Geology, geochemistry, geochronology and exploration footprints of the Christmas porphyry Cu-Mo deposit, Arizona

<p dir="ltr">The Christmas porphyry Cu-Mo deposit is located within the Banner mining district, Gila County, Arizona, USA. This deposit, hosted in Cretaceous Williamson Canyon volcanic and Paleozoic carbonate rocks, has historic production of 20 Mt at 1.2% Cu, a measured resource of 300 Mt at 0.480% Cu, 1.31 Mt Mo, 104 koz Au, 2,598 koz Ag and an indicated resource of 860 Mt at 0.311% Cu, 4.44 Mt Mo, 350 koz Au, and 6,471 koz Ag. Christmas lies within an extensive, well-studied, porphyry district that formed mostly during the Laramide Orogeny (~80 to 50 Ma).<br>Laramide magmatism in the Christmas area occurred from ~65 to 61 Ma. These medium- to high-K, calc-alkaline magmas display bulk-rock LREE and LILE enrichment and depletion in HFSE consistent with derivation from a subarc mantle melt. Bulk-rock geochemical data and Ti-in-zircon crystallisation temperatures demonstrate that tholeiitic basaltic magmatism of the Williamson Canyon volcanic host rocks evolved into intermediate, hornblende-bearing intrusive rocks (Ti-in-zircon crystallisation temperature of 674 ± 68 °C) and felsic intrusive rocks, which comprise the Christmas stock (crystallisation temperature of 620 ± 49°C), supporting the contention that the Laramide volcanic and intrusive rocks are different fractionation products of a common parental melt. New geochemical bulk-rock data and U-Pb zircon ages of the McDonald stock (63.71 ± 0.43 and 63.16 ± 0.65 Ma) and the early quartz diorite phase of the Christmas stock (63.31 ± 0.47 Ma) suggest that the McDonald stock may represent a less-fractionated phase from which the Christmas stock was derived. Bulk-rock Sr/Y, Cu and zircon REE plots demonstrate that the intrusions are adakite-like rocks and are prospective for porphyry-style mineralisation. However, an early aplite dyke (64.91 ± 0.43 Ma) and quartz-K-feldspar dyke (62.19 ± 0.53 Ma) data plot into infertile fields on REE zircon plots, suggesting a short window (62 to 64.5 Ma) of magmatism in the Christmas area that was most prospective for porphyry-style mineralisation. <br>Hypogene porphyry Cu mineralisation at Christmas produced an ore shell (bornite, molybdenite and chalcopyrite > pyrite) associated with K-silicate-quartz alteration and a quartz vein stockwork. These mineralisation features are spatially associated with the Christmas stock, a composite, multi?stage granodioritic to granitic stock, surrounded by peripheral, east-trending, sheeted dykes and hosted within Precambrian metasedimentary and metavolcanic rocks and Cretaceous-Tertiary volcanic and intrusive rocks. The potassic alteration domain is characterised by high bulk-rock Cu, Ag, S and ± Mo, low Mn, Zn and Ca values, and was overprinted by intermediate argillic alteration (chlorite-white mica?pyrite). Christmas-related skarn formed in carbonate rocks and produced Laramide garnet (66.00 ± 3.1 Ma) and epidote (42.00 ± 16 Ma) based on U-Pb dating. A Re-Os molybdenite age of 63.6 ± 0.3 Ma was obtained from a vein in prograde skarn. <br>Propylitic alteration at Christmas formed an outer alteration halo in the volcanic rocks, marked by the appearence of actinolite and epidote approximately 1 km laterally from the Christmas stock. The propylitic zone extends approximately 4 km from Christmas with the distal assemblage dominated by chlorite and lacking actinolite and epidote. A halo of clotty epidote alteration, centered on the Christmas stock developed, along with epidote veins in Williamson Canyon volcanic rocks at distances of 1 to 2.5 km from the stock. The halo of clotty epidote and the radial orientation of epidote veins are consistent with the Christmas intrusive complex being the heat and fluid source. Major and trace element data for epidote from Christmas do not show strong correlative trends with distance to deposit. However, multivariate principal component analysis (PCA) illuminates compositional differences between deposit-proximal and deposit-distal epidote where epidote <2 km of Christmas is enriched in LREE, As, Sb, and Na and depleted in HFSE (e.g., Ti, Nb, Zr, and Hf) and distal epidote shows the inverse of this trend (i.e., depleted LREE, As, Sb, and Na and enriched in HFSE). Deposit-proximal chlorite has high Mg, Al, Ti, and V, whereas deposit-distal chlorite is enriched in Si, Fe, Ca, Sr and metals (e.g., Mn, Pb, Zn and Cu). Chlorite defines a central, high-V and low- Co, Zn, and Mn geochemical domain that extends 1.5 km outwards and a larger, high-Ti and low-Sr domain that extends 2 km from Christmas. Chlorite formed during intermediate argillic alteration has high Al and Ti and low Si, Zn, and Cu, relative to propylitic chlorite. Chlorite proximitor ratios were developed using Christmas chlorite data with the [V/Co] and [V/Ca] long-range proximitor ratios have the highest R² values (0.56 and 