Abstract
Distal skarns form by the metasomatic reactions of a host rock induced by far-traveled hydrothermal fluids. Physical and structural characteristics and geochemical patterns of distal PbZn skarn bodies were studied at the Petrovitsa deposit in southern Bulgaria. Skarn bodies formed from the interaction of hydrothermal fluids with reactive host lithologies (marble and gneiss). These fluids were transported along sub-vertical feeder structures and lithological contacts. Epidote skarn developed in gneiss protolith, and pyroxene (johannsenite) skarn developed in marble. Detailed geological mapping, complimented by measurements of the internal structure of the skarn body using pyroxene growth versors, quantifies the propagation direction of the skarn body: 1) away from the major local fluid conduit (feeder structure), and 2) away from lithological contacts between aluminosilicate rock and marble. Such growth suggests that fluid flow was generally orthogonal to the skarn front propagation direction in the pyroxene skarn.Textural, mineralogical and geochemical data from skarn samples reveal multiple growth generations of major skarn calc-silicates epidote and pyroxene. The epidote skarn is characterized by limited spatial distribution and fine-grained epidote/clinozoisite growth associated with massive replacement and sulfide mineralization. The pyroxene skarn consists of acicular clinopyroxene crystals which form spheroidal aggregates with discrete growth banding. These bands are the physical representation of the cyclic fluid pulses which resulted in rhythmic skarn growth marked by geochemical banding. In situ geochemical analyses in the epidote skarn reveal early Al-rich epidote overprinted by Fe-rich epidote associated with higher Mn and Sr contents and irregular compositional banding. Clinopyroxene (Jo60–95) shows general increase in Na, Al, Mn, and REE + Y with distance from the feeder structure and lithologic contacts. These elements correlate with the distance traveled by the hydrothermal fluid from the feeder to the site of skarnification, which we define using a proxy based on the Al content of pyroxene crystals. This reflects an increasing degree of fluid “contamination” by interaction with the aluminosilicate host rocks and functions as a proxy for fluid transport distance. The spatial distribution of trace-elements in pyroxene on an outcrop scale is indicative of discrete pulses of hydrothermal fluid resulting in precipitation of skarn calc-silicates along the increasingly tortuous fluid pathway between the feeder structure and the skarn front, resulting in both the macro- and micro-scale chemical and textural variability of the skarn body.
Highlights
Skarns form from the fluid-induced replacement of reactive host li thologies by calc-silicate minerals as a function of host rock composi tion, fluid chemistry, and overall system temperature (Einaudi et al., 1981; Zharikov, 1970)
Distal skarns form by the metasomatic reactions of a host rock induced by far-traveled hydrothermal fluids
Physical Skarn Growth massive texture of the skarn body, no preference was given to crystal orientation relative to the skarn front, as was done in the previous study (Vezzoni et al, 2016)
Summary
Skarns form from the fluid-induced replacement of reactive host li thologies by calc-silicate minerals as a function of host rock composi tion, fluid chemistry, and overall system temperature (Einaudi et al., 1981; Zharikov, 1970). Metasomatic fluids preferentially destroy and replace reactive minerals (e.g., carbonates) via dissolution-precipitation reactions which allow for additional fluid flow and calc-silicate growth (Korzhinskii, 1968; Yin et al, 2017) This process generates zonation patterns (physical, chemical, and mineralogical) which are used to categorize skarns based on proximity to fluid source (proximal vs distal), timing of replacement stages (prograde vs retrograde), compo sition (calcic vs magnesian), and metal endowment (Einaudi and Burt, 1982). Skarn microstructures provide information on the local P-T-X conditions, yielding evi dence for mineral growth mechanisms during skarn formation (e.g., Ciobanu and Cook, 2004; Williams-Jones et al, 2010) These internal structures preserve information on the nature of the lithological contact between skarn and host rock — a feature directly relatable to the reac tivity of the host rock (Dipple and Gerdes, 1998). Interpreting paleofluid flow and cyclic skarn growth using field and petrographic obser vations remains the most efficient and cost-effective method of pre dicting skarn and ore body distribution
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