Frank, Mark R.
M.S. (Master of Science)
Department of Earth, Atmosphere and Environment
High-grade porphyry Cu-Au ore deposits are typically contained in the potassic alteration zone, which forms through the alteration of preexisting country rock by a magmatic volatile phase (MVP) exsolved from an igneous intrusion. The MVP is a H2O dominant phase that contains high concentrations of NaCl, KCl, FeCl2, HCl, sulfur (as H2S or SO2) and minor concentrations of metals. The minerals in the potassic alteration zone are predominantly K-feldspar and biotite with a shreddy texture. Shreddy biotite forms from the replacement of prismatic-tabular shaped hornblende crystals, resulting in a characteristic shreddy texture. The alteration of hornblende to biotite can neutralize the acidic component of the MVP which would lead to the deposition of Cu and Au; thus, experiments were conducted to constrain the acidity over which hornblende is stable in equilibrium with a high-salinity MVP at 500 oC and 50 MPa, 600 oC and 75 MPa, and 700 oC and 100 MPa. These temperatures and pressures equate to depths of about 2-7 km. The starting MVP contained NaCl, KCl, HCl, and H2O with a total salinity of 42 wt.% NaCleq, placing the MVP in the brine-only field at 500 oC and the brine + vapor field at 600 oC and 700 oC. The breakdown of hornblende and the formation of alteration products was observed in all conditions explored. Mineral run products were characterized petrographically, texturally, and compositionally. Hornblende altered to biotite at NaCl/HCl ratios in the MVP lower than 39.4 at 500 oC, 29.3 at 600 oC, and 13.6 at 700 oC. The identified reactions for hornblende breakdown were:
NaCa_2 Mg_2 Fe_3 (AlSi_7 O_22)(OH)_2 + KCl + 8 HCl ↔K(Mg,Fe)_3 AlSi_3 O_10 (OH)_2 + 2 FeCl_2 + 4 SiO_2 + NaCl + 2 CaCl_2 + 4 H_2 O
for lower NaCl/HCl ratios and, at greater NaCl/HCl ratios, hornblende broke down into anthophyllite through the following reaction:
Na(Ca,Na)(Mg)_5 (Si_8 O_22)(OH)_2 + 2 MgCl_2 ↔ Mg_7 Si_8 O_22 (OH)_2 + 2 NaCl +CaCl_2
The alteration of hornblende into biotite depletes the MVP of HCl and would force the precipitation of metals. The formation of anthophyllite is a product of a low HCl-bearing MVP, explaining its absence from high-grade porphyry ore deposits. Mineral stability fields were compared with K-feldspar stability fields determined in supercritical fluid- and brine-only experimental studies. This study is the first to demonstrate the mechanism through which K-feldspar and biotite could coexist with a high acidity MVP in the porphyry environment. While the results of this study cannot be directly compared to these studies, they provide context for conditions responsible for porphyry formation. The alteration of hornblende to biotite supports the proposed alteration reaction; thus, making it an important identifier for high ore grade deposits. The alteration of hornblende to anthophyllite, in contrast, is not a good indicator of elevated metal concentrations as this alteration sequence requires a MVP low in HCl concentration. Exploration efforts should focus on systems where shreddy biotite has replaced hornblende.
Lightner, James Mitchell, "Alteration Reactions in a Hornblende-biotite-anthophyllite-fluid System: Implications for Cu-au Ore Deposits" (2021). Graduate Research Theses & Dissertations. 7364.
Northern Illinois University
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