Enrichment characteristics and sources of the critical metal yttrium in Zhijin rare earth-containing phosphorites,Guizhou Province,China

Xingxiang Gong·Shengwei Wu·Yong Xia·Zhengwei Zhang·

Shan He1,2·Zhuojun Xie1,2·Jiafei Xiao1,2·Haiying Yang1,5·Qingping Tan1,2·Yi Huang3,4·Yuhong Yang3,4

Abstract Yttrium(Y)is a critical metallic element that is used widely in modern scientific,technological,and industrial applications.Phosphorites in the Zhijin area of Guizhou Province,China,are famous for Y enrichment,but the enrichment characteristics and sources remain unclear.Previous studies suggested that the sources of rare earth elements(REEs,La–Lu)and Y(REYs,La–Lu+Y)were related to hydrothermal deposition.However,this study presents evidence refuting that hypothesis,with major and trace elemental data collected with quadrupole–inductively coupled plasma–mass spectrometry(Q–ICP–MS)analysis.These data show that Y in Zhijin REYscontaining phosphorites has a normal distribution and is particularly enriched relative to other REYs.The Y enrichment degree is different at different∑REY intervals.Specifically,the Y enrichment degree is higher at lower∑REY values and lower at higher∑REY values.The REYs-containing phosphorites show features of primary phosphorites.Both REEs and Y have good correlations with P2O5 in the phosphorites with low REYs contents(total REYs＜535 ppm),whereas at high REYs contents(total REYs≥535 ppm),REEs have a good correlation with P2O5 but Y does not.Inconsistent enrichment processes of REYs are suggestive of complex sources of Y.Thus,seafloor hydrothermal fluids were not the direct source of Y.Normal seawater mixed with terrestrial sources might have contributed to the origin of Y here.This study could lead to improvements in Y mineral resource explorations and the situation involving the global REYs supply crisis.

Keywords Yttrium·Phosphorites·Rare earth elements·Trace elements·Enrichment processes

1 Introduction

The atomic number of yttrium(abbreviated as Y)is 39,the relative atomic weight is 89,the outer electron arrangement is 4d5s,and its common valence is 3.Because the chemical properties of Y are similar to those of rare earth elements(REEs),Y is classified as an REE.Y has been used widely in glass,alloys,televisions,and the iron and steel industries in recent decades,and nowadays,this metal has become a critical element in modern scientific,technological,and industrial applications.

Y often display itinerant properties of nature.In terms of a difference in stability or extraction properties,Y can occupy almost any position in the lanthanides.With different ligands,Y will display different properties.Y resembles a light REE for soft ligands;however,Y resembles a heavy REE for hard ligands(Borkowski and Siekierski 1992).

Many marine phosphorites are known for their enrichment of REYs(REEs and Y)during the early diagenesis phase(Hein et al.2016).The main mineral of phosphorites is francolite,and its composition can be summarized as(Ca,Na,Mg,Sr)(PO,CO,SO)F(McArthur et al.1980;McArthur 1986).The REYs such as REEand Y,as well as SiOcan replace Caand PO(REE+SiO=Ca+PO)and then enter the lattice of apatite by isomorphic substitution(Jiang et al.2020).

The REYs(especially Y)enrichment of Zhijin phosphorites in China has attracted the attention of numerous people.Many scholars(e.g.,Chen 2013,2019;Duan et al.2015;Guo et al.2017;Wu 2019;Xia et al.2019;Xu 2019;Zhang et al.2007a,2007b)have carried out studies on the relationship between REEs and P,and Y and P.These studies focused principally on the state of occurrence,geochemical characteristics,enrichment characteristics,and paleogeographic environment of REYs-bearing phosphorites.

Previous main views are as follows:

(1) In terms of occurrence state,REYs enriched principally in the sandy clastic phosphorites and the darkcolored parts of the layered structural phosphorites in the lower part of the Gezhongwu Formation(℈gz)(Guo et al.2017),which stratigraphic age considered to be between (522.9±8.6) -(535.2±1.7)Ma(Xia et al.2019).These REYs do not exist in the form of independent minerals,but are associated closely with apatite(Guo et al.2017),and existed principally in collophane in the form of isomorphism(Zhang et al.2007a,b).

(2) As far as the distribution patterns of Rare Earth are concerned,REYs showed negative Ce anomalies,positive Gd anomalies,Y enrichment,and other heavy rare earth elements(HREEs)depletion(Wu et al.2019;Wu 2019).

(3) REYs enrichment is related to the geographical microfacies.Xia et al.(2019)and Xu(2019)divided the paleogeographic environment of the Early Cambrian phosphorite-forming period in the Zhijin area into gentle slope basin,gentle slope beach,and gentle slope edge areas(the areas were also divided into front and back edges)according to the features of the rock structure and mineral size,grinding roundness,sorting characteristics,and so forth.The results showed that the different geographical microfacies have different degrees of REYs enrichment.

(4) Some signatures of hydrothermal deposition were showed(Guo 2017).

Previous studies have led to a certain understanding of the occurrence state and enrichment characteristics of REYs in phosphorites,and establish an important basis for this study.However,the enrichment characteristics and sources of Y remain unclear.To reveal the enrichment characteristics and sources of Y in the Zhijin area,the major and trace element concentrations of Y-rich phosphorite samples in this area were measured,and a cluster analysis and factor analysis of the REYs data was then carried out.This study allows us to gain insight into the enrichment characteristics and sources of Y,offer useful guidance for Y mineral resource explorations,and provide the possibility to ease the crisis of the global REYs(especially Y)supply crisis.

2 Geological background

The Yangtze platform is probably the largest phosphate basin in the world(Ilyin 1998).The Zhijin REYs-bearing phosphorite deposit is located at the southwestern end of the central Guizhou uplift in the passive fold belt in the southern part of the Upper Yangtze Old Continent.From old to new,the outcrop strata are as follows:Upper Sinian Dengying Formation,Lower Cambrian Gezhongwu Formation(℈gz),Niutitang Formation and Mingxinsi Formation,Lower Carboniferous Jiujialu Formation and Dapu Formation,Middle Permian Liangshan Formation,Qixia Formation,and Maokou Formation,Upper Permian Emeishan Basalt,Longtan Formation,Changxing Formation,and Dalong Formation,Lower Triassic Yelang Formation,Jialingjiang Formation,Middle Triassic Guanling formation,and Quaternary.Missing strata include the Paleozoic Upper Cambrian,Ordovician,Silurian,Devonian,and Mesozoic Jurassic and Cretaceous;magmatic rocks are not developed(Li et al.2008;Xia et al.2019;Yang et al.2013).The tectonic lines in the area are extended toward the northeast direction,and the structural deformations are dominantly faults,followed by folds(Li et al.2008).Previous studies(Xia et al.2019)have revealed that the relationship between the phosphorite deposition and structures in this area is not obvious,but the relationship between the phosphorite deposition and sedimentary paleogeographic settings is obvious.The Lower Cambrian Gezhongwu Formation(℈gz)is the regional phosphorite-bearing stratum.Host-Y minerals,namely,apatite,can be found in the Lower Cambrian Gezhongwu Formation(℈gz).

