assemblages. Cu coexists with Au, which indicates their simultaneous precipitation. Mass balance calculations show that ore‐forming elements are strongly enriched during the above‐mentioned three alteration stages.     

Quaternary Volcanic-Related Acid Hydrothermal Alteration in the Ugusu Silica Stone Deposit, Western Izu Peninsula, Central Japan: Geology and Alteration

Acid alteration areas accompanying Quaternary volcanoes are widespread in the western Izu Peninsula, central Japan. The Ugusu alteration area is the largest among them and is mined for “silica stone” at the silica body in the core of the alteration area. Silica zone defined by previous studies is subdivided into highly leached brecciated silica zone and residual silica zone, which extend along a NNW‐SSE–NNE‐SSW direction of fractures/faults. Fe‐rich, alunite, advanced argillic alteration, and intermediate argillic alteration zones occur toward the outside surrounding the two silica zones. The ascent of acid hydrothermal fluid would be responsible for the silica zones and surrounding alteration zones at an earlier stage, while the hydrothermal brecciation and silica veins were caused by a limited supply of silica‐saturated fluids at later stages. Based on the available mineral stability relations and fluid inclusion thermometries, the formation temperatures are estimated to be: >300°C for the residual silica zone; >290°C for the diaspore association in the advanced argillic alteration zone; and <260°C for the kaolinite association in the intermediate argillic alteration zone. The later stage quartz druses have been formed at 200–260°C. The Ugusu–Fukata acid hydrothermal systems were active at 1.5–1.2 Ma, which were temporally related to the Tanaba Andesite volcanism. Hydrothermal system at the Seikoshi gold–silver deposit survived until 0.7 Ma after the volcanism. In the western half of the Izu Peninsula, subduction of the Philippine Sea plate underneath the Suruga Trough caused nearly N–S‐trending maximum horizontal compressive stress (σ_Hmax) and the resultant formation of similarly trending alteration areas and Au‐Ag vein‐type deposits in the Ugusu‐Toi‐Seikoshi area. From a practical viewpoint, at the Ugusu silica stone deposit, the fracture‐controlled vertical morphology of the silica body provides an important guide for exploration. Because the alteration zones occur both in the lower and upper sides of the silica bodies, it is important to make sure to which side the alteration zones correspond.     

云南云龙铁厂锡矿床地质特征及找矿标志

云龙铁厂锡矿位于青藏滇缅"歹"字型构造收缩部位的澜沧江变质带中,属于东南亚锡矿带东支——滇泰锡矿带的北延部分之云龙锡矿带。矿化与燕山晚期—喜马拉雅期细粒二云母花岗岩有关,矿体多产在岩体舌状边缘及外接触带的层间剥离带或混合岩体内的构造裂隙中。矿石类型以锡石-电气石、石英型为主,锡石-硫化物型和锡石-角岩型次之,成因类型为混合岩化-高温热液矿床。以混合岩化、主干断裂及硅化、电气石化为本区重要找矿标志,通过对铁厂锡矿矿床地质特征及找矿标志的研究,对该区寻找同类型矿产具有积极的指导意义。     

