Thermobarometry of the Bleikvassli Zn-Pb-(Cu) deposit, Nordland, Norway

The Bleikvassli massive sulfide ore deposit is hosted by Proterozoic pelitic, quartzofeldspathic, and amphibolitic rocks of the Uppermost Allochthon of the Scandinavian Caledonides. Staurolite-garnet-biotite and kyanite-staurolite-biotite assemblages indicate that metamorphism reached the kyanite zone of the amphibolite facies. Geothermobarometry was conducted on rocks in and around the deposit using a variety of silicate and sulfide calibrations. Temperature determinations are most reliant on the garnet-biotite exchange reaction, with analyses obtained from 259 garnet rims and adjacent biotite. Results from nine calibrations of the garnet-biotite geothermometer are considered, but compositional limitations of many calibrations involving high Ca and Mn contents in garnet and Al^VI and Ti in biotite make many of the coexisting mineral pairs unsuitable. Average temperatures calculated from the two calibrations that most closely address the garnet-biotite compositions observed at Bleikvassli are 584 °C ± 49 °C and 570 °C ± 40 °C. The application of two calibrations of the garnet-staurolite geothermometer on a limited number of samples yields 581 °C ± 2 °C and 589 °C ± 12 °C, assuming a _H2O=0.84, based upon calculations of the modal proportions of gaseous species. Pressure determinations are less constrained. Phengite and plagioclase-biotite-garnet-muscovite geobarometers give average pressures of approximately 5.0 kbar and 8.5 ± 1.2 kbar, respectively. Pressures obtained from the sphalerite-hexagonal pyrrhotite-pyrite barometer average 7.7 ± 1.0 kbar. In consideration of these results, the peak metamorphic conditions at the Bleikvassli deposit are estimated to be 580 °C and 8 kbar. Received: 18 June 1997 / Accepted: 14 May 1998     

Lead-isotope study of the sulphide ore and alteration zone,Bleikvassli zinc-lead deposit,northern Norway

The Bleikvassli Zn-Pb deposit is located in the Uppermost Allochthon of the northern Norwegian Caledonides and is enclosed in amphibolite facies, multiply deformed supracrustal rocks. The stratiform orebody occurs stratigraphically above a sequence of gneiss and amphibolite and below a thick carbonate unit. The orebody, spatially associated with a footwall microcline gneiss that contains as much as 12wt K₂O, occurs in the lower part of the Mine Sequence which also comprises (kyanite-) mica schist and quartzo-feldspathic to siliceous rocks. The host rock lithology and the metal content of the Bleikvassli orebody are consistent with a SEDEX origin of the deposit. Field relationships and chemistry suggest that the microcline gneiss represents a potassic alteration of pelitic sediments related to the ore-forming process. A 464 ± 22 Ma Rb-Sr isochron for the microcline gneiss is interpreted to be a metamorphic age resulting from resetting of the Rb-Sr isotopic system during the Caledonian orogeny. The U-Pb in the whole rock shows evidence of recent mobilization of uranium and a partial or total resetting of the system during peak metamorphism. As with most SEDEX deposits, the lead isotope composition of the Bleikvassli ore plots close to the orogen growth curve. The geological setting of the ore and the lead — isotope compositions of the galenas indicate a Cambrian age of mineralization. However, the slope of the lead isotope data indicate an age of about 1000 Ma, which is also a maximum age of ore deposition. The lead isotope data for the galena, in conjunction with the compositions of the microcline gneiss during peak metamorphism, support a model whereby the microcline rock was formed as an alteration product by the ore forming fluid and the initial lead isotope composition of the microcline rock was similar to that of the galenas during ore deposition.     