The investigated area is located in the western margin of the Yangtze platform(Fig.1,modif ied according to Chen et al.2013).During the Early Cambrian period,the paleogeography was generally a gentle slope composed of carbonates and phosphates.Xia et al.(2019)studied the horizontal changing characteristics of sedimentary basins and found that,from west to east,the sedimentary basins can be roughly divided into the back edge of a gentle slope basin,the front edge of a gentle slope basin,gentle slope beach,the front edge of a gentle slope beach(Fig.2).These microfacies affected the enrichment of phosphorites and REYs,so that the upper and lower section of the prof ile shows different characteristics.The characteristics of the vertical change are as follows:the lower part is principally columnar,with strip and conical colloidal phosphate particles,and there are a small number of circular particles and biological particles;the sorting property and grinding roundness of the particles are medium to poor.Additionally,the upper part is principally granular particles,and the sorting property and grinding roundness of the particles are better in this location.As a result,the vertical hydrodynamic strength ref lected by the structural characteristics of the section differs such that the lower part’s strength is lower than that of the upper part,and laterally the hydrodynamic strength of the middle part of the basin is lower than that of the basin edge(Xia et al.2019;Xu 2019).

Fig.1 Early Cambrian Paleogeography Map in Yangtze Platform(Modif ied After Chen et al.2013)

Fig.2 Paleogeographic microfacies map of Zhijin REY-bearing phosphorite area(Modif ied according to Xu 2019).I Backward edge of gentle slope basin.II Gentle slope basin.III The front edge of gentle slope basin and the back edge of gentle slope beach.IV gentle slope beach.V The front edge of the slow slope beach

3 Characteristics of the mineral deposit

3.1 Characteristics of the strata-bearing ore

The series of REYs-containing phosphorites is principally located in the Gezhongwu Formation(℈gz)(Fig.3),which is underlay by the phosphorus-containing(REYs)Niutitang Formation(℈n)and overlay by carbonate dolostone of the Dengying Formation(Zdy).

Fig.3 Synthesis histogram of rock series-containing phosphorus(rare earth)(Modif ied After Meng et al.2016;Wu 2019))

The Dengying Formation(Zdy)is the direct base of the Gezhongwu Formation(℈gz)in the ore-bearing rock series.It is a medium-thick f ine-grained dolomite layer with colors of gray,light gray,and grayish-white.

The Niutitang Formation(℈n)is a f ine-grained feldspar-quartz sandstone,silty sandy carbonaceous mudstone,and tuberculate,lenticular siliceous phosphorite-REYs ore layer.In the Niutitang Formation(℈n),the concentrations of phosphorite and REYs change obviously.

The ore-bearing rock series is namely the Gezhongwu Formation(℈gz),which is the main phosphorus-containing(REYs)ore layer with a thickness of 0–33.73 m.The main features of the Gezhongwu Formation(℈gz)are as follows.

(1) Phosphorus-bearing dolomite(ore layer B).This layer is composed of gray-dark gray,medium to thick layered f ine-grained bearing-phosphorus dolomite.Its thickness ranges from 0 to 7.70 m(locally the section is not well developed).The POhas a concentration of 2.37%–18.67% (generally 5%–11%).The middle part of this layer developed only in the Guohua section and Dajia section of Xinhua phosphorite,where a continuous strip-shaped dolomitic phosphorite-REYs ore layer(namely ore layer B)is developed.For ore layer B,the monolayer thickness is 0.93–9.12 m,the average thickness is 3.14 m;the average POgrade is 18.67%,and the content of REEs(REEO)is 0.1%.

(2) Phosphorites with an unequal thick banded structure(ore layer A).It is a Dark gray medium to thick layered f ine-grained containing-phosphorus dolomite layer,with a thickness that ranges from 20 to 50 cm,interbedded with a dark gray thin layer of dolomitic containing-REYs phosphorite layer of a thickness of 2–15 cm.Thus,it forms a banded structure with unequal thickness.This layer is the main phosphorite(REYs)ore layer in the exploration area(ore layer A).In the mineral warrant area,the thickness of this layer is 2.08–31.57 m,with a typical value of 8 m.The original ore in this layer has a low POconcentration of 5.23%–24.94%,with an average POconcentration of 17.62%,an average REEOcontent of 0.1034%,and an average YOcontent of 0.0370%.Because of surface weathering,the POis relatively enriched and forms a strip-like dolomitic bearing-REYs phosphorite ore layer with high POconcentrations.The bottom of this layer is located in the exploration warrant area of the Motianchong Phosphorite Mine in the Guohua section of Xinhua phosphorite.At the bottom of this layer,the original ore is massive,the thickness is 0–14.67 m,the POconcentration is 12.12%–37.39%,and the REEOcontent is 0.142%.However,the bottom of this layer is poorly developed in the other areas(Meng et al.2016).

3.2 Ore characteristics

The REYs occur principally in phosphorites.There are nine ore sections(Meng et al.2016;Wu 2019),such as Xinhua,DaMaChang,GaoShan,DaJia,Guohua.

The REYs-containing phosphorite minerals are principally light gray,gray,gray-black,black,brown-yellow.Phosphorites usually show sandy-clastic(Fig.4C1),which are locally carbonaceous(Fig.4C1).Calcite,dolomite(Fig.4A2),and siliceous cementation(Fig.4B2)occur commonly.

3.3 Mineralogical characteristics

The main mineral of REYs-bearing phosphorite is apatite,the gangue mineral mainly consists of dolomite,quartz,and clay minerals.There is usually a growth-decline relationship between concentrations of ore minerals and concentrations of gangue minerals,that is,when the concentrations of apatite are high,the concentrations of dolomite,quartz,and clay minerals are usually low.In contrast,when the concentrations of apatite are low,the concentrations of dolomite,quartz,and clay minerals are usually high(Fig.4).

Fig.4 Characteristics of Early Cambrian phosphorite in Zhajin region.A dolomitic phosphorite.B siliceous phosphorite.C sand clastic phosphorite.D banded phosphorite.Number 1 for hand specimen.Number 2 for microscope.Number 3 for scanning electron microscope(SEM).Ap denotes apatite.Dol denotes dolomite.Qtz denotes quartz

In addition to the main ore minerals and gangue minerals,secondary minerals such as pyrite and hematite are often found in phosphorites.Zircon,rutile,barite,spinel,sphalerite,galena,siderite,pyrolusite,and other Cr-and Na-bearing minerals also appear locally in the phosphorites.

4 Sampling and testing methods

In the study,we selected representative Y-rich phosphorite samples from the Zhijin Gezhongwu Formation(℈gz)in Guizhou Province.All of the samples were collected from drillcore samples obtained in a borehole section,and the prof ile direction is mainly toward NW–SE orientation.The borehole numbers of samples were as follows:ZK1802,ZK2802,ZKX202,ZK2601,ZK2603,ZK2204,ZK2604,and ZKX001.A total of 150 phosphorite samples were collected.In general,the sampling distance was 1 m.

After sampling,the major elements were analyzed by melting the samples(melting temperature is at 1000°C)with lithium borate or lithium borate-lithium nitrate and then carrying out XRF analyses.The detection limit was 0.01%,and the relative standard deviation was better than 5%.

After decontamination and drying,the whole rock samples were ground into 200 mesh powder.Then,the trace element analyses were carried out.After high temperature(1000°C)and high-pressure reduction by the acidsolution method,the contents of trace elements were quantif ied by Q-ICP-MS analyses in the State Key Laboratory of Ore Deposit Geochemistry at the Institute of Geochemistry,Chinese Academy of Sciences(IGCAS).The model number of the apparatus was ELANDRC-e.The relative standard deviation was better than 10%(Chew et al.2016)(Figs.5,6).

Through these tests,15 REYs and 28 other major and trace elements were analyzed,including V,Cr,Co,Ni,Cu,Zn,Ga,Ge,As,Rb,Sr,Zr,Nb,Mo,Ag,Cd,Sb,Ba,W,Pb,Th,U,AlO,CaO,F,MgO,MnOand POwas determined.