Geology of the High Sulfidation Copper Deposits,Monywa Mine,Myanmar

The 50 km² Monywa copper district lies near the Chindwin River within the northward continuation of the Sunda‐Andaman magmatic arc through western Myanmar. There are four deposits; Sabetaung, Sabetaung South, Kyisintaung, and the much larger Letpadaung 7 km to the southeast. Following exploration drilling which began in 1959, production of copper concentrates from a small open pit started at Sabetaung in 1983. Since 1997, when resources totaled 7 million tonnes contained copper in 2 billion tonnes ore, a heap leach–electro‐winning operation has produced over 400,000 t copper cathode from Sabetaung and Sabetaung South. Ore is hosted by mid‐Miocene andesite or dacite porphyry intrusions, and by early mid‐Miocene sandstone and overlying volcaniclastics including eruptive diatreme facies which the porphyries intrude. District‐wide rhyolite dykes and domes with marginal breccias probably post‐date andesite porphyries in the mine area and lack ore‐grade copper. Host rocks to mineralization are altered to phyllic and advanced argillic hydrothermal assemblages within an outer chlorite zone; hypogene alunite is most abundant at Letpadaung and Kyisintaung. Most mineralization is structurally‐controlled with digenite‐chalcocite in breccia dykes, in steeply dipping NE‐trending sheeted veins, and in stockwork and low‐angle sulfide veins. A high‐grade pipe at Sabetaung grades up to 30% Cu, and much of the ore at Sabetaung South is in a NE‐trending zone of mega‐breccia and stockworked sandstone. The hydrothermal alteration, together with replacement quartz, alunite and barite in breccia dykes and veins, the virtual absence of vein quartz, and the presence of chalcopyrite and bornite only as rare veins and as inclusions within the abundant pyrite, indicate that the deposits are high sulfidation. Regional uplift, resistance to erosion and leaching of the altered and mineralized rocks have resulted in porous limonite‐stained leached caps over 200 m thick forming the Letpadaung and Kyisintaung hills. The barren caps pass abruptly downwards at the water table into the highest grade ore at the top of the supergene enrichment zone, within which copper grade, supergene kaolinite and cubic alunite decrease, and pyrite increases with depth; in contrast, marcasite is mostly shallow. Much of the copper to depths exceeding 200 m below the water table occurs as supergene digenite‐chalcocite and minor covellite. Disseminated chalcocite is mostly near‐surface and hence almost certainly supergene. We infer that during prolonged uplift at all four deposits, oxidation of residual pyrite at the water table generated enough acid to leach all the copper from earlier supergene‐enriched ore; below the water table the resulting acid sulfate solutions partly replaced enargite, covellite, chalcopyrite, bornite and pyrite with supergene chalcocite. Undeformed upward‐fining cross‐bedded conglomerates and sands of the ancestral Chindwin River floodplain overlie the margins of the Sabetaung deposits, form a major aquifer up to 40 m thick, and are a potential host for exotic copper mineralization. A mid‐Miocene pluton is inferred to underlie the Monywa deposits, but the possibility of porphyry‐type mineralization within the district is at best highly speculative.     

Geological Evolution of Selected Granitic Pegmatites in Myanmar (Burma): Constraints from Regional Setting,Lithology, and Fluid-Inclusion Studies

Pegmatite deposits commonly occur in the 1500 km long, N-S-trending, tungstentin-bearing granitoid belt in Myanmar. Pegmatites are emplaced as veins and dikes that cut granitoid, migmatite, granitoid gneiss, gneiss, and schist. The pegmatite veins and dikes are mostly 2 to 5 meters wide and 30 to 150 meters long, and some are traceable over a distance of 300 meters.

The pegmatites are composed of quartz, orthoclase, albite, microcline microperthite, and muscovite, with minor biotite, tourmaline, beryl, garnet, topaz, lepidolite, magnetite, wolframite, cassiterite, and rare columbite. They are commonly zoned, feldspars and muscovite being more abundant in the center and quartz more common at the margin. The zoning pattern is rather distinct in the pegmatite body, where tourmaline is present. The light-colored felsic minerals are confined to the core zone and the dark-colored tourmaline crystals to the outer zone.

Numerous fluid inclusions have been found in quartz, topaz, and beryl. Most of the inclusions are rounded to elliptical, with a variable degree of liquid filling. All inclusions are aqueous, two-phase (liquid and vapor) inclusions with no daughter minerals. Homogenization temperatures of 173 fluid inclusions were measured in this study.

Geothermometric studies indicate that the pegmatites were formed over a homogeniza-tion temperature range of 230° to 410°C. Salinities of fluid inclusions in pegmatite minerals yielded from 1.0 to 10.8 NaCl equiv. wt‰. Topaz and quartz single crystals (several cm across) from the Sakangyi pegmatite provide an opportunity to extract the fluids trapped in these minerals. The Na/K ratios of the fluid inclusions in two topaz samples were 3.0 to 4.9, and those of two quartz samples were 2.9 to 10.5, suggesting the presence of substantial potassium in the pegmatite-forming fluids. In this study, evidence for phase separation of the pegmatite-forming fluids was not observed. The post-magmatic, hydrothermal fluids responsible for the pegmatite veins evidently emanated from cooling S-type granitoids, with which they are spatially associated.     

Hydrothermal alteration and mineralisation of the Glen Eden Mo-W-Sn deposit: a leucogranite-related hydrothermal system,Southern New England Orogen,NSW, Australia