Mineralogy and textural relationships among sulphosalts and related minerals in the Bleikvassli Zn-Pb-(Cu) deposit, Nordland, Norway

Several distinct assemblages of Pb-Sb, Pb-As, Cu-Pb-Sb and Cu-Fe-Zn-Sn sulphosalts are identified in sulphide samples from Bleikvassli mine, Norway. Detailed optical microscopy and electron probe microanalysis have permitted investigation of textural relationships between minerals and compositional variations between different ore types. Tetrahedrite, typically containing 10–16?wt.% Ag (rare freibergite containing 25–30?wt.% Ag has also been identified in two samples), stannite (Cu₂(Fe>Zn)SnS₄), and meneghinite, CuPb₁₃Sb₇S₂₄, are widely distributed as trace constituents throughout massive pyritic and galena-rich ores. Native antimony and pyrargyrite occur in trace amounts in all ore types, as the breakdown products of earlier sulphosalts. Several distinct types of wall-rock mineralisation are present at Bleikvassli. Of considerable mineralogical interest are the coarse-grained sulphide mobilisates within the wall rock which contain a distinct?and characteristic suite of Pb-As sulphosalts:?tennantite?+?jordanite (Pb₁₄As₆S₂₃)?+?seligmannite (CuPbAsS₃) ± dufrenoysite (Pb₂As₂S₅). Bournonite (CuPbSbS₃) is the only Sb-bearing sulphosalt recognised in significant amounts within the mobilisates, meneghinite and tetrahedrite being conspicuously absent. These mobilisates display considerable Au enrichment; electrum can be confirmed, intimately associated with jordanite and tennantite. Appreciable Sb (up to 3?wt.%) is contained within galena in the mobilisates, in contrast to galena from massive ores which contains only negligible Sb. Contents of Ag and Bi in galena vary considerably in all ore types, but confirm earlier suggestions that galena is a major Ag-carrier at Bleikvassli. Boulangerite (Pb₅Sb₄S₁₁), jamesonite (FePb₄Sb₆S₁₄) and gudmundite (FeSbS) occur in trace amounts. Sn-sulphosalts are represented by kësterite, (Cu₂(Zn> Fe)SnS₄), but commonly zoned with respect to Zn/Fe ratio, in the mobilisates, rather than by stannite. A rare type of mobilisate, also in the wall rock, in which chalcocite and bornite are the main minerals, contains native Ag, stromeyerite (AgCuS), mckinstryite ((Ag,Cu)₂?S), Ag-free tetrahedrite, an unnamed Cu-Ag-Fe sulphide (Cu₃Ag₂FeS₄) and native Bi, myrmekitically intergrown with chalcocite. Although a comprehensive genetic model for the wall-rock mineralisation at Bleikvassli is largely impossible given the limitations in the present state of knowledge regarding mechanisms involved in remobilisation processes, a multi-stage model of remobilisation during regional metamorphism is considered to best explain the observations. An interplay of different solid- and liquid-state remobilisation mechanisms, in various combinations, is required to account for the macro- and microscopic observations. Remobilisation probably began during the earlier stages of metamorphism, with crystallisation and further remobilisation taking place during the entire metamorphic cycle, giving rise to the extensive chemical and mineralogical diversity observed today. Preserved mineral assemblages and their textural relationships reflect a complex sequence of replacement and decomposition reactions taking place during the latest phase of late-metamorphic crystallisation and subsequent cooling.     

A sulphur isotope study of the Broken Hill deposit,New South Wales,Australia

Sulphur isotopic compositions of sulphides within garnet-rich rocks and high-grade ore from the Broken Hill deposit, New South Wales, Australia, have been determined and show a range of values of –3.3 to +6.7 per mil. Thermochemical considerations, including the spread of values of ³⁴S, suggest that the deposit was derived from a mixed source of sulphur in which seawater, reduced by inorganic processes, mixed with magmatic sulphur or that sulphate from contemporaneous seawater was reduced biogenically at low temperatures. Thermochemical considerations also suggest that pyrrhotite formed by desulphidation of pyrite so that the original Fe-S-O assemblage was pyrite ± magnetite.³⁴S measurements show a broad range which is considered to be due to isotopic reequilibration during retrograde metamorphism and analytical and sampling technique. These data should not be used to indicate original temperatures of deposition or metamorphic temperatures associated with the various metamorphic events.     