Based on the tests for major and trace elements,analysis charts were drawn by using Mapgis and Photoshop.We used SPSS and Origin software to analyze the test data.The relevant parameters of the factor analysis were set as follows:(1)extraction method:principal component analysis;(2)analysis:correlation coeff icient matrix;(3)rotation:maximum variance;(4)convergence maximum:25 iterations;(5)factor analysis:regression analysis;(6)principal component analysis using the default method.

5 Enrichment characteristics of Y

5.1 Statistical characteristics

The samples collected in this study are phosphorites,which came from the same ore district and the same stratum in the Zhijin region.Previous studies(Yang et al.1995)have indicated that Y principally occurs here in apatite.Xu et al.(2019)found that the Y content is positively correlated with the POconcentration in phosphorites,and it was proposed that the Y content may be closely related to the chemical and mineral composition of the phosphorates in the phosphorites and the enrichment of Y may have resulted from the preferential absorption of Y relative to other lanthanides.As research has progressed,it is believed that the concentration of the major elements in geological samples obeys a normal distribution,and the logarithmic values of contents of trace elements obey a normal distribution on the whole(Stine 2016;Tan and Wang 2017;Tan et al.2018).To study the characteristics of the major elements,trace elements,and REYs in Zhijin phosphorites,the contents of trace elements and the total amount of REYs were log-transformed,while the concentrations of major elements were not log-transformed.

Statistical distribution features of the data can be expressed as frequency distribution histograms and normal Q-Q(quantile–quantile)graphs.The frequency distribution histograms and normal Q-Q plots of the sample data were drawn by using SPSS software.The conf idence intervals were set as 95%.The quartile and median of the samples were calculated according to∑REY scope,and these values amounted to 286 ppm(25%),535 ppm(50%),and 771 ppm(75%),respectively.According to the ∑ REY scope,the samples were divided into four sections,that is,∑REY values less than 286 ppm,values ranging from 286 to 535 ppm,values ranging from 535 to 771 ppm,and values more than 771 ppm.

The statistical results of the sample data are listed in Table 1,and the distribution pattern is shown in Fig.7.Here,the∑REY represents the total contents of the REEs and Y.δEu is the abnormity of europium calculated by the formula δEu=Eu/Eu*=Eu/(Sm×Gd)(Taylor and McLennan 1985),andδY is the abnormity of yttrium calculated by formulaδY=2Y/(Dy+Ho)(Fazio et al.2007)(Y inserted between Dy and Ho based on radius sizes).Er/Nd,Y/Ho,and(La/Sm)are ratios of certain REEs,where the subscript N denotes Normalized to Post-Archean Australian shale(PAAS)(McLennan 1989)values.

Fig.7 REY distribution pattern of Zhijin phosphorite-rare earth

Table 1 Some elemental Characteristics of Early Cambrian phosphorite in Zhijin Region

The statistical characteristics of the samples are shown in Fig.5.Figure 5A1,B1 presents the frequency distribution histograms of the contents of REYs and Y in the phosphorite,respectively.Figure 5A2,B2 presents the normal Q-Q graphs of the contents of REYs and Y in the phosphorite,respectively.The data for REYs and Y of the samples within the Q-Q diagram was located near the straight line of Q-Q graphs.The data for REYs and Y of the samples showed normal distribution characteristics.

Fig.5 The characteristic plots of lg(∑REY)and lg(Y).A1 frequency plot of lg(∑REY).A2 nomal Q-Q plot of lg(∑REY).B1 frequency plot of lg(Y).B2 nomal Q-Q plot of lg(Y)

5.2 Enrichment characteristics of yttrium

In this study,the enrichment degree of Y can be expressed by the percentage of Y in∑REY(Y/REY),the Y anomaly(δY),the Y/Ho ratio,and the REYs distribution pattern.The percentage of Y was found to be very high relative to other REYs,but this percentage was not constant.With∑REY values increasing from low to high,the percentage of Y in∑REY gradually decreased.When the∑REY value rose from less than 286 to 286–535 ppm,535–771 ppm,and greater than 771 ppm,the percentage of Y(median)decreased from 30.96%to 29.55%,27.13%,and 26.14%,respectively.As the∑REY value changed from the smallest value(less than 286 ppm)to the largest value(greater than 771 ppm),the percentage of Y in∑REY(median)decreased by 16%(Fig.6).This finding suggests that the Y enrichment degree decreases with increasing∑REY values.

Fig.6 Y/∑REY(%),δY and Y/Ho(medians)variation plots of different∑REY intervals

On the whole,the Zhijin phosphorite shows a positive Y anomaly,but this anomaly was found to vary.When∑REY increased from low to high,the Y anomalyδY decreased.When∑REY rose from less than 286 ppm to 286–535 ppm,535–771 ppm,and greater than 771 ppm,the Y anomalyδY(median)decreased from 1.87 to 1.64,1.39,and 1.18,respectively.From the smallest∑REY interval to the largest∑REY interval,the Y anomalyδY(median)decreased by 37%(Fig.6).This indicates that with an increase in∑REY,the abnormal changes of Y relative to Dy and Ho decrease,and the behavior of Y tends to be consistent with that of Dy and Ho.

The trend in which the Y anomaly decreased with the∑REY value was also reflected in the Y/Ho data.When the∑REY rose from less than 286 ppm to 286–535 ppm,535–771 ppm,and greater than 771 ppm,the Y/Ho ratio(median)decreased from 48.90 to 43.60,35.82,and 30.41,respectively.When∑REY increased from the smallest value(less than 286 ppm)to the largest value(greater than 771 ppm),the Y/Ho ratio(median)decreased by 38%(Fig.6).This finding indicates that,with the increase in∑REY,the abnormal change of Y relative to Ho becomes smaller,and the behavior of Y becomes gradually consistent with that of Ho.

The degree of variation of Y anomalies also can be illustrated in the REYs distribution patterns.Table 2 shows some PAAS-normalized values of REYs selected from random samples.According to Table 2,the REYs distribution pattern plots of Zhijin phosphorite were obtained(Fig.7).As shown in the plots,Zhijin REYs-bearing phosphorites are generally characterized by a cap-shaped REYs distribution pattern,negative Ce anomalies,no Eu anomalies,middle rare earth elements(MREEs)enrichment,and Y special enrichment.The diagram also revealed that,relative to Dy and Ho,when the∑REY value was smaller(as shown in samples ZK2601-121,ZK2204-137.9,and ZK2603-177.5),the Y anomaly was larger,and when the∑REY value was larger(as shown in samples ZK2603-181 and ZK1802-166.8),the Y anomaly was smaller.This finding indicates that the degree of Y enrichment decreases with the increase in∑REY.

As a consequence,whether from the perspective of percentages of Y in∑REY(Y/REY)and the Y anomalies(δY)or the Y/Ho ratios and the REYs distribution patterns,Zhijin REYs-bearing phosphorites show Y-enrichment characteristics,but the enrichment degree of Y decreases as the∑REY value increases from low to high.

6 Cluster analyses and factor analyses

6.1 Cluster analyses

Figure 8 presents a chart of the cluster analysis of different REYs contents of Zhijin phosphorites.In the chart,the horizontal coordinate is the square Euclidean distance,and the vertical coordinate is the combination characteristic of different elements.By using the square Euclidean distance 5 as the upper limit,the square Euclidean distance between 0 and 5 is indicative of the close relationship between the elements,i.e.,the farther from the square Euclidean distance 5,the more estrangement there is between the elements.

As shown in Fig.8,for the phosphorite,when the total REYs content was less than 286,the square Euclidean distance between PO,F,REE,and Y was between 0 and 5,which suggests that the enrichment of REE and Y were closely related to the PO.This is consistent with the conclusion of some of the previous discussions in this article that suggested that the enrichment of REYs is closely related to the apatite concentration.Namely,a higher apatite concentration is associated with a higher REYs content.

