Evgeniy Bastrakov

Volatile exsolution at the Dinkidi Cu-Au porphyry deposit, Philippines: A melt-inclusion record of the initial ore-forming process

Immiscible phases derived from degassing silicate magmas are considered to be precursors of metal-bearing hydrothermal fluids in porphyry deposits. The development of melt-inclusion techniques provides a window into this critical period of porphyry formation, when the cooling, decompression, and crystallization of silicate melts result in the formation of immiscible phases. The record of magmatic to hydrothermal evolution is presented using inclusions in clinopyroxene phenocrysts from the syenitic Balut dike, one of the host-rock lithologies for mineralization at the Dinkidi Cu-Au porphyry deposit, Philippines. Primary inclusions include silicate glass, multiphase aggregates comprising salts, silicates, sulfates, carbonates, sulfides and oxides, and highly saline aqueous fluids. Various analyses, including in situ laser ablation inductively coupled plasma-mass spectrometry of the multiphase inclusions, determined elevated concentrations of Cl, S, As, Tl, K, Na, and a number of metals, including those that form ore-grade deposits (e.g., Cu) and those that do not (e.g., Mo, Pb, Zn, and W) at the Dinkidi porphyry deposit. Silicate melt and multiphase salt-rich inclusions in clinopyroxene are interpreted as having originally formed as immiscible phases at magmatic temperatures.

K2GWB: Utility for generating thermodynamic data files for The Geochemist's Workbench ® at 0-1000 ° C and 1-5000 bar from UT2K and the UNITHERM database

The Geochemists Workbench^(R) (GWB) is a versatile geochemical modelling software package capable of reaction path modelling, equilibrium fluid speciation and activity diagramming, although it is restricted by the availability of thermodynamic (logK) data files with a temperature range of 0-300^oC and pressures on the critical curve for water. The K2GWB (logK to GWB) utility is used to produce a usable Geochemists Workbench data file from a generic logK and system file generated by the UT2K utility complementing the UNITHERM thermodynamic database system. The GWB data files can be generated for pressure-temperature conditions within the bounds of the modified Helgeson-Kirkham-Flowers model (0-1000^oC, 1-5000bar). Data can be generated with any combination of pressure-temperature logK grids required, allowing the Geochemists Workbench to be used to model a wide range of geological scenarios under polythermal and polybaric conditions, as well as the generation of activity diagrams over a wide range of pressures and temperatures. A comparative HCh and Geochemists Workbench model using the same thermodynamic dataset was run to benchmark test the validity of the new data files. Modelling the cooling of 2molal NaCl-H2O fluid in equilibrium with a ganodiorite-type rock from 600 to 300^oC gave close to identical results between the two codes, validating the thermodynamic data conversion process described here. An investigation of the speciation of Au across a temperature-pressure gradient from 550^oC and 1500bar to 150^oC and saturated vapor curve pressures has been used to illustrate the usefulness of coupling thermodynamic data file generation at wide temperatures and pressures from a central database and the functionality of The Geochemists Workbench.

The relevance of fluid inclusions to mineral systems and ore deposit exploration

Fluid inclusions provide the only direct samples of palaeofluids that may be related to mineralisation processes. In order to apply fluid inclusion data to study fluid flow on a larger scale, we have used a mineral systems approach, which regards a mineral deposit as part of a much larger system and considers all the processes that are involved in mobilising ore components from a source, transporting and accumulating them in a more concentrated form and then preserving them throughout the subsequent geological history. This not only enables a better understanding of fluid flow processes but also enables fluid inclusions to provide an exploration target that is much larger than the ore deposit itself.As an example, a study of fluid inclusions associated with gold mineralisation in the Tanami Region of Northern Australia was used to determine the temperatures and compositions of the ore fluids. Once the parameters of the mineralising fluids were established, the study was then expanded to a region of central Australia covering almost 100,000 km2. It was concluded that a high temperature (320–360 °C), low salinity fluid containing CO2 and other gases was circulating in the northern part of this region at around 1720 Ma. This suggests the circulation of an orogenic gold style fluid and indicates that this region has potential for other orogenic gold deposits. In the southern part of this region, a lower temperature (120 to 190 °C), high salinity fluid with no detectable gases was present and appears to represent circulation of a basinal brine.In the second example, fluid inclusion data from Cu-U-Au- Ag-REE prospects in the Olympic Copper-Gold Province in South Australia were used to constrain geochemical modelling of the mineralisation processes. By combining the inversion-generated, 3-D geophysical maps with geochemical and magnetic susceptibility values derived from the modelling, it has been possible to divide the range of observed magnetic susceptibilities into divisions that represent the various alteration assemblages within this region. This approach allows us to use geochemical modelling to relate alteration assemblages, and hence, the predicted sites of mineralisation, to the geophysical expressions of the mineral deposit.