大陆俯冲带流体活动:超高压变质岩矿物水含量和稳定同位素制约

对超高压变质岩中含水矿物和名义上无水矿物的地球化学研究,极大地深化了我们对大陆碰撞带地壳俯冲和折返过程中流体体制的认识。就流体体制和化学地球动力学来说,有关研究在大别-苏鲁造山带进行的最为详细,因此已经成为研究大陆俯冲带变质的典型地区。本文以大别-苏鲁造山带为对象,从矿物水含量的角度,结合稳定同位素论述了大陆俯冲带流体活动。超高压变质岩中名义上无水矿物含有大量的水,以结构羟基和分子水形式存在。名义上无水矿物中结构羟基和分子水出溶与含水矿物分解共同构成了折返过程中退变质流体的主要来源。名义上无水矿物所释放的水以富集轻的氢氧同位素为特征,而含水矿物分解则提供了富集D的流体来源。折返过程中,名义上无水矿物降压脱水存在亏损D的分子水的优先丢失和不同形式水之间的相互转化。不同岩性的水含量差异导致了它们在折返过程中不同的流体活动行为。大陆板块俯冲和折返过程中,在不同矿物、不同岩性以及板片不同部位之间存在水的再分配;板片的一部分作为富水流体的源,而另一部分可能作为汇。     

福建省紫金山矿田五子骑龙铜矿床流体包裹体研究

五子骑龙浆控高温热液型铜矿床位于紫金山矿田北东侧,产于紫金山复式花岗岩内,含矿岩体为燕山早期黑云母二长花岗岩。矿石构造类型主要有脉状和网脉状、浸染状。根据矿物组合与脉体穿插关系,将脉体分为4个阶段。阶段1为绢云母化-迪开石化-硅化蚀变带的石英±钾长石脉;阶段2为被明矾石化-硅化叠加的绢云母化-迪开石-硅化蚀变带的石英-斑铜矿-黄铜矿-蓝辉铜矿-黄铁矿脉±铜蓝;阶段3为石英-铜蓝-黄铁矿脉体;阶段4为明矾石化-硅化蚀变带中的石英±石膏±方解石脉。阶段1发育WV类、C类和少量WL包裹体,阶段2发育WV类、C类和WL类包裹体,阶段3发育WL类和少量WV类包裹体,阶段4只发育WL类包裹体。含子矿物多相包裹体(S型)仅见于隐爆角砾岩体中花岗闪长斑岩的石英斑晶中。阶段1均一温度集中在362~570℃之间,盐度为4%~19.92% NaCleqv,流体体系为NaCl-CO₂-H₂O体系。阶段2均一温度集中在306~390℃,盐度为0.35%~13.94% NaCleqv,流体沸腾现象显著,CO₂等挥发份逸失。阶段3均一温度集中在233~308℃,盐度为0.18%~14.67% NaCleqv。阶段4均一温度降至132~230℃,盐度降至0.88%~6.16% NaCleqv。总体而言,流体从初始的高温NaCl-CO₂-H₂O体系演化为最终的低温NaCl-H₂O体系,期间发生了流体沸腾作用、CO₂等挥发份逸失、金属硫化物沉淀、大气降水混入等。     

http://www.sciencedirect.com/science/article/pii/S1674987111000788

This paper presents numerical investigation on the ore-forming fluid migration driven by tectonic deformation and thermally-induced buoyancy force in the Chanziping ore district in South China. A series of numerical scenarios are considered to examine the effect of meteoric water precipitation, the dip angle of the faults, unconformity surface, and thermal input on the ore genesis. Our computations reveal that the downward basinal fluid flow driven by extensional stress mixes with the upward basal fluid driven by the thermal input from depth at the junction of two faults at a temperature of about 200 C, triggering the precipitation of the Chanziping uranium deposit.     

“Basin scale” versus “localized” pore pressure/stress coupling - Implications for trap integrity evaluation