0.41, respectively) and an effectiveness up to 3,500 m from Christmas. Short-range [Ti/Sr] and [Ti/Mn] proximitor ratios work ≤ 2 km of Christmas. <br>Hydrothermal epidote and chlorite veins, rare clots, and selective grain replacement were also mapped in the Williamson Canyon volcanic rocks around an eastern dyke swarm, 2 km east of Christmas and over 4 km south of Christmas (a.k.a., the southern alteration domain). The southern alteration domain epidote is distinct in the Christmas epidote dataset with relatively high Fe and low Al values and elevated metal contents (e.g., Cu, Zn, Pb, Co and Ni). Furthermore, the PCA-identified ‘ore-proximal’ epidote composition (enriched in LREE, As, Sb, and Na and depleted in HFSE) occurs in epidote adjacent to the McDonald stock and within the eastern dyke swarm. Ore-proximal epidote and elevated bulk-rock Cu values occur within the southern alteration domain. This epidote alteration consists of east-trending sheeted vein arrays, inconsistent with Christmas-related veins, suggesting a common but unidentified additional heat and fluid source in the south of the study area. ‘Proximal’ and distinct epidote compositions in the southern alteration domain warrant further exploration to discern the heat and fluid source of this alteration. <br>The Christmas magmatic-hydrothermal complex was dismembered by post-mineralisation, normal faulting associated with regional Tertiary Basin-and-Range tectonism. The Christmas and Joker faults bisected the Christmas intrusion and displaced its eastern half, ore-body and attendant hydrothermal alteration down-dip ~870 to 1,100 m. Heat regression models identified Christmas as a potential heat and fluid source using epidote (R² up to 0.50) and chlorite (R² up to 0.80) datasets that were filtered to remove alteration from interpreted secondary heat and fluid sources (i.e., southern alteration domain, eastern dykes and the McDonald stock). Restoration of Christmas samples to pre?Tertiary faulting locations reduced the variance in the predicted heat sources of both chlorite (R² up to 0.95) and epidote (R² up to 0.55) heat regression models. These models infer that an estimated displacement of 1,000 to 1,167 m along the Joker and Christmas faults best fits mineral chemistry heat regression modelling. Adjusting the calculated, pre-faulting sample locations back to present?day locations while retaining their modelled heat sources, moved the predicted heat and fluid source northeast of the Christmas stock and open pit, mapped potassic alteration and buried bulk-tonnage Cu mineralisation. <br>Calcite is present in the propylitic-altered volcanic rocks, and also in the Paleozoic carbonates around Christmas, occurring in veins and as replacements. Calcite UV fluorescence corresponds very well to calcite Mn content and highlights three distinct groups: (1) low Mn calcite (<10 ppm) veins that fluoresced green and white; (2) moderate Mn calcite (10 – 100 ppm) veins that fluoresced green and white; and (3) high Mn calcite (>200 ppm) that fluoresced red/pink and blue. This red/pink and blue fluorescence also correlates to high REE and low Sr values in calcite that was found proximal to Christmas and also in the southern alteration domain. The highest δ¹³C and δ¹⁸O values (closest to that of ‘unaltered limestone’) were produced by calcite from smaller, less-productive hydrothermal centres in the Chilito fault block and the phyllic-altered dykes. Christmas ore-proximal calcite data fall within theoretical δ¹³C and δ¹⁸O fields for skarn calcite, suggesting increased magmatic water-rock interaction. Calcite veins in the southern alteration domain have some of the lowest δ¹⁸O values (<10‰; indicative of mineralised rocks or increased meteoric water-rock interaction. Isotopic calcite data in the Christmas dataset do not distinguish alteration from highly productive and weakly productive systems as effectively as the trace element chemistry. However, it successfully differentiates calcite in the southern alteration domain from calcite near Chilito and late phyllic dykes. A U-Pb calcite age (72.9 ± 1.7 Ma) from the southern alteration domain is 10 My older than magmatic and alteration ages generated around, and in, the Christmas porphyry deposit. Nearby ~73-68 Ma intrusions and associated vein-hosted, greisen and porphyry mineralisation support the potential of an older hydrothermal event in the southern alteration domain. <br>The integration of bulk-rock and mineral chemistry, U-Pb dating, and isotopic analyses has clearly discriminated propylitic and skarn alteration of productive and that of less-productive systems in the study area. The development and testing of mineral chemistry vectoring, screening and heat regression modelling at Christmas have created new, and refined pre-existing, exploration methods in green and white rocks that could aid exploration in the porphyry province in the southwestern USA. </p>