Fig.8 Clustering analysis of Zhijin phosphorites

When the REYs content in phosphorites was between 286 and 535 ppm,the square Euclidean distance between PO,F,CaO,REE,and Y was 0–15,where the PO,F,CaO components represent the apatite composition,thereby denoting that the REE and Y enrichment was closely related to the apatite concentration.However,compared with the phosphorites with the content of REYs less than 286 ppm,the square Euclidean distance between element Y and PO,F,Sr,REE was 5–15,which indicated that the correlations between Y and PO,F,Sr,REE had decreased.

When the content of REYs in phosphorites was 535–771 ppm,the square Euclidean distance between PO,F,REE,CaO was between 0 and 10,and the results suggested that while there were close relations between elements,there was no correlation between Y and PO.When the content of REYs in phosphorites was more than 771 ppm,the results also indicated that there was no correlation between element Y and PO.

Therefore,the Y enrichment is inconsistent with the REE enrichment.For the Zhijin phosphorites with a lower REYs content(total REYs＜535 ppm),both REEs and Y showed good correlations with PO,whereas for those with a higher REYs content(total REYs≥535 ppm),REEs also had a good correlation with PO,but Y showed no correlation with PO.

Samples La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu ZK2601-121 0.62 0.20 0.43 0.50 0.54 0.65 0.77 0.71 0.72 1.66 0.77 0.82 0.82 0.67 0.61 ZK2204-137.9 0.73 0.27 0.52 0.62 0.64 0.88 0.86 0.84 0.77 1.67 0.91 0.90 0.73 0.73 0.54 ZK2603-177.5 0.75 0.25 0.49 0.57 0.57 0.87 1.08 0.77 0.89 2.01 0.87 0.97 0.81 0.74 0.54 ZK2603-182.5 1.47 0.50 1.03 1.19 1.24 2.12 2.46 1.80 1.75 3.78 1.84 1.81 1.43 1.13 0.94 ZKX202-601.5 1.60 0.59 1.12 1.23 1.21 1.40 1.76 1.59 1.78 3.46 2.14 1.90 1.80 1.16 1.05 ZKX202-603 1.65 0.59 1.25 1.36 1.40 1.65 1.88 1.71 1.75 3.21 2.00 1.72 1.72 1.23 1.18 ZK2603-189 2.93 0.82 2.13 2.43 2.65 3.59 4.45 3.41 3.23 6.86 3.38 3.24 2.77 2.09 1.78 ZK2802-151 3.01 1.06 2.59 2.77 3.03 3.47 3.92 3.64 3.80 5.96 4.16 3.89 3.51 2.19 2.18 ZKX202-600 3.01 1.10 2.60 2.96 3.17 3.39 3.98 3.72 3.65 5.67 4.16 3.49 3.06 1.92 1.78 ZK2204-124.8 4.01 1.52 3.45 4.09 4.59 5.42 6.57 5.66 5.32 7.82 5.22 4.85 3.88 2.82 2.18 ZK2603-181 4.32 1.24 3.23 3.66 3.95 6.12 7.07 5.08 4.85 9.57 4.80 4.63 3.73 2.68 2.25 ZK1802-166.8 3.77 1.44 3.99 4.59 5.57 6.68 6.84 6.19 6.20 7.70 6.90 6.04 4.99 3.65 3.07

6.2 Factor analyses

The results of the cluster analysis indicated that for Zhijin phosphorites with a lower REYs content(＜535 ppm),both REEs and Y had good correlations with PO.For those with a higher REYs content(≥535 ppm),REEs also had a good correlation with PO,but Y showed no correlation with PO.Therefore,based on the cluster analysis,two-factor analyses were respectively carried out for the phosphorites with a lower REYs content(＜535 ppm)and higher REYs content(≥535 ppm),which allowed for the study of the differences in REYs enrichment of the phosphorites with different REYs contents.

Results for the Kaiser–Meyer–Olkin(KMO)tests and Bartlett spherical tests of the factor analysis data for Zhijin phosphorites with a lower REYs content(＜535 ppm)and higher REYs content(≥535 ppm)are shown in Table 3.As a rule,for factor analysis,the best effect occurs when KMO statistics values are greater than 0.9,and results are acceptable when KMO statistics values are between 0.7 and 0.9,but results are not suitable when KMO statistics values are less than 0.5(George and Mallery 2019).In this study,for the factor analyses of the phosphorite samples with a lower REYs content(＜535 ppm)and higher REYs content(≥535 ppm),the KMO values were 0.752 and 0.841,respectively,which is considered suitable for factor analysis.For phosphorites with both a lower REYs content(＜535 ppm)and higher REYs content(≥535 ppm),the signif icance values of Bartlett spherical tests of the factor analyses were less than 0.01.This indicates that there are signif icant correlations between variables.

Tables 4 and 5 present the explanation lists of total variance for the factor analyses of the Zhijin phosphorite samples with a lower REYs content(＜535 ppm)and higher REYs content(≥535 ppm),respectively.The explanation lists of total variance consist of three parts,namely,the various interpretations of the eigenvalues of initial factors,the extracted load factors,and the rotating load factors.

Table 3 KMO and Bartlett testing

The variance interpretations of the eigenvalues of initial factors provide the variances explained by each factor and their sum of accumulation.In observations of the eigenvalues of initial factors of the phosphorites with a lower REYs content(＜535 ppm),the eigenvalues of the f irst seven factors were found to be all greater than 1,and the cumulative variance interpreted reached more than 80%;hence,extraction of these seven factors can best explain the information contained in the original variables.By the same token,for the phosphorites with a higher REYs content(≥535 ppm),the eigenvalues of the f irst six factors were found to be all greater than 1,and the cumulative variance interpreted reached more than 83%;hence,the extraction of these six factors can best explain the information contained in the original variables.The variance interpretation of the extracted load factors is namely the interpretation for the total variance by the extracted factors with a characteristic value greater than 1,and their values were the same as those of the initial eigenvalues.The variance interpretation of the rotation load factors represents the contribution value obtained by new factor variances obtained by the rotation of factors in the last column.Compared with the non-rotation factors,the variance contribution value of each common factor changes,but the f inal cumulative contribution rate of the variance remains unchanged.

Component Initial characteristic values Quadratic sum of extracted load Quadratic sum of revolved load Total Variance(%) Cumulation(%) Total Variance(%) Cumulation(%) Total Variance(%) Cumulation(%)1 9.846 33.952 33.952 9.846 33.952 33.952 7.525 25.948 25.948 2 5.107 17.609 51.561 5.107 17.609 51.561 5.286 18.226 44.174 3 2.799 9.653 61.214 2.799 9.653 61.214 3.124 10.774 54.948 4 1.759 6.064 67.278 1.759 6.064 67.278 2.363 8.147 63.095 5 1.423 4.907 72.186 1.423 4.907 72.186 1.991 6.866 69.961 6 1.318 4.543 76.729 1.318 4.543 76.729 1.745 6.016 75.977 7 1.171 4.038 80.766 1.171 4.038 80.766 1.389 4.789 80.766 8 0.931 3.211 83.978 9 0.749 2.583 86.560 10 0.587 2.025 88.586 11 0.492 1.698 90.283 12 0.411 1.416 91.699 13 0.368 1.269 92.968 14 0.339 1.170 94.138 15 0.287 0.990 95.128 16 0.241 0.830 95.958 17 0.208 0.717 96.675 18 0.194 0.670 97.345 19 0.176 0.607 97.952 20 0.150 0.517 98.469 21 0.111 0.383 98.851 22 0.096 0.330 99.181 23 0.077 0.265 99.445 24 0.065 0.226 99.671 25 0.035 0.121 99.792 26 0.026 0.088 99.880 27 0.015 0.053 99.933 28 0.012 0.043 99.976 29 0.007 0.024 100.000

Tables 6 and 7 show the matrices of component loads after rotation of the factor analyses data for Zhijin phosphorite samples with a lower REYs content(＜535 ppm)and higher REYs content(≥535 ppm),respectively.Using this matrix can help us better explain the meaning of the factors.Factor load is the correlation coeff icient between a factor and a variable.For a variable,the greater the absolute value of the load after rotation,the closer the relationship between the variable and its corresponding factor,and thus,the factor can also represent the variable.Therefore,during the study,the authors tagged the values that had an absolute value of the load greater than 0.5 in Tables 6 and 7 by roughening the texts of the values,and a summary is provided for the elements explained by each factor.

