Gold-bearing breccias of the Rain mine, Carlin trend, Nevada

The Rain mine includes two mined-out open pits that contained 36.4 t (1.17 Moz) gold, averaging 1.8 g/t gold, and underground reserves, including unde rground production, estimated at 4.9 t (157,000 oz) gold averaging 7.7 g/t. Rain orebodies are localized in a breccia complex within the hanging wall of the Rain fault and hosted within the Mississippian Webb Formation immedi ately overlying the contact with the Devonian Devils Gate Limestone. The ore host includes four texturally and genetically distinct breccia type s: (1) crackle breccia; (2) hydrothermal breccia; (3) tuffisite with accret ionary lapilli; and (4) collapse breccia. Crackle breccia forms a capping o ver multistage hydrothermal breccias that are cut by tabular- to pipe-shape d tuffisite dikes, with some containing accretionary lapilli. Pre- and syno re hydrothermal breccias formed during at least three episodes of convectiv e fluidization, followed by quartz sulfide-barite cementation. High-grade g old was deposited as a late phase along the upper portion of the hydrotherm al breccia mass and extended into the crackle breccia zone. Collapse brecci as occur along the floor of the composite breccia mass and have irregular u pper and lower contacts. The lower contact occurs on a dissolution boundary with the Devils Gate Limestone. Matrix-supported, heterolithic, hydrothermal breccias at Rain consist of se dimentary rock fragments composed of sandstone, siltstone, mudstone, limest one, and conglomerate. Some fragments contain as much as 8 percent introduc ed biotite in veinlets and/or fragment matrix replacements. The veinlets co nsist of euhedral quartz, biotite, sphalerite, and pyrite. Barite constitut es as much as 60 percent of the hydrothermal breccias in the form of fragme nts and as a cement to the breccias. Quartz replacement of fragments and as a breccia cement is pervasive. The total sulfide content in unoxidized ore s is less than 5 volume percent. The Rain orebody resulted from five interpreted stages of development: (1) structural preparation along the right-lateral oblique Rain fault system an d conjugate left-lateral oblique northeast-striking faults; (2) multiple ep isodes of hydrothermal breccia formation, with high-grade gold deposition i mmediately following the last brecciation event; (3) late channelized and f luidized rock fragments and fine clays forming tuffisite bodies with accret ionary lapilli; (4) postmineral extensional reactivation of structures; and (5) collapse brecciation resulting from postore supergene acidic fluid pen ding on and dissolving the upper Devils Gate Limestone. The age of the Rain orebody is poorly constrained. Et is older than 22 Ma supergene alunite, b ut no maximum age constraints other than the Mississippian host rock are kn own.