El Creston is a porphyry-moly deposit with proven and probable mineral reserve of 146,705,000 tonnes grading 0.077% Mo (+ Cu±Zn±Ag as byproducts). It is located in the Mexican part of the Basin and Range Province, where a metamorphosed Proterozoic basement, mainly composed of a metagranite body (Creston metagranite) intruded in a sequence of phyllites, quartzites, gneisses and metavolcanics, rests as a roof pendant on the Tertiary Sonoran batholith, which includes several intrusions mostly ranging in composition from granite-granodiorite to quartz monzonite. The deposit is related to the emplacement of porphyry stocks in the basement (particularly, the Creston metagranite). The mineralized system underwent a complex tectonic history, the most relevant structures being low-angle normal faults (dipping N35/30°) with horizontal displacement. A polyphasic hydrothermal evolution is testified by several types of hydrothermal veins and breccias. The following main veins types have been recognized (older to younger, even if some reversals surely occur): - Qtz + Kfs ± Ab ± Mlb (molybdenite) ± Phl (phlogopite) ± Ms ± Py ± Rt veins (QTZ-KF). - Qtz + Kfs + Phl ± Mlb veins (QTZ-PHL). - Bt + Mgt veins (BT-MT). - Qtz + Ms + Rt ± Py ± Sp ± Ccp ± Td-Tn (tetrahedrite-tennantite s.s.) veins (QTZ-MU). - Zn-Sd (zincian-siderite) ± Ms ± Sp ± Ccp ± Td-Tn veins (ZN-SID). Different types of breccia occur, including pre-mineralization intrusive breccias and hydrothermal breccias. Among the 7 alteration facies identified, the most significant are: - Qtz + Kfs + Ab ± Ph ± Zrn ± Py (KF), connected with the QTZ-KF veins; - Bt + Mgt ± Ms ± Zrn ± Ep (MT); - Qtz + Mu + Py ± Ccp ± Rt (MU), related to the QTZ-MU veins. Apart from an early (relict) albitization stage, the KF and (often overprinting) MU are the strongest alteration facies; the envelope of MT alteration around the KF alteration likely represents its slightly lower T equivalent. Mineralization is mostly connected with the KF veins and related alteration. At least locally, however, molybdenite enrichments are found in Py-Ms-bearing assemblages and at the contact between the two alteration facies. Strong enrichments also occur in the quartz (+ Kfs ± Phl) matrix of hydrothermal breccias. A reconnaissance fluid inclusion study performed in quartz from the QTZ-KF and MU veins led to identification of the following populations: - L+V+several solid phases (Phl, Ms, Cal, Mgt, Tur, Ccp…): these are the earliest inclusions, only occurring in QTZ-KF veins; - L+V±Ccp: these are the most abundant inclusions, connected with the MU alteration; - L+V+Hl±Hem: rarely occurring in QTZ-MU veins, or QTZ-KF veins with MU overprinting; - V+L±Ccp: VLC inclusions surely not due to necking are rare. Often in samples containing the halite-bearing inclusions, they may be coeval with them. A striking feature of the fluid inclusions picture is the paucity of hypersaline inclusions. This observation and microthermometry/Raman preliminary data suggest that in the QTZ-KF veins fluids with low salinity and moderate CO2 content, of likely magmatic derivation, were trapped above the solvus in the H2O-NaCl-CO2 system, thus implying a great depth of formation for the deposit. Also during cooling the fluid remained above the solvus, and only episodically unmixed: cooling and, probably, water-rock reaction, more than fluid immiscibility, played an important role for molybdenite precipitation at El Creston.
El Creston porphyry-Mo deposit (Sonora State, Mexico): mineralization, alteration pattern and first fluid inclusions data
ROSSETTI, Piergiorgio;
2011-01-01
Abstract
El Creston is a porphyry-moly deposit with proven and probable mineral reserve of 146,705,000 tonnes grading 0.077% Mo (+ Cu±Zn±Ag as byproducts). It is located in the Mexican part of the Basin and Range Province, where a metamorphosed Proterozoic basement, mainly composed of a metagranite body (Creston metagranite) intruded in a sequence of phyllites, quartzites, gneisses and metavolcanics, rests as a roof pendant on the Tertiary Sonoran batholith, which includes several intrusions mostly ranging in composition from granite-granodiorite to quartz monzonite. The deposit is related to the emplacement of porphyry stocks in the basement (particularly, the Creston metagranite). The mineralized system underwent a complex tectonic history, the most relevant structures being low-angle normal faults (dipping N35/30°) with horizontal displacement. A polyphasic hydrothermal evolution is testified by several types of hydrothermal veins and breccias. The following main veins types have been recognized (older to younger, even if some reversals surely occur): - Qtz + Kfs ± Ab ± Mlb (molybdenite) ± Phl (phlogopite) ± Ms ± Py ± Rt veins (QTZ-KF). - Qtz + Kfs + Phl ± Mlb veins (QTZ-PHL). - Bt + Mgt veins (BT-MT). - Qtz + Ms + Rt ± Py ± Sp ± Ccp ± Td-Tn (tetrahedrite-tennantite s.s.) veins (QTZ-MU). - Zn-Sd (zincian-siderite) ± Ms ± Sp ± Ccp ± Td-Tn veins (ZN-SID). Different types of breccia occur, including pre-mineralization intrusive breccias and hydrothermal breccias. Among the 7 alteration facies identified, the most significant are: - Qtz + Kfs + Ab ± Ph ± Zrn ± Py (KF), connected with the QTZ-KF veins; - Bt + Mgt ± Ms ± Zrn ± Ep (MT); - Qtz + Mu + Py ± Ccp ± Rt (MU), related to the QTZ-MU veins. Apart from an early (relict) albitization stage, the KF and (often overprinting) MU are the strongest alteration facies; the envelope of MT alteration around the KF alteration likely represents its slightly lower T equivalent. Mineralization is mostly connected with the KF veins and related alteration. At least locally, however, molybdenite enrichments are found in Py-Ms-bearing assemblages and at the contact between the two alteration facies. Strong enrichments also occur in the quartz (+ Kfs ± Phl) matrix of hydrothermal breccias. A reconnaissance fluid inclusion study performed in quartz from the QTZ-KF and MU veins led to identification of the following populations: - L+V+several solid phases (Phl, Ms, Cal, Mgt, Tur, Ccp…): these are the earliest inclusions, only occurring in QTZ-KF veins; - L+V±Ccp: these are the most abundant inclusions, connected with the MU alteration; - L+V+Hl±Hem: rarely occurring in QTZ-MU veins, or QTZ-KF veins with MU overprinting; - V+L±Ccp: VLC inclusions surely not due to necking are rare. Often in samples containing the halite-bearing inclusions, they may be coeval with them. A striking feature of the fluid inclusions picture is the paucity of hypersaline inclusions. This observation and microthermometry/Raman preliminary data suggest that in the QTZ-KF veins fluids with low salinity and moderate CO2 content, of likely magmatic derivation, were trapped above the solvus in the H2O-NaCl-CO2 system, thus implying a great depth of formation for the deposit. Also during cooling the fluid remained above the solvus, and only episodically unmixed: cooling and, probably, water-rock reaction, more than fluid immiscibility, played an important role for molybdenite precipitation at El Creston.
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