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| Titel |
Cu-Mo-Au Partitioning and Ore Mineral Solubility: Constraints on the Role of Temperature, Pressure, and Volatile Fugacities |
| VerfasserIn |
Brian Tattitch, Jon Blundy |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098014
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| Publikation (Nr.) |
 EGU/EGU2014-13649.pdf |
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| Zusammenfassung |
| The complex conditions under which volatile phases are exsolved from arc magmas play a
key role in the formation of Porphyry Copper Deposits (PCD). Specifically, the efficiency by
which ore metals are removed from melts by these “proto-ore fluids”, metal ratios in the
fluid(s), the mass of metals available for transport, and ultimately deposit grades are affected
by several key intensive variables and volatile fugacitites. Previous experiments have
shown that fluid salinity [1] as well as sulfur content [2] can strongly affect the ore
metal partitioning. In addition, more subtle parameters such as cation ratios (e.g
Na/K/H – [3]) along with melt ASI and fH2S/fSO2, may influence both Cu and Au.
We present experiments designed to constrain the role of key variables (T, P, fO2,
fH2S, fSO2, -
Clv) in influencing metal partitioning on a suite of ore metals in
equilibrium.
In order to simulate complex, natural proto-ore fluids, we have performed CuFeS2-MoS2
saturated, Cu-Au-Mo-Re partitioning experiments using pumice from the Cardones
Ignimbrite in northern Chile. These experiments examine fluid-melt-crystal partitioning and
ore mineral solubility for a magmatic system similar to those found in major PCD.
Experiments were performed at 100 & 200 MPa and super-solidus conditions (810 oC) in
oxidized (NNO + 2) and reduced (NNO + 0.75) environments with respect to sulphur, as well
as at near-solidus conditions (740 oC and NNO + 0.75). Observed Cu concentrations are
consistent with Cl-dependent partitioning, modified by the presence of sulfur. The
vapour/melt partitioning of Cu is enhanced by increased fH2S at reduced conditions at 810
oC, but returns to Cl-dependence for high fSO2 at oxidized conditions (DCuv-m decreases
from 40 ± 20 to 10 ± 5). In contrast, molybdenum partitioning remains constant with
changing fO2 (DMov-m = 3 ± 1 and 4 ± 2 respectively). This corresponds to
a change in the vapour Cu/Mo ratio from ~40:1 down to ~1:1. The decrease in
DCuv-m is likely a result of the changes to available ligands in an H2S dominated fluid
compared to an SO2 dominated fluid. The effect of oxidation may be stronger for
Cu relative to Au as the Cu/Au ratio in the vapour drops from ~100:1 down to
~30:1 at higher fO2. At 740 oC, near the solidus for the Cardones, the solubility
of Mo in the silicate melt is much lower, indicating that temperature and/or melt
properties will exert a strong control on Mo availability during volatile exsolution.
Supercritical experiments (200 MPa) have recently been completed to evaluate ore-mineral
solubility, and the influence of T and fO2, for fluids of median salinity compared to
vapours and brines at 100MPa. Evaluating changes to ore metal ratios (Cu/Mo,
Cu/Au, Mo/Re etc.) will allow us to evaluate complex ore metal behaviour during the
progression of magmatic volatiles from deep one-phase systems, through fluid unmixing,
continued exsolution down to the solidus, and finally sub-solidus transport and
deposition.
References:
[1] Candela and Holland (1984) GCA, 48, 373-380
[2] Simon et al. (2006) GCA, 70, 5583-5600
[3] Zajacz et al. (2011) GCA, 75, 2811-2827 |
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