Abstract Today's field trip examines “Carlin-style” gold deposits hosted within Permian-aged sediments of the Copperton anticline. We will visit two gold mines: the now exhausted Barneys Canyon deposit and the active Melco mine. Representative core will be available at the respective sites. The excursion will focus on the structural controls and alteration related to gold mineralization in this group of deposits. Weather permitting, a traverse will be made along the axis of the Copperton anticline from the Melco haul road south to Bingham Canyon. The traverse will pass down section through a sequence of unmineralized Permian to Pennsylvanian siliceous clastic rocks and end in the Bingham pyrite halo. This road log begins at the intersection of the new Bingham Highway with Highway U-111, approximately two miles east of the town of Copperton. The route from downtown Salt Lake City to this junction may be followed on the Day Two Road Log for the Bingham Canyon field trip. The route heads southwest toward Copperton, northwest on a private road toward the Kennecott Utah Copper concentrator, and then continues northwest to the Barneys Canyon and Melco gold mines. 0.0 Begin at the junction of the new Bingham Highway and U-111. Head southwest, toward Copperton, on the new Bingham Highway. 0.5 Dead ahead are the waste-rock piles from the Bingham Canyon mine. 1.5 Turn right, onto a private road, through the entrance gate to the Copperton concentrator. 2.6 Turn right at the fork in the road. Bingham's Copperton concentrator is to the left. The

Abstract The Melco deposit is the largest of five gold mines that constitute the 1.5 million ounces Barneys Canyon project. The deposits form an arc along the nose of the north-trending Copperton anticline. Gold mineralization formed in structurally prepared zones near Theologically contrasting formational contacts. Bedding plane slip during Mesozoic folding caused some deformation. The Melco deposit, located on the NW flank of the Copperton anticline, was localized in a structurally complex zone at the intersection of a N 55° E-trending strain corridor and N 30° E-striking high-angle faults. The latter faults provided conduits for Tertiary-age gold-bearing fluids and late stage carbon-pyrite-orpiment-realgar introduction. In cross section, gold distributions resemble an upright goblet with a stem but no base. These sediment-hosted deposits contain micron-sized gold as native metal and as inclusions within the arsenic-rich rims of zoned pyrite grains. The deposits are anomalous in As-Sb-Tl-Hg, and contain low silver and base metal values. Bleaching and decalcification are the most pronounced alteration effects. Silicification is present but subtle. The deposits lie at the outer edge of an asymmetrical pyrite-Au-As halo developed around the Bingham Canyon porphyry system. There is a growing body of evidence that these sediment-hosted gold deposits represent a distal precious metal event genetically related to late stage mineralization at Bingham Canyon, which lies 5 miles to the south.

Integrated geological and geochemical studies of the Barneys Canyon gold deposit in the Oquirrh Mountains of north-central Utah suggest that compressional tectonism and metamorphism are Jurassic in age. Detailed geologic mapping, clay mineralogy, and fluid-inclusion analyses together with Jurassic K/Ar age determinations indicate that deformation at Barneys Canyon was contemporaneous with regional Jurassic metamorphism recognized in the southern Oquirrh Mountains by Wilson and Parry (1990b). The Barneys Canyon gold deposit occurs on the crestal region of the Copperton anticline which is interpreted as a fault-bend fold. Bedding-plane gouges formed within the Barneys Canyon sedimentary sequence during flexural slip folding. Clay minerals formed in the gouges and in the Barneys Canyon gold deposit are kaolinite, illite, and some minor interstratified illite-smectite. The distribution of illite and kaolinite shows that the orebody is associated with illite alteration surrounded by a halo of more kaolinitic material. Illite crystallinity suggests that a lower-temperature (retrograde) zone is associated with the orebody. Fluid-inclusion analyses from quartz and barite show a range of homogenization temperatures from 130–400 °C with two weak modes at 225 °C and 345 °C. Kaolinite and quartz are unstable with respect to pyrophyllite at the higher temperatures. No pyrophyllite has been observed at Barneys Canyon restricting the kaolinitic alteration to the lower-temperature range. The formation temperature of illite is not constrained. The bedding-plane gouges contain illite, kaolinite (minor), quartz, carbonate, and as much as 1.5 ppm Au. The illites yielded K/Ar ages of 147 Ma and 159 Ma consistent with K/Ar ages of heavy metal bearing illite veins in the southern Oquirrh Mountains described by Wilson and Parry (1990b). This interpretation extends Jurassic deformation to north-central Utah from areas to the west where Jurassic magmatism and tectonics have previously been described.

Abstract Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast–elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same lithologic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes and a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry dikes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system.