Table 6 Component load matrix after rotation of Zhijin phosphorites with low rare earth content(total REY content less than 535 ppm)

Table 7 Component load matrix after rotation of Zhijin phosphorites with high rare earth content(total REY content more than 535 ppm)

For Zhijin REYs-bearing phosphorite samples with a lower REYs content(total REYs＜535 ppm),elements correlated with factor 1(F1)included REE,PO,F,Sr,Y,Th,CaO,U,and Ge;elements correlated with factor 2(F2)included Rb,AlO,Nb,Zr,Ga,and Cr;elements correlated with factor 3(F3)included Cu,Ag,Sb,and As;elements correlated with factor 4(F4)included Zn,Cd,and Pb;elements correlated with factor 5(F5)included As and Mo;elements correlated with factor 6(F6)included Ni,W,and Ba;and the element correlated with factor 7(F7)was V.

In REYs-bearing phosphorites with a low REYs content,the correlation coeff icients between F1 and F,PO,CaO,REE,and Y were 0.935,0.939,0.726,0.959,and 0.899,respectively,which are values representative of a certain degree of REYs enrichment during the diagenesis of phosphorites.The correlation coeff icient between F1 and MgO was-0.507,which was weak relative to the other elements,so the concentration of dolomite was relatively high during the formation process of phosphorite.The correlation coeff icients between F2 and Rb,AlO,Nb,Zr,and Ga were 0.928,0.919,0.904,0.865,and 0.673,respectively,which are values considered to represent the addition process of terrestrial detrital f lux.The correlation coeff icient between F2 and MgO was-0.437,which represents the leaching of dolomite during the addition of terrestrial detrital f lux.However,the correlation was relatively weak(the absolute value was less than 0.5),which indicates that the addition from the terrestrial detrital f lux was not strong.Because of the high concentration of dolomite during the diagenesis,as well as the additional weak terrestrial detrital f lux,the concentration of apatite in phosphorite was relatively low,so the content of REYs was relatively low.

Component Initial characteristic values Quadratic sum of extracted load Quadratic sum of revolved load Total Variance(%) Cumulation(%) Total Variance(%) Cumulation(%) Total Variance(%) Cumulation(%)1 13.204 45.532 45.532 13.204 45.532 45.532 8.519 29.374 29.374 2 4.249 14.652 60.184 4.249 14.652 60.184 5.694 19.634 49.009 3 2.981 10.280 70.464 2.981 10.280 70.464 3.281 11.315 60.323 4 1.590 5.483 75.947 1.590 5.483 75.947 3.017 10.403 70.726 5 1.177 4.059 80.006 1.177 4.059 80.006 2.471 8.521 79.247 6 1.090 3.757 83.763 1.090 3.757 83.763 1.310 4.517 83.763 7 0.929 3.202 86.965 8 0.599 2.066 89.031 9 0.457 1.577 90.608 10 0.409 1.411 92.019 11 0.367 1.266 93.285 12 0.299 1.032 94.317 13 0.260 0.896 95.213 14 0.229 0.791 96.004 15 0.213 0.734 96.738 16 0.169 0.583 97.321 17 0.150 0.519 97.840 18 0.138 0.477 98.317 19 0.104 0.358 98.675 20 0.093 0.322 98.996 21 0.080 0.275 99.272 22 0.063 0.218 99.490 23 0.047 0.164 99.654 24 0.027 0.093 99.746 25 0.025 0.085 99.831 26 0.023 0.079 99.910 27 0.014 0.048 99.958 28 0.009 0.031 99.989 29 0.003 0.011 100.000

For Zhijin REYs-bearing phosphorites with a high REYs content(total REYs≥535 ppm),the elements correlated with factor 1(F1)included Rb,AlO,Nb,Zr,Ga,Cr,Th,Ge,V,and MgO;the elements correlated with factor 2(F2)included MgO,PO,F,Sr,CaO,REE,and U,the elements correlated with factor 3(F3)included Zn,Cd,Mo,Ni,and As;the elements correlated with factor 4(F4)included W,Co,Ba,and Cu;the elements correlated with factor 5(F5)included Cu,Ag,Pb,and Sb;and the element correlated with factor 6(F6)was Y.

For Zhijin phosphorite with a high REYs content,the correlation coeff icients between F1 and Rb,AlO,Nb,Zr,Ga,Cr,Th,Ge,V,and MgO were 0.912,0.911,0.907,0.894,0.854,0.865,0.743,0.740,0.724,and-0.665,respectively,which are values considered to represent the addition from the terrestrial detrital f lux.The correlation coeff icient between F1 and MgO was-0.665,which indicates that the correlation was strong and further demonstrates that the addition from the continental detrital f lux was stronger relative to phosphorites with a low REYs content;this led to more leaching of dolomite.The correlation coeff icients between F2 and MgO,PO,F,Sr,CaO,REE,and U were-0.584,0.971,0.964,0.849,0.823,0.800,and 0.582,respectively,and it is thought that REE enrichment occurred during the diagenesis of phosphorite.The correlation coeff icient between F2 and MgO was-0.584,and the correlation was strong;hence,the concentration of dolomite was relatively low during the formation process of phosphorites.Because of the low concentration of dolomite during the diagenesis process of phosphorite and the relatively strong addition process of continental detrital f lux,the concentration of apatite in the phosphorite is relatively high,and thus,the REYs are more enriched.Nevertheless,for phosphorites with a high REYs content,the correlation between F6 and Y was 0.897,Y enrichment was correlated with F6;while REEs enrichment was correlated with F2(the correlation between F2 and REEs was 0.800).This indicates that Y enrichment processes are different from other REEs.

The results of the cluster analysis suggested that,for phosphorites with a low REYs content(total REYs＜535 ppm),there was a good correlation between REEs,Y,and PO;for phosphorites with a high REYs content(total REYs≥535 ppm),there also was a good correlation between REEs and PO,but there was no correlation between Y and PO.Moreover,the factor analysis also showed that Y was enriched congruously with REEs during the formation of phosphorites with a low REYs content(total REYs＜535 ppm);however,for the phosphorites with a high REYs content(total REYs≥535 ppm),Y showed an inconsistent enrichment process compared with the other REEs.As a consequence,it may be concluded that superposition and enrichment activities were occurring during the accumulation of REYs in the Zhijin phosphorites,that is,the accumulation of REYs could have been caused by diagenesis,and the addition of continental detrital f lux resulted in the leaching of MgO in dolomite,which in turn led to the enrichment of REYs in phosphorites.The inconsistent enrichment processes of REYs may be indicative of more complex Y sources than previously proposed.

7 Discussion

7.1 Identif ication of the original deposition parameters

From material sources to sedimentary mineralization,material-forming minerals must go through a series of processes,including pre-sedimentary source material release,separation,transport,accumulation,sedimentary diagenesis,and reworking of post-diagenesis(Ye 1963).Presently,it is a complex problem to distinguish the preand post-diagenesis enrichment of materials.The identif ications for original deposition parameters can be used to solve this problem.The identif ications involve both REEs parameters and major element parameters.

