The Bingham Canyon Porphyry Cu-Mo-Au Deposit. II. Vein Geometry and Ore Shell Formation by Pressure-Driven Rock Extension

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.