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The Woodjam district is a cluster of porphyry Cu-Au deposits of Early Jurassic age (~196 Ma) and is located in the Quesnel terrane in central British Columbia. Porphyry centers include the Southeast zone Cu-Mo porphyry, the Deerhom and Megabuck Au-Cu porphyries, and the Takorn and Three Firs Cu-Au porphyries. The Takomkane batholith, which intruded strata of the Nicola Group and is host to the Southeast zone, has characteristics of a calc-alkalic Cu-Mo porphyry. The Deerhorn, Megabuck, and Takom deposits are centered on narrow monzonite bodies with pencil-like geometries that intruded the Nicola Group volcanic sequence. These small volume intrusions have characteristics of high K calc-alkalic intrusions. The Southeast zone, Deerhorn, Megabuck, and Takom deposits have similar ages and their intrusive units can be divided into two groups with distinct geochemical characteristics. Differences between alteration and mineralization of the deposits are attributed to the magmatic evolution of the system as well as differences in the depth of emplacement and preservation. Based on stratigraphic relationships, the Deerhorn and Megabuck deposits are interpreted to be emplaced at the shallowest structural level in the district and have the highest Au/Cu ratios and the lowest temperature alteration assemblages. The Southeast zone is hosted by the felsic units in the Takomkane batholith and represents the deepest parts of the hydrothermal system. Cu-Au mineralization in the Takom deposit shows characteristics intermediate between the Southeast zone and the Deerhorn deposit. Based on stratigraphic interpretations the difference in depth of emplacement between the Southeast zone and Megabuck is about 1,400 m, with Takom being emplaced about 400 m above the Southeast zone. The Woodjam district illustrates the variety of styles of porphyry mineralization that can form over a restricted time interval (<1 m.y.) within a single district. [ABSTRACT FROM AUTHOR]

Porphyry-style Cu (Au, Mo) mineralization is widely associated with igneous suites with Sr/Y > 40 and low heavy rare earth element (HREE) content. This geochemical signature is commonly attributed to garnet or amphibole fractionation in the lower crust where mantle derived melts evolve into intermediate composition arc suites. However, the presence of a residuum related to mineralized porphyry systems where amphibole and garnet are stable is generally inferred from geochemistry and has nowhere been documented directly. We here report on two occurrences of late Miocene garnet-bearing granodiorite porphyry at the Tesorito Au-Cu prospect in the Quinchia district and El Poma Au-Cu prospect in the Middle Cauca Au-Cu porphyry belt in Colombia. These rocks provide proof that garnet was stable in the lower crust at the time of porphyry mineralization. At both studied sites, gamet-bearing I-type granodiorite porphyries were intruded by intramineralization garnet-free plagioclase phyric granodiorite porphyries that were cut by quartz and K-feldspar veins and affected by biotite alteration. Garnet-bearing porphyries predate intramineralization porphyries by 0.4 to 2 m.y. Garnet occurs as unzoned grains, commonly included in large plagioclase phenocrysts, that in some cases also contain inclusions of igneous epidote; the latter indicating pressures of >1 GPa for early crystallized phases. Large plagioclase phenocrysts exhibit two growth stages separated by a resorption boundary. High-Al amphibole phenocrysts as well as titanite and magnetite formed after the first stage but prior to the second stage of plagioclase growth at pressures of ca. 0.9 GPa. Whole-rock geochemical compositions of garnet-bearing porphyries are consistent with only limited garnet fractionation whereas some of the intramineralization porphyries have Y <11 ppm and Sr/Y >40, which is, together with other trace element indicators, taken as evidence for significant garnet fraction at depth during porphyry mineralization. HREE content of zircon in garnet-bearing porphyries are higher than that of intra-mineralization porphyries, which supports the interpretation that garnet fractionation in the lower crust was significant during the formation of the intramineralization porphyries. The phase relationships and geochemical data reflect a geodynamic change from an environment that permitted rapid ascent of oxidized garnet-bearing porphyries prior to Au-Cu mineralization to a regime that favored melt evolution in the lower crust where garnet was a residual phase and at the same time allowed the formation of large mid- to upper-crustal magma chambers. The latter are widely inferred to be essential for porphyry mineralization to occur. [ABSTRACT FROM AUTHOR]