The Glen Eden Mo-Sn-W deposit in north-eastern New South Wales, Australia, is an example of a leucogranite-related, low-grade, large-tonnage hydrothermal system. It occurs in the southern part of the New England Orogen and is hosted within Permian felsic volcanic rocks, intruded at depth by dykes of porphyritic microleucogranite (Glen Eden Granite). The deposit is hosted within a pipe-like quartz-rich greisen breccia body about 500 m in diameter, surrounded by a greisen zone several hundred metres across, zoning out into altered volcanic rocks. The dominant ore minerals, largely hosted as open space fillings and disseminations in quartz and quartz-rich greisen, are molybdenite, wolframite and cassiterite; they are accompanied by minor to trace amounts of muscovite, fluorite, topaz, siderite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, bismuth, bismuthinite, joseite A, cosalite, galenobismutite, beryl, anatase and late-stage dickite and kaolinite. Two types of breccia are recognised: (1) greisenised volcanic rock fragments (quartz + muscovite), cemented by hydrothermal quartz ± K-feldspar ± ore minerals, and (2) fragments of hydrothermal quartz ± cassiterite ± wolframite enclosed in quartz ± clay. In both types of breccia and in stockwork veins, there is evidence of early precipitation of Mo-Sn-W phases, followed by Bi minerals and base metal sulfides (± fluorite, siderite).Breccia formation and associated hydrothermal alteration (greisen, potassic, argillic, propylitic) are interpreted to be related to devolatilisation of the highly fractionated Glen Eden Granite of early Triassic age (240±1 Ma based on ⁴⁰Ar/³⁹Ar geochronology of greisen muscovite) as well as to fluid mixing with meteoric waters. The breccia pipe could have formed in part by rock dissolution and collapse, as well as by explosive degassing of boiling fluids. Fluid inclusion evidence is consistent with boiling, with breccia pipe formation and mineralisation having mainly occurred at 250–350 °C from fluids with salinity of 0.4–9 wt% NaCl equivalent in the dilute types and 30–47 wt% NaCl equivalent in the hypersaline types. Stable isotopic evidence (O, D, C, S) indicates a strong magmatic contribution to the hydrothermal fluids and metals in the breccia. The ¹⁸O values of quartz decrease outward from the breccia pipe (10.6–12.3 in the pipe to 3.4–8.7 in the peripheral quartz) indicating that there has been mixing with isotopically light (high latitude) meteoric fluids, mainly after formation of the breccia pipe.     

The Donoso copper-rich,tourmaline-bearing breccia pipe in central Chile: petrologic,fluid inclusion and stable isotope evidence for an origin from magmatic fluids

The copper-rich, tourmaline-bearing Donoso breccia pipe is one among more than 15 different mineralized breccias in the giant (>50 million metric tonnes of copper) Miocene and Pliocene Río Blanco-Los Bronces copper deposit in the high Andes of central Chile. This breccia pipe, bracketed in age between 5.2 and 4.9 Ma, has dimensions of 500 by 700 m at the current surface 3,670 m above sea level. Its roots have yet to be encountered, and it is >300 m in diameter at the depth of the deepest drill holes. The Donoso breccia is, for the most part, monolithic, containing clasts of the equigranular quartz monzonite pluton which hosts the pipe. It is matrix supported, with between 5 and 25% of the total rock volume consisting of breccia-matrix minerals, which include tourmaline, quartz, chalcopyrite, pyrite, specularite, and lesser amounts of bornite and anhydrite. An open pit mine, centered on this breccia pipe, has a current production of 50,000 tonnes of ore per day at an average grade of 1.2% copper, and copper grade in the breccia matrix is significantly higher. Measured '¹⁸O for tourmaline and quartz from the matrix of the Donoso breccia at different levels of the pipe range from +6.9 to +12.0‰, and measured 'D in tourmaline ranges from -73 to -95‰. Temperatures of crystallization of these minerals, as determined by the highest homogenization temperatures of highly saline fluid inclusions, range from 400 to >690°C. When corrected for these temperatures, the stable isotope data indicate that fluids from which these breccia-matrix minerals precipitated were magmatic, with '¹⁸O between +5.6 to +9.1‰ and 'D between -51 to -80‰. These isotopic data preclude participation of a significant amount of meteoric water in the formation of the Donoso breccia. They support a model in which brecciation is caused by expansion of magmatic fluids exsolved from a cooling pluton, and breccia-matrix minerals, including copper sulfides, precipitated from the same magmatic fluids responsible for brecciation. Sericitic alteration of clasts in the breccia was also caused by these magmatic fluids. Different types of fluid inclusions imply that several different magmatic fluids were involved in formation of the Donoso breccia. These include high-temperature, highly saline, non-boiling fluids, trapped in inclusions that homogenize by halite dissolution, which probably exsolved from a magma cooling under relatively high (>1 kbar) lithostatic pressure conditions, consistent with geologic constraints. Other high-temperature, highly saline fluids are trapped in inclusions that homogenize by vapor-bubble disappearance and are spatially associated with vapor-rich inclusions, suggesting either phase separation (boiling) or simultaneous separation of immiscible brine and vapor from a magma cooling at lower hydrostatic pressure conditions. Both types of high-temperature, highly saline fluids circulated intermittently, as pressure fluctuated between lithostatic and hydrostatic conditions because of episodes of sealing and rebrecciation.     

