Host rocks, veins and alteration in reduced intrusion-related gold (RIRG) deposits display similar features and distribution across the Tombstone gold belt (TGB). The primary intrusive rocks hosting gold at these deposits are felsic to intermediate, silica-saturated, alkalic to calcic, metaluminous to weakly peraluminous magnesian (calc-alkaline) granodiorite, quartz-monzonite, monzonite and syenite. Preliminary field and petrographic observations suggest that distribution of veins, vein textures, and their correlative vein selvedge alteration assemblages show similar distribution patterns to porphyry deposit models on a more spatially restricted but observable (drill core) scale. At each of the deposits examined on four properties, sulphide-poor (3–5% sulphide minerals) quartz veins were commonly observed proximal to the causative intrusion, whereas sulphide-rich (>10% sulphide minerals) veins tend to be more distal, though are commonly observed using the same fluid pathways as earlier veins. Early sulphide-poor quartz veins commonly exhibit potassic and locally sodic alteration in vein selvedges, whereas sulphide-rich veins typically coincide with phyllic (sericitic) alteration. Sinuous veins with diffuse boundaries suggest emplacement within a higher temperature ductile regime closer to the centre of the mineralizing intrusion, whereas sharp, straight-sided quartz sulphide-rich veins are more typical and emplaced within a moderate temperature brittle regime in the intrusion carapace and hornfelsed country rocks.
