BADDELEYITE GEOCHRONOLOGY; PROVING ALLOCHTHONOUS STRATIGRAPHY AT THE TURQUOISE RIDGE GOLD DISTRICT, NORTHERN NEVADA USA

Nevada Gold Mines’ Turquoise Ridge District (TRD) hosts large Carlin-Type Gold Deposits in Northern Nevada. TRD lies on the east flank of the Osgood Range near two Miocene rift margins. TRD stratigraphy was previously described as para-autochthonous and distal to the Eastern Nevada Paleozoic Carbonate Sequence (ENPCS). Detailed mapping, downhole structural data, 4-acid geochemistry, and baddeleyite geochronology indicate the TRD has two lithotectonic domains in a structurally-younging order, 1) Lower Duplex Domain (LDD) comprises primarily ductiley deformed Cambrian Preble and Lower Comus formations, terrigenous mudstones and slope limestones with OIBs overlain by the thick MORB North Pillow Basalt (NPB); 2) Upper Slivers Domain (USD) comprises a structurally-younging order of isoclinal folded mylonitic mudstone, Middle Comus platform limestones and OIBs, Upper Comus mudstones and OIB-affinity tuffaceous sediments over a carbonate reef sequence. New ²⁰⁶Pb/²³⁸U baddeleyite (SIMS) ages of structurally lowest to highest mafic rocks are as follow: 451±38Ma NPB (LDD), 510±55Ma OIB-gabbro (USD), and a newly recognized syenite body (USD) at 514±65Ma. This further confirms the out-of-sequence, allochthonous nature of the tectono-stratigraphy which was later thrusted during the Antler and Sonoman orogenies.

The differences between TRD and ENPCS stratigraphy are, 1) TRD rocks exhibit pervasive ductile strain, duplexing, and thrusted older on younger stratigraphy suggesting large transport distances; 2) There is no stratigraphic equivalent in the ENPCS of the Upper Comus tuffaceous rocks which ceased carbonate production in the TRD; 3) OIB, MORB and syenite bodies within the TRD indicate a more dynamic tectonic setting than a passive rift margin which defines the ENPCS. Collective observations within the TRD are incompatible with a para-autochthonous interpretation. The allochthonous nature of the TRD is related to early Antler orogenic modes of accretion west of the ENPCS.