Experimental studies on the origin of iron deposits; and mineralization of Sierra La Bandera, Chile

The occurrence of actinolite in magnetite deposits of possible magmatic origin has prompted an experimental investigation of the upper-thermal stability of actinolite. Experiments were done on the compositional re-equilibration of natural tremolite to a more Fe-rich actinolite over the range of 900--600°C, 1--4 kbar, at the Ni-NiO oxygen buffer, for 1--2 weeks. At 4 kbar/900--800°C, tremolite shows strong re-equilibration with overgrowths of an Fe-rich but low-Ca (1.7 > Ca > 1.4) actinolite. At 800--700°C, tremolite shows weak re-equilibration but nucleation of Fe-rich cummingtonite is strong. Similar results were observed at 1 kbar, with tremolite showing re-equilibration to low-Ca actinolite at 790--600°C with cummingtonite nucleation at 800°C and below.; Additional univariant reversal experiments were done on the thermal decomposition of natural actinolite (Fe# = 0.22) from Pleito Melon, Chile, indicating the breakdown boundary to be at 780°C/1kbar and 850°C/4 kbar. Considering only amphiboles with Ca > 1.7 apfu, the thermal stability of actinolite is observed to decrease in a linear manner over the P--T range investigated.; Additional experiments were done in order to investigate if actinolite is stable in the presence of a P-Fe-rich magma. The results indicate that tremolite can coexist with a P-Fe-magma, but it did not re-equilibrate to actinolite; instead, orthopyroxene at high temperatures (>800°C/1 kbar) or cummingtonite at low temperatures (<800°C/1 kbar) were the equilibrium phases present.; Additional experiments to investigate the effects of phosphorous and volatiles on the melting behavior of andesite show that when more than ∼12% of pure H3PO4 is added to the andesite, it produces an immiscible (P-Fe-rich) liquid with an approximate composition of 30% P 2O5, 23.3% FeO, 8.9% CaO, 5.9% MgO, 5.7% Al2O 3, 2% SiO2, 1.8% TiO2, 1.2% Na2O, and 0.3% K2O.; The estimated lower temperature limit of a P-Fe-rich magma is about 600°C at 4 kbar and 650°C at 1 kbar. These results and the thermodynamic modeling of the stability of actinolite gives a window of at least 150°C at 1 kbar where actinolite with a Fe# ≤ 0.3 can coexist with a P-Fe-rich melt. The fractionation of P-Fe-rich immiscible magma from andesite can result in the formation of Nelsonites and Kiruna-type deposits.