Magmatic to hydrothermal transition at the Elatsite ... Conference/BOR... · Magmatic to...

Magmatic to hydrothermal transition at the Elatsite porphyry Cu-Au-(PGE) deposit,

Srednogorie zone, Bulgaria

Elitsa Stefanova

Zoltán ZajaczThomas DriesnerChristoph HeinrichParaskev Petrov

Introduction

• The formation of porphyry-type copper deposits is related to volatiles released from upper crustal magma reservoirs

• Primary magmatic volatiles are highly efficient in: sequestering economic metals from the crystallizing magmatransporting them to the place of ore mineral precipitation

• In the Elatsite deposit, assemblages of co-existing silicate melt inclusions, saline brine inclusions and vapor inclusions in early magmatic-hydrothermal quartz veins are presented

• They offer a unique chance to reconstruct the compositional evolution of the melt and volatile phases

• Co-existence of fluid inclusions with silicate melt inclusions suggests that they represent the primary magmatic volatile phase released from the crystallizing magma

Outline

• Samples and methods

SEM-CL

Microthermometry of fluid inclusions

Homogenization experiments of silicate melt inclusions

Electron microprobe analyses

LA-ICPMS analyses of fluid and silicate melt inclusions

• Results

• Conclusions

• What follows next

Samples and methods – SEM-CL and microthermometry

All silicate melt inclusions and magmatic fluid inclusions analyzed in the present study occur in euhedral quartz crystals from pre-ore stage quartz veins

SEM-CL• is an efficient method for visualizing microtextures in quartz, which can not be observed using optical microscopy • textural correlation of successive quartz types and fluid inclusion assemblages

Microthermometry• Microthermometric measurments were carried out on the fluid inclusions to determine their salinity. We need this information as an internal standard toquantify the LA-ICPMS analyses

Methods – Homogenization experiments and EMPA

Electron microprobe analyses (EMPA)• Have been conduced on the rehomogenized melt inclusions to determine theirmajor element contents• These data were subsequently used to quantify the element concentrations in silicate melt inclusions analyses, obtained by LA-ICPMS

Homogenization of silicate melt inclusions• Silicate melt inclusions were rehomogenized under confining pressure in cold sealpressure vessels • Doubly polished quartz chips were placed into Ag Pd capsules, which were not welded • Quartz crystals were heated to 730°C under 150 MPa confining pressure for 120 hours and quenched subsequently

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Methods – LA-ICPMS

LA-ICPMS of silicate melt inclusions• We analyzed SMI ablated with their host mineral (quartz)• Quantification of SMI analyses was done using the method of Halter et al. (2002)• As an internal standard we used the average Al O of the melt inclusions, determined by EMPA

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Results - CL textures of quartz and correletion with fluid and melt inclusions assemblages

Results – SMI and fluid inclusions description, microthermometry

• SMI are recrystalized

• Brine inclusions – 50-70 vol% liquid,vapor, halite and silvite. They may contain anhydrite and two opaquephases (Cpy, hematite)

Salinity up to 55 wt% NaCl equivalent

• Vapor inclusions – liquid and more than70 vol% vapor and an opaque daughterphase.

No microthermometry data could be obtained because of their small liquid content

To avoid potential destruction of fluid inclusionshomogenization experiments were conduced only till 500°C.

Results – Homogenization experiments of SMI

Results - LA-ICPMS analyses

• LA-ICPMS data of brine inclusions show that they are Cu-rich - maxima of 9122 ppmPb is up to 2000 ppm and Zn content - 3300 ppm

• The concentration of Cu, Pb and Zn in the recrystalized silicate melt inclusions are highly variable (Cu 3-3657 ppm; Pb 11-797 ppm; Zn 5-1153 ppm)• The elevated concentrations of these fluid compatible elements in the SMI can be explained by the heterogeneous entrapment of fluid with the melt

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Conclusions

• Magmatic-hydrothermal quartz veins with co-existing SMI, saline brine inclusions and vapor inclusions

• Co-existence of fluid inclusions with silicate melt inclusions suggests that they represent the primary magmatic volatile phase released from the crystallizing magma

• After the heating experiments several composite inclusions consisting of silicate glassand brine or vapor were observed. Heterogeneous entrapment of silicate melt and brineor silicate melt and vapor

• Clear evidence for the co-existence of the three immiscible phases in the system

• Brine inclusions are Cu-rich and have high content of Pb and Zn

• The concentration of Cu, Pb and Zn in the recrystalized silicate melt inclusions are highly variable

• The elevated concentrations of these fluid compatible elements in the SMI are due to the heterogeneous entrapment of fluid with the melt

What follows next

• Additional LA-ICPMS analyses of SMI, brine and vapor inclusions

• Quantitative determination of flui/melt partition coefficients of elements of economic importance at Elatsite

• After LA-ICPMS analyses of fluid inclusions homogenization experiments will be performed on inclusions from the same assemblages that were not ablated

• To estimate P and T conditions at inclusion entrapment

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