Origin of the Moroccan Touissit-Bou Beker and Jbel Bou Dahar Supergene Non-Sulfide Biomineralization and its Relevance to Microbiological Activity, Late Miocene Uplift and Climate Changes

Through integration of Pb-Zn ± Cu non-sulfide mineralogy, texture, and stable isotope (C, O, S) geochemistry, the world-class Touissit- Bou Beker and Jbel Bou Dahar Mississippi Valley-type districts of the Moroccan Atlasic system have been investigated in order to gain insights into the origin and processes that contributed to the formation of the base metal non-sulfide mineralization. In both districts, direct replacement (“red calamine”) and wallrock replacement (“white calamine”) ores are observed. Based on the mineral assemblages, ore textures, and crosscutting relations, three distinct mineralizing stages are recognized. The earliest, pre-non-sulfide gossanous stage was a prerequisite for the following supergene stages and constituted the driving force that ultimately promoted the leaching of most base metals such as Zn and Cu and alkalis from their rock sources. The following two stages, referred to as the main supergene “red calamine” and late “white calamine” ore stages, generated the bulk of mineable “calamine” ores in the Touissit-Bou Beker and Jbel Bou Dahar districts. Stable isotope compositions (d13CV-PDB, d18OV-SMOW, d34SCDT) support a three-stage model whereby metals were released by supergene acidic fluids and then precipitated by bacteria and archaea-mediated metal-rich meteoric fluids due to a decrease in temperature and/or increase of fO2. Oxygen isotope thermometry indicates decreasing precipitation temperatures with advancing paragenetic sequence from 33° to 18 °C, with wet to semi-arid to arid climatic conditions. The close spatial relationships between coexisting sulfide and non-sulfide mineralization along with stable isotope constraints suggest that the oxidation of sulfides occurred concurrently after the main stage of the Alpine orogeny between 15 Ma and the present. More importantly, the current data show for the first time the involvement of biologically controlled activity as the major driving process that triggered both oxidation and deposition of supergene mineralization at Jbel Bou Dahar and Touissit-Bou Beker districts. Conclusions drawn from this study therefore have implications for supergene Mississippi Valley-type (MVT) -derived non-sulfide deposits worldwide and account for the prominent role of biological processes in the genesis of this category of ore deposits.

Radiogenic Pb Enrichment of Mississippi Valley-Type Metallic Ore Deposits, Southern Ozarks: Constraints Based on Geochemical Studies of Source Rocks and Their Diagenetic History

Southern Ozark Mississippi Valley-type ores are enriched in radiogenic Pb, with isotopic signatures suggesting that metals were supplied by two end-member components. While the less radiogenic component appears to be derived from various shale and sandstone units, the source of the more radiogenic component has not yet been identified. Analyses of cherts from the Early Ordovician Cotter Dolomite and tripolitic chert from the Early Mississippian Boone Formation contain highly radiogenic Pb, with isotopic ratios comparable to those of ores. However, most samples have lower 208Pb/204Pb and 207Pb/204Pb for a given 206Pb/204Pb compared to ores. These relationships demonstrate that the enriched Pb isotopic values of the ore array cannot be related to the host and regional lithologies sampled, suggesting that the source of high ratios may lay further afield. The slope of the linear trend defined by the Pb isotope ratios of ores corresponds to an age of about 1.19 Ga. Therefore, an alternative for the linear array is the involvement of Precambrian basement in supplying ore Pb. Rare earth element patterns show that diagenetic processes involving the action of groundwater and hydrothermal fluids affected the sampled lithologies to various degrees, with Cotter Dolomite having experienced the highest degree of alteration.

Zn-Pb deposits as a clue for recognising reactivated structures in the Picos de Europa area (NW Spain)

<p>The Picos de Europa Region constitutes one of the outermost areas of the Cantabrian Zone, the foreland and thrust belt of the Variscan orogen in NW Iberia. It constitutes a thrust imbricate formed of Carboniferous limestones that was emplaced towards the S-SW during the latest Pennsylvanian. During the Permian and throughout the Mesozoic, the area was subjected to extension, as attested by the scarce remnants of contemporary sedimentary successions. During the N-S Cenozoic Alpine convergence between Iberia and Eurasia, the Picos the Europa Massif was deformed under shallow crustal conditions through the reactivation of previous structures.</p><p>Zn-Pb ores, in the form of sphalerite and galena, are abundant in the central and eastern sections of the Picos de Europa Massif, where they formed as Mississippi Valley-type deposits. Although a direct dating of the minerals has not been performed to date, indirect attempts have been made based on field observations and paleomagnetic studies that have resulted in a broad span of age estimations comprised between Permian and Cenozoic times. Our ongoing research includes the study of Pb isotopes within galena samples in several localities in the Picos de Europa. The measured Pb isotopic ratios (206Pb/204Pb = 18.604–18.771, and 207Pb/204Pb = 15.686–15.707) are comparable to those of other Mississippi-Valley-type and Sedex-type ore deposits situated further east in the Basque-Cantabrian Basin. This basin was formed throughout the Mesozoic as an extensional basin, and the associated ores have been dated through ore-typology (syn-sedimentary Sedex-type deposits), metallogenic data, and other geological criteria. The similarity of the isotopic ratios in these deposits and our samples from the Picos de Europa Massif suggests a similar ore formation age, around the Lower Cretaceous, based on the interpretation of a comparable Pb crustal source.</p><p>The ores from Picos de Europa are largely associated with kilometre-scale faults that have acted simultaneously as fluid conduits and zones of preferential mineralisation. Many of the studied localities display significant deformation of the ore deposits, suggesting subsequent fault reactivation events following precipitation. Thus, the age of the deposits is useful for determining the relative timing of fault reactivation. There are two main mineralised fault systems: faults trending W-E with a variable dip, and a subvertical NW-SE-trending set. Faults from the first system were originally developed as Variscan thrusts and in some cases reactivated as normal and/or, subsequently, reverse faults during the Alpine orogenic cycle (e.g. the Cabuérniga Fault System). In contrast, the age and kinematics of the second fault system are more debated. Zn-Pb deposits from the Ándara and Liordes mining districts constitute illustrative examples of ore precipitation and subsequent brittle deformation along the San Carlos N118E-trending subvertical fault and the Liordes N117E-trending high-angle fault. While the San Carlos Fault accommodated an oblique but mainly dextral strike-slip displacement during ore deformation, the Liordes Fault acted as a dextral oblique fault with a larger reverse component, likely as a result of its slightly different dip angle. The last activity on these structures post-dates the Lower Cretaceous, suggesting a clear linkage with the Alpine orogeny.</p>