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Major, Minor and Trace Element Composition of Pyromorphite-Group Minerals As Recorder of Supergene Weathering Processes From

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Major, Minor and Trace Element Composition of Pyromorphite-Group Minerals As Recorder of Supergene Weathering Processes From 1 Major, minor and trace element composition of pyromorphite-group minerals as 2 recorder of supergene weathering processes from the Schwarzwald mining 3 district, SW Germany 4 5 6 REVISION 1 7 8 9 10 Gregor Markl*, Michael A. W. Marks, Johannes Holzäpfel, Thomas Wenzel 11 12 13 14 15 16 17 Mathematisch-Naturwissenschaftliche Fakultät, Fachbereich Geowissenschaften, 18 Universität Tübingen, Wilhelmstraße 56, D-72074 Tübingen, Germany. 19 20 *corresponding author: [email protected] 21 22 23 24 25 26 27 Keywords: pyromorphite, mimetite, vanadinite, fluorophosphohedyphane, hedyphane, 28 trace elements, solid solution, “hydroxylmimetite” 29 30 31 Abstract 32 33 More than 150 samples of pyromorphite, mimetite, vanadinite and minerals of the 34 hedyphane-group (which collectively are summarized here under the term PyGM for 35 pyromorphite-group minerals) from the Schwarzwald mining district, SW Germany, 36 have been analyzed by electron microprobe and LA-ICP-MS. In this largest study of its 37 kind, the relations of PyGM composition to host rock and fluid compositions and the 38 amount of solid solution between the various endmembers were investigated. In 39 addition, we report the colors of the many analyzed mineral compositions. The most 40 important results are: 41 - pyromorphite and mimetite are completely miscible; 42 - at conditions of the oxidation zone in ore deposits, the solvus between pyromorphite 43 and vanadinite appears to asymmetrical with up to about 3 mol% of vanadinite 44 component in pyromorphite and up to about 39 mol% of pyromorphite component in 45 vanadinite in cases where both minerals coexist; 46 - due to a lack of suitable samples, the solvus between vanadinite and mimetite could 47 not be completely constrained, but we report vanadinite analyses with up to 12 mol% 48 mimetite and 8 mol% pyromorphite component; 49 - there is complete miscibility between pyromorphite and phosphohedyphane and 50 between mimetite and hedyphane; 51 - F-rich varieties appear only to exist in hedyphanes and phosphohedyphanes, while 52 pyromorphites, mimetites and vanadinites are Cl- or OH-dominated; 53 - we report for the first time analyses suggesting the occurrence of an OH-endmember 54 corresponding to mimetite; 55 - while pyromorphites are preferably green and mimetites yellow, this is not at all a 56 diagnostic feature, as many exceptions exist; major elements are not correlated at all 57 with the color of a specific crystal; 58 - most PyGM are strongly zoned and display e. g. significant variations in Ca which are 59 likely to be related to variations of the compositions of various fluid pulses from which 60 the crystals formed; 61 - PyGM composition is generally uncorrelated with host rock composition, but PyGM 62 enrich other metals like REE, Cr, Sb, Bi or U up to a factor of 106; therefore, they can be 63 regarded as very sensitive recorders of the metal inventory of an oxidation zone and 64 they even record metals only present as traces in the primary ore deposit very reliably; 65 - REE patterns of PyGM show significant variability even at one location; this may 66 suggest that each zone of a PyGM crystal records the REE pattern of a single fluid pulse 67 or it may indicate fractionation of the REE during PyGM growth; in the absence of 68 conclusive data, the former possibility appears the more likely one; 69 - PyGM are extremely efficient filters for heavy metals from supergene solutions in and 70 in the vicinity of ore deposits. 71 72 Introduction 73 Pyromorphite-group minerals (PyGM) commonly form during supergene weathering of 74 Pb-bearing ore deposits. The website mindat.org currently lists more than 3000 75 localities worldwide, with either pyromorphite, mimetite or vanadinite, the respective 76 phosphate, arsenate and vanadate endmembers of the general composition Pb5X3Cl 3- 3- 3- 77 occur (X=PO4 , AsO4 and VO4 ), plus some dozens of localities for hedyphane-group 78 minerals (hedyphane, phosphohedyphane and fluorphosphohedyphane) which have the 3- 3- 79 composition Ca2Pb3X3(Cl, OH, F) with X being either PO4 or AsO4 (Pasero et al., 2010). 80 Hence, these minerals are ubiquitous and potentially sensitive indicators for processes 81 occurring during oxidation and weathering of ore deposits. 82 The extent of solid solution between pyromorphite, mimetite and vanadinite has 83 been a long-lasting controversy. Based on the analysis of natural samples, the existence 84 of complete solubility between pyromorphite and mimetite in nature (some of these 85 intermediate, As-rich members even had their own variety name "campylite"; Palache et 86 al., 1951) has long been known (e.g., Denen, 1959; Wondratschek, 1963; Förtsch and 87 Wondratschek, 1965; Cockbain, 1968). The variety "endlichite" (= arsenian vanadinite), 88 describes approximately 1:1 solid solutions of vanadate and arsenate endmembers 89 (Palache et al., 1951). 