云南昆阳磷矿黑色页岩微量元素特征及其地质意义*
2014-04-10徐林刚BerndLEHMANN张锡贵郑伟孟庆田
徐林刚 Bernd LEHMANN 张锡贵 郑伟 孟庆田
1. 中国地质科学院矿产资源研究所,国土资源部成矿作用与资源评价重点实验室,北京 1000372. 中国地质大学地质过程与矿产资源国家重点实验室,北京 1000833.
昆阳磷矿是我国下寒武统黑色页岩中赋存的最大的磷块岩矿床。为了探讨磷块岩及上覆黑色页岩的源区性质及古海洋的氧化还原环境,本文对梅树村组中谊村段的磷块岩、石岩头段的黑色页岩以及筇竹寺组玉案山段的砂质黑色页岩的地层剖面进行了岩石地球化学研究。发现昆阳剖面磷矿区的下寒武统黑色页岩Th-Zr和Th-Y/Ho比值以及Y-Y/Ho比值与陆源碎屑物质更为接近,石岩头段黑色页岩样品的稀土元素总量平均为174×10-6,与大陆壳稀土总量的平均丰度基本一致。稀土配分模式也十分类似,说明陆源碎屑物质来源比海水自生来源占的比重更大。受陆源碎屑物质的影响,Ni/Co,V/Cr和V/(V+Ni)等氧化还原敏感元素比值并不能很好的指示古海洋的沉积环境。但石岩头段黑色页岩的氧化还原敏感元素相对大陆壳具有一定程度的富集,并且富集程度不强烈,Ce具有轻微的负异常,反映了黑色页岩可能形成于次氧化环境。
氧化还原敏感元素;微量元素;稀土元素;昆阳磷矿;云南
1 引言
图1 华南下寒武统黑色页岩型多金属镍钼矿、钒矿、磷矿、重晶石矿和石煤矿分布图Fig.1 Spatial distribution of polymetallic Ni-Mo, vanadium, phosphorite, barite and stone coal deposits hosted in the Early Cambrian black shales
图2 昆阳磷矿地质图(据韩豫川等,2012)Fig.2 Simplified geological map of the Kunyang phosphorite deposit (after Han et al., 2012)
在我国华南地区,沿扬子地台南缘沉积了一套以页岩、泥岩和硅质岩为主的富含化石(海绵针骨、节肢动物、浮游生物以及细菌遗迹等)的早寒武世黑色页岩层序,分布范围西起云、贵、川,东至苏、浙、皖,在空间上呈北东向延伸长达1600km(Fanetal., 1984)。该套黑色页岩保存完好,层序稳定,富含钼、镍、铬、钒、金、铀等多种元素。尤其特殊的是,这套黑色页岩还赋存了一系列与海相沉积作用密切相关的矿产,包括多金属镍钼矿、钒矿、磷矿、重晶石矿以及石煤矿等,著名的矿产比如遵义-张家界地区多金属镍钼矿、湖南古丈钒矿、贵州江古钒矿、新晃-大河边重晶石矿、贵州织金磷矿以及云南昆阳磷矿等(图1)。对于华南下寒武统黑色页岩的沉积环境及成矿过程,一直以来是研究热点,并取得了丰硕的研究成果(杨勤生, 2001; 范德廉等, 2004; Steineretal., 2005;杨卫东等,2005)。然而,相对于多金属镍钼矿的大量研究成果(Maoetal., 2002; Jiangetal., 2006; Lehmannetal., 2007; 王敏, 2004; 皮道会, 2007; 陈兰, 2006; Xu, 2011; Xuetal., 2011, 2013),同样赋存在黑色页岩中的其他矿种的研究则较少(胡能勇等, 2010; 庞艳春等, 2011)。对于下寒武统黑色页岩形成环境,目前普遍认为经历了从静水还原环境到次氧化环境的演化过程(Goldbergetal., 2007; Xuetal., 2012; Pietal., 2013),本文针对云南昆阳磷矿的黑色页岩层序进行研究,以确定昆阳磷矿层上覆的黑色页岩的物源性质以及形成时期的古海洋氧化还原环境,为进一步对比整个华南早寒武世古海洋沉积环境及沉积成矿过程提供支撑。
2 地质背景
昆阳型磷矿泛指位于扬子地台西南缘,形成于滇东地区下寒武统成磷带的海相沉积型磷块岩矿床,地理位置上包括北起永善-镇雄,南至蒙自-广南之间的广阔范围。磷矿石总储量约占全国总储量的30%,是中国最大的磷矿床矿集区(韩豫川等,2012)。昆阳磷矿是我国下寒武统地层中赋存的最大磷块岩矿床,位于昆明市西南的滇池西岸,矿区位于近东西向的香条冲背斜两翼(图2),出露的地层包括中新元古界昆阳群、新元古界灯影组、下寒武统梅树村组、筇竹寺组、中泥盆统海口组以及石炭系。背斜轴部为新元古界灯影组,南北两翼由下寒武统含磷岩系以及顶板和底板岩系组成,外侧则分布着泥盆系-石炭系碎屑岩夹薄层碳酸盐岩。含磷层位包括两层,背斜南侧磷矿层产状一般160°~190°,倾角10°~30°,平均厚度11.6m,上磷矿层厚度较大,平均5.8m,下磷矿层厚度平均3.5m。上下磷矿层之间为一层白色含磷斑脱岩,厚度变化在0~1.3m之间(杨帆等,2011)。南北两翼的含磷岩系存在差异,南翼的含磷岩系厚度较小,一般在11~13m,上下矿层之间的夹层为含磷粘土岩,矿石中的白云石含量较少,品位较富。北翼的含磷岩系厚度较大,在31~67m之间,上下矿层之间的夹层由南翼的含磷粘土岩相变为含磷白云岩,矿石品位偏低(韩豫川等,2012)。
昆阳磷矿下寒武统黑色页岩包括梅树村组和筇竹寺组,构成一个完整的沉积旋回, 筇竹寺组上覆于梅树村组上部,构成含磷岩系的顶板,梅树村组含磷岩系可细划分为上部的中谊村段和下部的小歪头山段。含磷岩系下伏于梅树村组石岩头段(八道湾段)之下,中谊村段为主要含磷岩系,小歪头山段为含磷岩系的底板。小歪头山段上覆于新元古界灯影组之上。根据区域地层对应和野外观察,将所研究的剖面划分为8个岩性段,各岩性段特征见表1和图3。
表1云南省昆阳磷矿地层剖面岩性特征
Table 1Lithological descriptions of the geological profile in the Kunyang phosphorite deposit, Yunnan Province
地层单元地层厚度(m)岩性描述13.5下磷矿层,中下部为粒屑磷块岩或砾状磷块岩,上部为白云质硅质条带磷块岩21.5斑脱岩,灰白色,含磷含海绿石,产少量小壳动物化石,主要矿物包括伊利石、蒙脱石,以及石英-长石晶屑,含少量白云母和黄铁矿37.0上磷矿层,矿石矿物主要由碳氟磷灰石组成,显示为非晶质碳氟磷灰石胶磷矿.具有结核状、鲕状等原生沉积特征.含石英、硅质岩岩屑等陆源碎屑矿物,伴生有白云石、伊利石、蒙脱石、高岭土和黄铁矿等自生矿物,另外还有少量海绿石、绿泥石、电气石等41.5灰白色致密坚硬白云岩,含燧石条带及扁豆体50.5斑脱岩,灰白色,含磷含海绿石,产少量小壳动物化石,主要矿物包括伊利石、蒙脱石,以及石英-长石晶屑,含少量白云母和黄铁矿627黑色页岩,层薄层状,夹数层粉砂质微晶白云岩,局部见黄铁矿团块,粒度达数厘米765白云质粉砂岩,下部为灰黑色厚层状粉砂岩夹黑色薄层粉砂质页岩,上部为深灰-灰白色厚层状白云质粉砂岩夹粉砂质隐晶白云岩814粉砂质黑色页岩,主要为黑色薄-中层泥质粉砂岩以及粉砂质黑色页岩,该层上部为含澄江动物群化石层位
梅树村剖面寒武系地层出露良好,曾是前寒武纪-寒武纪界线全球标准层型剖面和点位的三个候选剖面之一(Cowie, 1985),产丰富的小壳动物化石,“澄江动物群”的产出层位相当于剖面上筇竹寺组玉案山段(Steineretal., 2007)。Compstonetal. (2008) 利用SHRIMP U-Pb法对石岩头组下部和中谊村段两层磷块岩之间的凝灰岩层进行定年,获得了分别为525.1±1.9Ma和539.4±2.9Ma的年龄,Zhuetal. (2009)利用高精度离子探针技术,对中谊村段两层磷块岩层之间的凝灰岩层定年,获得了SIMS锆石U-Pb年龄为535±1.7Ma,与Compstonetal. (2008)获得的年龄一致,为区域岩层对比提供了依据(图3)。
