Ebook: Metallogeny and Petrogenesis of Lamprophyres in the Mid-European Variscides
Based on a worldwide database of lamprophyres, N.M.S. Rock (1991) noted that “... Lamprophyres are a missing element in the traditional ‘granites + mineralization’ maxim which should no longer be ignored ...”.The Erzgebirge or “Ore Mountains” province is a key locality in the European Variscides to observe the important relationships between granites, lamprophyres, and a spectacular array of spatially overlapping mineralization types. Lamprophyre dikes and associated magmatic rocks were systematically mapped in the mining fields of the Erzgebirge and surrounding areas. Data are presented that unarguably demonstrate the important role played by lamprophyric rocks in the Erzgebirge ore province. Three intrusive stages of lamprophyres indicate significant mantle-associated magmatic pulses during the Permo-Carboniferous. Sn-W-Mo, Ag-base metal, and U mineralization events show a close spatial association to post-collisional lamprophyric and small granitic-rhyolitic intrusions which post-date widespread late-collisional granites. The metallogenetic importance of volatile-rich lamprophyres in the Erzgebirge is further affirmed by the overlapping temporal relationship between lamprophyres and post-collisional granitic-rhyolitic intrusions, as determined by modern radiometric dating and age data of lithostratigraphic unites of the Sub-Erzgebirge basin. The model presented in this book is important for exploration for Sn, W, Mo, Ag, Cu, Zn, Pb, In, and U mineralization in the Bohemian massif and comparable ore deposit provinces worldwide.
The main goal of this contribution is to document the possible importance of post-collisional lamprophyre intrusions for late-Variscan ore mineralization in the Erzgebirge (Germany and Czech Republic). Located at the northwestern border of the Bohemian massif in the Internal Variscides, the Erzgebirge is one of the most important silver, tin, tungsten, uranium, lead, zinc, copper, indium, bismuth, cobalt, and fluorite ore deposit provinces in Europe with a mining history of more than 800 years. Because of the close spatial association between late-Variscan ore mineralization and Permo-Carboniferous lamprophyric and rhyolitic intrusions the Erzgebirge is one of the key localities to study the genetic relationship of epigenetic Sn-W-Mo, Ag-base metal, and U deposits with post-collisional, mantle derived magmatic pulses.
Lamprophyres and granitic/rhyolitic rocks from various ore districts of the Erzgebirge were collected from underground and surface outcrops, and drill cores. Shoshonitic/ultrapotassic mafic volcanics were collected in surface outcrops of the Sub-Erzgebirge basin. Data were obtained for petrographic-mineralogical studies (n = 117), major, trace and rare earth element analyses (n = 115), Nd-Nd (n = 42), Sr-Sr (n = 42), and Pb-Pb isotope analyses (n = 65). Representative samples were dated using K/Ar (n = 8), Ar/Ar (n = 7), Pb-Pb single zircon evaporation (n = 4), and U-Pb SHRIMP methods (n = 2). Additionally, felsic (sub)volcanic rocks of the Erzgebirge and the Sub-Erzgebirge basin were analyzed by geochemical (n = 21), and radiogenic isotope methods (n = 17).
Most lamprophyric dikes (thickness from 0.5 - 30 m) occur in areas of crosscutting deep fault zones which also act as conduits for post-collisional rhyolitic intrusions and host postmagmatic Sn-W-Mo, Ag-base metal, and U deposits. The lamprophyres in the Erzgebirge have been divided into three main groups (LD1-LD3) using criteria of petrography and geochemistry, and relatively age relationships to late-Variscan volcano-plutonic activity and mineralization phases:
• LD1: late-collisional (pre-‘type Eibenstock’ granite age) transitional type kersantitic/spessartitic lamprophyres; they predate all epigenetic mineralizations in the Erzgebirge (or they are unrelated to polymetallic mineralization events in the Erzgebirge);
• LD2: post-collisional (post-‘type Eibenstock’ granite age) mica-lamprophyres, that predate Sn-polymetallic and Ag-base metal ore bodies;
• LD3: post-collisional feldspar-phyric kersantitic lamprophyres, that postdate Sn-polymetallic mineralizations and predate Ag-base metal ores.
The significant petrographical, geochemical, and structural similarities between some lamprophyres (especially subtypes LD2a and LD2c) as well as (sub)volcanic rhyolitic intrusions in the Erzgebirge and the Stephanian-Lower Rotliegend potassium-rich mafic and rhyolitic series in the Sub-Erzgebirge basin indicate a genetic link of the lamprophyres with bimodal magmatism in both areas with an age range from 315 to 290 Ma. Regional extension leading to lithospheric thinning, the development of shear zones, the formation of faultcontrolled basins, and decompression melting of updoming asthenosphere probably was an important factor for the Permo-Carboniferous bimodal magmatism in the Erzgebirge and Sub-Erzgebirge basin.
The high concentration of LILE (Cs, Rb, K, Ba, Sr) and HFSE (Sc, Zr, Hf, Nb, Ta, Ti, P, Th, Y, LREE) and the enriched Nd and Sr isotope composition of the lamprophyre intrusions and associated volcanics in the Erzgebirge indicate melting of a metasomatically enriched mantle. The higher abundances of LILE, HFSE and LREE of some lamprophyres (LD2-type) and shoshonitic/ultrapotassic volcanics in the Sub-Erzgebirge basin in comparison to the average contents of calc-alkaline lamprophyres worldwide (Rock, 1991) may be a function of smaller degrees of partial melting, derivation from a more extensively metasomatized mantle, or a function of different fluid compositions. High concentrations of LILE and HFSE, and the ‘primitive’ bulk geochemistry (high mg#, Cr, Ni, Co, and Sc contents) exclude significant crustal contamination as the cause of this enrichment in lamprophyres of the Erzgebirge and shoshonitic/ultrapotassic volcanics of the Sub-Erzgebirge basin.
High volatile contents (CO2 up to 7.3 wt.%, H2O+ up to 7.92 wt.%, F up to 4,600 ppm, Cl up to 1,700 ppm, S up to 3,160 ppm, P2O5 up to 1.33 wt.%) in whole rock samples and high H2O+, F, and Cl contents of early magmatic phlogopites and apatites are typical for postcollisional lamprophyres in the Erzgebirge (especially LD2) and shoshonitic/ultrapotassic mafic volcanics in the Sub-Erzgebirge basin. This, the frequent occurrence in ore deposit districts and the significant spatial and time relationship to the emplacement of post-collisional granitic/rhyolitic rocks and late-Variscan Sn-W-Mo, polymetallic Ag-base metal, and U mineralizations show the metallogenetic importance of post-collisional lamprophyres in the Erzgebirge. It seems therefore, that the emplacement of volatile-rich lamprophyric and rhyolitic intrusions in times of intracontinental rifting and related fast extension was an important factor for the ore deposition in the Erzgebirge.