CLAY MINERALOGY AND DISTRIBUTION IN THE ALTAR PORPHYRY COPPER (AU-MO) DEPOSIT, SAN JUAN, ARGENTINA. IMPLICATIONS FOR THE FUTURE MINE DEVELOPMENT

LAURA MAYDAGÁN; AGNES IMPICCINI,; FRANCHINI MARTA

Río de Janeiro

Conferencia; XV International Clay Conference; 2013

AIPEA

Altar is a telescoped porphyry Cu-(Au-Mo) (802 mT, 0.42% Cu and 0.059 g/t Au, cut-off 0.30% Cu equivalent) located in the Cordillera Principal of San Juan Province, Argentina, 25 km north of Los Pelambres and Pachón deposits. The mineralization at Altar is hosted in a complex stockwork with different generations of veinlets and the fertile subvolcanic intrusions (porphyries 2, 3, and 4) present a similar sequence of alteration and mineralization episodes. The drill-holes of the Altar mining district, with lengths between 300 and 1000 m, allow us to examine the extensive record of alteration, their variations with depth and mineralogy. Biotite ± potassic dioctahedral phyllosilicates (with quartz ± anhydrite ± K feldspar ± albite ± rutile) of the potassic alteration occurs as patches at shallow and intermediate levels affecting the porphyries and the wall rocks and is massive at depth. Secondary biotite occurs as aggregates of very fine brownish plates replacing previous igneous ferromagnesian silicates (biotite and edenite). Representative biotite crystals have the compositional range (K1.89-1.63 Na0.23-0.03 Ca0.16-0.00 (Al0.10-0.00 Mg4.56-3.18 Fe2.12-0.50Ti0.63-0.19Mn0.06-0.00) (Si5.85-5.,46 Al2.,67-2.14) O10 (OH)4. K is by far the dominant cation in the interlayer with minor Na and traces of Ca. Mg predominates over Fe (arbitrarily considered as Fe2+) in the octahedral site which is completed by small amounts of Ti, Al, and Mn2+. Biotite grains have Mg/(Mg+Fe) (XMg) values between 0.60 and 0.90 and F content (0.29-3.39 % F). The structural formulas of the secondary biotites fall in the phlogopite field. The high Mg and TiO2 contents suggest high fO2 and temperature of the hydrothermal fluids during the biotite formation. Phlogopite may originate from edenite by an increase of the aK2SO4 and/or H2SO4 in the hydrothermal fluids. Chloritic alteration (chlorite + quartz + dioctahedral potassic phyllosilicates ± rutile) is found in patches at deep, intermediate, and shallow levels, transitional between the potassic and phyllic alteration types. The chlorite octahedral cation vary between 11.93 and 11.44 per O20(OH)16 formula unit and the tetrahedral cation composition is between (Si6.03 Al1.97) and (Si5.62 Al2.38) with an overall composition of (Al3.29-2.44 Mg7.41-5.69 Fe2+2.44-1.95) (Si6.03-5.62 Al2.38-1.97) O20 (OH)16.  Thus, all are trioctahedral chlorites and, based on Fe2+/(Fe2+ + Mg) and Si contents, can be classified as Fe-clinochlore and Fe-Al clinochlore. Chlorite may have been formed due to  the phlogopite destabilization and a decrease of the K2SO4 in the hydrothermal fluids at intermediate values of  a(K+)/a(H+) as the temperature of the hydrothermal system decreased.   Potassic dioctahedral phyllosilicates (quartz ± tourmaline ± rutile) assemblage of the phyllic alteration affects all lithologies. It forms a continuous halo surrounding the potassic core and the chloritic alteration patches. At depth it is controlled by fractures and forms alteration halos around veins. The analytical results from microprobe analyses of representative crystals are K1.86-1.21Na0.13-0.06 (Al3.93-3.36Mg0.65-0.04Fe0.25-0.00Ti0.03-0.0)(Si6.61-6.18Al1.87-1.40)O10(OH)4. Unlike muscovite, the total charge of these micas is less than 2 and the Si the total value in the tetrahedral site exceeds 6. This charge difference is compensated by the loss of K. With regard to the theoretical composition of muscovite, the micas analyzed have higher concentrations of Na2O, CaO, MgO, FeO, TiO2 and traces of Cl. Moreover, they are not typical illite since the charge values in the interlayer are more scattered and satisfied by Na, besides K and contain Mg and Fe in the octahedral site, in addition to Al. The progressive cooling of the hydrothermal fluids and the lowering of the fluid pH to less than 4.5, seem to have controlled the formation of the phyllic halo. Kaolin minerals (polytype not differentiated) ± quartz ± rutile of the advanced argillic alteration occurs at depth in sectors and fractures that cut the porphyries and the wall-rocks. It is poorly developed and generally overlaps zones with phyllic alteration, although it has been recognized cutting potassic alteration zones and forming vein halos. Hydrothermal fluids with lower temperature and pH should have caused hydrolysis and the leaching of the alkali phyllosilicate cations. Recently, the project has been acquired by the Stillwater Mining Company, which is conducting a comprehensive drilling campaign, metallurgical testwork program, and conducting preliminary economic evaluations for a copper-gold mining and concentrating project at Altar. The presence of these aluminosilicate minerals in copper flotation causes different problems, thus, the mineralogy and distribution of phyllosilicates will have to be considered in the upcoming metallurgy.