A sulphur isotopic study of the Navia gold belt (Spain)

The Navia gold belt is located in the West Asturian-Leonese Zone of the Iberian Variscan Orogen. The host rocks of the mineralization are quartzites, sandstones and black shales of Cambro-Ordovician age. The gold belt extends along 35 km and has five major veins: Penedela, Encarnita, Fornaza, Carmina and S. Jose. The ores belong to at least four associations having contrasting mineralogies and textures. The δ³⁴S values for individual mineral phases reflect the polyphase metallogenic history. The older association (Stage 1) is Fe-Mn-rich and is made up of spessartine, grunerite-dannemorite and quartz, with magnetite, pyrrhotite and chalcopyrite as metallic phases. The mineralization of Stage 1 is followed by the As-rich Stage 2 with quartz, arsenopyrite and pyrite. The δ³⁴S values for pyrite range from 14.9 to 19.9 per mil (n = 16), and for arsenopyrite from 13.2 to 17.3 per mil (n = 7). The observed isotopic homogeneity likely implies isotopic equilibrium at the scale of the gold vein. Stage 3 contains a coarse-grained base metal sulphide-rich association. The δ⁴S values for sphalerite range from 16.4 to 20.6 per mil (n= 16), and for galena from 17.0 to 18.7 per mil (n = 11). δ³⁴S_sp > δ³⁴S_gl suggests that the sulphur isotopic fractionation of the ore-forming system had reached equilibrium. The youngest crosscutting mineral association (Stage 4) consists of Pb-Sb sulphosalts, bornite, electrum and quartz. The δ³⁴S values for sulphosalts range from 9.7 to 15.8 per mil, showing the lightest results of the Navia sulphides.The relatively tight clustering of δ³⁴S values of the Au-related sulphides, and the results of fluid inclusions and paragenetic studies, can be interpreted to indicate that the hydrothermal fluids of the last three stages were dominated by H₂S. In the H₂S predominant field, sulphide minerals precipitating from solutions would exhibit δ³⁴S values similar to the δ³⁴S_ΣS value of the ore fluid. The heavy δ³⁴S_ΣS of the Navia fluids is consistent with leaching of sulphur from the host rocks. The main sulphur source could be diagenetic pyrite from the siliciclastic rocks of the Cabos and Luarca Formations, which exhibit δ³⁴S values from 8.3 to 21.2 per mil. An additional sulphur-source in Stage 3 would be the leaching of disseminated sphalerite and galena present in Cambrian carbonates.     

A comparative isotopic study of the Lady Loretta zinc-lead-silver deposit

Measurements of the ³⁴S/³²S ratios in sulphides from the slightly metamorphosed Lady Loretta deposit show the sphalerite and galena to be in isotopic equilibrium. Pyrite in immediate association with these sulphides is not isotopically related. A similar distribution of sulphur isotopes had previously been noted in the even less altered McArthur deposit, for which a dual sulphur source was postulated. This fresh isotopic evidence from Lady Loretta now suggests that such a genesis for stratiform sulphides from the Proterozoic is not uncommon.     

云南鲁春VMS 锌铅铜多金属矿床的成矿金属来源

鲁春VMS 锌铅铜多金属矿床产于金沙江构造带内鲁春-红坡牛场伸展裂谷盆地中,是三江地区典型的火山成因块状硫化物矿床,其含矿层位为双峰式火山岩系中的流纹质火山--沉积岩系。通过研究该矿床的主成矿元素、双峰式火山岩和矿石的稀土元素特征,对其成矿金属来源、赋矿火山岩及构造环境进行研究表明,鲁春多金属矿床属Zn --Pb --Cu 型火山成因块状硫化物矿床,形成于碰撞造山后在薄陆壳( 陆缘弧) 基底上伸展而成的裂谷盆地环境; 矿石的主成矿元素含量特征w ( Zn) /w ( Pb + Zn) 均值为0. 64,与日本黑矿和四川呷村矿床较为接近; ΣREE 为( 15. 99 ～ 144. 43) × 10 － 6,平均73. 99 × 10 － 6,LREE/ HREE 为3. 59 ～ 11. 40,平均6. 30,呈典型的LREE 富集型; δEu 为0. 13 ～ 0. 46,平均0. 28,Eu 负异常明显,与矿区流纹岩极为相似。矿体与流纹岩空间上的密切关系以及地球化学特征的一致性表明,成矿金属元素源自下伏的长英质岩系。     

