Structural Control of Oxidation Anomalies Above Buried Mineral Bodies

Oxidation anomalies appear to form as a consequence of subtle
oxidation of reduced bodies in the subsurface. An oxidation-reduction
cell develops between a reduced body and the nearest source of
oxygen, the atmosphere. In most cases, a reduced pole propagates to
the surface, forming a Òreduced chimneyÓ above a reduced body. The
oxidizing poles of this cell form around the sides of the reduced body
and this chimney, producing conditions that are amenable to formation
of volatile halides of a large number of trace elements.
Commonly, the surficial geochemical anomaly formed by one of
these cells takes the form of an asymmetrical halo, which is represented
by highs for several elements included in the Òoxidation suiteÓ
Ñ Cl, Br, I, As, Sb, Mo, W, Re, Se, Te, V, U, and Th. Other elements
also may migrate into halos as volatile halides, including Au, Hg, the
rare earth elements, and high-field strength elements such as Zr, Nb,
Hf, Ta, and Ti. In strongly developed oxidation cells, base metals and
lithophile elements also migrate into these asymmetrical halos. At the
center of the halo is a Òcentral lowÓ Ñ a profound low for most of
the anomalous trace elements in the halo Ñ which is the surficial
footprint of the reduced chimney in the subsurface. Abundant evidence
is present from a large number of both research and exploration
soil grids that fracture patterns in bedrock, alluvium, barren
cover, or cap rocks play a major role in facilitating the migration of
trace elements into these Òoxidation haloÓ anomalies.
Locally, a structural feature in the subsurface may dominate the
morphology of an oxidation anomaly to such a degree that it masks
or eliminates the halo pattern. A number of cases have been
observed where the signature of a fault crosses the central low of an
oxidation anomaly with little interruption. In these cases, the halo
pattern is nonetheless still apparent. In a few other cases, high contrast
oxidation suite anomalies follow linear features in the subsurface.
At the Meikle Au deposit a large post-ore fault zone provides
abundant passageways for migration of trace elements to the surface.
Consequently, a high contrast anomaly is present for several
oxidation-suite elements directly over the fault, but within the
bounds of the orebody. A much lower contrast oxidation halo brackets
the flanks of the Meikle ore deposit. A more dramatic example is
the Equator prospect in the western United States, where all oxidation
suite and high-field strength elements define a large mineralized
structure in the subsurface. Within the scale of the sampling grid, the
anomaly pattern for this apparently mineralized fault results from a
number of high contrast apical anomalies strung out along the trace
of the structure. A few points along the trace of the structure are
defined by small central lows that are discernible among data for
iodine as well as that of several other oxidation-suite and high-field
strength elements, such as Zr.