Gold in Porphyry Deposits:
Porphyry Deposits are defined as large tonnage mineral deposits with, variously, valuable amounts of copper, molybdenum, gold and silver and combinations of one or more of those metals. Related metals may include iron (magnetite), titanium, rhenium and zinc. For discussion purposes deposits are further defined as alkalic and calc-alkalic, each of which has distinctly different characteristics; a few deposits have aspects of both.
Alkalic porphyries usually have copper and gold and little or no molybdenum; associated rock types are silica-poor and dioritic to gabbroic in composition and are rich in potassic minerals, biotite, potassic feldspar, chlorite and actinolite. Gold content is usually sufficiently high to be a substantial factor in determining the economic viability of a deposit. Deposits are usually small to medium size, from 50 to 200 million tonnes with <1 g/tonne gold and from 0.5% to 2% copper. Examples from British Columbia include Copper Mountain, Red Chris and Mt Polley deposits.
Calc-alkalic porphyry deposits are distinguished by copper and molybdenum mineralization hosted by siliceous rocks: granite, quartz monzonite and monzonites. Typical metal values are in the ranges of 0.3% to 1.5% copper, 0.02% to 0.6% molybdenum and 0.08 to 0.15 g/tonne gold. Deposits vary in size from large (150 M tonnes) to very large (1.0 to >3.5 MM tonnes). Examples from British Columbia include Gibraltar, Huckleberry and Valley Copper. The super-giant Bingham, Utah, deposit qualifies as an intermediate type with copper, gold and molybdenum.
The Hat deposit currently being explored by Doubleview Capital Corp. was identified by geophysical and geochemical surveys, followed by drilling. The deposit, on the basis of its alteration and mineralization is classified as an alkalic gold-copper deposit. Work to date has partially outlined the Lisle Zone, potentially a multi-hundred million tonne deposit with, speculatively, 0.3 to 0.6 g/tonne gold and 0.40% to 0.55% copper. Silver content is a small but likely additive component.
Hat Project drill holes have consistently returned long intercepts of alkalic porphyry-type mineralization, including drill hole H-22: * m with 0.* g/t gold and 0.*% copper; and drill hole H-23, with * m of *g/t gold and 0.*% copper. Drilling to date totals more than 10,000 metres in 30 drill holes.
Mineralization is hosted in andesitic volcaniclastic formations and gabbroic to dioritic intrusive rocks, with an apparent concentration of values proximal to the contact between the types. The genetic model implies that a gabbroic intrusion was emplaced into a volcanic pile with similar chemical composition. Emplacement was both forceful, driven by upward buoyancy of the gabbro, and passive, by assimilation of bounding materials. Effusions from the intrusion penetrated and altered the volcanic rocks and contributed gold, copper and iron from the parent melt.
Further work, including drilling, detailed geologic mapping, and induced polarization geophysical surveys, will continue to explore the Lisle Zone that remains unlimited in all directions. As presently defined, the Zone has dimensions in excess of * metres in the east-west direction, * metres, north-south, and * metres in thickness. Several nearby technically-promising areas remain to be tested. The interpretation is that the drill holes have cut the outer alteration shell of a large hydrothermal system. Additional drilling will determine if that system has a core of more intensely altered and mineralized material.