Hypozonal lode gold deposits: A genetic concept based on a review of the New Consort, Renco, Hutti, Hira Buddini, Navachab, Nevoria and The Granites deposits

• Hypozonal orogenic gold deposits formed at approx. 500–700 °C and 2–7 kbar.
• Hypozonal alteration assemblages replace peak metamorphic minerals and form in syn- to post-peak metamorphic shear zones.
• Gold mineralization forms an integral part of the terrane evolution during uplift.
• Hypozonal orogenic gold systems appear to be a Precambrian orogenic feature.

The mineral system of hypozonal lode gold deposits has been a matter of scientific discussion in recent years, mainly centring around two models: (1) syn- to post-peak metamorphic hydrothermal mineralization (hypozonal orogenic gold) and (2) pre-peak metamorphic mesothermal mineralization and subsequent metamorphic overprint.Here we review the evidence for and against the existence of hydrothermal gold mineralization in high-temperature metamorphic terranes. The New Consort, Renco, Hutti, Hira Buddini, Navachab, Nevoria and The Granite deposits are considered to represent hypozonal orogenic gold deposits based on petrographic and field observations. Gold mineralization is hosted in syn- to post-peak metamorphic shear zones. Ore and hydrothermal alteration assemblages replace the metamorphic mineral assemblages of the host rocks and follow the same retrograde PT path. This indicates that the mineralization forms an integral part of the terrane evolution from syn- to post-peak metamorphism to final exhumation.Comparison with other hypozonal deposits indicates that the hypozonal orogenic gold system is active at ca. 500–700 °C and 2–7 kbar, and forms a Ca–Se–Cu–Ni–Co–S-dominated enrichment compared to the mesozonal counterparts. The hypozonal deposits appear to be restricted to Precambrian terranes in variable host rocks ranging from amphibolite and banded-iron formation to marble and granite. The fluid source for hypozonal gold is metamorphic, magmatic or a combination of both. A complex tectonometamorphic evolution of the high-grade metamorphic host terranes is regarded critical for forming the deposits by: (1) generating permeability in shear zones; (2) creating PT gradients that promote fluid migration; (3) inverting the metamorphic gradient by thrusting or extensional shearing; and (4) initiating crustal anatexis and granite emplacement as source of heat and ore fluid. Hypozonal orogenic gold deposits form in syn- to post-peak metamorphic shear zones of high-grade terranes in the centre or foreland of Precambrian accretionary and collisional orogens during the evolved collision stage, when active subduction ceased or moved outboard and the terranes are uplifted.

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