3 样品采集、分析测试
本次研究样品采自昆阳磷矿区露天剖面,样品涵盖了梅树村组中谊村段磷块岩、斑脱岩,石岩头段黑色页岩以及筇竹寺组玉案山段粉砂质黑色页岩。采样间距随岩性变化有所不同,采样的重点是石岩头段黑色页岩(图3)。剖面总厚度98.3m,样品总计15件,包括1件斑脱岩、1件磷块岩、11件黑色页岩以及2件粉砂质黑色页岩。
主量元素的测试工作在德国汉诺威地质科学与自然资源研究所(Federal Institute for Geosciences and Natural Resources)完成,分析采用X射线荧光光谱分析法(XRF),分析误差小于3%。微量元素的测试在德国克劳斯塔尔工业大学(Technical University of Clausthal)完成。微量元素样品溶样在PicoTrace高压溶样系统中完成,首先称取100~120mg粉末样品,然后用1mL去离子水稀释,使反应器中不至于很干燥。加入3mL高浓度HF静置24h,以去除样品中的硅质。加入3mL HNO3,然后加热至180℃并持续20h。待温度降低到60℃后,将高压溶样系统连接上盛有NaOH溶液的反应瓶,使蒸发的酸液得到中和。继续加温到180℃并持续4h,待温度降低后在样品容器中加入32%的HCl 5mL,盖上溶样容器的盖子然后加压,加热容器到180℃并持续19h。将上述溶样程序重复两次以达到样品完全溶解的目的。最后在溶样容器中加入1mL HCl溶液,然后再加入5mL去离子水,再次加入1mL HCl溶液以及10mL去离子水,然后盖上盖子等待样品全部溶解,待样品全部溶解后将溶样转移至溶样瓶中并用0.5m/L的稀盐酸反复清洗反应容器3次。最后将待测样品在Perkin-Elmer/Sciex ELAN 6000 ICM-MS分析仪中进行测试,分析误差在5%以内。
4 分析结果
样品的主量元素和微量元素组成见表2。由于黑色页岩和磷块岩等样品含有大量有机质,导致样品烧失量较高,最高达23.9%(K-18)。磷块岩(K-3)具有高CaO,高P2O5,低Al2O3的特征,含量分别为48.2%,30.3%和0.41%。斑脱岩(K-2)中因含有少量磷结核,化学成分与磷块岩类似,但是其CaO,P2O5的含量比磷块岩低,Al2O3含量比磷块岩高,分别为29.5%,20.6%和6.52%。除了K-10(14.8m)外,大部分黑色页岩样品(14.3~39.3m)化学成分相对稳定,SiO2含量在48.3%到66.1%间变化,平均为59.8%。Al2O3含量为10.4%~13.8%,平均12.6%。Fe2O3T含量变化于2.8%到4.4%间,平均为3.6%。MgO含量为1.8%到6.5%之间(平均4.2%)。黑色页岩具有较低的CaO含量,1.2%~9.3%(平均3.6%)。P2O5含量低,在0.3%到0.7%之间,平均含量为0.4%。相比之下,K-10具有较低的SiO2,Al2O3, Fe2O3T, 分别为43.2%, 6.03%, 1.92%, 但是具有较高的gO和CaO含量,分别达到9.09%和13.9%。玉案山段2个砂质黑色页岩样品显示区别于石岩头段黑色页岩的特征,具有低SiO2(42.1%和35.6%),Al2O3(8.42%和6.54%)和高CaO(17.6%和15.3%)的特征。
表2昆阳剖面样品主量(wt%)和微量(×10-6)元素分析结果
Table 2Major (wt%) and trace (×10-6) element compositions of samples from the Kunyang profile
样品号K-18K-19K-17K-15K-14K-13K-8K-12K-7K-11K-6K-10K-9K-3K-2样品位置(m)98.395.339.320.319.318.317.316.816.315.815.314.814.384.5岩性砂质黑色页岩黑色页岩磷块岩斑脱岩SiO235.642.160.159.957.960.063.060.858.364.048.343.266.17.7033.6TiO20.393.450.660.690.690.740.710.660.640.680.530.230.310.070.06Al2O36.548.4212.612.912.413.813.412.512.513.310.46.0312.60.416.52Fe2O3T3.583.894.444.054.124.183.593.152.852.83.011.924.170.280.99MnO0.170.010.050.050.050.050.040.040.050.030.10.120.010.040.01MgO10.71.355.294.64.944.053.433.854.242.916.549.091.793.230.80CaO15.317.63.513.064.042.551.833.664.462.349.3113.91.1948.229.5Na2O0.020.070.060.060.060.050.060.060.060.060.040.020.030.100.17K2O2.112.63.824.664.525.215.244.854.805.484.062.716.140.173.67P2O50.5414.40.440.300.480.360.280.340.490.320.710.600.2930.320.6SO30.850.200.781.041.161.270.941.251.060.861.700.911.220.330.43Cl0.020.020.020.020.020.020.010.020.020.020.020.020.010.020.02F0.091.130.060.090.120.060.110.160.160.110.180.100.162.961.97LOI23.94.727.898.229.287.477.268.4910.16.8915.021.15.807.442.41Sum99.8100.099.899.799.899.899.899.899.799.899.899.999.9101.2100.7CIA0.270.290.630.620.590.640.650.590.570.630.440.270.63 As16.057.010.020.019.020.016.021.023.026.032.015.051.05.0012.0Ba252983455562520563586499477562389210417195359Bi<4<4<4<4<4<4<4<4<4<4<4<4<4<5<5Co6.008.009.0016.010.014.014.09.009.011.09.003.007.00<3<3Cd0.24 