In petroleum industry, the difference between pore pressure (P_p) and minimum horizontal stress S_h (termed the seal or retention capacity) is of major consideration because it is often assumed to represent how close a system is to hydraulic failure and thus the maximum hydrocarbon column height that can be maintained. While S_h and P_p are often considered to be independent parameters, several studies in the last decade have demonstrated that S_h and P_p are in fact coupled. However, the nature of this coupling relationship remains poorly understood. In this paper, we explore the influences of the spatial pore pressure distribution on S_h/P_p coupling and then on failure pressure predictions and trap integrity evaluation. With analytical models, we predict the fluid pressure sustainable within a reservoir before failure of its overpressured shale cover. We verify our analytical predictions with experiments involving analogue materials and fluids. We show that hydraulic fracturing and seal breach occur for fluid pressure greater than it would be expected from conventional retention capacity. This can be explained by the impact of the fluid overpressure field in the overburden and the pressure diffusion around the reservoir on the principal stresses. We calculate that supralithostatic pressure could locally be reached in overpressured covers. We also define the retention capacity of a cover (RC) surrounding a fluid source or reservoir as the difference between the failure pressure and the fluid overpressure prevailing in shale at the same depth. In response to a localized fluid pressure rise, we show that the retention capacity does not only depend on the pore fluid overpressure of the overburden but also on the tensile strength of the cover, its Poisson’s ratio, and the depth and width of the fluid source.     

3-D Mohr circle construction using vein orientation data from Gadag (southern India) – Implications to recognize fluid pressure fluctuation

In this paper orientations of quartz veins from the Archaean age lode-gold bearing region of Gadag (southern India) are used to determine the relative stress and fluid pressure (P_f) conditions by constructing 3-D Mohr circle. Anisotropy of magnetic susceptibility (AMS) analysis of the host massive metabasalt reveals that the magnetic foliation is NW–SE striking, which is related to early NE–SW compression (D1/D2 deformation) that affected the region. The quartz veins have a wide range of orientations, with NW–SE striking veins (steep northeasterly dips) being the most prominent. Vein emplacement is inferred to have taken place under NW–SE compression that is known to have caused late deformation (D3) in the region. It is argued that the NW–SE fabric defined the pre-existing anisotropy and channelized fluid flow during D3. The permeability was initially low, which resulted in high P_f (>σ₂). 3-D Mohr circle analysis indicates that the driving pressure ratio (R′) was 0.94, a condition that favoured fracturing and reactivation of fabric elements (foliations and fractures) having a wide range of orientations. This led to an increase in permeability and fluid flowed (burped) into the fractures. Resulting vein emplacement and sealing of fractures led to a reduction of P_f (<σ₂). It is argued that at this low P_f, NW–SE oriented structures continued to remain susceptible to reactivation and vein emplacement, while fractures of all other orientations were inactive and remained sealed. As a consequence, the study area has a cluster of NW–SE oriented veins. R′ is calculated to be 0.07 from 3-D Mohr circle analysis at low P_f, when fractures with NW–SE orientation only were susceptible to dilation. However, it is envisaged that any emplacement of veins in these fractures would have sealed them, thus reducing the permeability and initiating the next cycle of rise in P_f (>σ₂). Thus, it is concluded that the quartz veins in the Gadag region are a consequence of an interplay between conditions that fluctuated from P_f > σ₂ to P_f < σ₂.     

An SVM-based machine learning method for the separation of alteration zones in Sungun porphyry copper deposit

Sungun porphyry copper deposit is in East Azarbaijan province, NW of Iran. There exist four hypogene alteration types in Sungun: potassic, propylitic, potassic–phyllic, and phyllic. Copper mineralization is essentially associated more with the potassic and less with the phyllic alterations and their separation is, therefore, quite important. This research has tried to separate these two alteration zones in Sungun porphyry copper deposit using the Support Vector Machine (SVM) method based on the fluid inclusion data, and seven variables including homogenization temperatures, salinity, pressure, depth, density and the Cu grade have been measured and calculated for each separate sample. To apply this method, use is made of the radial basis function (RBF) as the kernel function. The best values for λ and C (the most important SVM parameters) that perform well in the training and test data are 0.0001 and 1, respectively. If these values for λ and C are applied, the phyllic and potassic alteration zones in the training and test data will be separated with an accuracy of about 95% and 100%, respectively. This method can help geochemists in separating the alteration zones because classifying and separating samples microscopically is not only very hard, but also quite time and money consuming.     

新疆白杨河大型铍铀矿床成矿流体特征及矿床成因初探

新疆和布克赛尔县白杨河大型铍铀矿床是亚洲最大的铍矿床,其矿体产出于晚石炭世微晶花岗斑岩与上泥盆统塔尔巴哈台组(D3t)中酸性火山岩的接触带上,该矿床为典型的热液矿床.萤石作为白杨河铍铀矿床中与成矿作用密切相关的脉石矿物,发育大量气液包裹体.通过对萤石中流体包裹体的显微测温,获得了其主成矿期包裹体的均一温度,为237～372℃(n=40),平均为308.5℃,反映该矿床为中高温热液矿床.萤石以相对富集中稀土(MREE) ((La/Yb)N=3.18～3.55),强烈亏损Eu (δEu=0.01),REE分布模式呈“V”字型为特征.萤石锶钕同位素数据显示出较高的锶同位素初始比值(0.7106),以及正εNd(t)值(3.50～2.95),相似于微晶花岗斑岩的锶钕同位素组成((87Sr/86Sr)i=0.7143～0.7466,εNd(t)=4.06～5.29),由此提出成矿流体主要来自微晶花岗斑岩岩浆分异的晚期热液.主成矿期的萤石包裹体显示高温、低盐度的特征,推断成矿流体源于微晶花岗斑岩晚期岩浆热液与大气降水的混合.     