The Hemlo gold deposit,Ontario: A geochemical and isotopic study

The Hemlo deposit, near Marathon, Ontario, is one of the largest gold deposits in North America. It is stratiform within Archean metamorphosed volcano-sedimentary rocks. The main ore zone is composed of pyritic, sericitic schist, and massive barite. This is the first report of stratiform barite in the Archean of North America, but other occurrences have since been found west of Hemlo. The mineralization is substantially enriched in Au, Mo, Sb, Hg, Tl and V and lacks carbonate. Because of metamorphism and deformation of the body its genesis is uncertain.${}^{87}\text{Sr}{}^{86}\text{Sr}$ of .7017 for barite from the deposit is similar to that of the sedimentary barite west of Hemlo and to initial ratios of contemporaneous volcanic rocks. At the base of the main ore zone, barite with δ^34S of +8 to +12%. was deposited with ~0%. pyrite. Upward, both barite and pyrite get isotopically lighter, with minimum values for pyrite, to ?17.5%, in non-baritic schist forming the upper part of the ore zone. In drill section, Au grades correlate with the isotopic composition of pyrite. This, and the association of fractionated sulphide with sulphate, suggests that Au, pyrite and barite were deposited contemporaneously. The linked, asymmetric distributions of S minerals and isotopic distributions, which are continuous from section to section, and the isotopic similarity of the Hemlo and western barites are consistent with a syngenetic depositional model.Two sources for the S minerals are considered. In the first, exogenous sulphate from a restricted basin were partially reduced in a geothermal system to form ³⁴S-depleted sulphide. In the second, the sulphate and sulphide are of magmatic-hydrothermal origin. Sulphate and fractionated sulphide are uncommon in Archean rocks, but one or both occur with unusual frequency in major Archean gold deposits. Hydrothermal fluids of moderately high , containing sulphate and permitting isotopic fractionation between oxidized and reduced S species, may have favoured the dissolution, transport and precipitation of Au.     

A stable isotope study of the Palaeoproterozoic Tallberg porphyry-type deposit,northern Sweden

The Tallberg deposit is situated in the Skellefte District in northern Sweden. It is a Palaeoproterozoic equivalent of Phanerozoic poryphyry-type deposits. The mineralization is situated within the Jörn granitoid complex and is associated with intrusive quartz-feldspar porphyries. The granitoids are coeval with mainly felsic volcanic rocks hosting several massive sulphide deposits. The alteration is generally of a mixed phyllic-propylitic type, but areas or zones associated with high gold grades exhibit phyllic alteration. Ore minerals are pyrite, chalcopyrite, sphalerite, magnetite, and trace amounts of molybdenite. In this stable isotope study, quartz, sericite, and chlorite from the alteration zones were sampled. The magmatic quartz has a ¹⁸O composition of + 6.2 to +6.7 whereas the quartz in the hydrothermal alteration zones have values ranging from +7.5 to +10.6. The calculated temperatures for this fractionation range from 430° to 520°C. The sericites have ¹⁸O ranging from +4.6 to +8.2 (average +6.6) and D -31 to -54 (average -41). Chlorites range from ¹⁸O +4.2 to +7.7 and D from –34 to –44. The range of ³⁴S of 11 pyrite samples is +3.8 to +5.5 with an average of +4.6 ± 0.5, suggesting a relatively homogeneous sulphur source, probably of magmatic origin. Modelling waters in equilibrium with the minerals indicates early magmatic fluids with ¹⁸O of 6.5. This fluid mixed with a low ¹⁸O and high D fluid, which is tentatively identified as seawater. The ¹⁸O signature of sericite and chlorite also indicates significant water-rock exchange, explaining the positive ¹⁸O values for the waters in equilibrium with the hydrated minerals.     

Possible Mesozoic age of Ellenville Zn-Pb-Cu(Ag) deposit,Shawangunk Mountains,New York

Ore textures, epithermal open-space filling of Permian structures of the Alleghanian orogeny, and largely postorogenic mineralization of the Ellenville, New York, composite Zn-Pb-Cu(Ag) vein system, provide permissive evidence for post-Permian mineralization. Isochron ages determined by ⁴⁰Ar/³⁹Ar laser-fusion techniques for K-bearing liquid inclusions in main-stage quartz from the Ellenville deposit additionally suggest a Mesozoic time of mineralization, associated with extensional formation of the Newark basin. The best ⁴⁰Ar/³⁹Ar total-fusion age range is 165 ± 30 to 193 ± 35 Ma. The Mesozoic ⁴⁰Ar/³⁹Ar age agrees with that of many other dated northern Appalachian Zn-Pb-Cu(Ag) deposits with near-matching lead isotope ratios, and adds new evidence of Jurassic tectonism and mineralization as an overprint to Late Paleozoic tectonism at least as far north as Ellenville (lat. 41°43N).     