7.1.1 Identif ication of the REEs parameters

The identif ication of the REEs parameters focused on two aspects,i.e.,the Ce anomaly and the Er/Luratios of the distribution patterns.

(1) Ce anomaly.The distribution pattern of REEs has characteristic Ce anomalies,which may be from two aspects,that is,the judgment of a real Ce anomaly and the correlations between the Ce anomaly and parameters correlated with REEs.

First,we discuss the real Ce anomaly.Shields and Stille(2001)believe that,relative to Pror Nd,some phosphorites will show greater Laenrichment,which results in uncertainty in the calculation of Ce anomalies.Bau and Dulski(1996)think the problem can be solved by the equation Pr/Pr*=2 Pr/(Ce+Nd).Since no chemical factor will result in Nd or Pr anomalies,the existence of real Ce anomalies will result in values of Pr/Pr*that are not less than 1.Based on the relationship between the Ce anomaly(δCe,calculated here by the formulaδCe=Ce/Ce*=Ce/(0.5La+0.5Pr)(Bau and Dulski 1996)and the Pr/Pr*value,Bau and Dulski(1996)divided the Ce and La anomalies into f ive situations(Fig.8).According to Bau and Dulski(1996),we investigated the relationship betweenδCe and Pr/Pr*in the Zhijin phosphorites and found that the Ce anomaly of Zhijin phosphorites is located in the IIIb domain,which belongs to the real Ce anomaly range(Fig.9);thus,the real Ce anomalies indicate that the phosphorites were formed under oxidizing conditions.

Fig.9 δCe-Pr/Pr*map of Zhijin phosphorites,which domain partition according to Bau and Dulski(1996).Domain I no Ce anomalies and no La anomalies.Domain IIa positive La anomalies,no Ce anomalies.Domain IIIb negative La anomalies,no Ce anomalies.Domain IIIa positive Ce anomalies.Domain IIIb negative Ce anomalies

Second,we discuss the correlations between the Ce anomaly and REEs parameters.Shields and Stille(2001)believe that,during the reworking of post-diagenesis,with the enrichment of REYs,the deviation of the REYs distribution pattern between seawater and phosphorites will increase,and this will keep the REYs distribution patterns of phosphorites away from the inf luence of oxidized seawater.As a consequence,the phosphorites affected by postdiagenetic reworking will show a good correlation between δCe and the REYs content,as well asδEu and Dy/Sm.However,for original phosphorites,the correlation betweenδCe and REYs will be poor.Morad and Felitsyn(2001)proposed that if(La/Sm)does not correlate with δCe and(La/Sm)is greater than 0.35,the Ce anomalies in apatite represent the original signature of seawater.

For Zhijin phosphorites,there were no correlations detected betweenδCe and∑REY,δCe and Dy/Sm,and δCe and(La/Sm)(Fig.10),and the(La/Sm)values were above 0.5(Fig.10D),thus indicating that the Ce anomalies of Zhijin phosphorites were the original features of seawater,and the whole REYs series was only slightly affected by post-diagenetic reworking.

Fig.10 The scatter plots ofδCe vs.∑REY(A),δCe vs.δEu(B),δCe vs.DyN/SmN(C)andδCe vs.(La/Sm)N(D)

(2) The Er/Luratios of the REYs distribution pattern.Higher Er/Luratios result from HREEs depletion.Shields and Stille(2001)studied Meishucun and Maidiping phosphorites in South China,which belong to the same Early Cambrian epoch,and the results showed that the Meishucun phosphorites exhibited consistent HREEs depletion from Er to Lu with higher Er/Luratios(1.73–3.37),while the Maidiping phosphorites had lower Er/Luratios(1.67–1.72).Shields and Stille(2001)believe that the REEs content of the Maidiping phosphorites with lower Er/Luratios may have originated from late weathering rather than the rare earth exchange of early diagenesis,while those of Meishucun phosphorites with higher Er/Luratios may have originated from the rare earth exchange of early diagenesis.The Er/Luratios of Zhijin phosphorites were 1.26–2.59,with an average of 1.99 and a median of 1.97(Fig.11A),which are values similar to those of Meishucun phosphorites,thus indicating that REYs of Zhijin phosphorites originated from the rare earth exchange of early diagenesis,and the inf luence of post-diagenetic reworking was very small.

7.1.2 Identif ication of the major element parameters

Identif ication of the major element parameters principally involved MgO(%),CaO/PO,and(CaO+MgO)/PO,which are indicators that can be used to distinguish reworked phosphorites from original phosphorites.Ge(1994)argues that MgO(%)values less than 1.46,CaO/POvalues less than 1.5,and(CaO+MgO)/POvalues less than 1.54 are the proxies of weathered phosphorites;otherwise,the data are indicators of original phosphorites.The values of MgO(%),CaO/PO,and(CaO+MgO)/POin Zhijin phosphorites indicated that the phosphorites have original properties(Fig.11B,C,D).

Fig.11 The scatter diagrams of ErN/LuN vs.∑REY(A),MgO(%)vs.∑REY(B),CaO/P2O5vs.∑REY(C)and(CaO+MgO)/P2O5vs.∑REY(D).The upper limit of weathering according to Ge(1994)

In general,Zhijin phosphorites have the signature of original phosphorites in terms of either the parameters of REYs or the parameters of major elements.Additionally,the inf luence of post-diagenetic reworking on the distribution patterns of REYs is very small.Therefore,the conclusion can be reached that the REYs in the Zhijin phosphorites came from original sources.

7.2 Y sources

7.2.1 Effect of seaf loor hydrothermal activity Eu anomalies.

Positive Eu anomalies can be indicative of a hydrothermal source for REYs and are usually used as indicators for a reductive seaf loor hydrothermal supply(Douville et al.1999;Yi et al.1995).The REYs distribution pattern of the modern Mid-Ocean Ridge is typical of positive Eu anomalies(Fig.13),but its REYs content is low,with REYs content values within the range of 470.06–1630.21 uppm(median of 1019.79 uppm,mean of 1040.02 uppm)and Y content values within the range of 55.84–158.70 uppm(median of 97.09 uppm,mean of 103.88 uppm)(Table 8,data are from Bau and Dulski 1999).Despite the low content of Y,a steady f lux of seaf loor hydrothermal f luids directly provided a source for Y in the Mid-Ocean Ridge.

The Eu anomalies of Zhijin phosphorite indicate that there was no direct supply source of reductive submarine hydrothermal f luids for Y.The values ofδEu in the Early Cambrian Zhijin phosphorites were between 0.03–1.25,but usually,around 1.0,as shown in Table 1 and Fig.12A.Since there generally was no Eu anomaly in the Early Cambrian Zhijin phosphorites,and there was no correlation betweenδEu and Y(Table 1 and Fig.12A),it can be inferred that there was no direct supply of reductive seaf loor hydrothermal f luids to Y.

Table 8 REY values of Mid-Ocean Ridges(data from Bau and Dulski 1999)

Representative

elements

of seaf loor

hydrothermal activity and ratio of seaf loor hydrothermal elements.