0.340.380.170.880.170.210.170.180.190.180.180.210.37Cr31.023471.018311613177.014436511528433.061.024.06.00Cs<318.04.008.007.008.008.0010.08.009.008.00<35.00<4<4Cu92.019.033.041.028.033.032.032.038.026.038.016.020.010.012.0Ga9.0010.017.017.016.018.018.016.019.017.013.07.0019.0<27.00Hf8.008.007.007.00<6<6<6<69.00<6<6<6<6<7<7Li13.56.8922.731.240.918.29.8928.531.146.149.752.071.338.138.6Mo7.0021.0<37.005.004.004.005.007.005.007.003.009.00<3<3Nb6.0062.010.010.012.011.011.013.012.010.09.008.043.03.007.00Ni59.015.031.064.060.068.058.053.058.055.062.022.027.0<3<3Pb6.0024.08.0012.012.024.023.025.019.026.039.021.01225.0015.0Rb59.063.010110810711711211011511489.045.071.010.033.0Sb<9<10<7<7<7<7<7<7<7<7<8<8<6<13<11Sc8.0016.012.012.012.014.012.012.012.012.011.06.005.00<34.00Sn<4<54<4<4<4<4<4<4<4<4<4<4<5<5Sr10331462.057.062.079.052.063.068.053.097.082.025.0575725Ta5.0<55.0<4<4<4<4<4<4<4<4<4<4<5<5Th14.042.021.018.020.020.018.019.019.017.019.013.025.08.0022.0U8.0084.04.0013.010.013.011.014.014.013.018.010.012.030.015.0V4220197649255392164254657252379146271328.0W<4<4<4<4<4<4<4<4<4<4<4<4<4<5<4Zn28.033.048.051.055.060.047.034.012834.010829.01146.0011.0Zr12725519717319318719818418216915376.019221084.0Mo/Sc0.881.31 0.580.420.290.330.420.580.420.640.501.80 V/Sc5.2512.68.0854.121.328.013.721.254.821.034.524.354.2 2.00U/Sc1.005.250.331.080.830.930.921.171.171.081.641.672.40 3.75Th/U1.750.505.251.382.001.541.641.361.361.311.061.302.080.271.47V/Mo6.009.57 92.751.098.041.050.893.950.454.148.730.1 Ni/Co9.831.883.444.006.004.864.145.896.445.006.897.333.86 V/Cr1.350.861.373.552.2.02.992.131.761.802.191.334.424.441.331.33V/(V+Ni)0.420.930.760.910.810.850.740.830.920.820.860.870.91
图3 昆阳剖面地层柱状图及采样位置Fig.3 Stratigraphic column of the Kunyang profile showing the Early Cambrian sedimentary sequences and sample locations in the Kunyang phosphorite deposit
昆阳剖面底部的斑脱岩和磷块岩微量元素组成与石岩头段黑色页岩具有明显差异,显示出亏损氧化还原敏感元素的特征,其Mo,Ni,Co含量均小于3×10-6,V含量也明显低于大陆壳丰度。而U含量分别为30×10-6和15×10-6,明显高于大陆壳丰度,甚至高于上覆黑色页岩。相对于大陆壳,石岩头段黑色页岩样品均富集Mo,V,U,Th等,但是富集程度比较低,其富集系数均在10以内。Mo含量为3×10-6~9×10-6,平均5.4×10-6;V含量在97×10-6~657×10-6之间,平均320×10-6;U含量为4×10-6~18×10-6,平均12×10-6;Th含量13×10-6~25×10-6,平均19×10-6。这4种元素相对于大陆壳丰度的富集系数分别为4.9,3.3,4.3和1.8。石岩头段黑色页岩中的Co,Cr,Ni,Sc含量则与大陆壳丰度基本一致,其Co含量在3×10-6~16×10-6之间,平均10.1×10-6;Cr含量33×10-6~365×10-6,平均144×10-6;Ni含量为22×10-6~68×10-6,平均50.7×10-6;Sc含量变化于5×10-6~14×10-6之间,平均10.9×10-6。这4种元素相对于大陆壳丰度的富集系数分别为0.6,1.6,1.1和0.8。玉案山段的2个砂质黑色页岩样品显示出比较明显的差异,K-19具有高Mo(21.0×10-6),U(84.0×10-6),V(201×10-6),Th(42×10-6)的特征,而Ni的含量比较低,仅为15.0×10-6。上覆的K-18砂质黑色页岩中微量元素则与石岩头段黑色页岩比较相似,含7×10-6Mo,59×10-6Ni,8×10-6U,42×10-6V和14×10-6Th。
昆阳剖面样品的稀土元素组成见表3。中谊村段斑脱岩和磷块岩具有较高的∑REE,分别为436×10-6和313×10-6,Ce负异常明显,其Ce/Ce*(Ce/Ce*=2/(LaPAAS+PrPAAS))比值分别为0.78和0.35。Eu也具有负异常的特征,Eu/Eu*(Eu/Eu*=2/(SmPAAS+GdPAAS))比值分别为0.44和0.84。Y/Ho和Pr/Pr*值分别为39.4和62.9、1.12和1.27。石岩头段黑色页岩的∑REE为141×10-6~206×10-6,平均174×10-6,与大陆壳平均稀土总量(169×10-6)基本一致。具有微弱的Ce负异常,Ce/Ce*比值为0.81~0.92,平均0.85。Eu/Eu*比值在0.44和1.03之间。整体上Ce/Ce*和Eu/Eu*比值在石岩头段有由低变高的趋势,而Y/Ho的变化趋势则相反,除了最底部的K-9具有较低的Y/Ho比值(25.4)以外,从底部到顶部Y/Ho的比值从33.5(K-10)降低到27.3(K-17)。Pr/Pr*比值比较稳定,变化范围在1.04~1.11之间,平均1.08。玉案山段砂质黑色页岩的∑REE变化范围很大,底部的K-19 ∑REE明显高于玉案山段黑色页岩,为848×10-6,而上覆砂纸黑色页岩(K-18)的∑REE则比较低,为137×10-6。两个样品均具有Eu正异常,其Eu/Eu*的比值分别为1.21和1.12。Ce/Ce*比值变化不明显,分别为0.98和0.95。Y/Ho和Pr/Pr*值分别为31.6和28.3以及0.93和1.01。
5 讨论
5.1 风化作用和成岩作用对样品化学组分的影响
由于样品采自露天剖面,必须考虑风化作用对样品化学成分的影响,本文对石岩头段黑色页岩和玉案山段砂质页岩样品的化学蚀变指数(CIA)进行计算,其计算方程为CIA=Al2O3/(Al2O3+CaO+Na2O+K2O) (Nesbitt and Young, 1982), 大部分石岩头段样品CIA的变化范围在0.44到0.65之间,与新疆塔里木地区新鲜的黑色页岩样品的化学蚀变指数基本一致(刘兵等, 2007; Yuetal., 2009),说明本次研究所采集的样品虽然来自露天剖面,但风化作用对样品化学组分并没有太大影响。另外部分样品(K-10,K-19和K-18)的CIA值比较低,可能反应了源区化学组分的不同对样品化学蚀变指数有一定的影响。
表3昆阳剖面样品稀土元素含量分析结果(×10-6)
Table 3Rare earth element compositions of samples from the Kunyang profile (×10-6)
