Abstract

The polymetallic Madem Lakkos sulfide deposit in northern Greece is hosted within marble of the Mesozoic (?) Kerdylia Formation, a high-grade metamorphic complex composed of migmatitic biotite gneiss interlayered with marble, hornblende gneiss, and amphibolite. The Kerdylia Formation is invaded by a variety of foliated and nonfoliated intermediate to felsic intrusions of Tertiary age. The Madem Lakkos deposit is long-believed to have formed from a single epigenetic hydrothermal replacement event related to Tertiary magmatism, but this research has recognized the presence of three different and distinct ore types in the deposit that resulted from a much longer and more complex genetic history.Based on ore mineralogy, textures, and geochemistry, the Madem Lakkos ores can be characterized as (1) massive sulfide ore, (2) disseminated sulfide ore, and (3) skarn ore. The massive pyrite-sphalerite-galena ore exhibits abundant and well-developed metamorphic structures and textures that indicate the ore has been metamorphosed to upper amphibolite grade, at temperatures of at least 600 degrees C, together with its marble and gneissic host rocks. These textures include foliated-lineated galena and sphalerite, slip planes and deformation twinning in galena and sphalerite, and granoblastic annealing-recrystallization features with the development of 120 degrees triple-point junctions in galena, sphalerite, and pyrite. Despite its metamorphism, this ore preserves a generally stratiform nature with sharp, unaltered host-rock contacts, a regional and stratigraphic association with chemical and possible evaporitic metasedimentary rocks, compositional layering, and metal zoning that are consistent with formation as a sedimentary massive sulfide deposit.Disseminated sulfide ore, the most abundant type in the deposit, consists of complex veins and irregular manto-type impregnations in altered marble that are composed of pyrite, sphalerite, tennantite, chalcopyrite, arsenopyrite, galena, seligmannite, boulangerite, and minor amounts of a wide variety of additional sulfominerals in a quartz-sericite-manganiferous carbonate gangue. Disseminated sulfide ore transects and has reacted with the earlier massive sulfide ore and does not exhibit evidence of metamorphism. Euhedral zoned crystals with mineral and fluid inclusions, open-space fillings, and complex textural relationships are characteristic of this ore type and indicate that it formed through the replacement of marble by reaction with hydrothermal solutions. Disseminated sulfide ore is enriched in Cu, As, Mn, Sb, and Bi in comparison with the massive sulfide ore.Skarn ore contains pyrite, chalcopyrite, scheelite, and minor amounts sphalerite, galena, and Pb-Bi sulfominerals in a calc-silicate assemblage of gangue minerals that includes andradite-grossularite garnet, diopside, calcite, quartz, epidote, and minor chlorite, actinolite, and magnetite. Textures similar to those found in the disseminated sulfide ore and an absence of metamorphic features are characteristic of the skarn ore. Highly saline fluid inclusions in quartz from the skarn ore suggest that high-temperature, low-pressure porphyry copper-type magmatic fluids were involved in generation of this ore. Skarn ore does not exhibit a spatial relationship to igneous rocks in the mine but may be related to porphyritic quartz diorite stocks a few kilometers to the south that have halos of propylitic and phyllic alteration and porphyry copper-type mineralization.The different ore types are characterized by a very similar lead isotope composition ( 206 pb/ 204 pb = 18.78-18.82, 207 pb/ 204 pb = 15.67, 208 Pb/ 204 pb = 38.88-38.92), which lies within the restricted field of igneous rocks from northern Greece. Although this resemblance between ore and igneous rock lead has been used to support a magmatic origin for the Madem Lakkos and related sulfide deposits, the uniform isotopic composition of all lead in this tectonically active region weakens this argument. If, as is proposed, the massive sulfide ore was initially deposited as a synsedimentary body within the Kerdylia Formation, the modern model age of the lead strongly suggests that mineralization took place only a short time before the rocks were metamorphosed.The superposition of multiple ore types having different mineralogic and chemical compositions, textures, metamorphic grades, and apparent ages indicates a complex, multistage, polygenetic origin for the Madem Lakkos deposit. An interpretation consistent with this evidence is that synsedimentary massive sulfide ore was deposited as a stratiform body within a sequence of probably Mesozoic shallow-water platform carbonate and clastic-volcaniclastic sediments, possible evaporitic sediments, and lesser amounts of volcanic rocks. This ore and its host rocks were metamorphosed to upper amphibolite grade during Cretaceous-Tertiary regional metamorphism.Coregional, post-tectonic intrusions generated heat and magmatic fluids that produced skarn and skarn ore by replacement of marble at temperatures above 360 degrees C. A continuing but cooling convective hydrothermal system mixed magmatic fluids with meteoric water. These hydrothermal fluids permeated marble and massive sulfide ore peripheral to the skarn ore, reacting with them and extensively altering the marble to form disseminated sulfide ore. Massive sulfide ore and related chemical sedimentary rocks were partly dissolved by and incorporated into the hydrothermal solutions, thereby contributing Pb, Zn, Fe, Mn, Ag, Au, and minor amounts of other constituents to the hydrothermal system. Fe, Cu, W, As, Sb, and Bi were probably magmatic contributions.

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