The element contents of Ag,As,and Sb and the element ratio of Al/(Al+Fe+Mn)can be used as indicators for seaf loor hydrothermal deposition(Hekinian and Fouquet 1985;Marchig et al.1982;Adachi et al.1986).The Ag and As contents of seaf loor hydrothermal sediments in modern Pacif ic Mid-Ocean Ridge areas are high,with an Ag content that ranges from 5 to 186 ppm(average of 37 ppm)and As content that ranges from 45 to 1253 ppm(average of 252 ppm)(Hekinian and Fouquet 1985).Marchig et al.(1982)suggested that the contents of As and Sb can be used to distinguish seaf loor hydrothermal sedimentary activity from normal sedimentary activity.The contents of As and Sb for seaf loor hydrothermal sedimentary are higher,and the average values of As and Sb contents are more than 100 ppm and 7 ppm,respectively.In contrast,those of normal deposition is lower,and the average values of As and Sb contents are 10 ppm and 2–3 ppm,respectively(Marchig et al.1982).An Al/(Al+Fe+Mn)ratio,as a percentage of mass,can also be used as a proxy of seaf loor hydrothermal sediments.A larger Al/(Al+Fe+Mn)ratio is indicative of fewer seaf loor hydrothermal deposits(Adachi et al.1986).

Representative elements of the hydrothermal activity of Zhijin phosphorites indicated that submarine hydrothermal f luids did not directly supply the Y source.For the Early Cambrian Zhijin phosphorites,the Ag contents(ppm)changed from 0.052 to 4.25 with a median of 0.707 and an average of 0.884,the As contents(ppm)changed from 1.89 to 247 with a median of 7.30 and an average of 12.89,the Sb contents(ppm)changed from 0.92 to 115 with a median of 5.78 and an average of 8.70,the Al/(Al+Fe+Mn)ratios changed from 0.08 to 0.72 with a median of 0.30 and an average of 0.32.This indicates that,when taking into account either the contents of representative elements Ag,As,and Sb of seaf loor hydrothermal activity or the ratio Al/(Al+Fe+Mn)of seaf loor hydrothermal elements,the Early Cambrian Zhijin phosphorite displays normal deposition characteristics rather than seaf loor hydrothermal deposition characteristics.As far as both the contents of seaf loor hydrothermal elements and the ratios of seaf loor hydrothermal element are concerned,the values were not correlated with Y(Fig.12),which indicates that the seaf loor hydrothermal supply had little effect on the source supply of Early Cambrian Zhijin phosphorites,and the seaf loor hydrothermal f luids were not a direct supply source for Y.

Fig.12 Scattered plots of the relationship between Y andδEu,the contents and rations of seaf loor hydrothermal elements of Zhijin bearing-rare earthphosphorites.A Y vs.δEu;B Y vs.As;C Y vs.Sb;D Y vs.Ag;E Y vs.Al/(Al+Fe+Mn)

7.2.2 Oxidative seawater as a primary source

The oxidation background is important for understanding the source of the metallogenic material.The abundance of small-shell fossils in the Early Cambrian Zhijin phosphorites is ref lective of the oxidation background of the oceans at that time.Zhang and Cui(2016)proposed that in each historical geological period,there was a minimum demand for oxygen to maintain the survival of organisms.During the Ediacara–Cambrian transition,the anoxic ocean shrank and oxygen levels continued to rise,and the oxygen level,which maintained biodiversity,was at least 25%of the present atmospheric level(Zhang and Cui 2016).The oxidation background not only was important for oxygen and nutrient inputs(such as phosphorus enrichment)but also brought about an important material basis for diagenetic mineralization at the same time when the Cambrian biological explosion was triggered.

The characteristic REYs data of oxidized seawater,terrestrial phosphorites,and Mid-Ocean Ridge and Zhijin phosphorites are shown in Table 9.Among them,the sample ZKX001-648 is Zhijin phosphorite,which is negative Ce anomaly and positive Y anomaly,no Eu anomaly or weak positive Eu anomaly(data from this study).M79/11 is an oxidized seawater sample(data from McArthur and Walsh 1984),which has an obvious negative Ce anomaly.NY-BC-SIM3 is a terrigenous source sample(data from McArthur and Walsh 1984),which has the characteristics of MREEs enrichment and HREEs depletion.And HT-5 is a Mid-Ocean Ridge sample(data from Bau and Dulski 1999),which is typical of positive Eu anomaly.

According to Table 9,we plotted the charts of REYs distribution patterns of oxidized seawater,terrestrial phosphorites,and Mid-Ocean Ridge and Zhijin phosphorites(Fig.13).As shown in Fig.13,the typical rare earth distribution pattern of oxidized seawater showed a negative Ce anomaly,no obvious Eu anomaly,and LREEs depletion relative to HREEs(curve M79/11 in Fig.13,data according to McArthur and Walsh 1984).Meanwhile,some terrestrial phosphorites(curve NY-BC-SIM3 in Fig.13,data according to McArthur and Walsh 1984)and Mid-Ocean Ridge phosphorites(curve HT-5 in Fig.13,data according to Bau and Dulski 1999)showed distribution patterns of rare earth with HREEs depletion.

Table 9 REY characteristic data of oxidized seawater,continental phosphorite,Mid-Ocean Ridge and Zhijin phosphorite(ppm)

Fig.13 Comparison of patterns of rare earth distribution.The sample ZKX001-648 is Zhijin phosphorite and its data comes from this study.The M79/11 belongs to the oxidized seawater REY model,NY-BCSIM3 belongs to the land-based REY model,data from the McARTHUR and WALSH(1984).HT-5 mid-ocean ridge REY model,data from the Bau and Dulski(1999)

The REYs distribution pattern of Zhijin phosphorites indicated that the Y may have originated from seawater as well as from the addition of terrestrial materials.The REYs distribution patterns in Early Cambrian Zhijin phosphorites were characterized by a negative Ce anomaly,no Eu anomaly,positive Y anomaly,MREEs enrichment,and HREEs depletion(curve ZKX001-648 in Fig.13).Compared to the REYs distribution pattern of the seawater-type,terrestrial materials-type,and Mid-Ocean Ridge-type,the REYs distribution pattern of Early Cambrian Zhijin phosphorite was similar to the characteristics of oxidative seawater in the La-Er section,but the Tm-Lu segment was similar to the characteristics of terrestrial materials(Fig.13).Because Zhijin phosphorite is of an original character and had no seaf loor hydrothermal supply,the REYs may have come principally from seawater,but there may have been the superimposition of terrestrial sources.That is,the source was possibly a mixture of seawater and terrestrial sources.

7.2.3 Terrestrial source

The Y/Ho ratios of Early Cambrian Zhijin phosphorites also demonstrate that the REYs may have come from a mixture of seawater and terrestrial sources.Y/Ho is an important indicator for distinguishing terrigenous and seawater origins(Abedini and Calagari 2017;Bau et al.1997).The Y/Ho values of 28 and 60 are REYs features of terrigenous and seawater origins,respectively(Abedini and Calagari 2017;Bau et al.1997;Bau and Dulski 1999;Nozaki et al.1997).The Y/Ho values of the Zhijin phosphorites ranged from 72.18 to 15.82,thus spanning the scopes of terrestrial sources and seawater characteristics.The Y/Ho(median)varied regularly as the total REYs content varied regularly from low to high.That is,when the total REYs content was less than 535 ppm,the median Y/Ho value was 43.60 and the Y/Ho values converged,whereas when the total REYs content was more than 535 ppm,the median Y/Ho value was 30.41 and the Y/Ho values were diffuse.The trend of Y/Ho changes with the total REYs content showed that as the total REYs content increased from low to high,the Y/Ho ratio gradually decreased from the values approximate to the seawater toward the values approximate to the terrigenous source(Fig.14A).This indicates that when the total REYs content was low,the sedimentary environment was similar to the ocean,the source for REYs was approximate to seawater;however,when the total REYs content was high,the sedimentary environment was similar to the terrestrial environment,and the source for REYs elements was extensive but approximate generally to a terrestrial origin.