样品号K-18K-19K-17K-15K-14K-13K-8K-12K-7K-11K-6K-10K-9K-3K-2La17.810133.030.532.731.337.140.337.233.136.621.621.059.264.3Ce38.923162.455.859.355.862.967.360.453.660.837.632.934.3110Pr5.0028.97.446.997.516.857.528.418.036.568.225.153.938.0816.3Nd21.513928.126.029.025.226.230.130.023.631.620.213.934.165.1Sm5.4640.25.244.786.094.814.025.325.943.966.454.553.265.8613.8Eu1.3311.21.060.981.290.970.741.041.180.741.380.620.321.331.33Gd5.6545.94.644.245.74.383.334.865.713.396.294.673.428.7814.6Tb0.856.280.720.660.850.690.570.760.870.550.950.790.781.242.35Dy5.133.34.474.0.04.974.233.724.905.423.695.815.036.538.6814.7Y28.518124.122.927.425.023.029.933.623.239.334.040.4138117Ho1.015.730.880.831.000.890.831.011.110.791.211.021.592.192.96Er2.8313.52.622.462.752.642.473.043.152.393.432.885.516.497.65Tm0.391.480.390.370.380.380.390.450.470.360.480.400.900.770.93Yb2.437.952.552.382.522.492.502.883.002.343.052.456.123.995.11Lu0.361.080.380.360.390.390.390.440.450.360.450.370.870.510.71Y/Ho28.331.627.327.627.527.927.729.530.229.532.633.525.462.939.4Ce/Ce*0.950.980.920.880.870.880.870.840.810.840.810.820.830.350.78Pr/Pr*1.010.931.041.081.061.081.091.101.111.081.101.091.081.271.12Eu/Eu*1.121.211.011.031.031.000.950.960.950.951.010.630.440.840.44∑REE137848178163182166176201197159206141141313436
图4 昆阳剖面样品Th-Zr,Th-Y/Ho关系图解Fig.4 Correlation diagrams of Th vs. Zr and Th vs. Y/Ho of samples from the Kunyang phosphorite deposit
图5 昆阳剖面样品Y-Y/Ho关系图解(底图据Bau and Dulski, 1996)Fig.5 Correlation diagram of Y vs. Y/Ho of samples from the Kunyang phosphorite deposit (after Bau and Dulski, 1996)
沉积岩中的化学组分一般有三个主要来源:陆源碎屑物质、生物作用以及自生海水来源(Piper, 1994)。为了更好的确定氧化还原敏感元素的富集规律,首先需要确定黑色页岩中是否有陆源碎屑来源的组分。常规的方法是利用一些在海水中滞留时间比较短的不相容元素,因为这些元素可以不经过在海水中滞留而直接在沉积物中富集,因此更接近陆壳的组分。但是,Johnson and Grimm (2001)指出,部分这类元素的富集往往与源区性质以及沉积物的粒度有关系(比如Al2O3)。本文采用Th作为衡量陆源碎屑组分的指标,因为沉积物的粒度对Th的影响比较小。由于黑色页岩的成岩过程对Zr,Y/Ho的影响比较小,因此利用Th与Zr和Y/Ho的比值可以有效的指示陆源碎屑对成岩的贡献 (Schröder and Grotzinger, 2007)。图4中Th与Zr和Y/Ho的关系图显示两者不存在明显的相关关系,说明黑色页岩中元素的来源并非单一的陆源碎屑物质,部分元素为海水自生来源。自然界中Y和Ho具有非常相似的地球化学性质,因此在很多地质过程中两者一同迁移或沉淀。Bau and Dulski (1996)研究发现,火山岩和碎屑沉积物中的Y/Ho的比值约为28,而海水的Y/Ho比值为44~74。利用这一规律,Xuetal. (2013)认为华南遵义地区下寒武统黑色页岩可能有部分陆源碎屑物质来源。本研究中除了中谊村段磷块岩(K-3)具有海水化学沉积的特点外,石岩头段黑色页岩和玉案山段粉砂质黑色页岩均具有明显的陆源碎屑物质来源的特点(图5)。综合Th-Zr和Th-Y/Ho比值以及Y-Y/Ho比值,昆阳磷矿区的下寒武统黑色页岩可能主要来源于陆源碎屑物质,海水自生来源虽然也是重要的来源之一,但是可能不如陆源碎屑物质占的比重大。
5.2 稀土元素地球化学
图6 昆阳剖面样品标准化稀土元素配分模式图(太古代澳大利亚页岩采用Mclennan, 1989;海水采用Nozaki, 1997)Fig.6 PAAS-normalized REE distribution patterns of samples for the Kunyang phosphorite deposit (PAAS values after Mclennan, 1989; Seawater values after Nozaki, 1997)
图7 昆阳剖面样品Eu/Eu*-Ba含量(×10-6)关系图解Fig.7 Correlation diagram of Eu/Eu* vs. Ba (×10-6) of samples from the Kunyang phosphorite deposit
图8 昆阳剖面Ce和Eu异常演化特征Fig.8 Ce and Eu anomalies of samples from the Kunyang phosphorite deposit
由于昆阳剖面石岩头段黑色页岩的稀土元素化学组成比较一致(表3),本文在稀土元素配分模式图中11个石岩头段黑色页岩样品采用平均值,为了方便对比,平均大陆壳和现代海水的稀土元素配分模式一并列出(图6)。中谊村段磷块岩、石岩头段黑色页岩和玉案山段砂质黑色页岩的轻重稀土富集程度差异不明显,总体上呈平坦型,仅玉案山段黑色页岩(K-19)中稀土较富集。稀土元素中只有Ce和Eu是价态可变元素,因此在很多地质过程中Ce和Eu会表现出异常的特征,影响Ce和Eu异常的因素很多,比如氧化还原状态的变化,热液叠加作用以及风化作用等。昆阳剖面不同岩性段样品的稀土元素地球化学存在差异。磷块岩样品显示出明显的Ce负异常和Y正异常特征,与现代海水的稀土元素配分模式基本一致(图6)。由于现代海水处于氧化状态,海水中Ce呈Ce4+,并且以CeO2的形式与铁锰氧化物一起沉淀下来,导致了现代海水具有明显的Ce负异常,磷块岩(K-3)具有明显Ce负异常、Y正异常的特征反应了磷块岩海相化学沉积的特点。ICP-MS测试技术中Ba元素的存在可能会导致Eu产生异常,本研究中Ba元素与Eu异常的相关系数仅为0.25,说明Ba含量的变化并没有对Eu异常形成干扰(图7)。