90 In the crystal structure of the pyromorphite group minerals two different M sites 91 (two M1 and three M2 sites) with slightly different coordination (9-fold and 7-9-fold, 92 respectively) exist, which plays a role for the hedyphane-group described below (Pasero 93 et al., 2010). The X site is tetrahedrally coordinated. As Pb-O bond lengths do not change 94 much with tetrahedral substitution, Dai and Hughes (1989) inferred that incomplete V-P 95 solid solution at ambient temperature is a consequence of the size difference between V 96 and P rather than a consequence of the differential distortion of the tetrahedra. Baker 97 (1966) demonstrated the existence of a complete solid solution between pyromorphite, 98 mimetite and vanadinite at 60-80°C based on an X-ray powder diffraction study on 99 twenty-five synthetic compounds grown in this temperature range and recent 100 experimental work showed that complete miscibility along the pyromorphite-vanadinite 101 and the pyromorphite-“fluorpyromorphite” joins exists at high temperatures 102 (Chernorukov et al., 2010; Knyazev et al., 2011). To our knowledge, however, no 103 intermediate compositions between phosphate and vanadate endmembers have been 104 reported from natural samples up to now. Baker (1966) suggested that this simply 105 reflects the availability of the respective anions during mineral formation: phosphate 106 and arsenate are relatively common in the oxidized zone of lead deposits and a wide 107 range on phosphate/arsenate ratios are therefore to be expected. According to Baker 108 (1966), however, vanadium is less commonly associated with Pb mineralizations, 109 although the extensive vanadinite and endlichite occurrences in Morocco, Mexico and 110 Arizona argue against this statement. In addition, in the present contribution we report 111 data showing up to 39 % pyromorphite component in natural vanadinite samples. 112 Members of the hedyphane-group (hedyphane, phosphohedyphane and 113 fluorphosphohedyphane) are characterized by high Ca contents and crystal-chemical 114 studies showed that Ca and Pb are ordered at the two different sites M1 and M2 (e.g., 115 Rouse et al., 1984). Based on the analysis of natural samples, Kempf et al. (2006) showed 116 that Ca and Pb contents show considerable variation but do not exceed the ideal ratio of 117 2:3. These authors suggested a complete solid solution series among the endmembers 118 pyromorphite-phosphohedyphane and mimetite-hedyphane but the absence of a solid 119 solution series between the joins phosphohedyphane-hedyphane and chlorapatite- 120 turneaureite (Ca5(AsO4)3Cl) in natural systems. 121 In principle, the crystallographic site typically occupied by Cl should also 122 incorporate OH and F. However, a naturally existing F-dominant analogue has been only 123 described (and IMA-approved) for phosphohedyphane (Kempf and Housley, 2011; 124 Pasero et al., 2010). OH-endmembers are not defined to date (Pasero et al., 2010), but 125 Dunn et al. (1985) described OH-bearing hedyphane, while OH-dominated 126 phosphohedyphane is indicated by data from Stalder and Rozendaal (2002). In the 127 present contribution, we report analyses supporting the existence of an OH-endmember 128 of mimetite. 129 Pyromorphite, mimetite and vanadinite have very low solubility products (log K 130 values of around -75 to -86; Nriagu, 1973; Flis et al., 2007; Bajda 2010; Gerke et al., 131 2009). For that reason, the use of pyromorphite for treatment and remediation of Pb- 132 contaminated sites by adding phosphate to the soil was suggested (e.g., Ruby et al., 133 1994; Stanforth and Qui, 2001; Eusden et al., 2002; Basta and McGowen, 2004). 134 The study of Burmann et al. (2013) used the oxygen isotope composition of 135 phosphate in pyromorphites from some of the localities also used in the present study to 136 decipher the source of the phosphate. The authors came to the conclusion that most of 137 the phosphate has been biologically modified and that no purely inorganic phosphate (e. 138 g. from apatite dissolution in the host rocks of ore deposits) can be identified in 139 pyromorphites. 140 The present contribution presents a comprehensive dataset of color and 141 geochemical composition from more than 150 samples of PyGM from hydrothermal 142 veins in a large continental mining district, the Schwarzwald (SW Germany). We use 143 major, minor and trace element compositions (including rare earth elements; REE) to 144 detect correlations between host rock composition, primary mineral association of a 145 given hydrothermal vein and fluid source with PyGM geochemistry in order to explore 146 the potential of this ubiquitous mineral group as recorder for fluid-driven processes 147 during weathering of ore deposits. 148 149 Samples and geological background 150 The Schwarzwald in Southern Germany is a low mountain range about 50 by 100 km 151 large (Fig. 1). It hosts about 1000 hydrothermal veins, which have been mined since 152 Neolithic and Roman times (Metz et al., 1957; Bliedtner & Martin, 1986); only one mine 153 (the Clara mine near Wolfach) is still exploiting barite and fluorite.
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