GOLD MINERALIZATION IN THE RAIN-RAILROAD DISTRICT, SOUTHERN CARLIN TREND, NEVADA: VARIATIONS IN DISSEMINATED SEDIMENTARY ROCK-HOSTED GOLD DEPOSIT STYLES

A curious feature of the Carlin trend that sets it apart from the other major gold belts of northern Nevada is that a large part cuts obliquely across the leading edge of the Roberts Mountain allochthon (RMA). This thrust front coincidentally subdivides the trend into two parts, the world famous northern Carlin trend (NCT), which formed in host rocks lying beneath the siliciclastic RMA, and a more loosely defined southern Carlin trend (SCT), which possibly formed in host rocks at more shallow, even near surface, levels beneath an Eocene basin environment.

Both segments have a similar density of gold deposits, although there are important differences in the style, size, and grade of sedimentary rock-hosted Au deposits between the two trends. Deposits of the NCT only host classic Au-only Carlin type deposits (CTDs), whereas the deposits of the SCT contain both classic CTDs as well as atypical “Carlinesque” Au-Ag deposits. These latter deposits tend to be smaller and lower grade, have higher Ag:Au ratios, possess different ore textures, and are hosted in rocks younger in age and structurally higher than seen in the classic CTDs to the north.

Cross cutting relationships and U/Pb zircon dating shows that Carlinesque deposits of the SCT formed after ~44-40 Ma clastic sedimentary rocks and, in other cases, after 39-37 Ma silicic to intermediate dikes. These age constraints are consistent with the 42-36 Ma age of gold mineralization for classic CTDs of the NCT. The comparable timing between both styles of disseminated deposits as well as their similarity in ore geochemistry (As, Tl, Sb, Ba), strongly suggest that they are genetically linked to the same type of hydrothermal system.

Where classic CTDs and Carlinesque gold deposits may differ, is in their depth of emplacement. Detrital zircon dating of mineralized Eocene sedimentary rocks suggest that the Carlinesque deposits formed near Eocene paleosurfaces present throughout the SCT. This is shallower than the 1-2 km depths proposed for classic CTDs of the NCT, which entirely lacks Eocene surficial rocks. Therefore, the size, precious metal ratios, and textural differences observed between the two types of disseminated deposits may simply reflect an alteration and geochemical zonation associated with varying depths of emplacement.