江西城门山铜矿铼-锇同位素年龄研究

城门山铜矿中，侵入到花岗闪长岩中的石英斑岩，其Ｒｅ－Ｏｓ法同位素测年结果为１４０×１０６ａ，进而确定了该区二次岩浆侵入活动时间相隔约１０×１０６ａ，在这期间内，岩浆由中酸性演变为酸性；与其相应的成矿作用则从夕卡岩型铜、硫矿化演变为细脉浸染型铜、金矿化，构成了一个演化序列。在细脉浸染型辉钼矿化作用中，石英细脉中辉钼矿比石英斑岩中浸染状辉钼矿形成略早，二者在空间上呈正消长，而整个成矿过程大约经历了４×１０６～３×１０６ａ。     

斑岩-矽卡岩型矿床成矿作用过程中的Cu同位素地球化学行为初步研究：以冬瓜山矿床为例

本文初步报道了长江中下游铜陵矿集区内冬瓜山斑岩-矽卡岩型Cu-Au矿中硫化物、石英闪长岩体和赋矿围岩的Cu同位素组成特征。其中,硫化物的δ65Cu变化范围为-0.54‰~0.95‰,变化范围较大,可达1.5‰,表明高温成矿体系下铜同位素发生分馏,铜同位素具有示踪高温成矿作用过程的潜力。不同空间位置的黄铜矿的铜同位素组成呈现出空间分带特征,表现为从岩体→斑岩型矿体→近岩体矽卡岩→矽卡岩型矿体,随着远离岩体,黄铜矿的铜同位素组成逐渐变重。导致斑岩-矽卡岩型矿床铜同位素出现空间分带的主要原因是矿化过程中铜同位素发生分馏。并且,对于冬瓜山矿床来讲,导致铜同位素组成空间分带的分馏不是发生在Cu在气-液两相之间分配的过程中,而是发生在硫化物从流体中沉淀出来的过程中。在硫化物的沉淀过程中,铜的重同位素优先在流体中富集,轻同位素在沉淀中富集,随着流体向外迁移,硫化物沉淀的进行,残余热液流体会逐渐富集铜的重同位素。硫化物的铜同位素组成可以用来反演和指示成矿流体的迁移方向。     

A new sulphur isotopic study of some Iberian Pyrite Belt deposits: evidence of a textural control on sulphur isotope composition

The sulphide deposits of the Iberian Pyrite Belt (IPB) represent an ore province of global importance. Our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores. Measured ³⁴S values range from −26 to +10‰ mostly for massive and stockwork ores, in agreement with data previously published. In situ laser ³⁴S analyses reveals a close correlation of ³⁴S with texture. Primary diagenetic textures are dominated by relatively low ³⁴S (−8‰ to −2‰), whereas stockwork feeder textures are dominated by higher ³⁴S (∼+3‰ to +5‰). Intermediate textures (mainly coarse textures in stratiform zones) have intermediate ³⁴S, although they are mostly dominated by the high ³⁴S component. Rare barite has a homogeneous ³⁴S around +18‰, which is consistent with direct derivation from Lower Carboniferous seawater sulphate. A dual source of sulphide sulphur in the IPB deposits has been considered. A hydrothermal source, derived from reduction of coeval seawater sulphate in the convective systems, is represented by sulphide in the feeder zones. Here variations in ³⁴S are caused by variations in the extent of the sulphate reduction, which governs the SO₄:H₂S ratio. The second end-member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface, as reflected in the primary textures. A distinct geographical variation in ³⁴S and texture from SW (more bacteriogenic and primary textures) to NE (more hydrothermal textures and ³⁴S) which reflects a variation in the relative input of each source was likely controlled by local geological environments. Given that the sulphur isotope characteristics of the IPB deposits are unlike most VMS and Kuroko deposits, and noting the dominance of a mixed reduced sedimentary and volcanic environment, we suggest that the IPB could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types. Received: 8 October 1997 / Accepted: 14 May 1998     