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.     

Fluid inclusion evidence for hydrothermal fluid evolution in the Darreh-Zar porphyry copper deposit,Iran

The Darreh-Zar porphyry copper deposit is associated with a quartz monzonitic–granodioritic–porphyritic stock hosted by an Eocene volcanic sedimentary complex in which magmatic hydrothermal fluids were introduced and formed veins and alteration. Within the deepest quartz-rich and chalcopyrite-poor group A veins, LVHS2 inclusions trapped high salinity, high temperature aqueous fluids exsolved directly from a relatively shallow magma (0.5 kbar). These late fluids were enriched in NaCl and reached halite saturation as a result of the low pressure of magma crystallization and fluid exsolution. These fluids extracted Cu from the crystallizing melt and transported it to the hydrothermal system. As a result of ascent, the temperature and pressure of these fluids decreased from 600 to 415 °C, and approximately 500–315 bars. At these conditions, K-feldspar and biotite were stabilized. Type A veins were formed at a depth of ∼1.2 km under conditions of lithostatic pressure and abrupt cooling. Upon cooling and decompressing, the fluid intersected with the liquid–vapor field resulting in separation of immiscible liquid and vapor. This stage was recorded by formation of LVHS1, LVHS3 and VL inclusions. These immiscible fluids formed chalcopyrite–pyrite–quartz veins with sericitic alteration envelopes (B veins) under the lithostatic–hydrostatic pressure regime at temperatures between 415 and 355 °C at 1.3 km below the paleowater table. As the fluids ascended, copper contents decreased and these fluids were diluted by mixing with the low salinity-external fluid. Therefore, pyrite-dominated quartz veins were formed in purely hydrostatic conditions in which pressure decreased from 125 bars to 54 bars and temperature decreased from 355 to 298 °C. During the magmatic-hydrothermal evolution, the composition and P–T regime changed drastically and caused various types of veins and alterations. The abundance of chalcopyrite precipitation in group B veins suggests that boiling and cooling were important factors in copper mineralization in Darreh-Zar.     

河南省新县姚冲钼矿床流体包裹体研究

河南省新县姚冲钼矿床产于大别造山带,属于陆-陆碰撞体制的斑岩型矿床,其流体成矿过程可以分为早、中、晚三个阶段,分别以石英+钾长石±黄铁矿±磁铁矿、石英±钾长石+辉钼矿±其他硫化物和石英±碳酸盐±萤石组合为标志.热液石英和萤石中发育纯CO2包裹体(PC型)、CO2-H2O型包裹体(C型)、水溶液包裹体(W型)和含子晶多相包裹体(S型).早阶段石英中发育纯CO2包裹体、CO2-H2O型包裹体和含子晶多相包裹体,中阶段的石英则发育CO2-H2O型包裹体、水溶液包裹体和含子晶多相包裹体,在晚阶段的无矿石英脉中发育水溶液包裹体和少量的CO2-H2O型包裹体,石英-碳酸盐-(萤石)脉石英与萤石中只发育水溶液包裹体.早阶段流体包裹体的均一温度为277～ 380℃,集中于300～ 360℃,盐度变化于3.0％～10.3％ NaCleqv之间.中阶段包裹体均一温度介于185 ～ 351℃之间,集中在260～ 320℃,盐度介于2.4％ ～9.3％NaCleqv;晚阶段包裹体均一温度为139 ～245℃,盐度介于0.7％ ～6.3％ NaCleqv之间.中阶段多相包裹体中常见黄铜矿和其他透明子矿物,表明流体具有还原性、过饱和的特征,是矿石矿物沉淀的主要阶段.估算早、中阶段流体捕获压力分别集中于47 ～ 131MPa和26 ～118MPa,所对应的成矿深度分别约为4.7km和2.6～4.2km.上述流体包裹体的研究表明姚冲钼矿床的初始成矿流体具有高温、高盐度、富CO2的特征,同时预测了深部找矿潜能.