Eu正异常一般形成于酸性、中高温(>250℃)还原性热液流体(Bau, 1991),而Eu负异常一般出现在高度演化的岩浆岩中,由于Eu在斜长石中富集,而相对于其他矿物是不相容元素,随着岩浆岩演化过程中斜长石的不断析出,造成了Eu的负异常。中谊村段斑脱岩样品(K-2)具有明显的Eu负异常,可能是斑脱岩中仅含少量斜长石造成。石岩头段黑色页岩样品的稀土元素含量变化不大,其平均值无论是在稀土总量上还是在配分模式上与大陆壳稀土元素都十分类似(图6),只是与大陆壳相比,石岩头段黑色页岩具有轻微的Ce负异常的特征,说明了石岩头段黑色页岩的化学成分主要来自于陆源碎屑物质,而弱Ce负异常却反映部分物质是海水自生沉积而成,这与Th-Zr和Th-Y/Ho比值以及Y-Y/Ho比值的结果一致。玉案山段底部粉砂质黑色页岩的稀土元素总量明显高于石岩头段黑色页岩,并且具有明显的Eu正异常(Eu/Eu*=1.21),可能反映了热液活动的影响,热液叠加作用一方面携带了更多的稀土元素,另一方面其酸性、中高温等特征导致了Eu/Eu*正异常。类似的Eu/Eu*正异常特征也在Sedex型(Maetal., 2004)和VMS型(Relvasetal., 2006)等热液矿床中普遍存在。
现代海水中由于氧化作用,Ce以Ce4+的形式沉淀下来而相对于其相邻的La和Pr元素亏损。由于Ce具有氧化还原敏感性,被广泛应用于古海洋的氧化还原环境重建(German and Elderfieldetal., 1990; Jiangetal., 2007),Ce异常除了与环境的氧化还原性有关外,还与诸多因素比如微生物作用、海水的酸碱度、海水深度以及古海洋的地质时代有关系。图8中Ce异常曲线显示,中谊村段磷块岩具有明显的海水沉积成因的Ce负异常特征,石岩头段黑色页岩整体上呈弱Ce负异常,且从石岩头段底部向上Ce负异常逐渐变小。这种轻微的负异常可能是继承了古海水中Ce负异常的结果。Shields and Stille (2001)认为华南早寒武世黑色页岩的Ce负异常可能反映了早寒武世古海洋处于缺氧的环境。Jiangetal. (2007) 则通过磷块岩REE特征认为磷块岩形成的早期古海洋处于相对氧化的环境,这种环境更利于保存海水Ce负异常的特征,而晚期的稀土元素则经历了再活化作用,而且其环境可能相对更加还原。昆阳剖面石岩头段黑色页岩由下到上Ce负异常逐渐变轻也一定程度上体现了古海洋还原程度逐渐减弱的趋势。Eu/Eu*从石岩头段没有异常到玉案山段明显正异常反映了热液活动在石岩头段并不强烈,而在玉案山段则非常强烈(图8)。
5.3 氧化还原敏感元素及其比值
黑色页岩中的氧化还原敏感元素含量及其比值,比如Ni/Co,V/Cr和V/(V+Ni),被广泛应用于古海洋沉积环境研究中(Dill, 1986; Hatch and Leventhal, 1992; Jones and Manning, 1994; Rimmer, 2004; Tribovillardetal., 2006; Dumoulinetal., 2011)。虽然不同学者对于这些元素及其比值所反映的沉积环境的划分方法有所差异,但是都承认一个基本事实:随着氧化程度的逐渐增加,Ni/Co,V/Cr和V/(V+Ni)的比值均逐渐下降。这种规律与元素本身的化学性质关系密切,Ni、Co、V和Cr的还原相溶解度极低。Hatch and Leventhal (1992)和Jones and Mannning (1994)通过系统研究认为,Ni/Co<5指示氧化环境,5~7指示缺氧环境,>7指示的是还原环境;而V/Cr<2为氧化环境,2~2.45指示缺氧环境,而>4.25为还原环境;Lewan (1984)研究发现形成于还原环境的黑色页岩其V/(V+Ni)比值均大于0.5。昆阳剖面石岩头段黑色页岩的Ni/Co比值均在5左右,而V/Cr比值除两个样品大于4.25外其他样品均在2~3之间(图9),根据Hatch and Leventhal (1992)和Jones and Manning (1994)给出的划分方案,Ni/Co和V/Cr比值均指示昆阳剖面石岩头段为氧化或轻度缺氧环境。然而石岩头段V/(V+Ni)比值在0.74~0.92之间,与Lewan (1984)所认为的还原环境黑色页岩V/(V+Ni)比值均大于0.5一致,这与V/Cr和Ni/Co比值所得出的结论相矛盾。前人的研究认为自生成因的氧化还原敏感元素对沉积环境的指示作用更有意义,而昆阳磷矿黑色页岩有大量陆源物质的加入,因而对沉积环境的指示作用产生一定影响。昆阳剖面石岩头段黑色页岩样品的氧化还原敏感元素相比于大陆壳具有一定程度的富集,但是富集程度都不强烈,反映了黑色页岩可能形成于次氧化环境,与Ce具有轻微的负异常特征所反映的次氧化沉积环境一致,这种环境下金属元素的自生作用不甚明显,陆源物质的加入则成为主导。
图9 昆阳剖面样品的氧化还原敏感元素及其比值演化趋势图Fig.9 Mo, Ni, U, V/Cr, Ni/Co, and V/(V+Ni) data of samples from the Kunyang phosphorite deposit
6 结论
昆阳磷矿层赋存于梅树村组黑色页岩中,通过对梅树村组中谊村段的磷块岩、石岩头段的黑色页岩以及筇竹寺组玉案山段的砂质黑色页岩的岩石地球化学研究认为:
(1)样品的化学蚀变指数(CIA)指示本次研究所采集的样品虽然来自露天剖面,但风化作用对样品化学组分并没有太大影响。
(2)磷矿区下寒武统黑色页岩Th-Zr和Th-Y/Ho比值以及Y-Y/Ho比值更接近于陆源碎屑物质,石岩头段黑色页岩样品的稀土元素总量和配分模式与大陆壳稀土元素十分类似,海水自生来源虽然也是来源之一,但是所占比重可能不如陆源碎屑物质大。
(3)由于陆源物质的影响,Ni/Co,V/Cr和V/(V+Ni)等比值并不能很好的指示古海洋的沉积环境。石岩头段黑色页岩的氧化还原敏感元素相比于大陆壳具有一定程度的富集,但是富集程度都不强烈,Ce具有轻微的负异常等特征均反映了黑色页岩可能形成于次氧化环境。
致谢野外工作得到了云南省地质矿产勘查开发局和昆阳磷矿的大力支持;分析测试工作在德国汉诺威地质科学与自然资源研究所和德国克拉斯塔尔工业大学实验室完成;在此一并表示感谢。
Bau M. 1991. Rare-earth element mobility during hydrothermal and metamorphic fluid-rock interaction and the significance of the oxidation state of europium. Chemical Geology, 93(3-4): 219-230
Bau M and Dulski P. 1996. Distribution of yttrium and rare-earth elements in the Penge and Kuruman iron-formations, Transvaal Supergroup, South Africa. Precambrian Research, 79(1-2): 37-55
Chen L. 2006. Sedimentology and geochemistry of the Early Cambrian black rock series in the Hunan-Guizhou area, China. Ph. D. Dissertation. Guiyang: Institute of Geochemistry, Chinese Academy of Sciences, 1-103 (in Chinese)
