title: Geologic and geochemical vectors to mineralisation at the Resolution porphyry Cu-Mo deposit, Arizona
Creators: name: Phillips, JJ
Publisher: name: University of Tasmania
Year: 2019

description: The Resolution porphyry Cu-Mo deposit, located within the Superior mining district, Arizona, has a resource of 1,787 Mt at 1.54% Cu and 0.035% Mo. It formed during the Late Cretaceous to Paleocene Laramide Orogeny and is one of the largest and highest grade porphyry Cu deposits in North America. Since that time, Tertiary gravels and volcanic rocks related to Basin and Range extension buried the deposit under approximately 1.5 km of cover. This deep cover prevented the discovery of Resolution until the 1990s, despite historical mining of the nearby Magma and Silver King epithermal deposits since 1875. A significant component of the pre-mineralisation sequence in the Superior district are the Proterozoic basaltic lava flows and dolerite sills of the Mesoproterozoic Apache Group. The dolerite sills intruded between ca. 1,110 and 1,075 Ma, coeval with Midcontinent rift magmatism. They can be geochemically subdivided into four discrete suites using immobile element geochemistry, due to the different degrees of crustal contamination and fractionation. Generally, the dolerites have a slightly more enriched mantle signature than the older Apache Basalt. This is interpreted to reflect the distal influence of a mantle plume-head responsible for Midcontinent rift magmatism. The dolerite sills are derived from an enriched, shallow mantle source, similar to the Nipigon and Pigeon River suites of the Midcontinent rift. Two suites show evidence for significant contamination by the lower crust. One of these contaminated dolerite suites also retained significantly elevated levels of PGEs (~20 ppb combined Pt + Pd) compared to the other sills (Pt and Pd below detection limit). The intrusion of Mesoproterozoic dolerite sills into the water-rich, salt-bearing Apache Group stratigraphy caused mobilisation of significant volumes of evaporite-derived hypersaline brine. Widespread K-feldspar - chlorite alteration of Apache Basalt occurred at this time, part of a previously undocumented phase of significant Mesoproterozoic potassium metasomatism within the Proterozoic volcanic rocks, centred on E-striking faults. This metasomatic event produced several protolith-controlled alteration assemblages in the Proterozoic rocks of the Superior district, including calc-silicate skarn assemblages within the Pinal Schist, patchy hematite - quartz alteration in sandstones, magnesian and magnetite skarn alteration of dolomitic rocks, epidote – calcite – chlorite alteration of the basaltic rocks, and widespread actinolite – chlorite – biotite – albite alteration in the dolerite sills. U-Pb dating of hydrothermal apatite in four epidote – calcite – chlorite-altered Apache Basalt samples returned ages ranging from 1,247 ± 52 Ma to 1,113 ± 55 Ma, implicating the intrusion of the dolerite sills as a major driver for Proterozoic hydrothermal activity in the Superior district. The apatite ages also correlate with uraninite and galena ages of uranium mineralisation from elsewhere in the region, suggesting that this was a basin-wide phenomenon. A laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) epidote U-Pb geochronology method was developed as part of the current study in conjunction with apatite U-Pb geochronology. When applied to epidote in the Proterozoic basalts, the epidote U-Pb method provides similar precision to apatite (~5%), although initial results show a 10% offset in final age between apatite (e.g., 1,151 ± 48 Ma ) and epidote (e.g., 1,266 ± 57 Ma) due to the lack of a matrix matched epidote standard. The incorporation of \(^{206}\)Pb and \(^{207}\)Pb into routine LA-ICP-MS trace element analysis methods for epidote has enabled collection of geochronological and corresponding trace element data, although the uncertainties are larger (~10%). Using the routine trace element method, a composite age for all Proterozoic epidote analyses yielded an age of 1,183 Ma ± 23 Ma. This age confirms that there was a Proterozoic episode of epidote alteration in the district, broadly coeval with the intrusion of the dolerite sills. Argon-Ar geochronology conducted on muscovite-altered samples from Silver King (75.70 ± 0.12 Ma and 74.31 ± 0.11 Ma) and the Magma vein (68.22 ± 0.11 Ma and 67.39 ± 0.29 Ma) indicate a temporal distinction between those Laramide epithermal deposits and the ca. 65 Ma Resolution porphyry deposit. This result confirms that alteration around the Silver King stock is significantly older than the main mineralising event in the district, and supports a possible genetic distinction between Resolution and Magma, as suggested by previous authors. Epidote and chlorite LA-ICP-MS analyses from throughout the district were classified using supervised machine learning techniques including random forests and discriminant projection analysis, in order to discriminant