Ultramafic nodules from Seiland,northern Norway

Peridotitic inclusions were emplaced in synorogenic basic and ultrabasic dykes during the later evolutionary stages of the Seiland petrographic province. Tectonite and cumulate types are recognized. The former show penetrative textural evidence of a series of pre-incorporation mineral assemblages due to partial equilibration of the source along a decreasing P-T gradient. Unmixing of aluminous pyroxenes and reaction with spinel resulted in the appearance of minor plagioclase, which later was replaced by hydrous minerals. The metamorphic fabric eliminates an accumulative relationship with the transporting magma. The rare cumulate-type nodules, however, retain evidence of poikilitic textures and primary plagioclase; orthopyroxene and spinel are due to subsolidus reaction of olivine and plagioclase.A high-temperature alpine peridotite rising diapirically through the upper mantle is postulated to be the parent of the tectonite-type nodules. This intrusion may be the source of the synorogenic magmatism and positive gravity anomaly of the Seiland province.     

安哥拉Bonga铌矿床成矿碳酸岩岩石地球化学和C-O同位素研究

安哥拉Bonga碳酸岩型铌矿床位于Parana'-Angola-Etendeka碱性岩-碳酸岩火成岩省东部,是一个孤立产出的中心式岩栓,侵入于元古宙花岗岩基底中。岩石地球化学研究表明,Bonga岩体由钙碳酸岩和少量的镁碳酸岩组成,岩体成分从钙碳酸岩向镁碳酸岩演化。矿物组合上,钙碳酸岩以方解石为主,副矿物有磷灰石、磁铁矿、烧绿石和少量稀土矿物;镁碳酸岩以白云石为主,烧绿石含量降低,稀土矿物含量增高。富钙碳酸岩(摩尔比值Ca O/Ca O+Mg O+Fe O+Mn O0.83)中Nb含量较高,变化于148.1×10~(-6)~8394×10~(-6),平均为2127×10~(-6),∑REE变化于1441×10~(-6)~9452×10~(-6),平均为2791×10~(-6),LREE/HREE变化于16.7~58.3,平均为25.0;富镁碳酸岩(摩尔比值Ca O/Ca O+Mg O+Fe O+Mn O0.83)Nb含量降低,变化于300.9×10~(-6)~3910×10~(-6),平均为1502×10~(-6),∑REE升高,变化于1659×10~(-6)~18849×10~(-6),平均为7111×10~(-6),轻稀土更加富集,LREE/HREE增大,变化于19.1~114,平均为57.6。铌在碳酸岩浆演化的早期富集,铌矿化主要与富钙碳酸岩有关;稀土元素的富集相对较晚,主要与富镁碳酸岩有关。对碳酸岩碳氧同位素的瑞利分馏模拟计算(RIFMS模型)结果表明,Bonga碳酸岩的铌矿化(烧绿石沉淀)主要受岩浆作用控制,其温度不低于600℃。     

Holocene climate in the subarctic fjord Malangen, northern Norway: a multi-proxy study

A Holocene sedimentary record from the deep-silled Malangen fjord in northern Norway reveals regional changes in sedimentary environment and climate. Down-core analysis of two sediment cores includes multi-core sensor logging, grain size, x-radiography, foraminifera, oxygen isotopes, dinoflagellates, pollen, trace elements and radiocarbon datings. The cores are located just proximal to the submarine Younger Dryas moraine complex, and reveal the deglaciation after Younger Dryas and the postglacial evolution. Five sedimentary units have been identified. The oldest units, V and IV, bracket the Younger Dryas glacial readvance in the fjord between 12 700 cal. years BP and 11 800 cal. years BP. This is followed by deposition of glaciomarine sediments (units IV and III) starting around 12 100 cal. years BP. Glaciomarine sedimentation ceased in the fjord c. 10 300 cal. years BP and was replaced by open marine sedimentation (units II and I). A rapid stepwise warming occurred during the Preboreal. Onset of surface water warming lagged bottom water warming by several hundred years. The δ[Formula: See Text]O record indicates a significant, gradual bottom water cooling (c. 4°C) between 8000 and 2000 cal. years BP, a trend also supported by the other proxy data. Other records in the region, as well as GCM simulations, also support this long-term climatic evolution. Superimposed on this cooling were brief warmings around 6000 cal. years BP and 2000 cal. years BP. The long-term change may be driven by orbitally forced reduction in insolation, whereas the short-term changes may be linked to for example solar forcing, meltwater and NAO changes all causing regional changes in the North Atlantic heat transport.     