Fig.14 Scattered plots of Y/Ho,Er/Nd vs.∑REY in Zhijin phosphorites.A Y/Ho vs.∑REY.B Er/Nd vs.∑REY.Reference data for seawater,land and debris come from Abedini and Calagari(2017)

The Er/Nd values of Early Cambrian Zhijin phosphorites indicated that the origin for terrestrial clastic material may have played an important role in Y enrichment when the REYs content was high.Mao et al.(2015)believed that the Zhijin phosphorite is generally controlled by the surface of unconformity of old karst formed by ancient weathering.Ancient weathering not only resulted in the base of the phosphorite bed but also played a role in the deposition process of phosphorite that may have provided the origins for Y enrichment.The Er/Nd ratio of normal seawater is approximately 0.27,and the detritus can make the Nd preferentially concentrated relative to Er,thus reducing the Er/Nd value to less than 0.11(Abedini and Calagari 2017).The Er/Nd values of Zhijin phosphorites ranged from 0.087 to 0.14(median of 0.11).When the total REYs content was less than 286 ppm,the Er/Nd values ranged from 0.089 to 0.146(median of 0.125).When the total REYs content changed from 286 to 535 ppm,the Er/Nd values ranged from 0.096 to 0.141(median of 0.112).When the total REYs content changed from 535 to 771 ppm,the Er/Nd values ranged from 0.096 to 0.128(median of 0.110).When the total REYs content was more than 771 ppm,the Er/Nd values ranged from 0.087 to 0.128(median of 0.103)(Fig.15).The ratios of Er/Nd conf irmed the terrestrial detrital origin for REYs.Usually,the correlation between Er/Nd and Y was weak,but when the total REYs content was more than 771 ppm,Er/Nd and Y showed a stronger correlation(R=-0.545)(Fig.15D),thus indicating that the origin for terrestrial clastic material may have played an important role in the enrichment of terrestrial materials when the REYs content was high.

Fig.15 Scatter plots of Y vs.Er/Nd in Zhijin phosphorites with different total REY content

7.2.4 Offshore distance and water depth

Tian and Zhang(2016)believe that Mn/Ti and MnOconcentrations can represent the offshore distance and water depth,respectively.Specif ically,it has been suggested that offshore distances are associated with Mn/Ti values,and seawater depths are associated with MnOconcentrations(Tian and Zhang 2016).The Mn/Ti of 100 m offshore is equal to 0.1.From lakeside to deep water,the MnOconcentration will increase from 0.00094%to 0.051%.The Mn/Ti values of the Zhijin phosphorite samples were between 0.0527–34.76(median of 5.983),which denotes a median offshore distance of approximately 6 km.MnOconcentrations of the Zhijin phosphorite samples were greater than 0.01(median of 0.1100 and average of 0.1453),which suggests that the overall environment belonged to a deep-water body.The values of Mn/Ti and MnOwere negatively correlated with Y(Fig.16),which suggests that the greater the offshore distance and/or the deeper the seawater,the less Y enrichment occurred.In contrast,the smaller the offshore distance and/or the shallower the seawater,the more Y enrichment occurred.Additionally,smaller offshore distances and shallower seawater were more favorable for the transport of terrigenous materials involving Y into marine sediments,which is suggestive of the contribution of terrestrial detrital f lux to Y.This also conf irms the foregoing conclusions that some Y came from the source of terrigenous detrital f lux as discussed with the Er/Nd results.

Fig.16 Scatter plots of Y vs.Mn/Ti,Y vs.MnO2 in Zhijin phosphorites

The free property of Y allows it to enrich easily during the process of phosphorite formation.As far as the deposition process is concerned,REYs can be absorbed from either seawater or occluded in associated biological fragments and/or other clastic f luxes(McArthur and Walsh 1984)followed by reactivation into pore water during the formation of phosphorites.However,REYs must go through the processes of weathering,activation,transport,and placement before the accumulation and mineralization from the parent rock into the phosphorites.The addition of a pre-diagenesis terrigenous detrital f lux is a major geological driving factor in the sedimentary diagenesis process.McArthur and Walsh (1984) proposed that weathering will give priority to the removal of LREEs.Due to the preferential removal of LREEs,weathering may result in HREEs enrichment and LREEs depletion in debris in the parent rock area.In contrast,for the material components transported to seawater,the situation is reversed,namely,LREEs enrichment and HREEs depletion.The properties of Y are special;however,because of the different removal effects in different environments of the solution,Y sometimes behaves similarly to HREEs and sometimes similar to LREEs(Borkowski and Siekierski 1992;Quinn et al.2004).The properties of Y made it possible to approximate the nature of LREEs in the Early Cambrian Zhijin phosphorite,and Y shows even more covalent tendencies than LREEs.As a consequence,Y was highly enriched and deposited,and it became the richest rare element in this area.Consequently,the source for Y may have been from a series of original geological processes such as weathering,transport,placement,and so forth,but after sedimentary diagenesis,the enrichment of Y was rarely affected by post-diagenetic reworking.

In general,REYs in the Zhijin phosphorites showed an original signature,no Eu anomaly,no correlation between the hydrothermal elements and the ratios of hydrothermal elements with Y.Additionally,the paleogeography of Zhijin phosphorites was on the continental shelf,far from the Mid-Ocean Ridge,and thus,the seaf loor hydrothermal activity did not directly provide the source of Y.Studies have revealed that the modern ocean f loor is rich in rare earth especially Y(Takaya et al.2018).Up-swelling currents transport rare earths and Y from the bottom of the ocean to the shallow sea(Hein et al.2004),which results in the direct origin of some rare earth elements from seawater.Y in deep-sea mud exists principally in the form of phosphate and appears completely in apatite minerals,and the occurrence state of Y is similar to that in terrestrial phosphorites(Zhang et al.2018).Through the investigations of Y/Ho ratios,Er/Nd ratios,and so on,it is proposed that the enrichment of Y exists because of the addition of a terrigenous source.Hence,the source for Y was the result of a mixture of seawater and terrestrial sources.

8 Conclusions

From the above discussion,the following conclusions were drawn:

(1) According to the statistical analysis of the sample data,the Y in Zhijin rare earth-bearing phosphorite has normal distribution characteristics.

(2) In Zhijin rare earth-bearing phosphorite,Y is especially enriched relative to other REYs elements.But in different∑REY intervals,the enrichment degree of Y is different relative to its neighbor elements.When the value of∑REY is smaller,the enrichment degree of Y is larger relative to its neighbor elements Dy and Ho;when the value of∑REY is higher,the enrichment degree of Y is smaller relative to its neighbor elements Dy and Ho.

(3) The analysis of the parameters of major and trace elements suggests that Zhijin rare earth-bearing phosphorite has the signature of primary phosphorite.

(4) The results of the clustering and factor analysis show that,for the phosphorite with a low rare earth content(＜535 ppm),there is a good correlation between REEs,Y,and PO.For the phosphorite with a high rare earth content(≥535 ppm),there is also a good correlation between REEs and PO,but there is no correlation between Y and PO.The REYs enrichment in the Zhijin rare earth-bearing phosphorite revealed that superposition enrichment may occur for Y.The inconsistent enrichment processes of REYs may be correlated with more complex sources of Y.

(5) The data analysis of the Eu anomalies,representative elements of submarine hydrothermal activity and ratios of hydrothermal elements,REYs distribution patterns,and values of Y/Ho,Er/Nd,Mn/Ti,and MnOshow that submarine hydrothermal activity did not directly provide the source for Y,Y came from a mixture of seawater and terrestrial sources.

Acknowledgement

s We thank Prof.Jianzhong Liu for his helpful comments.This work is jointly supported by the National Natural Science Foundation of China(No.U1812402)and the Public Benef icial and Basic Geological Project from the Department of Land and Resources of Guizhou Province(No.2016-09-1).