the geochemical signatures of each generation of epidote and chlorite alteration associated with the multiple hydrothermal systems in the district. Proterozoic epidote is enriched in U, B, and HREEs and Proterozoic chlorite is depleted in Zn and Mn. These Proterozoic epidote and chlorite compositions represent mineral chemistry background for exploration in the region. Resolution-related epidote and chlorite were recognised in altered Apache Basalt as exposed in the Superior range front. The distal geochemical signature of Resolution is defined by elevated Pb and Sr in epidote, and Zn and Mn in chlorite. Epidotes related to Silver King were generally characterised by higher Sr, while those from Magma tended to show higher Ni, Cr, and V, when compared to epidote from Resolution. Chlorite from Silver King typically has higher Ti, Al and Ba, while chlorite from Magma is characterised by higher Ni and V values, compared to Resolution. The mineral chemistry footprint of Resolution defines an 4.2 km radius geochemical footprint around the deposit, and this alteration halo is partly exposed in the range front. Results from carbon and oxygen isotope analyses of samples taken along epithermal veins that crop out in the range front west of Resolution range from -9.5‰ to 0.8‰ (δ\(^{13}\)C) and and 10.5‰ to 23.7‰ (δ\(^{18}\)O) relative to VPDB and VSMOW respectively, revealing significant exchange of magmatic fluids with country rocks. The carbon isotopic system has been disturbed by the diverse sources of carbon in the Apache Group (including Proterozoic kerogens), whereas the oxygen isotopic signature is interpreted to preserve a distal cooling trend away from primary magmatic compositions close to Resolution. Over successive paragenetic stages, decreasing (δ\(^{18}\)O) compositions suggest increased degrees of mixing with meteoric fluids that had partially equilibrated with carbonate country rocks. At the district scale, the (δ\(^{18}\)O) values of main stage calcite define a 2.3 km depletion halo around Resolution. Sulfur isotope analysis of sulfides from Resolution ((δ\(^{34}\)S) -9.6 ‰ to 4.4 ‰) support fluid evolution under predominantly oxidising sulfate-stable conditions, with magmatic sulfur as the dominant sulfur source. Locally within the Mescal Limestone, there is evidence that some evaporitic sulfur may have been incorporated in the fluids ((δ\(^{34}\)S 4.4 ‰ and 0.59 ‰). Directly above the main mineralized zone at Resolution, sulfur isotopic compositions become increasingly negative with successive alteration stages (potassic average: -1.8 ‰, phyllic: -3.6 ‰, early advanced argillic: -3.8 ‰, late advanced argillic: -4.4 ‰), whereas laterally, sulfur isotopic compositions tended to increase due to exchange with more reduced country rocks under low temperature conditions (0.1 to 0.5 ‰). The zonation patterns defined by sulfur isotopes indicate that Resolution was the fluid source with outflow along major structures. There is also a westerly zonation toward more negative values (-5.0 to 1.5 ‰) along the Magma vein, implying the possibility of a second magmatic-hydrothermal fluid source to the west. A (δ\(^{34}\)S value of -5.9 ‰ 2 km east of Resolution (termed Resolution East) may indicate a seperate oxidised magmatichydrothermal fluid source between Resolution and Superior East. The Pb isotopic compositions of sulfides in the Superior district are particularly non-radiogenic, in accord with the rest of the southwest US, and support increasing lower crustal involvement in melts and/ or metal contributions over time. At the deposit scale, early stages of alteration and mineralisation have slightly more radiogenic Pb isotopic compositions, indicating magmatic metal contributions were mixed with some metals scavenged from the Proterozoic wallrocks. The less radiogenic Pb isotopic compositions of sulfides with time at Resolution imply decreasing amounts of wallrock Pb incorporation. The least radiogenic magmatic Pb isotopic composition of sulfide samples define a 0.75 - 1 km halo around Resolution. This extends out to approximately 2 km based on the Pb isotopic composition of epidote. At the district scale, Pb isotopic compositions of sulfides and epidote tend to get more radiogenic with greater distance from the main Resolution hydrothermal centre, as fluids exchanged with the radiogenic Proterozoic country rocks. Magma ores have a more radiogenic Pb isotopic composition than Resolution. The integration of mineral chemistry, isotopic and dating techniques has proven to be a powerful tool to discriminant propylitic alteration around giant porphyry deposits from other hydrothermal events that may have affected a district. This approach also shows potential for vectoring toward high temperature oxidised magmatic-hydrothermal fluid sources associated with porphyry mineralisation and has highlighted exploration targets in the Superior district that should be tested to assess the validity of this integrated geologic and geochemical approach.