陕西略阳煎茶岭金矿床成矿作用探讨

煎茶岭金矿床位于勉略宁三角区东北缘,矿体产于环绕超基性岩体分布的含金蚀变带中,属蚀变岩型矿床。由于对其成矿作用认识分歧较大,严重影响了后续的找矿勘探工作。从矿床地质特征入手,通过矿区岩矿石常量元素、微量元素、稀土元素以及硫、氢、氧同位素的测试分析,探讨其控矿因素和成矿机理。结果表明,矿床受叠加于接触带部位的韧脆性剪切带控制,成矿不仅与超基性岩有关,而且与花岗斑岩关系密切。花岗斑岩的侵入引起超基性岩的彻底蚀变,同时导致原岩中的金等成矿物质被释放出来。矿石、蚀变超基性岩、白云岩中黄铁矿的硫同位素组成与花岗斑岩中黄铁矿的硫同位素组成具有较好的一致性,并表现出一定的规律性,说明花岗斑岩提供了成矿所需的硫;矿石氢、氧同位素组成介于岩浆水与大气降水之间,且多数样品聚集在雨水线附近,暗示花岗斑岩提供了初始热液,随着成矿作用的进行,大气降水不断加入,从而形成复合热液。鉴于矿床与花岗斑岩的密切关系以及铬云母化的产出特点,根据同位素年龄,认为矿床形成于印支中期—燕山中期。     

Partial melting and melt segregation in footwall units within the contact aureole of the Sudbury Igneous Complex (North and East Ranges,Sudbury structure), with implications for their relationship to footwall Cu–Ni–PGE mineralization

We performed detailed field and drill core mapping of partial melting features and felsic rocks (footwall granophyres, FWGRs) representing segregated and crystallized partial melts within the contact aureole of the Sudbury Igneous Complex (SIC) in the 1.85 Ga Sudbury impact structure. Our results, derived from mapping within the North (Windy Lake, Foy, Wisner areas) and East Ranges (Skynner, Frost areas) of the structure, reveal that partial melting was widespread in both felsic and mafic footwall units up to distances of 500 m from the basal contact of the SIC. Texturally and mineralogically, significant differences exist between rocks formed by partial melting within and between localities. In general, however, melt bodies are dominated by different quartz-feldspar intergrowths (e.g. granophyric, graphic) and miarolitic cavities up to 5 cm in diameter. Major and trace element compositions of Wisner and Frost FWGRs imply that they crystallized from melts dominantly derived from partial melting of felsic Levack Gneiss and Cartier granitoid rocks, as well as from gabbroic rocks only at Frost. These results accord with our observations on in situ partial melting features and crystallized melt of microscopic scale in both felsic and mafic rocks. We conclude that partial melting occurred at a pressure of 1.5 ± 0.5 kbar and at temperatures up to 750°C in the Wisner area and up to 900°C in the Frost and Windy Lake areas. Segregations of partial melt into veins and dikes are present in all localities, and were promoted by deformation of the Sudbury structure in the Penokean orogeny as indicated by dominant strike directions. Whereas veins and dikes reflect brittle conditions during melt migration, sheared melt pods in the Sudbury breccia matrix indicate ductile conditions during their crystallization. Our results suggest a close genetic association of partial melting, melt segregation, and hydrothermal processes responsible for remobilization of Cu–Ni–PGE sulphides into and within the SIC footwall.     

The Graveyard Point Intrusion: an Example of Extreme Differentiation of Snake River Plain Basalt in a Shallow Crustal Pluton

The Graveyard Point intrusion is the only known example of awell-exposed differentiated mafic pluton associated with thelate Miocene–Pleistocene magmatism of the western SnakeRiver Plain (SRP). It is exposed in a 6 km by 4 km area adjacentto the Oregon–Idaho border, and exposures range in thicknessfrom 20 to 160 m. The thicker parts of the intrusion are stronglydifferentiated and contain a 25–60 m thick section ofwell-laminated cumulus-textured gabbros that grade upward intopegmatoidal ferrogabbro. Evolved liquids formed sheets of Fe-richsiliceous granophyre. At least two injections of magma are indicatedby abrupt discontinuities in the rock and mineral compositions,and by the lack of mass balance between the bulk intrusion andits chilled borders. The laminated gabbros are interpreted tohave formed from a tongue of augite and plagioclase crystalsthat were carried in with the second pulse of magma. Followingthe final emplacement of the intrusion, in situ differentiationproceeded through a two-stage process: the ferrogabbros areexplained as interstitial liquids forced out of the crystalmush by compaction, and the siliceous granophyres are interpretedto be residual liquids that migrated out of the partly crystallizedferrogabbros in response to the exsolution of volatiles. Becausethe geochemical trend inferred for the mafic to intermediatecomposition liquids in the Graveyard Point intrusion is similarto the trend for many western Snake River Plain lavas, the plutonmay be a good model for shallow sub-volcanic magma chamberselsewhere in the SRP. However, some western SRP lavas containanomalously high concentrations of P₂O₅ , which are best explainedby mixing within the active crustal mush column or with partialmelts of previously formed differentiated mafic intrusions. KEY WORDS: Snake River Plain; mafic intrusions; tholeiitic; sill; granophyre     

