With the following example

task sourcesZip(type:Zip) {
from sourceSets.main.java
classifier='sources'
}

artifacts{
archives sourcesZip
}

The generated ivy.xml is
<ivy-module version="1.0">
...
<publications>
<artifact name="test" type="jar" ext="jar" conf="archives"/>
<artifact name="test" type="zip" ext="zip" conf="archives" classifier="sources"/>
</publications>
...
</ivy>

The classifier element is not a valid attribute

--> It's missing the Maven namespace
<ivy-module version="2.0" xmlns:m="http://ant.apache.org/ivy/maven">

<artifact name="test" type="zip" ext="zip" conf="archives" m:classifier="sources"/>