Breccia-hosted copper-molybdenum mineralisation at Rio Blanco, Chile

The Rio Blanco-Los Bronces ore deposit is located at 33°l5'S in the Principal Cordillera of central Chile. It lies midway between the Los Pelambres and El Teniente porphyry copper deposits, which together define one of Chile's most economically significant metallogenic belts. Rio Batholith and surrounding Tertiary volcanic and volcaniclastic rocks at approximately 5-4 Ma. The bulk of the >50 million metric tonne copper resource is hosted by the Rio Blanco Magmatic Breccia and the Sur-Sur Tourmaline Breccia. The geometry of the Rio Blanco-Los Bronces system is asymmetrical, and appears to have been controlled by three principal fracture orientations that trend N, NW and NE. Two N-trending faults tenninate at the eastern and western boundaries of the Rio Blanco-Los Bronces deposit and have acted as pull-apart structures that allowed hydrothermal fluids and felsic magmas to be emplaced within the ore deposit. The N-trending faults are the largest and most deeply eroded fault orientations in the district and at Sur-Sur are occupied by Tounnaline Breccia. The phyllic altered Tourmaline Breccia is marginal to the potassic altered Rio Blanco Magmatic Breccia, which constitutes the core zone between the two N-trending fault terminations. Although the NWand NE-trending faults crosscut the Tounnaline Breccia, their fracture intensities are highest where strongly mineralised zones occur along the strike length of the breccia body, indicating that these fracture trends were active during brecciation and helped to localise fluid flow. Paragenetic studies indicate nine main stages of veins, breccia and porphyry emplacement in the Rio Blanco and Sur-Sur sectors of the ore deposit. The stages are: ( 1) magnetite-actinolite alteration; (2) potassic stockwork veins; (3) Rio Blanco Magmatic Breccia and Sur-Sur Tourmaline Breccia; ( 4) Feldspar Porphyry; ( 5) potassic stockwork veins; ( 6) Quartz Monzonite Porphyry and Don Luis Porphyry; (7) molybdenite stockwork veins; (8) chalcopyrite stockwork veins; and (9) D veins. Barren dacite and rhyolite intrusions cross-cut the ore deposit complex. A change from potassic to phyllic alteration defines the contact between the Rio Blanco Magmatic Breccia and Sur-Sur Tounnaline Breccia. The Rio Blanco Magmatic Breccia occupies a large volume of rock within the Rio Blanco, La Union, Don Luis and Sur-Sur sectors, and extends over a vertical interval of ~2 km. The Tourmaline Breccia lies transitionally outward from the Magmatic Breccia mainly in the Sur-Sur sector. There is a 100m thick gradational contact between deeplevel biotite breccia and shallow-level tourmaline breccia at Sur-Sur. Paragenetic studies ofbreccia cement minerals in the Rio Blanco Magmatic Breccia and Sur-Sur Tounnaline Breccia reveal a similar cement infill sequence involving initial biotite and/or tounnaline precipitation at clast margins, followed by sulfate (anhydrite) and oxide (specularite) mineral phases, and in tum by chalcopyrite and magnetite. Spatial zonation of breccia cement minerals occurs in the Rio Blanco and Sur-Sur breccias. Zonation of biotite and tourmaline coincides with zonation of potassic and phyllic alteration, respectively. Chalcopyrite is spatially associated with stage 3 Magmatic Breccia and biotite alteration in the Rio Blanco to Don Luis sectors, and with stage 3 Tounnaline Breccia and quartz-sericite alteration in the Sur-Sur sector. Outward from the potassic and phyllic altered zones; a propylitic assemblage occurs that is defined by chlorite alteration and pyrite-specularite breccia cement minerals. New 40 Arf39 Ar geochronology of hydrothermal biotite from the base of the Sur-Sur Tourmaline Breccia and whole rock sericite from the top of the Sur-Sur Tounnaline Breccia yielded ages of 4.78 ± 0.04 Ma and 5.42 ± 0.09 Ma, respectively. Both 87Sr/86Sr and aNd analyses for tounnaline and anhydrite from the Rio Blanco Magmatic Breccia and Sur-Sur Tounnaline Breccia range between 0.7040 and 0.7044, and +1.70 and +2.53, respectively. 206Pbf2°4Pb values for anhydrite cement in the Rio Blanco Magmatic Breccia and the Sur-Sur Tourmaline Breccia ranged between 17.558 and 18.479, 207Pbf204pb values ranged between 15.534 and 15.623, and 208Pb/204Pb values ranged between 37.341 and 38.412. The early-fonned anhydrite cement has Pb isotopic compositions that are less radiogenic than the sulfide ores and host rocks, and also has elevated initial Sr ratios compared to the host rocks. Pb and Sr in anhydrite are interpreted to have been sourced from rocks and/or waters external to the main magmatic-hydrothermal system. Oxygen/deuterium isotopes for tourmaline breccia cement minerals have 'magmatic' values, however recalculated values of propylitic-altered samples from previous workers indicate a meteoric water component of up to 25%. Zonation of sulfur isotope compositions occurs in the mineralised breccias, particularly at Sur-Sur. The Rio Blanco sector is characterised by sulfides with 834S values between -3.94 and +3.34 and sulfates between +10.07 and + 17.86 values. The Sur-Sur sector is characterised by sulfides with 834S values between -4.12 and +2.65 and sulfates between +11.15 and +13.39. These values are consistent with a magmatic-hydrothermal sulfur source. At Sur-Sur, the most negative ()34S compositions (834S < -3 per mil) are spatially associated with the highest copper grades and specularite cement. The Rio Blanco Magmatic Breccia and Sur-Sur Tourmaline Breccia contain co-existing low salinity liquid-rich and vapour-rich fluid inclusions and localised zones containing co-existing vapour-rich and hypersaline fluid inclusions. Homogenisation temperatures from >600 to 131 oc have been measured, but most are between 450° and 300°C. Complete salinity arrays from 0-69 wt.% NaCl equivalent were observed, and eutectic temperatures are commonly below -35°C. Minimum pressure estimates from fluid inclusions are between 48 and 368 bars. An average lithostatic formation depth of 200 m and a hydrostatic formation depth of 2300 m below the palaeo-surface have been calculated, indicating that up to approximately 1 km of erosion has occurred since breccia formation. The mineralised breccias in the Rio Blanco and Sur-Sur sectors are magmatic-hydrothermal in origin. They formed when magmatic-hydrothermal fluids (brine and gas) exsolved from deepseated magma and potentially mixed with an external water. Hydrostatic pressures catastrophically exceeded lithostatic load plus the tensile strength of the confining rocks leading to brecciation. At Sur-Sur, fault rupture along the Rio Blanco Fault may have been a trigger for magmatic-hydrothennal brecciation. Phase separation of magmatically-derived aqueous fluid occurred at the onset ofbrecciation, with a low density gas phase (carrying H20, S02, HCl and B20 3) separating physically from the dense copper-bearing brine. The gas phase fluxed through the breccia column first, where it condensed into exotic groundwaters of uncertain derivation, resulting in the deposition of oxide-stage cements (anhydrite, specularite, tourmaline) from a hybrid low salinity water. Subsequent upwelling of the magmatic-hydrothermal brine resulted in main stage sulfide deposition, possibly in part due to fluid mixing with the hybrid water.