The Arinem Te‐Bearing Gold‐Silver‐Base Metal Deposit,West Java,Indonesia

The vein system in the Arinem area is a gold‐silver‐base metal deposit of Late Miocene (8.8–9.4 Ma) age located in the southwestern part of Java Island, Indonesia. The mineralization in the area is represented by the Arinem vein with a total length of about 5900 m, with a vertical extent up to 575 m, with other associated veins such as Bantarhuni and Halimun. The Arinem vein is hosted by andesitic tuff, breccia, and lava of the Oligocene–Middle Miocene Jampang Formation (23–11.6 Ma) and overlain unconformably by Pliocene–Pleistocene volcanic rocks composed of andesitic‐basaltic tuff, tuff breccia and lavas. The inferred reserve is approximately 2 million tons at 5.7 g t^?1 gold and 41.5 g t^?1 silver at a cut‐off of 4 g t^?1 Au, which equates to approximately 12.5t of Au and 91.4t of Ag. The ore mineral assemblage of the Arinem vein consists of sphalerite, galena, chalcopyrite, pyrite, marcasite, and arsenopyrite with small amounts of pyrrhotite, argentite, electrum, bornite, hessite, tetradymite, altaite, petzite, stutzite, hematite, enargite, tennantite, chalcocite, and covellite. These ore minerals occur in quartz with colloform, crustiform, comb, vuggy, massive, brecciated, bladed and calcedonic textures and sulfide veins. A pervasive quartz–illite–pyrite alteration zone encloses the quartz and sulfide veins and is associated with veinlets of quartz–calcite–pyrite. This alteration zone is enveloped by smectite–illite–kaolinite–quartz–pyrite alteration, which grades into a chlorite–smectite–kaolinite–calcite–pyrite zone. Early stage mineralization (stage I) of vuggy–massive–banded crystalline quartz‐sulfide was followed by middle stage (stage II) of banded–brecciated–massive sulfide‐quartz and then by last stage (stage III) of massive‐crystalline barren quartz. The temperature of the mineralization, estimated from fluid inclusion microthermometry in quartz ranges from 157 to 325°C, whereas the temperatures indicated by fluid inclusions from sphalerite and calcite range from 153 to 218 and 140 to 217°C, respectively. The mineralizing fluid is dilute, with a salinity <4.3 wt% NaCl equiv. The ore‐mineral assemblage and paragenesis of the Arinem vein is characteristically of a low sulfidation epithermal system with indication of high sulfidation overprinted at stage II. Boiling is probably the main control for the gold solubility and precipitation of gold occurred during cooling in stage I mineralization.     

Endogreisen, brecciation and fluid activity at the Mount Bischoff Sn-deposit, north-west Tasmania, Australia

Abstract Endogreisens which replace K-feld-spar-quartz dykes in a Devonian (360 Ma) tin deposit at Mt Bischoff, north-west Tasmania, formed from the interaction of unusual solutions, probably derived from an underlying leucogranite pluton, porphyry dykes and limited quantities of local dolomitic country rock components. The intensity of greisenization and pH of the solutions increase inward to the greisenized dykes'cores and downward. The following types of greisen assemblages indicate increasing degrees of greisenization: 'sericite'muscovite + quartz ± tourmaline ± fluorite, topaz + quartz ± tourmaline ± fluorite, weberite, prosopite, ralstonite, Ca-ralstonite; and quartz ± topaz ± fluorite. Where the solutions interacted with dolomite, exogreisens consisting of topaz- or tourmaline-bearing assemblages were formed. The greisens were subsequently overprinted to varying degrees by siderite, sulphides and hydrous silicates (talc, serpentine, chlorite, micas).
The temperature during greisenization ranged from 180 to 414°C, based on fluid inclusions in topaz, quartz, fluorite, sellaite and cassiterite. The main greisen-forming event occurred at temperatures of 360±20°C. The fluids boiled intermittently. Their salinities ranged from 31.5 to 38.9 wt% total dissolved salts, consisting of Ca–K–Na–Fe–Cl±hydrocarbon species. Fluid inclusion data indicate that only 0.5–1.5 km of cover were present above this deposit at the time of formation.
The greisenized dykes were intruded by and intrude different stages of breccias. The breccias consist mainly of country rock and greisenized dyke fragments, with rock-flour and later tourmaline alteration. The Mt Bischoff greisen system is possibly part of a 'porphyry tin'style deposit formed at near-surface conditions (0.5–1.0 km).