Summary:
Olympias is an example of a polymetallic carbonate-replacement deposit. However, it is somewhat unusual due to its high gold content.
The Olympias deposit is located six km north of the Stratoni fault within the Kerdilion unit. Replacement-style sulphide orebodies are hosted by marble interlayered within a sequence of quartzo-feldspathic biotite-gneiss, amphibolite, and plagioclase microcline orthogneiss (Kalogeropoulos et al. 1989; Siron et al. 2016). The massive sulfide orebodies plunge shallowly to the southeast for over 1.8 km , subparallel to the orientation of F2 fold hinges and a locally developed L2 intersection lineation (Siron et al. 2018). The locations of the sulfide lenses, however, are largely controlled by strands of the ductile-brittle Kassandra fault and East fault and sub-horizontal shear zones that occur between the two faults.
The sulfide ore bodies are grouped into three major domains: East Zone, West Zone, and Flats Zone. Two smaller sub-zones, the Remnants and North Zones, are considered as part of the West Zone for the purpose of resource estimation. The East Zone ore occurs dominantly in the footwall to the steeply northeast-dipping East fault with the sulphide lenses hosted by marble at or below the upper gneiss-marble contact. The stratabound ore lenses dip shallowly to moderately to the northeast and strike for ~1.1 km. Ore lenses typically pinch and swell with thickness up to 10 m and widths up to 130 m. The West Zone ore bodies are controlled by the Kassandra fault and have a steep (~ 60°) NE dip that shallows at depth as it merges into the Flats. The West Zone ore has a strike extent of ~1.2 km and is up to 25 m thick with a down-dip extent up to 200 m. The Flats ore body extends to the east from the West Zone ore and dips shallowly to the northeast; however, in detail it comprises a series of stacked lenses that also extend to the west into the footwall of the Kassandra fault. The Flats ore bodies have a strike extent of over 1 km and are locally up to 15 m thick, although more commonly range from 3 to 10 m thick, and ore lenses are tens of meters wide to up to 100 m in places.
Sulphides associated with the Olympias deposit include coarse-grained, massive, and banded lenses dominated by variable amounts of sphalerite, galena, pyrite, arsenopyrite, chalcopyrite, and boulangerite. Accessory minerals include pyrrhotite, marcasite, stibnite, mackinawite, enargite, freibergite, bournonite, geocronite, cubanite, bornite and covellite (Nicolaou and Kokonis 1980; Kalogeropoulos and Economou 1987; Kalogeropoulos et al. 1989; Siron et al. 2016). Secondary ion mass spectroscopy studies indicate that Ag and Au primarily occur in solid solution with their respective host minerals; Ag in galena and Au in both arsenopyrite and pyrite.
Olympias Mineralization Types
Mine nomenclature classifies the mineralization into eight types. Types 1 to 3 are base metal-pyrite dominant, Types 6 and 7 are arsenopyrite-rich and variably silica bearing, Type 8 is manganeserich and Types 4 and 5 are sub-economic pyritic wall rock alteration with minor to semi-massive sulphides.
Types 1, 2, and 3 are gradational with one another and reflect endmembers of galena-sphalerite dominant (Type 1) to pyrite dominant (Type 2) to transitional mixed galena-sphalerite-pyrite (Type 3). Arsenopyrite is common in all three types but is not the dominant sulphide. These mineralization types typically occur as massive to banded sulphide zones with medium to coarse grained sphalerite-galena-pyrite-arsenopyrite and calcite gangue. Types 1 to 3 are dominant in the Flats.
Type 7 is arsenopyrite-rich and has the highest gold content. The mineralization is typically siliceous with massive to banded sulphide dominated by blocky to acicular arsenopyrite with lesser pyrite, galena, and sphalerite. Type 7 is locally gradational to Type 3, and banded zones commonly comprise intergrown Types 1 to 3 and 7. Type 7 is dominant in the East Zone.
Type 6 is a paragenetically younger quartz-rich sulfide assemblage that locally overprints the early replacement massive sulphide ore horizons. Type 6 can vary from banded siliceous zones to extensive intervals of grey siliceous matrix breccia that contain angular altered wall rock fragments. These quartz-rich sulphide bodies consist of interlocking, euhedral and growth-zoned quartz accompanied by interstitial arsenopyrite and boulangerite with subordinate pyrite, galena, and sphalerite. The breccia cement consists of dark gray chalcedonic quartz with disseminated, euhedral pyrite, fibrous boulangerite and bladed arsenopyrite. In places, Type 6 grades into Type 7, and commonly these quartz-rich ore types are surrounded by lower grade quartz-rhodochrosite alteration of the marble (Type 8). Types 6 and 8 are most commonly developed in the northern and eastern portions of the Olympias deposit, and in and around the Kassandra fault.