Compston W, Zhang ZC, Cooper JA, Ma GG and Jenkins RJF. 2008. Further SHRIMP geochronology on the Early Cambrian of South China. American Journal of Science, 308(4): 399-420
Cowie JW. 1985. Continuing work on the Precambiran-Cambrian boundary. Episodes, 8: 93-97
Dill H. 1986. Metallogenesis of Early Paleozoic graptolite shales from the Graafenthal Horst (Northern Bavaria-Federal Republic of Germany). Economic Geology, 81(4): 889-903
Dumoulin JA, Slack JF, Whalen MT and Harris AG. 2011. Depositional setting and geochemistry of phosphorites and metalliferous black shales in the Carboniferous-Permian Lisburne Group, northern Alaska. In: Dumoulin JA and Galloway JP (eds.). Studies by the U. S. Geological Survey in Alaska, 2008-2009: U.S. Geological Survey Professional Paper 1776-C, 64
Elderfield H, Upstill-Goddard R and Sholkovitz ER. 1990. The rare earth elements in rivers, estuaries, and coastal seas and their significance to the composition of ocean waters. Geochimica et Cosmochimica Acta, 54(4): 971-991
Fan DL, Yang RY and Huang ZX. 1984. The Lower Cambrian black shales series and the iridium anomaly in South China. Developments in Geoscience, 27thInternational Geological Congress, Moscow. Beijing: Science Press, 215-225
Fan DL, Zhang T and Ye J. 2004. Black Shales in China and Associated Metalogency. Beijing: Science Publishing House, 1-441 (in Chinese)
Goldberg T, Strauss H, Guo QJ and Liu CQ. 2007. Reconstructing marine redox conditions for the Early Cambrian Yangtze Platform: Evidence from biogenic sulphur and organic carbon isotopes. Palaeogeography, Palaeoclimatology, Palaeoecology, 254(1-2): 175-193
Han YC, Xia XH, Xiao RG, Wei SS, Yao CM, Yang JH, Tian LS, Lian W, Yuan CJ, Hao EH, Liang ZP and Wang BQ. 2012. The Chinese Phophate Deposits. Beijing: Geological Publishing House, 1-723 (in Chinese)
Hatch JR and Leventhal JS. 1992. Relationship between inferred redox potential of the depositional environment and geochemistry of the Upper Pennsylvanian (Missourian) stark shale member of the Dennis limestone, Wabaunsee country, Kansas, USA. Chemical Geology, 99(1-3): 65-82
Hu NY, Xia HD, Dai TG, You XJ, Bao ZX and Bao JM. 2010. Sedimentary vanadium deposit of Lower Cambrian black rock series in western Hunan. Contributions to Geology and Mineral Resources Research, 25(4): 296-302 (In Chinese with English abstract)
Jiang SY, Chen YQ, Ling HF, Yang JH, Feng HZ and Ni P. 2006. Trace- and rare-earth element geochemistry and Pb-Pb dating of black shales and intercalated Ni-Mo-PGE-Au sulfide ores in Lower Cambrian strata, Yangtze Platform, South China. Mineralium Deposita, 41(5): 453-467
Jiang SY, Zhao HX, Cheng YQ, Yang T, Yang JH and Ling HF. 2007. Trace and rare earth element geochemistry of phosphate nodules from the Lower Cambrian black shale sequence in the Mufu Mountain of Nanjing, Jiangsu Province, China. Chemical Geology, 244(3-4): 584-604
Johnson KM and Grimm KA. 2001. Opal and organic carbon in laminated diatomaceous sediments: Saanich Inlet, Santa Barbara Basin and the Miocene Monterey Formation. Marine Geology, 174(1-4): 159-174
Jones B and Manning DAC. 1994. Comparison of geochemical indeces used for the interpretation of palaeoredox conditions in ancient mudstones. Chemical Geology, 111(1-4): 111-129