A carbon and oxygen stable isotopic study of the siderite alteration in the Black Ridge gold deposit,Clermont, central Queensland

Gold mineralisation in the Black Ridge gold deposit, Clermont, central Queensland is associated with extensive siderite alteration and is most concentrated along a locally sheared unconformity between the Proterozoic? Anakie Metamorphics and the overlying basal conglomerates of the Permian Blair Athol Coal Measures. The carbon and oxygen isotopic compositions of 49 siderite specimens from all rock units show a systematic variation spatially related to the unconformity. The δ ¹³C values of siderite decrease with depth from about 0 to ?19‰, while the siderite δ ¹⁸O values of siderite from the overlying conglomerates are lighter (average 20.5‰) than those from the underlying Anakie Metamorphics (average 23.5‰); both isotopes show the largest variations in the samples from the unconformity itself. The data are best interpreted as the result of the mixing of two isotopically and thermally distinct fluids. The upper, cooler fluid has a marine signature and was derived from the pore water of the conglomerate. The lower, geothermal fluid, which carried most of the gold, was not locally derived, but was probably modified in the upper part of the Anakie Metamorphics as a consequence f geothermal gradients imposed by instrusive rocks common throughout the Clermont area. Mixing of the two fluids along the unconformity led to gold solubility in the geothermal fluid dropping more than four orders of magnitude causing gold mineralisation there.     

A late Precambrian tidal shelf deposit, the Lower Sandfjord Formation, Finnmark, North Norway

The Lower Sandfjord Formation is a 1.5 km thick late Precambrian sandstone. It is a remarkably homogeneous unit consisting largely (98%) of cross-bedded, texturally and mineralogically mature, coarse or medium sandstone, and is interpreted as a shallow marine deposit. This interpretation is based on the maturity, the exclusively tabular bed geometry, occasional sets of herring-bone cross-bedding and most importantly, the abundance of sheet-like pebble layers only 1–5 grain diameters thick and sometimes overlain by thin siltstone drapes. Various different types of compound cross-bedding, all of which show evidence of reversing currents, are interpreted as sub-tidal sandwaves. These sets range in thickness from 0.5 to 14 m, and in conjunction with the overall abundance of cross-bedding probably indicate strong tidal currents. A tide-dominated current regime is also considered essential to explain the derivation of such large quantities of sand from the contemporary coasts. It is suggested that sand transport offshore took place during the erosional transgression of abandoned delta lobes. However, the predominance of a single, easterly, mode in the palaeocurrent patterns suggests that the tidal currents were reinforced by some other current system. The predominantly unimodal palaeocurrent patterns and the coarse, sand-rich nature of the succession, taken together with the thickness do not superficially seem likely characteristics for a shallow marine sequence. Nevertheless this study appears to demonstrate that such deposits were formed on tidal shelves in at least late Precambrian time.     

The sulphur isotope systematics of the Taolin lead-zinc ore deposit,China

Sulphur isotope data from coexisting sulphides and sulphates from the Taolin Pb-Zn ore deposit have been used to estimate the temperatures of sulphur mineral precipitation. The data indicate that sulphide was the dominant sulphur species in solution at high temperatures and that sulphate was dominant at low temperatures. Also the data show that the δ³⁴S value of total sulphur in solution was close to zero at high temperatures (~325°C) but had high positive values (+15%.) at low temperatures (~250°C). We interpret this phenomenon in terms of the effects of mineral precipitation on the isotopic composition of the solution. The increase in the δ³⁴S value of total sulphur with decreasing temperature was brought about by the removal from the system, by precipitation, of isotopically light sulphides.