Geochemical Characterization of Magnetite and Geological Setting of the Iron Oxide .+-. Iron Silicate .+-. Iron Carbonate Rich Pb-Zn Sulfides from the La Union and Mazarron Stratabound Deposits (SE Spain)

From a mineralogical point of view the La Union ore field ( SE Spain) can be regarded as an oddity as some of its hydrothermal, stratabound type deposits display an extremely unusual paragenesis comprising magnetite +/- greenalite +/- minnesotaite +/- siderite and galena +/- sphalerite (IOSC - LZS). Recent mineralogical studies have shown that this paragenesis is also present at the neighboring Mazarron ore field. These ore fields share a similar geologic setting, involving metamorphic and sedimentary rocks (Paleozoic s. l. to Permian) hosting late Miocene high-K calc-alkaline volcanic and subvolcanic rocks. The latter have andesitic to dacitic composition, and triggered hydrothermal activity and ore deposition. This study discusses the detailed mineral chemistry of magnetite samples from Mazarron and La Union and provides some hints for the origin of the IOSC - LZS paragenesis. We performed electron microprobe (EPMA) analyses in magnetite samples from La Union and Mazarron to determine the contents of minor and trace elements (Zn, Ni, Mn, Cr, V, K, Ca, Ti, Al, Si, Mg). Given that some results fell below the detection limit for the EPMA instrumental conditions we used robust regression on order statistics (robust ROS), with the NADA package in R to deal with these data sets. The Ca + Al + Mn contents in magnetites from San Cristobal and Emilia are equivalent to the mean contents of those of IOCG, Kiruna, BIF, Cu porphyry, skarn, VMS, hydrothermal and clastic Pb- Zn deposits, but they are low in Ti + V and Ni + Cr. The principal components analysis indicates that Zn, Ni, Cr, V, K, Ca, Ti, and Mg are roughly collinear, therefore correlated, being also independent from Fe. Besides, the function discriminant analysis of data shows that the magnetites from Emilia and San Cristobal cluster in differentiated groups, thus probably reflecting some differences related to the distance to the magmatic source that triggered the hydrothermal system: proximal in the case of San Cristobal and distal in Emilia.