Genesis Of Skarn In Hanxing Iron Ore-concentrating Area And Its Metallogenic Indication Significance

The Hanxing area is located in the central part of the North China Craton,in the middle-south section of the Mesozoic Intraplate orogenic belt in the Taihang Mountains.The skarn in the Hanxing area is not only developed between the rock body and the carbonate rock,but also a considerable part is produced in the form of agglomerates in the iron ore body,and some skarn is produced in the form of veins.The boundary between the ore body and the surrounding rock is generally straight,and no skarn is seen.It is speculated that most of the skarns in the study area are heterogeneous skarns formed in the deep magma chamber.There are two main types of skarn in the study area: one is distributed around the Fushan complex which intrusion is diorite,There are mainly garnet skarn,diopside skarn,and epidote skarn and other calcium skarn;the other is distributed around the Wuan complex which intrusion is diorite-monzonite.There are fewer garnet skarns,mainly diopside skarn,phlogopite skarn,tremolite skarn and other magnesium skarn.There are many types of garnets in the study area,including andradite,grossular and high-titanium garnet-melanite.Andradite is produced from ore body skarn,formed in an alkaline,oxidized-weakly oxidized environment.It is relatively rich in iron and has an obvious ring zone.In the formation environment,the andradite have low ?REE concentrations with strong positive Eu anomalies in the core and edges.The difference between light and heavy rare earth and the Eu positive anomalies are more obvious in the edges,Trace elements are mainly enriched in U and depleted in Ba,Nb,and Sr.Grossular is produced in the surrounding rock skarn,formed under a slightly acidic,weakly reducing environment,and is relatively rich in calcium and aluminum.Hightitanium garnet-melanite is produced in vein skarn and crystallizes directly from the deep fluid at high temperature,high pressure,and high titanium,and forms in a rapidly rising environment with gradually decreasing P,T,and Ti.The REE patterns shows that the ?REE is very high in the formation environment,and the fractionation of light and heavy rare earth is not obvious.Eu anomaly is not obvious.Pyroxene can be seen in two types of diopside and hedenbergite in the study area.Diopside is mostly present in skarn,and the hedenbergite coexists with well-crystallized melanite,which is relatively rare.Melanite crystallizes directly from the fluid under the environment of deep high temperature,high pressure and high titanium,and rises rapidly with the ore-bearing fluid.The invading ore-bearing fluid replaces the garnet formed in the early stage,forming a garnet circle structure.A large amount of magnetite developed on the edge is formed later than the garnet,and the iron is derived from the ore-bearing fluid.The continuous addition of a large amount of volatiles in the shallow part forms a lowdensity "ore-containing solution-fluid",which rises to the shallow part to cut the surrounding rock and form a magnetite vein with sharp contact limits.From this,it can be seen that the Hanxing iron ore deposit was formed later than skarn,and a large amount of ore-bearing fluids participated,mainly because the "ore-bearing solutionfluid" penetrated into the mineralization along magmatic channels or weak structural belts.