Lehmann B, Nägler TF, Holland HD, Wille M, Mao JW, Pan JY, Ma DS and Dulski P. 2007. Highly metalliferous carbonaceous shale and Early Cambrian seawater. Geology, 35(5): 403-406
Lewan MD. 1984. Factors controlling the proportionality of vanadium to nickel in crude oils. Geochimica et Cosmochimica Acta, 48(11): 2231-2238
Liu B, Xu B, Meng XY, Kou XW, He JY, Wei W and Mi H. 2007. Study on the chemical index of alteration of Neoproterozoic strata in the Tarim plate and its implications. Acta Petrologica Sinica, 23(7): 1664-1670 (in Chinese with English abstract)
Ma GL, Beaudoin G, Qi SJ and Li Y. 2004. Geology and geochemistry of the Changba SEDEX Pb-Zn deposit, Qinling orogenic belt, China. Mineralium Deposita, 39(3): 380-395
Mao JW, Lehmann B, Du AD, Zhang GD, Ma DS, Wang YT, Zeng MG and Kerrich R. 2002. Re-Os dating of polymetallic Ni-Mo-PGE-Au mineralization in Lower Cambrian black shales of South China and its geologic significance. Economic Geology, 97(5): 1051-1061
Mclennan SM. 1989. Rare earth elements in sedimentary rocks: Influence of provenance and sedimentary processes. Reviews in Mineralogy and Geochemistry, 21: 169-200
Nesbitt HW and Young GM. 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature, 299(5885): 715-717
Nozaki Y. 1997. A fresh look at element distribution in the North Pacific. Eos (Transactions, American Geophysical Union)
Pang YC, Lin L, Zhu LD, Zhou YH and Ren CY. 2011. Features of Cambrian Niutitang Formation in the beidoushan ore district, Weng’an County, Guizhou Province. Geological Bulletin of China, 30(8): 1245-1250 (in Chinese with English abstract)
Pi DH. 2007. Geochemistry of Early Cambrian black rock series from Zunyi, Guizhou Province. Ph. D. Dissertation. Guiyang: Institute of Geochemistry, Chinese Academy of Sciences, 1-117 (in Chinese with English summary)
Pi DH, Liu CQ, Shields-Zhou GA and Jiang SY. 2013. Trace and rare earth element geochemistry of black shale and kerogen in the Early Cambrian Niutitang Formation in Guizhou Province, South China: Constraints for redox environments and origin of metal enrichments. Precambrian Research, 225: 218-229
Piper DZ. 1994. Seawater as the source of minor elements in black shales, phosphorites and other sedimentary rocks. Chemical Geology, 114(1-2): 95-114
Relvas JMRS, Barriga FJAS, Ferreira A, Noiva PC, Pacheco N and Barrga G. 2006. Hydrothermal alteration and mineralization in the Neves-Corvo volcanic-hosted massive sulfide deposit, Portugal. I. Geology, mineralogy, and geochemistry. Economic Geology, 101(4): 753-790
Rimmer SM. 2004. Geochemical paleoredox indicators in Devonian-Mississippian black shales, central Appalachian basin (USA). Chemical Geology, 206(3-4): 373-391
Schröder S and Grotzinger JP. 2007. Evidence for anoxia at the Ediacaran-Cambrian boundary: The record of redox-sensitive trace elements and rare earth elements in Oman. Journal of the Geological Society, London, 164(1): 175-187
Shields G and Stille P. 2001. Diagenetic constraints on the use of cerium anormalies as paleoseawater redox proxies: An isotope and REE study of Cambrian phosphorites. Chemical Geology, 175(1-2): 29-48
Steiner M, Zhu MY, Zhao YL and Erdtmann BD. 2005. Lower Cambrian Burgees Shale-type fossil associations of South China. Palaeogeography, Palaeoclimatology, Palaeoecology, 220(1-2): 129-152