La Bodega and La Mascota are Plio-Pleistocene high- to intermediate-sulfidation epithermal gold deposits hosted in Proterozoic gneisses and Mesozoic granites of the Santander massif, eastern Cordillera of Colombia and are part of the California-Vetas mining district. Coeval volcanic rocks are not recognized in the district and volumetrically minor granodiorite porphyries dated at 10.1 Ma are the only igneous rocks younger than Jurassic known from the vicinity of the epithermal deposits. As of 2010, La Bodega and La Mascota contained an inferred resource of 3.47 Moz Au, 19.2 Moz Ag, and 84.4 Mlbs Cu at a 2 g/t Au cut off. Mineralization exhibits NE-trending, NW-dipping structural control associated with the right lateral strike-slip La Baja fault. Mineralization at La Bodega is composed of vein networks and tectonic-hydrothermal breccias while at La Mascota it is largely contained in tabular, fault-controlled tectonic-hydrothermal breccia bodies. Alteration and sulfide assemblages can be assigned to six distinct hydrothermal episodes which comprise early porphyry-style phases and a late epithermal-style phase with up to four distinct stages. Porphyry phases comprise stages 1 and 2. Stage 1 is characterized by propylitie alteration with epidote, chlorite, calcite, speeularite veins, minor pyrite, and chalcopyrite. Molybdenite from veins collected 0.7 km southwest of La Mascota yielded an Re-Os age of 10.14 Ma which constrains the age of stage 1. Stage 2 is related to a separate, younger magmatic-hydrothermal system and characterized by phyllic alteration (sericite-illite, quartz, pyrite) associated with quartz-pyrite veins and its age is constrained by 40Ar/3!,Ar geochronology on hydrothermal muscovite (i.e., sericite) to -3.4 Ma. The epithermal phase comprises stages 3 through 6 and is related to multiphase hydrothermal breccia development and quartz-alunite alteration. Stage 3 is defined by presence of copper sulfides such as bornite, covellite, and chalcopyrite, stage 4 by wolframite, stage 5 by enargite, and stage 6 by sphalerite. Pyrite, quartz, and alunite are common to all epithermal stages. Residual vuggy quartz is scarce. Gold-silver mineralization took place during stages 3 through 5 with some gold introduced during stage 2. Alunite 40Ar/3flAr ages constrain epithermal mineralization to between ℓ2.6 and ℓ1.6 Ma. Primary fluid inclusion assemblages representative of hydrothermal stages 4 and 5 contain liquid and vaporrich inclusions and, together with quartz vein textures, suggest that boiling was an important ore-precipitating mechanism. Homogenization temperatures of these range from -143° and 238°C and salinities from 0.5 to 5.6 wt % NaCl equiv. Pyrite δ34S ranges from -16.9 to -11.3%c at La Mascota and -8.3 to -6.1%c at La Bodega. The strongly negative sulfur isotope signature can be explained by fluid boiling or, alternatively, by biogenic sulfur source from the Proterozoic paragneisses. Based on δ18O and δD data, alunite was precipitated from magmatic fluids and only stage 6 alunite shows clear evidence for mixing of the hydrothermal fluid with meteoric water. [ABSTRACT FROM AUTHOR]

Apatite is a common resistate mineral occurring in a range of host rocks and ore-related hydrothermal alteration assemblages. Apatite in several porphyry copper deposits in British Columbia has a unique set of physical and compositional characteristics that can be used to evaluate the chemical conditions of magmas that formed the causative intrusions or associated hydrothermal alteration. Apatite under visible light and SEM shows no notable variations between unaltered and altered varieties but cathodoluminescence reveals significant differences. Apatite in unaltered rocks displays yellow, yellow-brown, and brown luminescence, whereas in K silicate-altered rocks apatite displays a characteristic green luminescence. The green-luminescent apatite replaces yellow- or brown-luminescent apatite and locally overgrows it. Apatite occurring with muscovite (i.e., phyllic)-altered rocks displays characteristic gray luminescence. The chemistry of apatite, as determined by electron microprobe and laser ICP-MS analyses, directly reflects its alteration and luminescence. The unaltered yellow-luminescent apatite has high concentrations of Mn (0.3-0.5 wt % MnO) and a high Mn/Fe ratio (>1), whereas the brown-luminescent apatite has low Mn, but higher concentrations of S and REE + Y. The green K silicate alteration-related luminescence is caused by lower Mn/Fe ratios (ca. 1) along with depletions of other trace elements such as Cl, S, and Na. Gray-luminescent apatite occurring with muscovite-altered rocks results from significant Mn loss (<0.15% MnO) contemporaneous with depletion in Na, S, Cl, and REE during low pH phyllic alteration in calc-alkalic porphyry deposits. The correlation between apatite texture, luminescence, and chemical composition with the type and intensity of porphyry alteration offers a potentially fast and effective method to utilize it as an indicator for porphyry mineralization in a range of exploration materials including soils, regoliths, and heavy mineral concentrates from glacial and fluvial materials. [ABSTRACT FROM AUTHOR]