This study was undertaken to determine the source of iron in Comus Formation sedimentary rocks that were sulfidized during deposition of gold in the Megapit area of the Twin Creeks Carlin-type deposit. Sedimentary rocks in and near the Megapit contain ferroan dolomite, largely as overgrowths on iron-poor dolomite. Iron to form these overgrowths appears to have been released from mafic volcanic rocks that are interlayered with the sedimentary rocks. These igneous rocks have undergone two stages of hydrothermal alteration. The first stage involved formation of albite and iron-rich chlorite, possibly caused by interaction with seawater. The second stage involved destruction of the iron-rich chlorite by illite or sericite, which released iron to form ferroan dolomite in the sedimentary rocks. Comparisons show that transfer of iron from the igneous rocks to the sedimentary rocks can account for the present distributions of iron in these rocks. Relative to basalts, Comus Formation igneous rocks are enriched in iron and potassium. These results suggest that ferroan dolomite in sedimentary rocks is not solely a product of diagenetic processes and can form when iron is released from adjacent iron-bearing igneous rocks. Recognition of this additional mechanism for formation of ferroan dolomite expands the range of geologic settings that can be favorable for formation of gold deposits formed by sulfidation.

We report here an investigation of the distribution of Au, As, Sb, Hg, carbonates, K-Al silicates, and pyrite in the Twin Creeks Carlin-type gold deposit. The main objective of the study was to determine the nature and degree of correlation among these variables and use them to identify the process(es) that deposited gold. The study focused on deposit-scale variations in these parameters and was based, in part, on data from two large geochemical databases that were prepared by mine staff.

Country rocks at Twin Creeks include Ordovician-age interlayered calcareous shales and mafic igneous rocks, the overlying Leviathan allochthon, and the Pennsylvanian-Permian Etchart Formation that was deposited unconformably over these rocks. Most gold values are found in calcareous shales in the Ordovician sequence and in limestones in the Etchart Formation, although not all layers contain the same amount of gold. Strongest gold mineralization is not adjacent to faults but its general form and distribution suggest that gold-bearing solutions gained access to favorable layers along the faults. In the Ordovician sequence, gold values are highest in shales that have undergone maximum dissolution of carbonate minerals. Petrographic study shows that some gold is associated with adularia, but deposit-scale comparisons do not show a consistent relation between K/Al ratios and gold values. The distribution of antimony is similar to that of gold, whereas mercury is more concentrated than gold, and arsenic is more widely dispersed than gold.

The relation between gold, iron, and sulfide sulfur values shows that mineralization is concentrated in rocks that have gained sulfur, but not iron, to form gold-bearing arsenian pyrite. Thus, these rocks have undergone sulfidation rather than pyritization. The iron that underwent sulfidation came largely from preore, diagenetic(P) ferroan dolomite and was released into solution by decarbonation, a common form of alteration associated with Carlin-type deposits. The results of this study suggest that wall-rock iron content and decarbonation processes which liberate this iron are the most important factors controlling formation of Carlin-type gold deposits. New deposits should be sought where stratigraphic units containing abundant ferroan dolomite are cut by favorable structures.