Steiner M, Li GX, Qian Y, Zhu MY and Erdtmann BD. 2007. Neoproterozoic to Early Cambrian small selly fossil assemblages and a revised biostratifraphic correlation of the Yangtze Platform (China). Palaeogeography, Palaeoclimatology, Palaeoecology, 254(1-2): 67-99
Tribovillard N, Algeo TJ, Lyons T and Riboulleau A. 2006. Trace metals as paleoredox and paleoproductivity proxies: An update. Chemical Geology, 232(1-2): 12-32
Wang M. 2004. Geology, geochemistry and genesis of PGE-polymetallic deposits, southern China. Ph. D. Dissertation. Guangzhou: Zhongshan University, 1-156
Xu LG. 2011. Early Cambrian black shale and associated polymetallic Ni-Mo-PGE-Au mineralization, South China. Ph. D. Dissertation. Claufeld-Zellerfeld: Technical University of Clausthal, 1-369
Xu LG, Lehmann B, Mao JW, Qu WJ and Du AD. 2011. Re-Os age of polymetallic Ni-Mo-PGE-Au mineralization in Early Cambrian black shales of South China: A reassessment. Economic Geology, 106(3): 511-522
Xu LG, Lehmann B, Mao JW, Nägler TF, Neubert N and Böttcher ME. 2012. Mo isotope and trace element patterns of Early Cambrian black shales in South China: Constraints on the paleoenvironment. Chemical Geology, 318-319: 45-59
Xu LG, Lehmann B and Mao JW. 2013. Seawater as origin of polymetallic Ni-Mo-PGE-Au mineralization in Early Cambrian black shales of South China: Evidences from Mo isotope, PGE, trace element and REE geochemistry. Ore Geology Reviews, 52: 66-84
Yang F, Xiao RG and Xia XH. 2011. Sedimentary environment and geochemistry of the Kunyang phosphorite deposit in eastern Yunnan Province. Geology and Exploration, 47(2): 294-303 (in Chinese with English abstract)
Yang QS. 2001. Metallogenetic characteristics and prospecting in the black rock series of East Yuannan and the neighbourhood. Yunnan Geology, 20(1): 59-72 (in Chinese with English abstract)
Yang WD, Qi L and Lu XY. 1995. Geochemical characteristics and origin of phosphoric sedimentary formation REE in Early Cambrian in the eastern Yunnan. Bulletin of Mineralogy Petrology and Geochemistry, 12: 224-227 (in Chinese with English abstract)
Yu BS, Dong HL, Widom E, Chen JQ and Lin CS. 2009. Geochemistry of basal Cambrian black shales and cherts from the northern Tarim Basin, Northwest China: Implications for depositional setting and tectonic history. Journal of Asian Earth Sciences, 34: 418-436
Zhu RX, Li XH, Hou XG, Pan YX, Wang F, Deng CL and He HY. 2009. SIMS U-Pb zircon age of a tuff layer in the Meishucun section, Yunnan, Southwest China: Constraint on the age of the Precambrian-Cambrian boundary. Science in China (Series D), 52(9): 1385-1392
附中文参考文献
陈兰. 2006. 湘黔地区早寒武世黑色岩系沉积学及地球化学研究. 博士学位论文. 贵阳: 中国科学地球化学研究所, 1-103
范徳廉, 张焘, 叶杰. 2004. 中国的黑色岩系及其有关矿床. 北京: 科学出版社, 1-441
韩豫川, 夏学惠, 肖荣阁, 魏祥松, 姚超美, 杨金湖, 田升平, 连卫, 袁从建, 郝尔宏, 梁中朋, 王炳铨. 2012. 中国磷矿床. 北京: 地质出版社, 1-723
胡能勇, 夏浩东, 戴塔根, 游先军, 鲍正襄, 包觉敏. 2010. 湘西北下寒武统黑色岩系中的沉积型钒矿. 地质找矿论丛, 25(4): 296-302
刘兵, 徐备, 孟祥英, 寇晓威, 何金有, 卫巍, 米合. 2007. 塔里木板块新元古代地层化学蚀变指数研究及其意义. 岩石学报, 23(7): 1664-1670
庞艳春, 林丽, 朱利东, 周玉华, 任才云. 2011. 贵州瓮安地区北斗山矿区寒武系牛蹄塘组的特征. 地质通报, 30(8): 1245-1250
皮道会. 2007. 贵州遵义早寒武世黑色岩系地球化学研究. 博士学位论文. 贵阳: 中国科学院地球化学研究所, 1-117
王敏. 2004. 华南下寒武统黑色岩系铂多金属矿地质地球化学及其成因. 博士学位论文. 广州: 中山大学, 1-156
杨帆, 肖荣阁, 夏学惠. 2011. 昆阳磷矿沉积环境与矿床地球化学. 地质与勘探, 47(2): 294-303
杨勤生. 2001. 云南东部及临区黑色岩系内的矿床(化)特征与找矿设想. 云南地质, 20(1): 59-72
杨卫东, 漆亮, 鲁晓莺. 1995. 滇东早寒武世含磷岩系稀土元素地质地球化学特征及成因. 矿物岩石地球化学通报, 12(4): 224-227