巴泉铀矿床花岗斑岩中的交代钠长岩及其演化特征

巴泉铀矿床花岗斑岩的钠交代作用十分强烈。通过对各采矿中段的观察,精选45米中段5号穿脉剖面为例,进行岩石学的详细研究二作。钠交代作用在巴泉铀矿床是一次独立的地质作用。它发生在花岗斑岩成岩后,铀矿化之前。钠交代作用在花岗斑(?)中,空间分布上是连续的,在强烈部位花岗斑岩可能化成合钠长石78%的交代钠长岩。     

A deep drillcore from the Morokweng impact structure, South Africa: petrography, geochemistry, and constraints on the crater size

The Morokweng impact structure in South Africa was formed 145 Ma ago, at the time of the minor mass extinction that marks the Jurassic-Cretaceous boundary. Previous size estimates for the Morokweng impact structure ranged between 70 and 340 km, and those workers favoring a very large size speculated on the role that the Morokweng impact might have played in relation to this mass extinction. Consequently, the actual size of this impact structure has wide-ranging implications. Petrographic and geochemical analyses, combined with SHRIMP U-Pb single zircon chronology, of a >3400 m long drillcore from about 40 km west of the center of the Morokweng impact structure show that this borehole is most likely located outside the impact structure, limiting the maximum crater diameter to <80 km. While the formation of a <80 km impact structure in continental terrane probably did not engender global biological consequences, it is possible that a series of impact events at J/K boundary time could have caused environmental stress on a worldwide scale. The terrestrial impact crater record does list several smaller structures of similar ages to that of the Morokweng structure. The results of this study are also significant for South African Archean geology. Late Archean (2.6-2.8 Ga) granitoids in the crust of the Kaapvaal Craton are much more widespread than previously thought. The overprint of the Namaquan-Kibaran orogenesis at 1.2-0.9 Ga along the western and southern margin of the craton is significant even in the far North West Province of South Africa.     

Rocky relationships: the petroglyphs of the Murujuga (Burrup Peninsula and Dampier Archipelago) in Western Australia

The petroglyphs of the Murujuga (Burrup Peninsula and Dampier Archipelago) in Western Australia are of exceptional cultural value for the nation. It is Australia’s largest and most significant collection of aboriginal rock art essentially as petroglyphs, with the number estimated at more than one million engravings. The Murujuga is a textbook example of ancient and modern times colliding as it also hosts potentially polluting, major industrial complexes such as iron ore and salt ports; liquefied natural gas, liquid ammonia and ammonium nitrate plants; railway lines, pipelines and rock quarries. The work presented in this paper is underpinned by an annual monitoring study of 10 selected sites including two control sites located on Dolphin and Gidley islands and eight test sites located closer to the industrial areas. The main rock types of the Murujuga, gabbro and granophyre, have been affected by weathering consisting of a cm-thick leached zone capped by a skin of orange and red iron and manganese oxides. The petroglyphs were engraved by removing the few top millimetres of the iron oxide-rich layer and the contrast with the paler leached zone clearly showing the carved motifs. Phosphorus is abnormally enriched in the leached zone and the surface coating, with manganese exclusively on the surface coating, demonstrating the impact of bird droppings and ‘desert varnish’ on the rocks. The colour difference and the hematite–goethite ratio, measured with a field portable reflectance spectrometer, between the background and engraving provides a tool to estimate the relative age of the petroglyphs and the selection of the backgrounds. As the fine-grained granophyres were preferentially chosen from the coarser-grained gabbros to host the petroglyphs, the goethitic (or yellower) backgrounds were prepared as canvas before being engraved. Many petroglyphs register the food items of the area, which changed as the last ice age ended and sea levels rose. We hypothesise that based on their locations and colour difference the constraint for the oldest age of the petroglyphs exceeds 17 000?years BP.